Department of Biology, University College, London WC1E 6BT, United Kingdom
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ABSTRACT |
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New analgesic drugs are necessary because a number of pain states are untreatable. Genetic approaches to the identification of analgesic drug targets include mapping genes involved in human pain perception (e.g., trkA involved in hereditary neuropathies), identifying regulators of sensory neuron function in simple multicellular organisms and then investigating the activity of their mammalian homologs (e.g., POU domain transcription factors that specify sensory cell fate), as well as difference, expression, and homology cloning of receptors, ