1 Laboratory of Cell Physiology, Department of Neuroscience, Division of Physiology, Uppsala University, Biomedical Center, SE-75123 Uppsala, Sweden; and 2 Department of Neurobiology, A. I. Virtanen Institute for Molecular Sciences, University of Kuopio, BioTeknia, FIN-70210 Kuopio, Finland
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Orexin A and orexin B are hypothalamic peptides that act on their targets via two G protein-coupled receptors (OX1 and OX2 receptors). In the central nervous system, the cell bodies producing orexins are localized in a narrow region within the lateral hypothalamus and project mainly to regions involved in feeding, sleep, and autonomic functions. Via putative pre- and postsynaptic effects, orexins increase synaptic activity in these regions. In isolated neurons and cells expressing recombinant receptors orexins cause Ca2+ elevation, which is mainly dependent on influx. The activity of orexinergic cells appears to be controlled by feeding- and sleep-related signals via a variety of neurotransmitters/hormones from the brain and other tissues. Orexins and orexin receptors are also found outside the central nervous system, particularly in organs involved in feeding and energy metabolism, e.g., gastrointestinal tract, pancreas, and adrenal gland. In the present review we focus on the physiological properties of the cells that secrete or respond to orexins.
sleep; feeding; calcium; neuron; neuroendocrine regulation
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
OUR KNOWLEDGE of the orexinergic system was initiated in 1998 by two independent groups and approaches (reviewed in Refs. 26, 157). In January 1998, the group of Sutcliffe published a study (37) in which they predicted that a particular rat hypothalamic mRNA species ("clone 35"; Ref. 50) would code for a precursor peptide 130 amino acids (aa) in length, each molecule of which would generate two separate peptides of 39- and 29-aa lengths. The peptides were named hypocretins, on the basis of their hypothalamic localization and their proposed sequence similarity to the secretin family of peptides; however, only orexin B bears any significant resemblance to secretin. The precursor peptide would thus be preprohypocretin and the two final peptides hypocretin (hcrt)-1 and hcrt-2. Simultaneously with the work of the group of Sutcliffe, the group of Yanagisawa (154) isolated a hypothalamic 33-aa peptide that activated the orphan receptor HFGAN72. This receptor was found also to respond to another peptide, 28 aa in length. A second receptor was cloned on the basis of its sequence similarity to HFGAN72, and this receptor also responded to both isolated peptides with Ca2+ elevation. On the basis of the peptide sequences, Sakurai and coworkers (154) cloned the cDNA for the precursor peptide (130 aa in the rat). The peptides were named orexins, because they increased food intake in nonfasted rats when injected into the lateral ventricle. The precursor peptide thus became preproorexin, the 33- and 28-aa final peptides orexin A and orexin B, respectively, and the receptors OX1 and OX2 receptors. Both preprohypocretin and preproorexin were found preferentially in the rat brain and more specifically in the hypothalamus (37, 154). Cellular and systemic responses to the peptides were observed in the rat. Application of hcrt-2 increased postsynaptic current frequency in rat hypothalamic neurons in culture (37), and orexin A and -B increased food intake in nonfasted rats (154).
It soon became clear that the peptides isolated by the two groups were, in principle, identical. However, there was an error in the peptide sequences deduced by de Lecea and coworkers (37), so that most authors refer to the accepted sequences by Sakurai et al. (154) and not to de Lecea et al. (37) even when they are using the hypocretin nomenclature. Therefore, the current praxis makes preproorexin = preprohypocretin, orexin A = hcrt-1, orexin B = hcrt-2, OX1 receptor = hcrtr-1, and OX2 receptor = hcrtr-2. There is an ongoing argument concerning which nomenclature should be used. To avoid confusion with the sequences we use the orexin nomenclature throughout this review.
Today there is a wealth of information concerning the anatomic architecture of the orexinergic system as well as the systemic effects of orexins. The most prominent and well-demonstrated effect of orexins is undoubtedly regulation of sleep/wakefulness (reviewed in Refs. 74, 172, 190). Although many other responses have been shown, their physiological significance is less clear. Effects on feeding behavior have been demonstrated in a multitude of studies, but the results are somewhat contradictory. The current view is that orexins may rather regulate short-term appetite, energy metabolism, and feeding-associated processes (e.g., gastrointestinal functions, mastication). Orexins also affect hormonal secretion, the most well-demonstrated effects being those on the hormones involved in the stress response and energy metabolism such as glucocorticoids and norepinephrine. At the same time, information concerning the regulatory mechanisms of orexinergic systems and their targets on a cellular level is lagging behind. In this review, we try to create a comprehensive picture of orexins and their proven and putative physiological role. The main emphasis is placed on functional properties and interactions between the orexin-producing and orexin-responding cells.
![]() |
OVERVIEW OF OREXINS AND OREXIN RECEPTORS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Genetics and Chemistry of Orexins
Mammalian (human, pig, dog, rat, mouse) preproorexin is composed of 130-131 aa (37, 41, 73, 154, 155). The human preproorexin gene (chromosome 17) has been thoroughly characterized (155). It is composed of two exons, the latter of which includes the whole of the coding region of the final peptides. The 3.2-kb promoter region of the preproorexin gene seems to be enough to direct expression of the downstream gene to the lateral hypothalamus in the mouse (155). This has been utilized to cause selective depletion of orexinergic neurons in transgenic mice expressing preproorexin-ataxin-3 fusion protein (60). In these mice, orexinergic neurons are progressively lost postnatally. For transcription, the most essential part has been suggested to be the 450 bp most proximal to the gene (185).Cleavage of one molecule of preproorexin and further modification leads
to production of one molecule each of orexin A and orexin B; however,
HPLC detection of orexins in the human brain suggests two- to fivefold
higher levels of orexin B than orexin A in several areas of the central
nervous system (CNS) (34, 35, 125, 126). Mammalian orexin
A in rat is a 33-aa peptide with two intrachain disulfide bridges, and
orexin B is a 28-aa linear peptide (Fig.
1A). There is a substantial
sequence identity in the COOH termini of these peptides. In addition,
orexin B and secretin contain an identical stretch of seven amino
acids. Orexin A and -B are likely to be COOH-terminally amidated, and
the NH2-terminal glutamine residue of orexin A may be
modified to a pyroglutamoyl residue (154); however, the
posttranslational modifications have been experimentally verified only
in the rat. Orexin A in human, pig, dog, rat, and mouse are
identical (Fig. 1B), whereas orexin B in pig and dog
differs by one amino acid and orexin B in rat and mouse by two amino
acids from human orexin B (Fig. 1C). The three-dimensional structure of orexin A is not known, but orexin B has
been determined to consist of two -helices at a 60-80° angle
to each other (100).
|
Orexin A seems to have much higher stability than orexin B in the
physiological milieu (89), which may explain why orexin A
is more readily detected in cerebrospinal fluid (CSF) than orexin B
(148). Orexin A also displays much higher lipid solubility than orexin B, probably making orexin Ain contrast to orexin B
blood-brain barrier permeant (89). Orexin
immunoreactivity has been found in putative endocrine release sites of
hypothalamus, neurohypophysis, and pancreas (see below) offering the
interesting possibility that orexins could also act as hormones between
the CNS and the periphery. Yet indirect evidence suggests that orexin A
in plasma originates from peripheral sources (32), and no brain penetration of intravenous orexin A has been seen in the rat
(11).
Orexins from Xenopus laevis have also been described (163). In this species, preproorexin is rather similar to that in humans (overall identity = 56%). Xenopus orexin A is 31 aa long, lacking two NH2-terminal amino acids seen in human orexin A, and it contains 6 aa substitutions compared with human orexin A (Fig. 1B). Xenopus orexin B is 28 aa long, and it contains 7 aa substitutions compared with human orexin B (Fig. 1C). Both Xenopus orexins are supposed to be posttranslationally modified in the same way as the rat orexins (disulfide bridges and COOH-terminal amidation) except that Xenopus orexin A does not contain the NH2-terminal pyroglutamoyl residue.
Orexin Release
Orexin A and preproorexin mRNA concentrations show circadian variation in rats in a 12:12-h light-dark cycle (47, 176, 196). Preproorexin mRNA and orexin A concentration in the hypothalamus reach a maximum around the light onset or somewhat later and a minimum at the light offset or somewhat later. Measurements of orexin A in the pons and in the intracisternal space also show diurnal variation, but the time points are shifted compared with the hypothalamus (47, 176). In contrast, Mondal et al. (126) did not find any circadian variation in hypothalamic orexin A or -B concentration; the time points (5 h after dark and 5 h after light onset) may have been unfortunately chosen (in the light of results in Ref. 176). There appear to be bilateral connections between suprachiasmatic nucleus and orexinergic neurons in the rat posterior hypothalamus (1, 36, 64, 121, 125), which may indicate reciprocal regulation of the circadian rhythms.Orexin Receptors
Two receptors responding to orexin stimulation have been cloned. These receptors, called OX1 and OX2 receptors, form a subclass of their own under the class A G protein-coupled receptors (class A
|
![]() |
CELLULAR RESPONSES TO OREXINS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the original studies on orexin receptors two cellular responses were identified: recombinantly expressed orexin receptor strongly elevated Ca2+ in Chinese hamster ovary (CHO)-K1 cells (154), whereas in the hypothalamic neurons increased action potential frequency was observed (37). Many systemic responses were described thereafter but the molecular mechanisms of these have not in most cases been investigated in detail.
Ca2+ Elevation
Both OX1 and OX2 receptors elevate Ca2+ when recombinantly expressed in CHO cells (67, 107, 138, 154, 168). Because orexin receptors caused Ca2+ elevations of high magnitude in this cell line devoid of voltage-gated Ca2+ channels, they were thought to couple to the phospholipase C
|
What then is the general role of Ca2+ influx in orexin receptor-mediated cellular responses? In neuronal cells, Ca2+ influx has been observed by fura 2 measurements (181, 182-184) and is also suggested by electrophysiological measurements (44, 70). Because orexins may depolarize neurons via block of K+ channels (see Depolarizing actions), some Ca2+ influx may even result from an indirect activation of voltage-gated Ca2+ channels. The situation may be more complex at other sites as discussed below.
In CHO cells, Smart et al. (168) and Lund et al.
(107) discovered that removal of extracellular
Ca2+ caused a significant (8- to 100-fold) drop in the
potency of orexin A for the OX1 receptor (Fig.
4A). In other respects, the results of Smart et al. (168) suggest qualities expected
from Gq-dependent signaling, that is, sensitivity to the
phospholipase C inhibitor U-73122, rapid and transient spike, and
extracellular Ca2+-dependent secondary phase (capacitative
Ca2+ entry). In contrast, a more detailed analysis
of Ca2+ signaling in CHO cells indicated that the
dependence of the Ca2+ response on extracellular
Ca2+ is due to the activation of a Ca2+ influx
pathway (107). The Ca2+ response was abolished
when the membrane potential was held close to the reversal potential of
Ca2+ (Fig. 4B), and a Ca2+ elevation
could be observed by returning to a negative holding potential
(107). In addition, orexin A activated an influx of Mn2+ ions in conditions where no intracellular release
occurred. The primary response to OX1 receptor stimulation
in CHO cells is thus receptor-operated Ca2+ influx
(107). This Ca2+ influx amplifies
phosphatidylinositol-specific phospholipase C, so that in the absence
of Ca2+ influx, IP3 production and
Ca2+ release occur with a potency 100 times lower than in
the presence of Ca2+ influx. Ca2+ elevation
alone is not sufficient to activate phospholipase C in these cells,
suggesting that Ca2+ influx acts in concert with a G
protein-mediated mechanism. The identity of this Ca2+
influx pathway and its activation mechanism by the orexin receptors remains unresolved; however, the potency of pharmacological blockers suggests that it is a different molecular entity than the
store-operated Ca2+ channel in CHO cells (96).
It thus seems that both in neuronal and nonneuronal cells orexin
receptors activate both Ca2+ influx and the
phosphatidylinositol-specific phospholipase C pathway, a view that is
supported by a study in which orexin receptors are heterologously
expressed in a variety of different cell types (Holmqvist, Åkerman,
and Kukkonen, unpublished observation). In a recent study,
orexin A and -B equipotently increased norepinephrine release from rat
cerebrocortical slices. When extracellular Ca2+ was
removed, the maximum responses to both peptides were lowered by 50%
and the EC50 values shifted to >10-fold higher
concentrations (66). Although the response mechanisms have
not been clarified with direct Ca2+ measurements, the
overall pattern is very similar to what is seen with recombinant CHO
cells (107).
|
Functional experiments indicate that in addition to Ca2+ release and activation of store-operated Ca2+ channels, there are several types of Ca2+ influx pathways activated by G protein-coupled receptors (reviewed in Ref. 7). Although in most cases the mechanisms have not been clarified, these pathways may be activated directly by G protein subunits or second messengers. These pathways may be identical or related to the transient receptor potential (TRP) ion channel family (reviewed in Ref. 29). The TRP genes encode a family of over 20 proteins. Many of the TRP gene products form Ca2+-permeable nonselective cation channels, and some members of the TRP (super)family even have inherent protein kinase or ADP-ribose pyrophosphatase domains. TRP channels are widely expressed in mammalian cells, and they are activated after Gq-coupled receptor stimulation but the activation mechanisms are in many cases unknown. It will be interesting to see whether any of the TRP channels can be held responsible for the orexin receptor-mediated Ca2+ influx.
Electrophysiological Responses
Methodological considerations. Characterization of receptor-mediated electrical responses in neuronal preparations is performed with some common electrophysiological techniques. In many situations in which intracellular recordings are difficult, extracellular electrodes can be more practical to use and they are likely to induce much less cellular damage. On the other hand, these response measurements are usually limited to the registration of action potential frequency. Measurements with intracellular electrodes (normal or microelectrodes) can be performed with feedback amplification allowing voltage or current clamp or in a "free-running" mode, in which case voltage changes are monitored. These latter methods, most importantly, offer better possibilities for investigation of the mechanisms underlying the electrical activity than the extracellular electrodes.
Recordings from neurons are usually performed in slices or similar preparations, in which cells are synaptically coupled. The cell measured can thus be the target for upstream signals rather than responding directly to orexins themselves. The usual way of eliminating the secondary effects far upstream of the measured cell is to use tetrodotoxin (TTX). By blocking voltage-gated Na+ channels this hinders action potential propagation in most neurons. However, TTX will not, for instance, block direct effects of orexins on the presynaptic terminal most immediately upstream of the cell investigated. Therefore, TTX does not exclusively separate pre- and postsynaptic effects. Presynaptic voltage-gated Ca2+ channelsIncreased synaptic activity.
The most common electrophysiological response to orexins appears to be
increase in spontaneous or evoked action potential frequency, as
observed with extracellular or intracellular electrodes. Increased
action potential frequency has been measured in the hypothalamus [rat
tuberomamillary nucleus, magnocellular preoptic nucleus, lateral
hypothalamic glucose-sensitive, -responsive, and -indifferent neurons
(for definition, see Responses of orexinergic cells to feeding
stimuli), arcuate nucleus, and paraventricular nucleus; Refs.
8, 42, 44, 103,
147, 159, 164, 165, 195], brain stem (rat locus ceruleus , substantia nigra
pars reticulata, and dorsal motor nucleus of vagus and mouse
laterodorsal tegmental nucleus; Refs. 15, 19,
56, 70, 75, 93,
170), spinal cord (rat preganglionic sympathetic neurons
of intermediolateral cell column of thoracic and lumbar cord; Ref.
3), and peripheral neurons (guinea pig ileal S-type
submucosal neurons; Ref. 92). In contrast, orexin A
suppresses the firing rate of glucose-responsive neurons in rat
ventromedial hypothalamic nucleus, antagonizing the effect of glucose
(164). Action potentials and their enhancement with
orexins are blocked by TTX (3, 44, 54, 70, 75, 165, 170,
195) but usually not by synaptic isolation with low Ca2+-high Mg2+ (8, 42, 93, 147) or
voltage-gated Ca2+ channel block with Cd2+
(170). The latter data therefore suggest that orexins
would directly affect the postsynaptic neurons at some sites, with
reservations for the methodological shortcomings (see above). It should
also be noted that the putative presynaptic block has only been applied in a minority of studies. Interestingly, a TTX-insensitive response is
observed in rat locus ceruleus. Hypocretin 2 (orexin B) depolarizes and increases spontaneous firing frequency in locus ceruleus slices (70). In the presence of TTX these spontaneous spikes
vanish, but orexin B still causes a small depolarization (3.4 mV) and electrical discharges, although these spikes are slower than the spontaneous spikes. These spikes may be caused by Ca2+
channels or TTX-insensitive Na+ channels.
Depolarizing actions.
Depolarization or inward current has often been measured in response to
orexin application. This depolarization also results, when
investigated, without exception in increased action potential frequency, and often the depolarization has been studied in the presence of TTX to block this "disturbance" (Fig.
5, A and B). Depolarizations of a maximum magnitude of 3-10 mV have been
observedmainly with free-running electrodes
in the hypothalamus (rat
tuberomamillary nucleus, paraventricular nucleus, and lateral
hypothalamic glucose-sensitive neurons; Refs. 8,
44, 103, 159, 165),
in the brain stem (rat locus ceruleus and dorsal motor nucleus of
vagus; Refs. 56, 70, 75,
80, 170), in the spinal cord (rat
intermediolateral cell column; Ref. 3), and in the
periphery (guinea pig ileal S-type submucosal neurons; Ref.
92). Inward currents have been observed in rat locus
ceruleus (150 pA; Ref. 170), dorsal motor nucleus of vagus
(30 pA; Ref. 75), and superficial dorsal horn (35 pA; Ref.
54) and in mouse laterodorsal tegmental nucleus (20 pA;
Ref. 19).
|
Summary of orexin effects on electrical activity of neurons. Thus orexins most often increase neuronal activity via presynaptic effects (increased transmitter release via unknown mechanisms) and via postsynaptic depolarization (block of K+ channels, activation of cation channels and electrogenic transporters?). Some of the effects such as the putative K+ channel inhibition are typical for Gq-coupled receptors (see, e.g., Refs. 59, 65, 112, 187). Other mechanisms may be secondary to the Ca2+ elevation or may depend on the activation of nonselective cation channels like the TRP channels. On particular neurons, orexins may also cause inhibition via increased inhibitory transmitter release or via decreased postsynaptic effects of excitatory transmitters. It is unclear which direct or indirect roles Ca2+ transients play in electrical responses to orexins, and not much effort has been put into investigation of this to date. Independent of the method of measurement of electrical activity, the effects of orexins usually have a slow onset of action (10 s-4 min) and the recovery is even slower (see, e.g., Refs. 3, 9, 19, 44, 54, 75, 93, 103, 159, 165, 195). Most studies have been performed with slices, a preparation in which perfusion of orexins in and out may be slow. However, because rather superficial neurons are patched, there is no obvious reason why the perfusate would not reach the neurons immediately. Furthermore, similar slow onset (10- to 20-s lag, 1 min to maximum) and delayed offset (>2 min to complete decay after removal) are seen in dissociated neurons (195). Therefore, it is possible that orexins affect ion channels via, e.g., phosphorylation rather than direct G protein interaction. Some results indicate involvement of protein kinase A and -C in orexin responses (93, 181, 183). Orexin B has been reported to reduce N-methyl-D-aspartate (NMDA)-induced current amplitude by 19% and to enhance GABA-induced current amplitude by 48% in dissociated rat nucleus accumbens cells (113); this could also possibly be a phosphorylation-dependent event.
Transmitter Release
As discussed in Increased synaptic activity, orexins may increase transmitter release as indicated by increased/decreased synaptic potential frequency. These are the most direct indications of presynaptic effects of orexins, and in the presence of TTX, rather certainly suggest a presynaptic site for orexin effects. Inhibition of K+-stimulated serotonin release by orexin A and -B from rat hypothalamic synaptosomes also directly shows a presynaptic site of action (140). There are also indirect indications of orexin-stimulated transmitter release from in vitro slice (norepinephrine from rat cerebral cortex; Ref. 66) or in vivo microdialysis (histamine from rat anterior hypothalamus; Ref. 79) studies. In these cases, it is impossible to tell whether orexins directly affect the releasing neurons or some upstream neurons; some complex mechanism is suggested by the delayed (by 6 min) response in rat cerebrocortical slices.Other Responses
cAMP elevation in response to orexin stimulation was reported in the rat and human adrenal cortices, putatively via OX1 receptors (110, 117). In contrast, in primary cultures of rat hypothalamic neurons, no elevation of cAMP is seen (183). Elevation of cAMP may occur by several mechanisms depending on the isoform of adenylyl cyclase expressed. Adenylyl cyclase isoforms are differentially activated by Ca2+, protein kinase C, and G proteinOrexin receptors may also couple to protein kinase cascades involved in
cell growth, differentiation, and death. These effects are clearly seen
when orexin receptors are expressed in CHO cells (S. Ammoun, L. Korhonen, L. Lindholm, K. E. O. Åkerman, and J. P. Kukkonen, unpublished observation). One of the most immediate indications is the rapid activation of mitogen-activated protein kinase
(MAPK) pathways seen on OX1 receptor stimulation (Fig. 6).
|
Pharmacology
OX1 and OX2 receptors were already shown in the original study to have different binding affinities for orexin A and -B (154). When expressed recombinantly in CHO cells, OX1 receptor has 10 times higher affinity for orexin A than for orexin B whereas OX2 receptor binds orexin A and -B with equal affinity. The same selectivity is seen with respect to Ca2+ elevation; thus orexin A activates OX1 receptors up to 10 times more potently than orexin B (138, 154, 168). Thus the evidence for selectivity of OX1 receptor and nonselectivity of OX2 receptor between orexin A and -B is based solely on the studies on Ca2+ elevation in recombinant CHO cells. It is difficult to find exclusive expression of one subtype in any tissue, but in some cases support for this selectivity can be found: orexin A is 5.5-fold more potent in increasing electrical activity of rat locus ceruleus, where OX1 expression dominates (170). In isolated human adrenocortical cells, where OX1 mRNA is also predominantly found, orexin A but not orexin B stimulates cortisol secretion (117). In tissues where OX2 expression dominates, such as rat tuberomamillary nucleus, orexin A and -B are essentially equipotent in stimulating the electrical activity. Orexin A and -B are also equally potent in releasing catecholamines from human pheochromocytoma cells, which only express OX2 mRNA (116). On the other hand, the putatively OX2-mediated adrenocorticotropic hormone (ACTH) release is 10-fold less sensitive to orexin B than to orexin A (13, 158). It is generally dangerous to rely on so-called selective agonists, because their efficacies can be very dependent on receptor and G protein expression levels. For these pharmacological reasons and because of the chemical differences between orexin A and -B (see Genetics and Chemistry of Orexins), conclusions on receptor subtype involvement based on potency of orexin variants (42, 113, 138, 158) should probably be avoided. Some studies report higher potency of orexin B than orexin A with respect to some responses (76, 85), which is unexplainable on the basis of recombinant pharmacology. Even if any pharmacological selectivity with respect to the interaction of orexin A and -B with the orexin receptor subtypes was to occur in the tissues, its physiological significance would be unclear.Molecular determinants required from orexin peptides for binding to and activation of orexin receptors have been investigated in CHO cells. Comparison of structures and activities of orexin A and -B suggests that the NH2-terminal amino acids are less important for receptor binding and activation than the COOH-terminal amino acids (33). Truncation of orexin A to orexin A15-33 reduces its potency for both OX1 and OX2 receptors 20- to 60-fold (33, 138), and further truncation successively abolishes the Ca2+ response, at least for OX1 (33). Mutation of cystine-forming cysteines to alanines in orexin A reduces the potency of the mutant peptides for both OX1 and OX2 receptors ~10-fold (138). However, it is difficult to judge whether this effect is due to the lost disulfide bridges or the exchanged cysteines themselves. One-by-one replacement of each amino acid in orexin-A15-33 with alanine (alanine scan) has revealed regions of particular importance for orexin A activation of the OX1 receptor, in particular leucines 19 and 20 and the most COOH-terminal aa 26-33 seem to be of great importance (33). Recently, we observed (S. Ammoun, T. Holmqvist, R. Shariatmadari, R. Oonk, M. Detheux, M. Parmentier, K. E. O. Åkerman, and J. P. Kukkonen, unpublished observations) that OX1 and OX2 receptors are approximately equally sensitive to NH2-terminal truncation and alanine scan of the orexin peptides. The amino acids of importance are largely the same as those described in Ref. 33. Change/removal of these amino acids affects both OX1 and OX2 receptors, but some interesting differences are seen in the potency of the response. Interestingly, none of the mutated peptides without activity are antagonists.
Xenopus orexin B has been proposed to have 10-fold higher affinity and potency than Xenopus orexin A for human OX2 receptor expressed in CHO cells (163). Both Xenopus orexin A and -B were equipotent on human OX1 receptor. Synthetic peptides (hcrt-1 and hcrt-2) based on the originally described hypocretin sequences (37) have been shown to be ~1,000-fold less potent than the corresponding orexin peptides with respect to Ca2+ elevation in recombinant CHO cells (167).
In more than 50% of the studies on cellular responses, rather high concentrations (>100 nM) of orexins were used to evoke response. Even the EC50 values, when determined, can be rather high [e.g., 200 nM for orexin A in rat locus ceruleus (increased firing); Ref. 170]. On the other hand, even subnanomolar concentrations evoke some responses (e.g., Ca2+ response in rat ventral tegmental neurons; Ref. 130). To resolve the mechanical basis for this, experiments could be performed in recombinant expression systems at different expression levels with measurement of different responses. Another reason for the high concentrations required in some tissues may be the use of orexin B at OX1 receptor or use of the hypocretins based on the original sequences, if the weak potencies reported in recombinant systems hold true in tissues (see above).
At the moment, no reliable in vivo selectivity can be obtained with known orexin receptor agonists. Recently, synthesis of an orexin receptor subtype-selective antagonist was reported (145, 169). This antagonist, SB-334867 (Fig. 1D), displays a 100-fold higher affinity for OX1 than OX2 receptor. This selectivity has been successfully utilized in in vivo experiments to elucidate the role of OX1 receptors in some systemic effects of orexins (see SYSTEMIC EFFECTS OF OREXINS). Unfortunately, no antagonist able to block OX2 receptors is available. Conclusions on the role of OX1 receptors in some physiological processes should not be based solely on SB-334867, because orexin receptor subtypes may have overlapping and interacting roles. However, SB-334867 is a uniquely useful tool in orexin receptor investigations. From this perspective, it is unfortunate that SB-334867 is not widely available for orexin receptor researchers.
Recently, a variety of peptide transmitters, including secretin and NPY variants, were reported to inhibit orexin A binding to membrane preparations from endogenous and recombinant cells expressing OX1 receptors, even with a nanomolar affinity (87). These results were interpreted to indicate that interaction between orexinergic and NPYergic systems (see Connections of orexinergic pathways with other feeding-regulating transmitters/hormones) could occur at the orexin receptor level. These results on binding could not, however, be verified in functional experiments or in binding experiments with intact cells (67, 169). Other peptides [secretin, pituitary adenylate cyclase-activating polypeptide (PACAP), NPY variants] do not display any detectable affinity or activity on OX1 or OX2 receptors (67, 169). As suggested in Ref. 67, it is more likely that the other peptides affect some intracellular process, for instance G protein binding to the receptor, which is required for high-affinity agonist binding, and therefore reduce orexin binding in some particular preparations. Therefore, secretin should not be used to define specific orexin A binding, because this can to lead to possibly erroneous conclusions (86, 188).
![]() |
DISTRIBUTION OF OREXINERGIC CELLS AND OREXIN RECEPTORS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Orexinergic Cells in CNS
The distribution of orexinergic cells has been investigated with molecular biological and immunological methods, which give rather similar results. The main peptide investigated has been orexin A, but comparison of the distribution of orexin A, orexin B, and preproorexin suggests extensive colocalization. Therefore, all the data have been pooled. Most of the data on orexin distribution originate from the rat but, when investigated, the other vertebrate species have given similar results.Orexinergic Cell Bodies in Rat CNS
In the rat brain, orexinergic cell bodies are found solely in the hypothalamus. Within the hypothalamus these cell bodies reside bilaterally symmetrically in the lateral parts, probably most abundantly in perifornical areas (17, 24, 31, 36, 37, 56, 58, 69, 70, 131). Orexinergic cells are interspersed among melanocyte-concentrating hormone (MCH)-ergic cells (24, 143). However, the overlap of the areas is not complete (17) and MCHergic cells are clearly different from orexinergic cells (17, 43, 58, 143), although they otherwise share some of the properties of orexinergic cells (1, 69, 71). Isolated orexinergic cell bodies have also been reported in median eminence, posterior, dorsal, and dorsomedial hypothalamus, and arcuate and subincertal nuclei (24, 31, 143).Orexinergic neurons are variable in size (diameter of cell body = 15-40 µm) and shape (spherical, fusiform, multipolar) (24, 31, 36, 131), and they have been assumed to number from 1,100 to 3,400 in the whole rat brain (61, 143). Because orexinergic cells have been shown to project to other orexinergic cells within the hypothalamus (69) there might be different populations of orexinergic cells within the hypothalamus.
Preproorexin mRNA and protein have even been detected in the ependymal cell layer (97, 143), and orexin A has been found in choroid plexi (31). It would be interesting to find out whether this expression occurs in the ependymal cells or, for instance, in neuronal stem cells.
Orexinergic Projections in Rat CNS
Orexinergic fibers have been visualized mainly by using orexin A immunohistochemistry. For orexin A, both smooth and varicose fibers have been observed (31, 143) but varicose fibers appear to be the ones most often seen, although this result might also be caused by the easier detection of this kind of processes. A few studies investigating orexin B distribution report projection sites similar to those for orexin A (31, 36).Despite the low number of orexinergic cell bodies in the hypothalamus,
orexinergic fibers project widely in the CNS. The most important
orexinergic projection areas are within the hypothalamus, two thalamic
nuclei, brain stem, and the whole length of the spinal cord (Table
1; see also Fig.
7). Interested readers are recommended to consult the original papers, some of which are very detailed (24, 31, 34, 36, 37, 61, 69-71, 97, 125, 126, 154, 182).
|
|
Orexins in CNS of Other Species
Only a few studies have been conducted in species other than the rat. However, when these studies have been performed, the results seem to be in good agreement with studies in the rat. In several vertebrate species (Table 2) orexinergic cells are found in hypothalamic areas similar to those in the rat. The cells have been assumed to number ~20,000 and 50,000-83,000 in dog and human brain, respectively (148, 178). The projection areas also appear similar in the rat and other species (Table 2); however, in most studies only limited areas were investigated, making full conclusions impossible. Also in these species, orexinergic neurites are often determined to be varicose.
|
Orexin Receptors in CNS
Similarly to orexins, orexin receptor distribution has been investigated with molecular biological and immunological methods. Almost all the data originate from the rat. Altogether, orexin receptors are found at the projection sites of orexinergic neurons (see above), which suggests that orexinergic neurons contact many different transmitter systems (see below). Most of the studies have not specified whether the expression is seen on neurons or other cells, although electrophysiological and other measurements, whenever performed, indicate activation of neurons. Levels of orexin receptors can but do not necessarily fully follow the density of orexinergic innervation. Only in a few cases have synaptic contacts been anatomically shown (48, 55, 70, 195). Localization of some orexinergic cells/processes close to the ventricular surfaces (24, 31, 97, 143) suggests that orexins could act in paracrine or endocrine fashion.Similar amounts of OX1 and OX2 mRNA are found
in the rat brain as a whole, and OX1 and OX2
receptor mRNAs mostly show similar distribution, although some
differences are seen (Fig. 7). Most interesting are the areas in which
essentially only one subtype is expressed (111, 180).
However, mRNA expression may not directly correspond to receptor
protein expression, as discussed by van den Pol et al.
(184). The correlation of receptor distribution with
different functions of the orexinergic system is shown in Table
3. While this article was under review,
studies on the expression of OX1 and OX2
receptor protein in the rat CNS were published (30, 64). A
particularly interesting finding is the expression of OX2
receptor protein in the molecular and granular layers of the cerebellum
(30); earlier studies had failed to detect any orexin
receptors or their mRNA in the cerebellum although orexinergic fibers
were projecting to the cerebellum. Otherwise, the immunohistochemical
studies suggest a distribution generally similar to that from the mRNA
measurements, although some differences are seen. For instance, greater
overlap of the expression of OX1 and OX2
receptors is suggested on the basis of immunological studies than of in
situ hybridization. As discussed by the authors, these differences may
in some degree be related to the fact that mRNA measurements visualize
the expressing cell bodies whereas immunohistochemistry indicates the
actual cellular localization of the protein. Both receptor mRNA and
protein measurements are semiquantitative rather than quantitative so
they do not allow absolute comparison of subtype expression within any
site.
|
Coexpression and Connections of Orexins With Other Transmitters
Despite the partial overlap of distribution of orexinergic and MCHergic cell bodies, there is no coexpression of these peptides in the same cells (Refs. 17, 58, 143; Figs. 8 and 9). In contrast, essentially all the orexin and prodynorphin expression colocalizes in the lateral hypothalamus (10, 27). Orexinergic neurons may also coexpress other transmitters such as galanin (58) and glutamate (1).
|
|
Vasoactive intestinal peptide (VIP)-, vasopressin-, and NPY-containing neurons innervate orexinergic neurons in the hypothalamus (Refs. 1, 69, 71; Figs. 8 and 9). On the other hand, orexinergic nerves from lateral hypothalamus innervate neuronal circuits, which utilize many different transmitters, for instance, norepinephrine, dopamine, serotonin, histamine, acetylcholine, vasopressin, VIP, somatostatin, corticotropin-releasing hormone (CRF), NPY/agouti-gene-related peptide (AGRP), proopiomelanocortin (POMC), cocaine/ amphetamine-regulated transcript (CART), GABA, MCH, and glutamate (Figs. 8 and 9; for the anatomy, see, e.g., Refs. 5, 31, 36, 131, 143), and even functional responses to orexins have been shown at some of these sites (e.g., Refs. 9, 18, 19, 42, 44, 49, 56, 70, 80, 93, 113, 130, 159, 165, 170, 183, 195; see CELLULAR RESPONSES TO OREXINS). Orexinergic neurons even make contact with other orexinergic neurons within the hypothalamus (5, 69).
Connections of orexinergic pathways with other feeding-regulating
transmitters/hormones.
Orexins were first isolated as appetite-increasing peptides, although
this view was later disputed. Several hypothalamic sites involved in
the regulation of feeding, such as the lateral hypothalamic area,
parvocellular paraventricular nucleus, ventromedial nucleus, and
arcuate nucleus, are innervated by orexinergic fibers from the lateral
hypothalamic area (see e.g., Ref. 36). Both
OX1 and OX2 receptors may be expressed at these
projection areas (111). There are abundant connections
between orexin and other feeding-regulating transmitters/hormones such
as leptin, NPY, MCH, POMC, CART, galanin, and AGRP (Figs. 8 and 9).
Almost all the orexin-positive cells also express leptin receptor, and
even STAT3the leptin receptor-activated promoter element
and orexin
are coexpressed (Refs. 58, 69, Fig. 8; see also Effects of orexins on
gastrointestinal tract and other organs of feeding/energy
metabolism). On the other hand, not all the leptin
receptor-positive cells in the lateral hypothalamic areas where
orexinergic cell bodies lie are orexin positive (58). Other cells that express leptin receptors are the MCHergic neurons in
the lateral hypothalamus and the NPY-/AGRP-, POMC- and CARTergic cells
in the arcuate nucleus (Refs. 69, 57,
58, 119; Fig. 9). Orexinergic fibers project
to NPY- and POMCergic cells in the arcuate nucleus (69).
On the other hand, NPY/AGRP- and POMCergic fibers, most likely from the
arcuate nucleus, project to orexin- and MCHergic cells of the lateral
hypothalamus (17, 43, 55, 69, 71). Orexinergic cells of
the lateral hypothalamus also contain other feeding-regulating
transmitters such as dynorphin and galanin (Refs. 10,
27, 58; Fig. 9), which may enhance the
effects of orexins on feeding.
Responses of orexinergic cells to feeding stimuli. Hypoglycemia induced by various means has been shown to increase c-fos (see below), orexin, and preproorexin mRNA expression in a subpopulation of the orexinergic cells of the lateral hypothalamus (16, 20, 22, 53, 99, 105, 127; see also Effects of orexins on gastrointestinal tract and other organs of feeding/energy metabolism). Fasting also abolishes the circadian variation in CSF orexin levels (47). The lateral hypothalamic area, as well as some other hypothalamic regions, contains neurons that are activated by decreased levels of glucose ("glucose-sensitive neurons") and neurons that are activated by elevated levels of glucose ("glucose-responsive neurons") (Ref. 101; Fig. 9). In the study of Muroya et al. (129), almost half of the histochemically defined orexinergic neurons of the rat lateral hypothalamus directly responded to lowering of extracellular glucose with an increase in cytosolic Ca2+; thus some of the glucose-sensitive neurons in lateral hypothalamus may be orexinergic. Liu et al. (103) identified glucose-sensitive, -responsive, or -indifferent populations among rat hypothalamic neurons that responded to orexin stimulation with depolarization and increased firing rate. In contrast, orexin A suppressed the firing rate of glucose-responsive neurons in rat ventromedial hypothalamic nucleus (164). Orexinergic neurons may also make contact and/or be contacted by glucose-sensitive or -responsive neurons in other brain areas such as lateral hypothalamus, ventromedial hypothalamus, arcuate nucleus, and the nucleus of the solitary tract (Ref. 101; see also anatomic studies on orexinergic projections).
Leptin is thought to be one of the major negative regulators of orexin expression in the hypothalamus (see Effects of Orexins on Feeding Behavior), probably both directly via leptin receptors on orexinergic cells and via leptin receptors on other feeding-controlling cells connected to orexinergic cells (see above). Attempts to elucidate leptin-orexin interaction by measurement of orexin and orexin mRNA levels have been made in rodent model systems with defective leptin signaling, such as fa/fa Zucker rats and db/db mice (devoid of functional leptin receptors) and ob/ob mice (devoid of leptin) (reviewed in Ref. 21). The results are, however, contradictory, and if any conclusion can be drawn, it is that leptin is not necessarily required to keep orexin expression low. However, as also often in knockout systems, it is difficult to evaluate the physiological role because of possible multiple disturbances, redundancy in signaling, and compensatory mechanisms.Orexins and Orexin Receptors Outside CNS
Orexin immunoreactivity or preproorexin mRNA has been found in a variety of peripheral organs, mainly in the rat. The cells expressing orexins have in some cases been identified and include both neurons and endocrine cells. Some projections, e.g., those in the pituitary and pineal gland, may originate from the neurons in hypothalamus, but most peripheral neurons appear to have peripheral origin. Orexin receptors/receptor mRNA have been mainly found in the same organs as orexins. The cell types expressing orexin receptors might include at least endocrine, muscle, and nerve cells (84, 92, 108, 117, 133). Even in the periphery, expression of the subtypes varies in different tissues.Orexin and orexin receptor immunoreactivity has been found in the
gastrointestinal tract and pancreas (rat, guinea pig, human; Refs.
92, 133; Tables
4 and 5).
Substantial levels of both orexin A and -B and OX1 and
OX2 mRNA and protein are found in the rat pituitary
(13, 35, 84), and Jöhren et al. (84) found preproorexin and OX1 mRNA in the rat testis. Both
OX1 mRNA and OX2 mRNA or immunoreactivity are
found in the rat and human adrenal glands (12, 84, 104, 108,
109), although the subtype expression in different layers is
somewhat disputed and there may be differences between species (see,
e.g., Ref. 117). Only OX1 mRNA has been found
in the kidney and thyroid and only OX2 mRNA in the lung
(84). Orexinergic fibers extend from the CNS toward the
rat pineal gland, and OX2 mRNA is detected in the pineal gland (121).
|
|
However, the expression of preproorexin or orexin receptor message in the stomach, gut, pancreas, lung, kidney, pituitary, or adrenal gland has been disputed (31, 84, 154). Some differences may be explained by the less sensitive methods (Northern blotting) failing to detect a signal when more sensitive methods (PCR, in situ hybridization) do. In contrast, it is very difficult to explain opposite receptor subtype expression reported for human adrenal cortex (117, 146).
![]() |
SYSTEMIC EFFECTS OF OREXINS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The location of orexinergic cells, their projections, and their contacts with other transmitter systems suggest a role in eating, sleep/wakefulness, and neuroendocrine and autonomic functions. It is sometimes difficult to judge on the basis of the available experimental evidence whether a particular systemic response to orexins is caused indirectly or directly on the target organs. In some cases, direct effects of orexins on isolated organs or even isolated cells have been demonstrated; we have been trying to highlight this kind of evidence.
Effects of Orexins on Feeding Behavior
In the original publication dealing with orexins, it was shown that intracerebroventricular injection of orexins induces feeding in rats (154). When injected into different hypothalamic sites in the rat, orexin A is effective only in a narrow area (perifornical area, lateral hypothalamus, and dorsomedial and paraventricular nuclei but not in preoptic area or arcuate or ventromedial nucleus) whereas orexin B is inactive (39, 173). Some of the hypothalamic nuclei possibly involved in feeding responses to orexins are listed in Table 3. Intraperitoneally injected OX1 receptor antagonist (SB-334867) inhibits baseline feeding, normal weight gain, and the feeding response to orexin injection in the hypothalamus (62, 150). The feeding-stimulating effect of fasting is blocked by intracisternally administered anti-orexin A antibodies (192) and intraperitoneal SB-334867 (62). This suggests that at least the OX1 receptor is involved in the natural regulation of feeding, but both orexin receptor subtypes are expressed in the nuclei involved in the regulation of feeding (Refs. 111, 180; Table 3). Orexin A increases c-fos expression in areas of energy regulatory sites within the hypothalamus, the limbic region, and the hindbrain (128). As mentioned above (see CELLULAR RESPONSES TO OREXINS), orexins belong to a regulatory network of peptides, including NPY, MCH, AGRP, galanin, POMC products, CART, and dynorphin, which regulate feeding behavior. Orexins appear to act upstream of NPY as antagonists to the NPY Y1 or Y5 receptors block orexin-stimulated feeding (38, 81).The degree to which orexins regulate long-term food intake is subject to debate; some data indicate rather that orexin-induced food intake is only an acute effect and is evened out by compensatory decrease in food intake within 24 h (see, e.g., Refs. 38, 63, 194). It can be speculated that part of orexin-induced food intake is secondary to increased wakefulness and activity and therefore following increased energy expenditure. A general view emerging from the different results is that orexins may participate in the short-term regulation of energy homeostasis by initiating feeding in response to falls in glucose, but the subsequent increase in glucose, leptin, and visceral signals would decrease orexin, which in its turn would terminate the behavior after food ingestion (Refs. 22, 53; reviewed in Refs. 189, 190). Mapping of orexinergic projections indicates that orexins could also be involved in the overall regulation of feeding. For instance, nucleus of the solitary tract, sensory trigeminal nucleus, and lamina 1 and 10 of the sacral cord receive afferents from the mouth and the gastrointestinal tract and the dorsal motor nucleus of vagus, motor trigeminal nucleus, intermediolateral cell column, and sacral parasympathetic nucleus give rise to nerves that innervate the gastrointestinal tract and muscles of mastication (31, 34, 36, 131, 143, 182, 198). Therefore, it is tempting to suggest that central orexinergic nerves could be involved in the primary eating process via sensory and motor innervation of the mouth and in the passage of food through the gastrointestinal tract via sensory, motor, and secretomotor innervation of the intestines. Peripheral orexinergic neurons and endocrine release of orexins from, e.g., enterochromaffin cells could affect intestinal motility, endo- and exocrine secretion, uptake, and sensory signaling in the gastrointestinal tract (see Effects of orexins on gastrointestinal tract and other organs of feeding/energy metabolism). Some direct data (6, 106, 124, 175, 186, 197) and indirect observations from narcoleptic patients and animal models (23, 60, 162, 193) suggest that orexins may affect metabolic rate or heat loss or related processes. An overall role in energy metabolism is also suggested by the effects of orexins on the release of hormones, especially adrenal steroids (see Orexins in Autonomic/Endocrine Functions).
Involvement of Orexins in Sleep/Wakefulness
Regulation of sleep/wakefulness is the most well-demonstrated systemic effect of the orexinergic pathways. At the anatomic level, orexinergic neurons are present and project to a multitude of nuclei at different levels of CNS involved in sleep/arousal. Orexin receptors/receptor mRNAs are observed in these same areas. OX1 and OX2 receptor mRNA expression within these areas suggests that both receptor subtypes are involved (Table 3).Injection of orexin A into rat cerebral ventricles or into the more specific CNS sites in the rat or cat increases wakefulness and decreases sleep (15, 45, 56, 72, 120, 144). Most markedly, the effects are seen as decreased rapid eye movement (REM) sleep and slow-wave sleep episode number and duration. A profound role of histaminergic systems in these processes is suggested by results that show that these effects of orexin A in the mouse are blocked by histamine H1 receptor antagonists or gene disruption (72, 195). Narcoleptic dogs with disrupted OX2 receptors have decreased brain histamine content (134), and OX2 receptor seems to be responsible for orexin effects on the tuberomamillary nucleus even in the rat (44, 195). Expression of c-fos has been shown to increase mainly in orexin-positive cells of the hypothalamus at the time of normal waking of rats, and c-fos expression is also increased by sleep deprivation and by the antinarcoleptic drugs metamphetamine and mofanadil (23, 46, 161).
Cellular responses to orexins have been observed in areas involved in sleep. Orexins excite neurons in rat locus ceruleus (56, 70, 80, 170), dorsal raphe nucleus (18), and tuberomamillary nucleus (9, 44) and in mouse laterodorsal tegmental nucleus (19) (see CELLULAR RESPONSES TO OREXINS). Even in these areas OX1 and OX2 receptors seem to have overlapping roles. Orexinergic systems in sleep and wakefulness as well as knockout phenotypes are further discussed in Narcolepsy, a Disorder of the Orexinergic System.
Orexins in Autonomic/Endocrine Functions
Orexinergic efferents project to particular areas in the brain stem and spinal cord, which suggest a direct involvement in autonomic afferents and efferents and pain transmission (11, 31, 34, 122, 182). Autonomic effects are manifested as increased blood pressure, heart rate, and intestinal motility (see below), gastric acid secretion (138, 177), and sympathetic nerve activity (98, 124, 166) in response to intracerebroventricular orexin A. When administered intravenously, orexin A induces analgesia in thermal nociception paradigms (11). This effect of orexin A is blocked by the OX1 receptor antagonist SB-334867Orexins may affect the water balance via effects on drinking, water/salt homeostasis, or mechanisms involved in blood pressure regulation, as suggested by the anatomy of the projections (24, 31, 34, 36, 37, 61, 69-71, 125, 126, 154, 182). In fact, intracerebroventricular orexin A and -B increase water intake in the rat (98, 149). Injection of orexin A or -B into particular brain stem sites increases mean arterial pressure and heart rate in the rat (3, 25, 156, 166). To some extent these effects may be mediated by increased release of hormones, such as (nor)epinephrine (3, 166).
Release of many hormones from hypothalamus has been shown to be affected by orexins. Orexins may regulate hypothalamo-hypophysial hormone secretion indirectly via neuronal circuits, but even a direct effect on the hypophysis is possible: orexins are found in median eminence (35, 125), and mRNA for orexin receptor subtypes is found in anterior and intermediate hypophysial lobes (35). Blanco et al. (13) identify coexpression of growth hormone and OX1 receptors in acidophil cells of adenohypophysis and coexpression of ACTH and OX2 receptors in pars intermedia and basophil cells of adenohypophysis. Orexins and orexin receptors are also found at low levels in the posterior hypophysial lobe. Altogether, both OX1 and OX2 mRNA are expressed in the areas involved in neuroendocrine regulation (111).
Stimulatory effect on the release on multiple levels of the hypothalamo-hypophysio-adrenal axis have been reported (see Orexinergic regulation of hypothalamo-hypophysio-adrenal axis and stress response). Orexin A injected intravenously or directly into the hypothalamic paraventricular nucleus decreases TSH release in the rat (123, 153), but no change has been seen in the plasma levels of thyroid hormones (123, 153). Intracerebroventricular orexin A decreases growth hormone release in the rat (56). Effects on the release of other hormones have also been reported, but the results are equivocal. Some release effects have been shown in hypothalamic explants in vitro: orexin A stimulates release of neurotensin, NPY, VIP, somatostatin, and luteinizing hormone-releasing hormone (LHRH) and decreases the release of TRH but has no effect on the release of dopamine, vasopressin, or MCH (123, 151, 152).
Orexinergic regulation of hypothalamo-hypophysio-adrenal axis and stress response. Intracerebroventricular administration of orexin A increases plasma levels of CRF, ACTH, corticosterone, vasopressin, and epinephrine in the rat (2, 56, 78, 82, 83, 85, 151, 152). CRF release has been suggested to be mediated by elevated NPY acting on Y1 receptors (83, 152), placing NPY downstream of orexins even in this system. Orexin A can even stimulate CRF release in vitro (151). A CRF receptor antagonist has been reported to block orexin-stimulated plasma corticosterone elevation in the rat (82, 159); on the other hand, orexins have been reported to inhibit CRF-stimulated ACTH release from cultured rat adenohypophysial cells (158).
Orexins have also been reported to stimulate release of corticosterone/cortisol (108, 110, 117) and aldosterone (108, 132) from rat, porcine, or human adrenal glands/adrenal cells in vitro. This release is independent of ACTH (110, 117). Corticosterone/cortisol release has been determined to co-occur with cAMP elevation and probably to depend on it [rat (110), human (117)]. This, together with the lack of additivity between orexin A and ACTH, suggests that these peptides utilize the same mechanism for glucocorticoid release (117). In contrast, no direct effect on rat adrenal slices was seen by Jaszberenyi et al. (82). In a study with long-term (7 day) exposure to orexin A or orexin B, plasma levels of both aldosterone and corticosterone but not CRF or ACTH were elevated in the rat (108). Release of epinephrine and norepinephrine from rat adrenal medulla is increased by intracerebroventricular orexins (166), and a similar although smaller effect is seen in cultured porcine adrenal medullar cells (132). Both OX1 and OX2 receptor mRNA have been suggested to be expressed in the rat adrenal medulla (104), although other studies present opposing views (84). Orexin-mediated increases in CRF, ACTH, corticosterone, aldosterone, vasopressin, and epinephrine release suggest that orexins could mediate stress responses. Interesting in this context, orexins increase particular stereotypic behavioral patterns, such as grooming, face washing, and burrowing, which may be related to activation of stress response (40, 45, 77, 78). Orexin A-induced grooming is inhibited by OX1 receptor block with SB-334867 (40), and a CRF receptor antagonist inhibits orexin-induced face washing and grooming by ~50% and the shift from rest to locomotor activity by 70-80% (78). Involvement of complex pathways is highlighted by inhibitory effects of dopamine and serotonin receptor antagonists (40, 115, 130). It should be noted that orexins increase wakefulness and/or activity of the rat, and it is therefore important to be able to separate these effects from the stress response.Effects of orexins on gastrointestinal tract and other organs of feeding/energy metabolism. In the gastrointestinal tract, both neurons and some endocrine cells may express orexins (92, 133). In neurons, orexin immunoreactivity is seen both in the submucosal and myenteric plexi. Orexinergic neurons in these plexi have been suggested to be sensory or secretomotor on the basis of the coexpression of other markers such as VIP and choline acetyltransferase (92, 133). The neurites project to other orexin-containing cells and to mucosa, muscle layers, and submucosal blood vessels. In the guinea pig, all the orexin-positive neurons of submucosal and myenteric plexi express leptin receptors as well (92). Fasting increases the number of orexin A-immunoreactive submucosal neurons in the guinea pig, most of which also become positive for phospho-cAMP response element binding protein (pCREB) (92). This, similar to c-fos expression, indicates activation of the neurons, suggesting that fasting activates both central and peripheral orexinergic systems.
Also, some endocrine cells in the gut and stomachNarcolepsy, a Disorder of the Orexinergic System
Narcolepsy is a disorder that in short can be characterized as abnormal inclusion of REM sleep features, such as muscle paralysis and hallucinations in the waking state and in the sleep/waking transition state. In addition, hypersomnia is often observed. Narcolepsy in humans is usually a sporadic disease; its strong association with some major histocompatibility complex antigens (especially HLA-DRB1*15, HLA-DQA1*0102, and HLA-DQB1*0602) has for a long time been suggested to relate it to some autoimmune process. Yet autoantibodies or signs of ongoing or previous inflammation have been hard to find (reviewed in Refs. 74, 94, 102, 141, 179). This could, however, be caused 1) by the fact that postmortem narcoleptic brains are usually investigated years after disease onset and 2) by the dispersed nature of the orexinergic cells. Orexin A is undetectable in the CSF of most human narcoleptic patients (32, 135), the number of orexinergic neurons in postmortem narcoleptic brains is much reduced (142, 178), and even signs of gliosis have been observed (178). At the same time MCHergic neurons, at the same brain areas, are present in normal numbers (142, 178). This, together with the animal model systems (see below), has led to the hypothesis of narcolepsy as a disease of orexinergic neurons. As further support for this, only one clearly debilitating preproorexin or orexin receptor mutation has been found in human subjects (Ref. 142; see below).In the mouse, depletion of the hypothalamus of orexins results in a narcoleptic phenotype. This has been accomplished by genetic disruption of the preproorexin gene (orexin knockout mouse; Ref. 23), genetic destruction of orexinergic cells in preproorexin-ataxin-3 knockin mouse (60), and toxic destruction of orexinergic cells by hypothalamic injection of orexin B-saporin conjugate (52).
In dogs, narcolepsy is mainly a familial disorder. Recently, familial canine narcolepsy of Doberman pinschers and Labrador retrievers was shown to be caused by mutations in the OX2 receptor gene (102). In both cases, the mutations cause premature amino acid chain termination. When expressed heterologously in HEK293 cells, the mutant receptors mainly remain in the intracellular compartments and neither bind orexin A nor activate Ca2+ mobilization (73). Point mutation Glu54Lys at the NH2 terminus/first transmembrane segment junction of the OX2 receptor also leads to reduced orexin A binding and 400-fold reduced potency for Ca2+ elevation when expressed in HEK293 cells (73). The 10 other reported mutations in preproorexin or OX2 receptor genes are thought to represent benign polymorphisms (73). Sporadic cases of canine narcolepsy have been suggested to be caused by lack of orexins (148).
OX1 or OX2 receptor gene abnormalities are not
considered to be responsible for human narcoleptic cases, and the 26 single-base changes (11 aa changes) found in healthy and narcoleptic
human subjects (Refs. 139, 142; see also
Genetics and Chemistry of Orexins) both in introns and all
over the coding regions are supposed to represent benign polymorphisms.
One other polymorphism concerns the reported sequence for the
OX1 receptors (aa 280; see Genetics and Chemistry of
Orexins), although this could be a sequencing error. One mutation,
Leu16Arg, found in the 5'-signal peptide region of the human
preproorexin gene probably results in early-onset, severe narcolepsy
(142). When transiently expressed in Neuro-2A cells, the
mutated peptide apparently accumulated in some tubular network,
presumably smooth endoplasmic reticulum (ER), in contrast to the
wild-type preproorexin, which appeared in vesicles. Another point mutation in the 5'-untranslated region may be involved in disease
process in the presence of a particular type of tissue antigen
(HLA-DR2; Ref. 51). Insertion of an adenosine at 593 (promoter region) has been found in one narcoleptic patient, although the effect of this on pathogenicity is unclear
(51).
Thus there is strong evidence that narcolepsy is caused by reduced orexinergic signaling via either destruction of orexinergic neurons or defective orexin peptides or receptors, although larger population studies are necessary to confirm this. OX2 receptor appears to be especially involved in the dog, which would promote the role of nuclei relying on this receptor subtype, such as tuberomamillary nucleus, although it is possible that OX1 and OX2 receptors have different distributions in rats and humans compared with dogs.
Orexinergic disorders may also provide some interesting information on the role of orexins in feeding. Patients with narcolepsy show somewhat increased body mass index compared with healthy subjects (162). Both of the two previously described kinds of orexin-transgenic mice are hypophagic, but preproorexin knockout mice are lean (23) as opposed to orexinergic neuron-deficient mice, which are obese (60). It is thus possible that preproorexin knockout mice develop some compensatory mechanisms for weight regulation whereas orexinergic neuron-deficient mice, which lose their orexinergic neurons postnatally (see Genetics and Chemistry of Orexins), will not be able to do this. Another interesting possibility was recently proposed by Chou et al. (27). Essentially all the mouse lateral hypothalamic orexinergic neurons express dynorphin (10, 27), removal of which in orexinergic neuron-deficient mice might be responsible for the more pronounced phenotype.
![]() |
FUTURE PERSPECTIVES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Although the anatomic architecture of the orexinergic system in the CNS is well characterized, there are several open questions as to the actual roles of orexins in central and peripheral functions. What are the mechanisms of action of orexins at the cellular level? Have all orexin receptors been identified? Are there presynaptic inhibitory orexin receptors in the brain? The limited access to orexin-responding neurons is an experimental problem. There are several limitations concerning the methods available for the characterization of the functional properties of these cells in slice preparations or acutely isolated cells. Model systems based on cultured neurons or even neuronal stem cells might provide an important improvement for studies of orexin signaling. The CHO cell lines available have provided insight in functions of the orexinergic system and have enabled screening for orexin receptor ligands, but all the data obtained may not be relevant for the nervous system. Recombinant models based on neurons or neuronlike cells might therefore provide further information about the signal pathways used by orexins.
What role do the orexin cells play in the regulation of feeding? Available information suggests that at least one population of the orexinergic cells in the lateral hypothalamus directly monitors and responds to changes in glucose concentration in their environment and may also directly respond to signals inhibiting feeding such as leptin. Are these orexinergic cells the ones that project to areas in brain involved in feeding behavior? Orexin receptors are expressed in high numbers in several regions within the hypothalamus. Do these regulate the release of other feeding-related neurotransmitters? The regulation of feeding seems to be complex, involving many other neuropeptides, and there seems to be a gross redundancy in the system. Blocking of the orexinergic system by various means will reduce feeding, but permanent deletion of the orexinergic system has marginal or controversial effects. Thus even an increase rather than the expected decrease in body mass may occur on deletions of different components of the orexinergic system.
The role of orexins in sleep seems clearer in light of the connection of the narcoleptic phenotype to orexin cell loss and the similar symptomatology occurring in functional orexin or orexin receptor loss. In this context it also appears quite clear that a major part of orexin effects is mediated via monoaminergic systems, which regulate cholinergic pathways involved in the initiation of REM sleep (reviewed in, e.g., Refs. 74, 90). Yet it is unclear how the activity of the orexinergic cells themselves is regulated. Use of orexin antagonists as hypnotics could prove rewarding because many commonly used hypnotics interfere with REM sleep and cause tolerance, addiction, and memory disturbances. An interesting question is whether there is an overlap between cells regulating feeding and sleep in the lateral hypothalamus. Is, for instance, feeding behavior associated with arousal effects of orexins? Possibilities in the future to devise better pharmacological methods for identification of different orexinergic neurons as well as for manipulating their function will hopefully give answers to these questions.
One of the most intriguing questions concerns what roles orexins play
in the periphery. Do they act as hormones/paracrine mediators? The
presence of orexins and their receptors in many peripheral organs such
as the intestine, endocrine pancreas, adrenals, and pituitary suggests
that this may be the case. Could orexins even transmit signals from the
periphery to the CNS? As discussed above, the few studies available
suggest separate "peripheral" and "central" orexinergic
systems. In some studies changes in central orexin concentrations are
not reflected in the periphery, and in narcoleptics devoid of central
orexins, orexins are still found in the plasma. This may indicate that
the plasma orexin levels reflect peripherally produced orexins. Are the
peripheral orexin receptors similar to the central? At least in the
adrenal cortical zona fasciculata/reticularis orexin receptors appear to couple to cAMP elevation and not to Ca2+ elevation. Does
this reflect different receptor subtypesas suggested by some
studies
or coupling to different signal pathways? Cell lines and
primary cultures from peripheral tissues should provide a lot of the
missing information in this context.
![]() |
ACKNOWLEDGEMENTS |
---|
We acknowledge van den Pol et al. (183) and Eriksson et al. (44), who have allowed reproduction of Figs. 3 and 5.
![]() |
FOOTNOTES |
---|
This study was supported by European Union Contract QLG3-CT-2002-00826 and grants from the Swedish Medical Research Council, the Cancer Research Fund of Sweden, the Lars Hierta Foundation, the Academy of Finland, and the Sigrid Juselius Foundation.
Address for reprint requests and other correspondence: J. Kukkonen, Dept. of Neuroscience, Div. of Physiology, Uppsala Univ., BMC, PO Box 572, SE-75123 Uppsala, Sweden (E-mail jkukkone{at}fysiologi.uu.se).
10.1152/ajpcell.00055.2002
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abrahamson, EE,
Leak RK,
and
Moore RY.
The suprachiasmatic nucleus projects to posterior hypothalamic arousal systems.
Neuroreport
12:
435-440,
2001[ISI][Medline].
2.
Al-Barazanji, KA,
Wilson S,
Baker J,
Jessop DS,
and
Harbuz MS.
Central orexin-A activates hypothalamic-pituitary-adrenal axis and stimulates hypothalamic corticotropin releasing factor and arginine vasopressin neurones in conscious rats.
J Neuroendocrinol
13:
421-424,
2001[ISI][Medline].
3.
Antunes, VR,
Brailoiu GC,
Kwok EH,
Scruggs P,
and
Dun NJ.
Orexins/hypocretins excite rat sympathetic preganglionic neurons in vivo and in vitro.
Am J Physiol Regul Integr Comp Physiol
281:
R1801-R1807,
2001
4.
Arihara, Z,
Takahashi K,
Murakami O,
Totsune K,
Sone M,
Satoh F,
Ito S,
Hayashi Y,
Sasano H,
and
Mouri T.
Orexin-A in the human brain and tumor tissues of ganglioneuroblastoma and neuroblastoma.
Peptides
21:
565-570,
2000[ISI][Medline].
5.
Backberg, M,
Hervieu G,
Wilson S,
and
Meister B.
Orexin receptor-1 (OX-R1) immunoreactivity in chemically identified neurons of the hypothalamus: focus on orexin targets involved in control of food and water intake.
Eur J Neurosci
15:
315-328,
2002[ISI][Medline].
6.
Balasko, M,
Szelenyi Z,
and
Szekely M.
Central thermoregulatory effects of neuropeptide Y and orexin A in rats.
Acta Physiol Hung
86:
219-222,
1999[Medline].
7.
Barritt, GJ.
Receptor-activated Ca2+ inflow in animal cells: a variety of pathways tailored to meet different intracellular Ca2+ signalling requirements.
Biochem J
337:
153-169,
1999[ISI][Medline].
8.
Bayer, L,
Colard C,
Nguyen NU,
Risold PY,
Fellmann D,
and
Griffond B.
Alteration of the expression of the hypocretin (orexin) gene by 2-deoxyglucose in the rat lateral hypothalamic area.
Neuroreport
11:
531-533,
2000[ISI][Medline].
9.
Bayer, L,
Eggermann E,
Serafin M,
Saint-Mleux B,
Machard D,
Jones B,
and
Muhlethaler M.
Orexins (hypocretins) directly excite tuberomammillary neurons.
Eur J Neurosci
14:
1571-1575,
2001[ISI][Medline].
10.
Bayer, L,
Mairet-Coello G,
Risold PY,
and
Griffond B.
Orexin/hypocretin neurons: chemical phenotype and possible interactions with melanin-concentrating hormone neurons.
Regul Pept
104:
33-39,
2002[ISI][Medline].
11.
Bingham, S,
Davey PT,
Babbs AJ,
Irving EA,
Sammons MJ,
Wyles M,
Jeffrey P,
Cutler L,
Riba I,
Johns A,
Porter RA,
Upton N,
Hunter AJ,
and
Parsons AA.
Orexin-A, an hypothalamic peptide with analgesic properties.
Pain
92:
81-90,
2001[ISI][Medline].
12.
Blanco, M,
Garcia-Caballero T,
Fraga M,
Gallego R,
Cuevas J,
Forteza J,
Beiras A,
and
Dieguez C.
Cellular localization of orexin receptors in human adrenal gland, adrenocortical adenomas and pheochromocytomas.
Regul Pept
104:
161-165,
2002[ISI][Medline].
13.
Blanco, M,
Lopez M,
Garcia-Caballero T,
Gallego R,
Vazquez-Boquete A,
Morel G,
Senaris R,
Casanueva F,
Dieguez C,
and
Beiras A.
Cellular localization of orexin receptors in human pituitary.
J Clin Endocrinol Metab
86:
1616-1619,
2001
14.
Bonini, JA,
Jones KA,
Adham N,
Forray C,
Artymyshyn R,
Durkin MM,
Smith KE,
Tamm JA,
Boteju LW,
Lakhlani PP,
Raddatz R,
Yao WJ,
Ogozalek KL,
Boyle N,
Kouranova EV,
Quan Y,
Vaysse PJ,
Wetzel JM,
Branchek TA,
Gerald C,
and
Borowsky B.
Identification and characterization of two G protein-coupled receptors for neuropeptide FF.
J Biol Chem
275:
39324-39331,
2000
15.
Bourgin, P,
Huitron-Resendiz S,
Spier AD,
Fabre V,
Morte B,
Criado JR,
Sutcliffe JG,
Henriksen SJ,
and
de Lecea L.
Hypocretin-1 modulates rapid eye movement sleep through activation of locus coeruleus neurons.
J Neurosci
20:
7760-7765,
2000
16.
Briski, KP,
and
Sylvester PW.
Hypothalamic orexin-A-immunopositive neurons express Fos in response to central glucopenia.
Neuroreport
12:
531-534,
2001[ISI][Medline].
17.
Broberger, C,
De Lecea L,
Sutcliffe JG,
and
Hokfelt T.
Hypocretin/orexin- and melanin-concentrating hormone-expressing cells form distinct populations in the rodent lateral hypothalamus: relationship to the neuropeptide Y and agouti gene-related protein systems.
J Comp Neurol
402:
460-474,
1998[ISI][Medline].
18.
Brown, RE,
Sergeeva O,
Eriksson KS,
and
Haas HL.
Orexin A excites serotonergic neurons in the dorsal raphe nucleus of the rat.
Neuropharmacology
40:
457-459,
2001[ISI][Medline].
19.
Burlet, S,
Tyler CJ,
and
Leonard CS.
Direct and indirect excitation of laterodorsal tegmental neurons by Hypocretin/Orexin peptides: implications for wakefulness and narcolepsy.
J Neurosci
22:
2862-2872,
2002
20.
Cai, XJ,
Evans ML,
Lister CA,
Leslie RA,
Arch JR,
Wilson S,
and
Williams G.
Hypoglycemia activates orexin neurons and selectively increases hypothalamic orexin-B levels: responses inhibited by feeding and possibly mediated by the nucleus of the solitary tract.
Diabetes
50:
105-112,
2001
21.
Cai, XJ,
Liu XH,
Evans M,
Clapham JC,
Wilson S,
Arch JR,
Morris R,
and
Williams G.
Orexins and feeding: special occasions or everyday occurrence?
Regul Pept
104:
1-9,
2002[ISI][Medline].
22.
Cai, XJ,
Widdowson PS,
Harrold J,
Wilson S,
Buckingham RE,
Arch JR,
Tadayyon M,
Clapham JC,
Wilding J,
and
Williams G.
Hypothalamic orexin expression: modulation by blood glucose and feeding.
Diabetes
48:
2132-2137,
1999[Abstract].
23.
Chemelli, RM,
Willie JT,
Sinton CM,
Elmquist JK,
Scammell T,
Lee C,
Richardson JA,
Williams SC,
Xiong Y,
Kisanuki Y,
Fitch TE,
Nakazato M,
Hammer RE,
Saper CB,
and
Yanagisawa M.
Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation.
Cell
98:
437-451,
1999[ISI][Medline].
24.
Chen, CT,
Dun SL,
Kwok EH,
Dun NJ,
and
Chang JK.
Orexin A-like immunoreactivity in the rat brain.
Neurosci Lett
260:
161-164,
1999[ISI][Medline].
25.
Chen, CT,
Hwang LL,
Chang JK,
and
Dun NJ.
Pressor effects of orexins injected intracisternally and to rostral ventrolateral medulla of anesthetized rats.
Am J Physiol Regul Integr Comp Physiol
278:
R692-R697,
2000
26.
Chicurel, M.
Neuroscience. The sandman's secrets.
Nature
407:
554-556,
2000[ISI][Medline].
27.
Chou, TC,
Lee CE,
Lu J,
Elmquist JK,
Hara J,
Willie JT,
Beuckmann CT,
Chemelli RM,
Sakurai T,
Yanagisawa M,
Saper CB,
and
Scammell TE.
Orexin (hypocretin) neurons contain dynorphin.
J Neurosci
21:
RC168,
2001
28.
Cikos, S,
Gregor P,
and
Koppel J.
Sequence and tissue distribution of a novel G-protein-coupled receptor expressed prominently in human placenta.
Biochem Biophys Res Commun
256:
352-356,
1999[ISI][Medline].
29.
Clapham, DE,
Runnels LW,
and
Strubing C.
The TRP ion channel family.
Nat Rev Neurosci
2:
387-396,
2001[ISI][Medline].
30.
Cluderay, JE,
Harrison DC,
and
Hervieu GJ.
Protein distribution of the orexin-2 receptor in the rat central nervous system.
Regul Pept
104:
131-144,
2002[ISI][Medline].
31.
Cutler, DJ,
Morris R,
Sheridhar V,
Wattam TA,
Holmes S,
Patel S,
Arch JR,
Wilson S,
Buckingham RE,
Evans ML,
Leslie RA,
and
Williams G.
Differential distribution of orexin-A and orexin-B immunoreactivity in the rat brain and spinal cord.
Peptides
20:
1455-1470,
1999[ISI][Medline].
32.
Dalal, MA,
Schuld A,
Haack M,
Uhr M,
Geisler P,
Eisensehr I,
Noachtar S,
and
Pollmacher T.
Normal plasma levels of orexin A (hypocretin-1) in narcoleptic patients.
Neurology
56:
1749-1751,
2001
33.
Darker, JG,
Porter RA,
Eggleston DS,
Smart D,
Brough SJ,
Sabido-David C,
and
Jerman JC.
Structure-activity analysis of truncated orexin-A analogues at the orexin-1 receptor.
Bioorg Med Chem Lett
11:
737-740,
2001[ISI][Medline].
34.
Date, Y,
Mondal MS,
Matsukura S,
and
Nakazato M.
Distribution of orexin-A and orexin-B (hypocretins) in the rat spinal cord.
Neurosci Lett
288:
87-90,
2000[ISI][Medline].
35.
Date, Y,
Mondal MS,
Matsukura S,
Ueta Y,
Yamashita H,
Kaiya H,
Kangawa K,
and
Nakazato M.
Distribution of orexin/hypocretin in the rat median eminence and pituitary.
Brain Res Mol Brain Res
76:
1-6,
2000[ISI][Medline].
36.
Date, Y,
Ueta Y,
Yamashita H,
Yamaguchi H,
Matsukura S,
Kangawa K,
Sakurai T,
Yanagisawa M,
and
Nakazato M.
Orexins, orexigenic hypothalamic peptides, interact with autonomic, neuroendocrine and neuroregulatory systems.
Proc Natl Acad Sci USA
96:
748-753,
1999
37.
De Lecea, L,
Kilduff TS,
Peyron C,
Gao X,
Foye PE,
Danielson PE,
Fukuhara C,
Battenberg EL,
Gautvik VT,
Bartlett FS,
Frankel WN,
van den Pol AN,
Bloom FE,
Gautvik KM,
and
Sutcliffe JG.
The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity.
Proc Natl Acad Sci USA
95:
322-327,
1998
38.
Dube, MG,
Horvath TL,
Kalra PS,
and
Kalra SP.
Evidence of NPY Y5 receptor involvement in food intake elicited by orexin A in sated rats.
Peptides
21:
1557-1560,
2000[ISI][Medline].
39.
Dube, MG,
Kalra SP,
and
Kalra PS.
Food intake elicited by central administration of orexins/hypocretins: identification of hypothalamic sites of action.
Brain Res
842:
473-477,
1999[ISI][Medline].
40.
Duxon, MS,
Stretton J,
Starr K,
Jones DN,
Holland V,
Riley G,
Jerman J,
Brough S,
Smart D,
Johns A,
Chan W,
Porter RA,
and
Upton N.
Evidence that orexin-A-evoked grooming in the rat is mediated by orexin-1 (OX1) receptors, with downstream 5-HT2C receptor involvement.
Psychopharmacology (Berl)
153:
203-209,
2001[Medline].
41.
Dyer, CJ,
Touchette KJ,
Carroll JA,
Allee GL,
and
Matteri RL.
Cloning of porcine prepro-orexin cDNA and effects of an intramuscular injection of synthetic porcine orexin-B on feed intake in young pigs.
Domest Anim Endocrinol
16:
145-148,
1999[ISI][Medline].
42.
Eggermann, E,
Serafin M,
Bayer L,
Machard D,
Saint-Mleux B,
Jones BE,
and
Muhlethaler M.
Orexins/hypocretins excite basal forebrain cholinergic neurones.
Neuroscience
108:
177-181,
2001[ISI][Medline].
43.
Elias, CF,
Saper CB,
Maratos-Flier E,
Tritos NA,
Lee C,
Kelly J,
Tatro JB,
Hoffman GE,
Ollmann MM,
Barsh GS,
Sakurai T,
Yanagisawa M,
and
Elmquist JK.
Chemically defined projections linking the mediobasal hypothalamus and the lateral hypothalamic area.
J Comp Neurol
402:
442-459,
1998[ISI][Medline].
44.
Eriksson, KS,
Sergeeva O,
Brown RE,
and
Haas HL.
Orexin/hypocretin excites the histaminergic neurons of the tuberomammillary nucleus.
J Neurosci
21:
9273-9279,
2001
45.
Espana, RA,
Baldo BA,
Kelley AE,
and
Berridge CW.
Wake-promoting and sleep-suppressing actions of hypocretin (orexin): basal forebrain sites of action.
Neuroscience
106:
699-715,
2001[ISI][Medline].
46.
Estabrooke, IV,
McCarthy MT,
Ko E,
Chou TC,
Chemelli RM,
Yanagisawa M,
Saper CB,
and
Scammell TE.
Fos expression in orexin neurons varies with behavioral state.
J Neurosci
21:
1656-1662,
2001
47.
Fujiki, N,
Yoshida Y,
Ripley B,
Honda K,
Mignot E,
and
Nishino S.
Changes in CSF hypocretin-1 (orexin A) levels in rats across 24 hours and in response to food deprivation.
Neuroreport
12:
993-997,
2001[ISI][Medline].
48.
Fung, SJ,
Yamuy J,
Sampogna S,
Morales FR,
and
Chase MH.
Hypocretin (orexin) input to trigeminal and hypoglossal motoneurons in the cat: a double-labeling immunohistochemical study.
Brain Res
903:
257-262,
2001[ISI][Medline].
49.
Gao, XB,
and
van den Pol AN.
Melanin concentrating hormone depresses synaptic activity of glutamate and GABA neurons from rat lateral hypothalamus.
J Physiol
533:
237-252,
2001
50.
Gautvik, KM,
de Lecea L,
Gautvik VT,
Danielson PE,
Tranque P,
Dopazo A,
Bloom FE,
and
Sutcliffe JG.
Overview of the most prevalent hypothalamus-specific mRNAs, as identified by directional tag PCR subtraction.
Proc Natl Acad Sci USA
93:
8733-8738,
1996
51.
Gencik, M,
Dahmen N,
Wieczorek S,
Kasten M,
Bierbrauer J,
Anghelescu I,
Szegedi A,
Menezes Saecker AM,
and
Epplen JT.
A prepro-orexin gene polymorphism is associated with narcolepsy.
Neurology
56:
115-117,
2001
52.
Gerashchenko, D,
Kohls MD,
Greco M,
Waleh NS,
Salin-Pascual R,
Kilduff TS,
Lappi DA,
and
Shiromani PJ.
Hypocretin-2-saporin lesions of the lateral hypothalamus produce narcoleptic-like sleep behavior in the rat.
J Neurosci
21:
7273-7283,
2001
53.
Griffond, B,
Risold PY,
Jacquemard C,
Colard C,
and
Fellmann D.
Insulin-induced hypoglycemia increases preprohypocretin (orexin) mRNA in the rat lateral hypothalamic area.
Neurosci Lett
262:
77-80,
1999[ISI][Medline].
54.
Grudt, TJ,
van den Pol AN,
and
Perl ER.
Hypocretin-2 (orexin-B) modulation of superficial dorsal horn activity in rat.
J Physiol
538:
517-525,
2002
55.
Guan, JL,
Saotome T,
Wang QP,
Funahashi H,
Hori T,
Tanaka S,
and
Shioda S.
Orexinergic innervation of POMC-containing neurons in the rat arcuate nucleus.
Neuroreport
12:
547-551,
2001[ISI][Medline].
56.
Hagan, JJ,
Leslie RA,
Patel S,
Evans ML,
Wattam TA,
Holmes S,
Benham CD,
Taylor SG,
Routledge C,
Hemmati P,
Munton RP,
Ashmeade TE,
Shah AS,
Hatcher JP,
Hatcher PD,
Jones DN,
Smith MI,
Piper DC,
Hunter AJ,
Porter RA,
and
Upton N.
Orexin A activates locus coeruleus cell firing and increases arousal in the rat.
Proc Natl Acad Sci USA
96:
10911-10916,
1999
57.
Håkansson, ML,
Brown H,
Ghilardi N,
Skoda RC,
and
Meister B.
Leptin receptor immunoreactivity in chemically defined target neurons of the hypothalamus.
J Neurosci
18:
559-572,
1998
58.
Håkansson, M,
de Lecea L,
Sutcliffe JG,
Yanagisawa M,
and
Meister B.
Leptin receptor- and STAT3-immunoreactivities in hypocretin/orexin neurones of the lateral hypothalamus.
J Neuroendocrinol
11:
653-663,
1999[ISI][Medline].
59.
Hamilton, SE,
Loose MD,
Qi M,
Levey AI,
Hille B,
McKnight GS,
Idzerda RL,
and
Nathanson NM.
Disruption of the m1 receptor gene ablates muscarinic receptor-dependent M current regulation and seizure activity in mice.
Proc Natl Acad Sci USA
94:
13311-13316,
1997
60.
Hara, J,
Beuckmann CT,
Nambu T,
Willie JT,
Chemelli RM,
Sinton CM,
Sugiyama F,
Yagami K,
Goto K,
Yanagisawa M,
and
Sakurai T.
Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity.
Neuron
30:
345-354,
2001[ISI][Medline].
61.
Harrison, TA,
Chen CT,
Dun NJ,
and
Chang JK.
Hypothalamic orexin A-immunoreactive neurons project to the rat dorsal medulla.
Neurosci Lett
273:
17-20,
1999[ISI][Medline].
62.
Haynes, AC,
Jackson B,
Chapman H,
Tadayyon M,
Johns A,
Porter RA,
and
Arch JR.
A selective orexin-1 receptor antagonist reduces food consumption in male and female rats.
Regul Pept
96:
45-51,
2000[ISI][Medline].
63.
Haynes, AC,
Jackson B,
Overend P,
Buckingham RE,
Wilson S,
Tadayyon M,
and
Arch JR.
Effects of single and chronic intracerebroventricular administration of the orexins on feeding in the rat.
Peptides
20:
1099-1105,
1999[ISI][Medline].
64.
Hervieu, GJ,
Cluderay JE,
Harrison DC,
Roberts JC,
and
Leslie RA.
Gene expression and protein distribution of the orexin-1 receptor in the rat brain and spinal cord.
Neuroscience
103:
777-797,
2001[ISI][Medline].
65.
Hill, JJ,
and
Peralta EG.
Inhibition of a Gi-activated potassium channel (GIRK1/4) by the Gq-coupled m1 muscarinic acetylcholine receptor.
J Biol Chem
276:
5505-5510,
2001
66.
Hirota, K,
Kushikata T,
Kudo M,
Kudo T,
Lambert DG,
and
Matsuki A.
Orexin A and B evoke noradrenaline release from rat cerebrocortical slices.
Br J Pharmacol
134:
1461-1466,
2001
67.
Holmqvist, T,
Åkerman KEO,
and
Kukkonen JP.
High specificity of human orexin receptors for orexins over neuropeptide Y and other neuropeptides.
Neurosci Lett
305:
177-180,
2001[ISI][Medline].
68.
Horn, F,
Weare J,
Beukers MW,
Horsch S,
Bairoch A,
Chen W,
Edvardsen O,
Campagne F,
and
Vriend G.
GPCRDB: an information system for G protein-coupled receptors.
Nucleic Acids Res
26:
275-279,
1998
69.
Horvath, TL,
Diano S,
and
van den Pol AN.
Synaptic interaction between hypocretin (orexin) and neuropeptide Y cells in the rodent and primate hypothalamus: a novel circuit implicated in metabolic and endocrine regulations.
J Neurosci
19:
1072-1087,
1999
70.
Horvath, TL,
Peyron C,
Diano S,
Ivanov A,
Aston-Jones G,
Kilduff TS,
and
van Den Pol AN.
Hypocretin (orexin) activation and synaptic innervation of the locus coeruleus noradrenergic system.
J Comp Neurol
415:
145-159,
1999[ISI][Medline].
71.
Horvath, TL,
Warden CH,
Hajos M,
Lombardi A,
Goglia F,
and
Diano S.
Brain uncoupling protein 2: uncoupled neuronal mitochondria predict thermal synapses in homeostatic centers.
J Neurosci
19:
10417-10427,
1999
72.
Huang, ZL,
Qu WM,
Li WD,
Mochizuki T,
Eguchi N,
Watanabe T,
Urade Y,
and
Hayaishi O.
Arousal effect of orexin A depends on activation of the histaminergic system.
Proc Natl Acad Sci USA
98:
9965-9970,
2001
73.
Hungs, M,
Fan J,
Lin L,
Lin X,
Maki RA,
and
Mignot E.
Identification and functional analysis of mutations in the hypocretin (orexin) genes of narcoleptic canines.
Genome Res
11:
531-539,
2001
74.
Hungs, M,
and
Mignot E.
Hypocretin/orexin, sleep and narcolepsy.
Bioessays
23:
397-408,
2001[ISI][Medline].
75.
Hwang, LL,
Chen CT,
and
Dun NJ.
Mechanisms of orexin-induced depolarizations in rat dorsal motor nucleus of vagus neurones in vitro.
J Physiol
537:
511-520,
2001
76.
Ichinose, M,
Asai M,
Sawada M,
Sasaki K,
and
Oomura Y.
Induction of outward current by orexin-B in mouse peritoneal macrophages.
FEBS Lett
440:
51-54,
1998[ISI][Medline].
77.
Ida, T,
Nakahara K,
Katayama T,
Murakami N,
and
Nakazato M.
Effect of lateral cerebroventricular injection of the appetite-stimulating neuropeptide, orexin and neuropeptide Y, on the various behavioral activities of rats.
Brain Res
821:
526-529,
1999[ISI][Medline].
78.
Ida, T,
Nakahara K,
Murakami T,
Hanada R,
Nakazato M,
and
Murakami N.
Possible involvement of orexin in the stress reaction in rats.
Biochem Biophys Res Commun
270:
318-323,
2000[ISI][Medline].
79.
Ishizuka, T,
Yamamoto Y,
and
Yamatodani A.
The effect of orexin-A and -B on the histamine release in the anterior hypothalamus in rats.
Neurosci Lett
323:
93-96,
2002[ISI][Medline].
80.
Ivanov, A,
and
Aston-Jones G.
Hypocretin/orexin depolarizes and decreases potassium conductance in locus coeruleus neurons.
Neuroreport
11:
1755-1758,
2000[ISI][Medline].
81.
Jain, MR,
Horvath TL,
Kalra PS,
and
Kalra SP.
Evidence that NPY Y1 receptors are involved in stimulation of feeding by orexins (hypocretins) in sated rats.
Regul Pept
87:
19-24,
2000[ISI][Medline].
82.
Jaszberenyi, M,
Bujdoso E,
Pataki I,
and
Telegdy G.
Effects of orexins on the hypothalamic-pituitary-adrenal system.
J Neuroendocrinol
12:
1174-1178,
2000[ISI][Medline].
83.
Jaszberenyi, M,
Bujdoso E,
and
Telegdy G.
The role of neuropeptide Y in orexin-induced hypothalamic-pituitary-adrenal activation.
J Neuroendocrinol
13:
438-441,
2001[ISI][Medline].
84.
Jöhren, O,
Neidert SJ,
Kummer M,
Dendorfer A,
and
Dominiak P.
Prepro-orexin and orexin receptor mRNAs are differentially expressed in peripheral tissues of male and female rats.
Endocrinology
142:
3324-3331,
2001
85.
Jones, DN,
Gartlon J,
Parker F,
Taylor SG,
Routledge C,
Hemmati P,
Munton RP,
Ashmeade TE,
Hatcher JP,
Johns A,
Porter RA,
Hagan JJ,
Hunter AJ,
and
Upton N.
Effects of centrally administered orexin-B and orexin-A: a role for orexin-1 receptors in orexin-B-induced hyperactivity.
Psychopharmacology (Berl)
153:
210-218,
2001[Medline].
86.
Kane, JK,
Parker SL,
Matta SG,
Fu Y,
Sharp BM,
and
Li MD.
Nicotine up-regulates expression of orexin and its receptors in rat brain.
Endocrinology
141:
3623-3629,
2000
87.
Kane, JK,
Tanaka H,
Parker SL,
Yanagisawa M,
and
Li MD.
Sensitivity of orexin-A binding to phospholipase C inhibitors, neuropeptide Y, and secretin.
Biochem Biophys Res Commun
272:
959-965,
2000[ISI][Medline].
88.
Karteris, E,
Randeva HS,
Grammatopoulos DK,
Jaffe RB,
and
Hillhouse EW.
Expression and coupling characteristics of the crh and orexin type 2 receptors in human fetal adrenals.
J Clin Endocrinol Metab
86:
4512-4519,
2001
89.
Kastin, AJ,
and
Akerstrom V.
Orexin A but not orexin B rapidly enters brain from blood by simple diffusion.
J Pharmacol Exp Ther
289:
219-223,
1999
90.
Kilduff, TS,
and
Peyron C.
The hypocretin/orexin ligand-receptor system: implications for sleep and sleep disorders.
Trends Neurosci
23:
359-365,
2000[ISI][Medline].
91.
Kimura, J,
Miyamae S,
and
Noma A.
Identification of sodium-calcium exchange current in single ventricular cells of guinea-pig.
J Physiol
384:
199-222,
1987[Abstract].
92.
Kirchgessner, AL,
and
Liu M.
Orexin synthesis and response in the gut.
Neuron
24:
941-951,
1999[ISI][Medline].
93.
Korotkova, TM,
Eriksson KS,
Haas HL,
and
Brown RE.
Selective excitation of GABAergic neurons in the substantia nigra of the rat by orexin/hypocretin in vitro.
Regul Pept
104:
83-89,
2002[ISI][Medline].
94.
Krahn, LE,
Black JL,
and
Silber MH.
Narcolepsy: new understanding of irresistible sleep.
Mayo Clin Proc
76:
185-194,
2001[ISI][Medline].
95.
Kreegipuu, A,
Blom N,
and
Brunak S.
PhosphoBase, a database of phosphorylation sites: release 2.0.
Nucleic Acids Res
27:
237-239,
1999
96.
Kukkonen, JP,
and
Åkerman KEO
Orexin receptors couple to Ca2+ channels different from store-operated Ca2+ channels.
Neuroreport
12:
2017-2020,
2001[ISI][Medline].
97.
Kummer, M,
Neidert SJ,
Johren O,
and
Dominiak P.
Orexin (hypocretin) gene expression in rat ependymal cells.
Neuroreport
12:
2117-2120,
2001[ISI][Medline].
98.
Kunii, K,
Yamanaka A,
Nambu T,
Matsuzaki I,
Goto K,
and
Sakurai T.
Orexins/hypocretins regulate drinking behaviour.
Brain Res
842:
256-261,
1999[ISI][Medline].
99.
Kurose, T,
Ueta Y,
Yamamoto Y,
Serino R,
Ozaki Y,
Saito J,
Nagata S,
and
Yamashita H.
Effects of restricted feeding on the activity of hypothalamic Orexin (OX)-A containing neurons and OX2 receptor mRNA level in the paraventricular nucleus of rats.
Regul Pept
104:
145-151,
2002[ISI][Medline].
100.
Lee, JH,
Bang E,
Chae KJ,
Kim JY,
Lee DW,
and
Lee W.
Solution structure of a new hypothalamic neuropeptide, human hypocretin-2/orexin-B.
Eur J Biochem
266:
831-839,
1999
101.
Levin, BE,
Dunn-Meynell AA,
and
Routh VH.
Brain glucose sensing and body energy homeostasis: role in obesity and diabetes.
Am J Physiol Regul Integr Comp Physiol
276:
R1223-R1231,
1999
102.
Lin, L,
Faraco J,
Li R,
Kadotani H,
Rogers W,
Lin X,
Qiu X,
de Jong PJ,
Nishino S,
and
Mignot E.
The sleep disorder canine narcolepsy is caused by a mutation in the hypocretin (orexin) receptor 2 gene.
Cell
98:
365-376,
1999[ISI][Medline].
103.
Liu, XH,
Morris R,
Spiller D,
White M,
and
Williams G.
Orexin a preferentially excites glucose-sensitive neurons in the lateral hypothalamus of the rat in vitro.
Diabetes
50:
2431-2437,
2001
104.
Lopez, M,
Senaris R,
Gallego R,
Garcia-Caballero T,
Lago F,
Seoane L,
Casanueva F,
and
Dieguez C.
Orexin receptors are expressed in the adrenal medulla of the rat.
Endocrinology
140:
5991-5994,
1999
105.
Lopez, M,
Seoane L,
Garcia MC,
Lago F,
Casanueva FF,
Senaris R,
and
Dieguez C.
Leptin regulation of prepro-orexin and orexin receptor mRNA levels in the hypothalamus.
Biochem Biophys Res Commun
269:
41-45,
2000[ISI][Medline].
106.
Lubkin, M,
and
Stricker-Krongrad A.
Independent feeding and metabolic actions of orexins in mice.
Biochem Biophys Res Commun
253:
241-245,
1998[ISI][Medline].
107.
Lund, PE,
Shariatmadari R,
Uustare A,
Detheux M,
Parmentier M,
Kukkonen JP,
and
Åkerman KEO
The orexin OX1 receptor activates a novel Ca2+ influx pathway necessary for coupling to phospholipase C.
J Biol Chem
275:
30806-30812,
2000
108.
Malendowicz, LK,
Hochol A,
Ziolkowska A,
Nowak M,
Gottardo L,
and
Nussdorfer GG.
Prolonged orexin administration stimulates steroid-hormone secretion, acting directly on the rat adrenal gland.
Int J Mol Med
7:
401-404,
2001[ISI][Medline].
109.
Malendowicz, LK,
Jedrzejczak N,
Belloni AS,
Trejter M,
Hochol A,
and
Nussdorfer GG.
Effects of orexins A and B on the secretory and proliferative activity of immature and regenerating rat adrenal glands.
Histol Histopathol
16:
713-717,
2001[ISI][Medline].
110.
Malendowicz, LK,
Tortorella C,
and
Nussdorfer GG.
Orexins stimulate corticosterone secretion of rat adrenocortical cells, through the activation of the adenylate cyclase-dependent signaling cascade.
J Steroid Biochem Mol Biol
70:
185-188,
1999[ISI][Medline].
111.
Marcus, JN,
Aschkenasi CJ,
Lee CE,
Chemelli RM,
Saper CB,
Yanagisawa M,
and
Elmquist JK.
Differential expression of orexin receptors 1 and 2 in the rat brain.
J Comp Neurol
435:
6-25,
2001[ISI][Medline].
112.
Mark, MD,
and
Herlitze S.
G-protein mediated gating of inward-rectifier K+ channels.
Eur J Biochem
267:
5830-5836,
2000
113.
Martin, G,
Fabre V,
Siggins GR,
and
de Lecea L.
Interaction of the hypocretins with neurotransmitters in the nucleus accumbens.
Regul Pept
104:
111-117,
2002[ISI][Medline].
114.
Matsumura, K,
Tsuchihashi T,
and
Abe I.
Central orexin-A augments sympathoadrenal outflow in conscious rabbits.
Hypertension
37:
1382-1387,
2001
115.
Matsuzaki, I,
Sakurai T,
Kunii K,
Nakamura T,
Yanagisawa M,
and
Goto K.
Involvement of the serotonergic system in orexin-induced behavioral alterations in rats.
Regul Pept
104:
119-123,
2002[ISI][Medline].
116.
Mazzocchi, G,
Malendowicz LK,
Aragona F,
Rebuffat P,
Gottardo L,
and
Nussdorfer GG.
Human pheochromocytomas express orexin receptor type 2 gene and display an in vitro secretory response to orexins A and B.
J Clin Endocrinol Metab
86:
4818-4821,
2001
117.
Mazzocchi, G,
Malendowicz LK,
Gottardo L,
Aragona F,
and
Nussdorfer GG.
Orexin A stimulates cortisol secretion from human adrenocortical cells through activation of the adenylate cyclase-dependent signaling cascade.
J Clin Endocrinol Metab
86:
778-782,
2001
118.
McGranaghan, PA,
and
Piggins HD.
Orexin A-like immunoreactivity in the hypothalamus and thalamus of the Syrian hamster (Mesocricetus auratus) and Siberian hamster (Phodopus sungorus), with special reference to circadian structures.
Brain Res
904:
234-244,
2001[ISI][Medline].
119.
Meister, B.
Control of food intake via leptin receptors in the hypothalamus.
Vitam Horm
59:
265-304,
2000[Medline].
120.
Methippara, MM,
Alam MN,
Szymusiak R,
and
McGinty D.
Effects of lateral preoptic area application of orexin-A on sleep-wakefulness.
Neuroreport
11:
3423-3426,
2000[ISI][Medline].
121.
Mikkelsen, JD,
Hauser F,
deLecea L,
Sutcliffe JG,
Kilduff TS,
Calgari C,
Pevet P,
and
Simonneaux V.
Hypocretin (orexin) in the rat pineal gland: a central transmitter with effects on noradrenaline-induced release of melatonin.
Eur J Neurosci
14:
419-425,
2001[ISI][Medline].
122.
Mintz, EM,
van den Pol AN,
Casano AA,
and
Albers HE.
Distribution of hypocretin-(orexin) immunoreactivity in the central nervous system of Syrian hamsters (Mesocricetus auratus).
J Chem Neuroanat
21:
225-238,
2001[ISI][Medline].
123.
Mitsuma, T,
Hirooka Y,
Mori Y,
Kayama M,
Adachi K,
Rhue N,
Ping J,
and
Nogimori T.
Effects of orexin A on thyrotropin-releasing hormone and thyrotropin secretion in rats.
Horm Metab Res
31:
606-609,
1999[ISI][Medline].
124.
Monda, M,
Viggiano A,
Mondola P,
and
De Luca V.
Inhibition of prostaglandin synthesis reduces hyperthermic reactions induced by hypocretin-1/orexin A.
Brain Res
909:
68-74,
2001[ISI][Medline].
125.
Mondal, MS,
Nakazato M,
Date Y,
Murakami N,
Hanada R,
Sakata T,
and
Matsukura S.
Characterization of orexin-A and orexin-B in the microdissected rat brain nuclei and their contents in two obese rat models.
Neurosci Lett
273:
45-48,
1999[ISI][Medline].
126.
Mondal, MS,
Nakazato M,
Date Y,
Murakami N,
Yanagisawa M,
and
Matsukura S.
Widespread distribution of orexin in rat brain and its regulation upon fasting.
Biochem Biophys Res Commun
256:
495-499,
1999[ISI][Medline].
127.
Moriguchi, T,
Sakurai T,
Nambu T,
Yanagisawa M,
and
Goto K.
Neurons containing orexin in the lateral hypothalamic area of the adult rat brain are activated by insulin-induced acute hypoglycemia.
Neurosci Lett
264:
101-104,
1999[ISI][Medline].
128.
Mullett, MA,
Billington CJ,
Levine AS,
and
Kotz CM.
Hypocretin I in the lateral hypothalamus activates key feeding-regulatory brain sites.
Neuroreport
11:
103-108,
2000[ISI][Medline].
129.
Muroya, S,
Uramura K,
Sakurai T,
Takigawa M,
and
Yada T.
Lowering glucose concentrations increases cytosolic Ca2+ in orexin neurons of the rat lateral hypothalamus.
Neurosci Lett
309:
165-168,
2001[ISI][Medline].
130.
Nakamura, T,
Uramura K,
Nambu T,
Yada T,
Goto K,
Yanagisawa M,
and
Sakurai T.
Orexin-induced hyperlocomotion and stereotypy are mediated by the dopaminergic system.
Brain Res
873:
181-187,
2000[ISI][Medline].
131.
Nambu, T,
Sakurai T,
Mizukami K,
Hosoya Y,
Yanagisawa M,
and
Goto K.
Distribution of orexin neurons in the adult rat brain.
Brain Res
827:
243-260,
1999[ISI][Medline].
132.
Nanmoku, T,
Isobe K,
Sakurai T,
Yamanaka A,
Takekoshi K,
Kawakami Y,
Goto K,
and
Nakai T.
Effects of orexin on cultured porcine adrenal medullary and cortex cells.
Regul Pept
104:
125-130,
2002[ISI][Medline].
133.
Näslund, E,
Ehrstrom M,
Ma J,
Hellstrom PM,
and
Kirchgessner AL.
Localization and effects of orexin on fasting motility in the rat duodenum.
Am J Physiol Gastrointest Liver Physiol
282:
G470-G479,
2002
134.
Nishino, S,
Fujiki N,
Ripley B,
Sakurai E,
Kato M,
Watanabe T,
Mignot E,
and
Yanai K.
Decreased brain histamine content in hypocretin/orexin receptor-2 mutated narcoleptic dogs.
Neurosci Lett
313:
125-128,
2001[ISI][Medline].
135.
Nishino, S,
Ripley B,
Overeem S,
Lammers GJ,
and
Mignot E.
Hypocretin (orexin) deficiency in human narcolepsy.
Lancet
355:
39-40,
2000[ISI][Medline].
136.
Novak, CM,
and
Albers HE.
Localization of hypocretin-like immunoreactivity in the brain of the diurnal rodent, Arvicanthis niloticus.
J Chem Neuroanat
23:
49-58,
2002[ISI][Medline].
137.
Nowak, KW,
Mackowiak P,
Switonska MM,
Fabis M,
and
Malendowicz LK.
Acute orexin effects on insulin secretion in the rat: in vivo and in vitro studies.
Life Sci
66:
449-454,
2000[ISI][Medline].
138.
Okumura, T,
Takeuchi S,
Motomura W,
Yamada H,
Egashira Si S,
Asahi S,
Kanatani A,
Ihara M,
and
Kohgo Y.
Requirement of intact disulfide bonds in orexin-A-induced stimulation of gastric acid secretion that is mediated by OX1 receptor activation.
Biochem Biophys Res Commun
280:
976-981,
2001[ISI][Medline].
139.
Olafsdottir, BR,
Rye DB,
Scammell TE,
Matheson JK,
Stefansson K,
and
Gulcher JR.
Polymorphisms in hypocretin/orexin pathway genes and narcolepsy.
Neurology
57:
1896-1899,
2001
140.
Orlando, G,
Brunetti L,
Di Nisio C,
Michelotto B,
Recinella L,
Ciabattoni G,
and
Vacca M.
Effects of cocaine- and amphetamine-regulated transcript peptide, leptin and orexins on hypothalamic serotonin release.
Eur J Pharmacol
430:
269-272,
2001[ISI][Medline].
141.
Overeem, S,
Mignot E,
Gert van Dijk J,
and
Lammers GJ.
Narcolepsy: clinical features, new pathophysiologic insights, and future perspectives.
J Clin Neurophysiol
18:
78-105,
2001[ISI][Medline].
142.
Peyron, C,
Faraco J,
Rogers W,
Ripley B,
Overeem S,
Charnay Y,
Nevsimalova S,
Aldrich M,
Reynolds D,
Albin R,
Li R,
Hungs M,
Pedrazzoli M,
Padigaru M,
Kucherlapati M,
Fan J,
Maki R,
Lammers GJ,
Bouras C,
Kucherlapati R,
Nishino S,
and
Mignot E.
A mutation in a case of early onset narcolepsy and a generalized absence of hypocretin peptides in human narcoleptic brains.
Nat Med
6:
991-997,
2000[ISI][Medline].
143.
Peyron, C,
Tighe DK,
van den Pol AN,
de Lecea L,
Heller HC,
Sutcliffe JG,
and
Kilduff TS.
Neurons containing hypocretin (orexin) project to multiple neuronal systems.
J Neurosci
18:
9996-10015,
1998
144.
Piper, DC,
Upton N,
Smith MI,
and
Hunter AJ.
The novel brain neuropeptide, orexin-A, modulates the sleep-wake cycle of rats.
Eur J Neurosci
12:
726-730,
2000[ISI][Medline].
145.
Porter, RA,
Chan WN,
Coulton S,
Johns A,
Hadley MS,
Widdowson K,
Jerman JC,
Brough SJ,
Coldwell M,
Smart D,
Jewitt F,
Jeffrey P,
and
Austin N.
1,3-Biarylureas as selective non-peptide antagonists of the orexin-1 receptor.
Bioorg Med Chem Lett
11:
1907-1910,
2001[ISI][Medline].
146.
Randeva, HS,
Karteris E,
Grammatopoulos D,
and
Hillhouse EW.
Expression of orexin-A and functional orexin type 2 receptors in the human adult adrenals: implications for adrenal function and energy homeostasis.
J Clin Endocrinol Metab
86:
4808-4813,
2001
147.
Rauch, M,
Riediger T,
Schmid HA,
and
Simon E.
Orexin A activates leptin-responsive neurons in the arcuate nucleus.
Pflügers Arch
440:
699-703,
2000[ISI][Medline].
148.
Ripley, B,
Fujiki N,
Okura M,
Mignot E,
and
Nishino S.
Hypocretin levels in sporadic and familial cases of canine narcolepsy.
Neurobiol Dis
8:
525-534,
2001[ISI][Medline].
149.
Rodgers, RJ,
Halford JC,
Nunes de Souza RL,
Canto de Souza AL,
Piper DC,
Arch JR,
and
Blundell JE.
Dose-response effects of orexin-A on food intake and the behavioural satiety sequence in rats.
Regul Pept
96:
71-84,
2000[ISI][Medline].
150.
Rodgers, RJ,
Halford JC,
Nunes de Souza RL,
Canto de Souza AL,
Piper DC,
Arch JR,
Upton N,
Porter RA,
Johns A,
and
Blundell JE.
SB-334867, a selective orexin-1 receptor antagonist, enhances behavioural satiety and blocks the hyperphagic effect of orexin-A in rats.
Eur J Neurosci
13:
1444-1452,
2001[ISI][Medline].
151.
Russell, SH,
Kim MS,
Small CJ,
Abbott CR,
Morgan DG,
Taheri S,
Murphy KG,
Todd JF,
Ghatei MA,
and
Bloom SR.
Central administration of orexin A suppresses basal and domperidone stimulated plasma prolactin.
J Neuroendocrinol
12:
1213-1218,
2000[ISI][Medline].
152.
Russell, SH,
Small CJ,
Dakin CL,
Abbott CR,
Morgan DG,
Ghatei MA,
and
Bloom SR.
The central effects of orexin-A in the hypothalamic-pituitary-adrenal axis in vivo and in vitro in male rats.
J Neuroendocrinol
13:
561-566,
2001[ISI][Medline].
153.
Russell, SH,
Small CJ,
Sunter D,
Morgan I,
Dakin CL,
Cohen MA,
and
Bloom SR.
Chronic intraparaventricular nuclear administration of orexin A in male rats does not alter thyroid axis or uncoupling protein-1 in brown adipose tissue.
Regul Pept
104:
61-68,
2002[ISI][Medline].
154.
Sakurai, T,
Amemiya A,
Ishii M,
Matsuzaki I,
Chemelli RM,
Tanaka H,
Williams SC,
Richardson JA,
Kozlowski GP,
Wilson S,
Arch JR,
Buckingham RE,
Haynes AC,
Carr SA,
Annan RS,
McNulty DE,
Liu WS,
Terrett JA,
Elshourbagy NA,
Bergsma DJ,
and
Yanagisawa M.
Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior.
Cell
92:
573-585,
1998[ISI][Medline].
155.
Sakurai, T,
Moriguchi T,
Furuya K,
Kajiwara N,
Nakamura T,
Yanagisawa M,
and
Goto K.
Structure and function of human prepro-orexin gene.
J Biol Chem
274:
17771-17776,
1999
156.
Samson, WK,
Gosnell B,
Chang JK,
Resch ZT,
and
Murphy TC.
Cardiovascular regulatory actions of the hypocretins in brain.
Brain Res
831:
248-253,
1999[ISI][Medline].
157.
Samson, WK,
and
Resch ZT.
The hypocretin/orexin story.
Trends Endocrinol Metab
11:
257-262,
2000[ISI][Medline].
158.
Samson, WK,
and
Taylor MM.
Hypocretin/orexin suppresses corticotroph responsiveness in vitro.
Am J Physiol Regul Integr Comp Physiol
281:
R1140-R1145,
2001
159.
Samson, WK,
Taylor MM,
Follwell M,
and
Ferguson AV.
Orexin actions in hypothalamic paraventricular nucleus: physiological consequences and cellular correlates.
Regul Pept
104:
97-103,
2002[ISI][Medline].
160.
Satoh, Y,
Uchida M,
Fujita A,
Nishio H,
Takeuchi T,
and
Hata F.
Possible role of orexin A in nonadrenergic, noncholinergic inhibitory response of muscle of the mouse small intestine.
Eur J Pharmacol
428:
337-342,
2001[ISI][Medline].
161.
Scammell, TE,
Estabrooke IV,
McCarthy MT,
Chemelli RM,
Yanagisawa M,
Miller MS,
and
Saper CB.
Hypothalamic arousal regions are activated during modafinil-induced wakefulness.
J Neurosci
20:
8620-8628,
2000
162.
Schuld, A,
Hebebrand J,
Geller F,
and
Pollmacher T.
Increased body-mass index in patients with narcolepsy.
Lancet
355:
1274-1275,
2000[ISI][Medline].
163.
Shibahara, M,
Sakurai T,
Nambu T,
Takenouchi T,
Iwaasa H,
Egashira SI,
Ihara M,
and
Goto K.
Structure, tissue distribution, and pharmacological characterization of Xenopus orexins.
Peptides
20:
1169-1176,
1999[ISI][Medline].
164.
Shiraishi, T,
Oomura Y,
Sasaki K,
and
Wayner MJ.
Effects of leptin and orexin-A on food intake and feeding related hypothalamic neurons.
Physiol Behav
71:
251-261,
2000[ISI][Medline].
165.
Shirasaka, T,
Miyahara S,
Kunitake T,
Jin QH,
Kato K,
Takasaki M,
and
Kannan H.
Orexin depolarizes rat hypothalamic paraventricular nucleus neurons.
Am J Physiol Regul Integr Comp Physiol
281:
R1114-R1118,
2001
166.
Shirasaka, T,
Nakazato M,
Matsukura S,
Takasaki M,
and
Kannan H.
Sympathetic and cardiovascular actions of orexins in conscious rats.
Am J Physiol Regul Integr Comp Physiol
277:
R1780-R1785,
1999
167.
Smart, D,
Jerman JC,
Brough SJ,
Neville WA,
Jewitt F,
and
Porter RA.
The hypocretins are weak agonists at recombinant human orexin-1 and orexin-2 receptors.
Br J Pharmacol
129:
1289-1291,
2000
168.
Smart, D,
Jerman JC,
Brough SJ,
Rushton SL,
Murdock PR,
Jewitt F,
Elshourbagy NA,
Ellis CE,
Middlemiss DN,
and
Brown F.
Characterization of recombinant human orexin receptor pharmacology in a Chinese hamster ovary cell-line using FLIPR.
Br J Pharmacol
128:
1-3,
1999
169.
Smart, D,
Sabido-David C,
Brough SJ,
Jewitt F,
Johns A,
Porter RA,
and
Jerman JC.
SB-334867-A: the first selective orexin-1 receptor antagonist.
Br J Pharmacol
132:
1179-1182,
2001
170.
Soffin, EM,
Evans ML,
Gill CH,
Harries MH,
Benham CD,
and
Davies CH.
SB-334867-A antagonises orexin mediated excitation in the locus coeruleus.
Neuropharmacology
42:
127-133,
2002[ISI][Medline].
171.
Sunahara, RK,
Dessauer CW,
and
Gilman AG.
Complexity and diversity of mammalian adenylyl cyclases.
Annu Rev Pharmacol Toxicol
36:
461-480,
1996[ISI][Medline].
172.
Sutcliffe, JG,
and
de Lecea L.
The hypocretins: excitatory neuromodulatory peptides for multiple homeostatic systems, including sleep and feeding.
J Neurosci Res
62:
161-168,
2000[ISI][Medline].
173.
Sweet, DC,
Levine AS,
Billington CJ,
and
Kotz CM.
Feeding response to central orexins.
Brain Res
821:
535-538,
1999[ISI][Medline].
174.
Switonska, MM,
Kaczmarek P,
Malendowicz LK,
and
Nowak KW.
Orexins and adipoinsular axis function in the rat.
Regul Pept
104:
69-73,
2002[ISI][Medline].
175.
Szekely, M,
Petervari E,
Balasko M,
Hernadi I,
and
Uzsoki B.
Effects of orexins on energy balance and thermoregulation.
Regul Pept
104:
47-53,
2002[ISI][Medline].
176.
Taheri, S,
Sunter D,
Dakin C,
Moyes S,
Seal L,
Gardiner J,
Rossi M,
Ghatei M,
and
Bloom S.
Diurnal variation in orexin A immunoreactivity and prepro-orexin mRNA in the rat central nervous system.
Neurosci Lett
279:
109-112,
2000[ISI][Medline].
177.
Takahashi, N,
Okumura T,
Yamada H,
and
Kohgo Y.
Stimulation of gastric acid secretion by centrally administered orexin-A in conscious rats.
Biochem Biophys Res Commun
254:
623-627,
1999[ISI][Medline].
178.
Thannickal, TC,
Moore RY,
Nienhuis R,
Ramanathan L,
Gulyani S,
Aldrich M,
Cornford M,
and
Siegel JM.
Reduced number of hypocretin neurons in human narcolepsy.
Neuron
27:
469-474,
2000[ISI][Medline].
179.
Thorpy, M.
Current concepts in the etiology, diagnosis and treatment of narcolepsy.
Sleep Med
2:
5-17,
2001[Medline].
180.
Trivedi, P,
Yu H,
MacNeil DJ,
Van der Ploeg LH,
and
Guan XM.
Distribution of orexin receptor mRNA in the rat brain.
FEBS Lett
438:
71-75,
1998[ISI][Medline].
181.
Uramura, K,
Funahashi H,
Muroya S,
Shioda S,
Takigawa M,
and
Yada T.
Orexin-a activates phospholipase C- and protein kinase C-mediated Ca2+ signaling in dopamine neurons of the ventral tegmental area.
Neuroreport
12:
1885-1889,
2001[ISI][Medline].
182.
Van den Pol, AN.
Hypothalamic hypocretin (orexin): robust innervation of the spinal cord.
J Neurosci
19:
3171-3182,
1999
183.
Van den Pol, AN,
Gao XB,
Obrietan K,
Kilduff TS,
and
Belousov AB.
Presynaptic and postsynaptic actions and modulation of neuroendocrine neurons by a new hypothalamic peptide, hypocretin/orexin.
J Neurosci
18:
7962-7971,
1998
184.
Van den Pol, AN,
Patrylo PR,
Ghosh PK,
and
Gao XB.
Lateral hypothalamus: early developmental expression and response to hypocretin (orexin).
J Comp Neurol
433:
349-363,
2001[ISI][Medline].
185.
Waleh, NS,
Apte-Deshpande A,
Terao A,
Ding J,
and
Kilduff TS.
Modulation of the promoter region of prepro-hypocretin by alpha-interferon.
Gene
262:
123-128,
2001[ISI][Medline].
186.
Wang, J,
Osaka T,
and
Inoue S.
Energy expenditure by intracerebroventricular administration of orexin to anesthetized rats.
Neurosci Lett
315:
49-52,
2001[ISI][Medline].
187.
Wickman, K,
and
Clapham DE.
Ion channel regulation by G proteins.
Physiol Rev
75:
865-885,
1995
188.
Wieland, HA,
Soll RM,
Doods HN,
Stenkamp D,
Hurnaus R,
Lammle B,
and
Beck-Sickinger AG.
The SK-N-MC cell line expresses an orexin binding site different from recombinant orexin 1-type receptor.
Eur J Biochem
269:
1128-1135,
2002
189.
Williams, G,
Harrold JA,
and
Cutler DJ.
The hypothalamus and the regulation of energy homeostasis: lifting the lid on a black box.
Proc Nutr Soc
59:
385-396,
2000[ISI][Medline].
190.
Willie, JT,
Chemelli RM,
Sinton CM,
and
Yanagisawa M.
To eat or to sleep? Orexin in the regulation of feeding and wakefulness.
Annu Rev Neurosci
24:
429-458,
2001[ISI][Medline].
191.
Xi, MC,
Morales FR,
and
Chase MH.
Effects on sleep and wakefulness of the injection of hypocretin-1 (orexin-A) into the laterodorsal tegmental nucleus of the cat.
Brain Res
901:
259-264,
2001[ISI][Medline].
192.
Yamada, H,
Okumura T,
Motomura W,
Kobayashi Y,
and
Kohgo Y.
Inhibition of food intake by central injection of anti-orexin antibody in fasted rats.
Biochem Biophys Res Commun
267:
527-531,
2000[ISI][Medline].
193.
Yamamoto, Y,
Ueta Y,
Date Y,
Nakazato M,
Hara Y,
Serino R,
Nomura M,
Shibuya I,
Matsukura S,
and
Yamashita H.
Down regulation of the prepro-orexin gene expression in genetically obese mice.
Brain Res Mol Brain Res
65:
14-22,
1999[ISI][Medline].
194.
Yamanaka, A,
Sakurai T,
Katsumoto T,
Yanagisawa M,
and
Goto K.
Chronic intracerebroventricular administration of orexin-A to rats increases food intake in daytime, but has no effect on body weight.
Brain Res
849:
248-252,
1999[ISI][Medline].
195.
Yamanaka, A,
Tsujino N,
Funahashi H,
Honda K,
Guan JL,
Wang QP,
Tominaga M,
Goto K,
Shioda S,
and
Sakurai T.
Orexins activate histaminergic neurons via the orexin 2 receptor.
Biochem Biophys Res Commun
290:
1237-1245,
2002[ISI][Medline].
196.
Yoshida, Y,
Fujiki N,
Nakajima T,
Ripley B,
Matsumura H,
Yoneda H,
Mignot E,
and
Nishino S.
Fluctuation of extracellular hypocretin-1 (orexin A) levels in the rat in relation to the light-dark cycle and sleep-wake activities.
Eur J Neurosci
14:
1075-1081,
2001[ISI][Medline].
197.
Yoshimichi, G,
Yoshimatsu H,
Masaki T,
and
Sakata T.
Orexin-A regulates body temperature in coordination with arousal status.
Exp Biol Med (Maywood)
226:
468-476,
2001
198.
Zhang, J,
and
Luo P.
Orexin B immunoreactive fibers and terminals innervate the sensory and motor neurons of jaw-elevator muscles in the rat.
Synapse
44:
106-110,
2002[ISI][Medline].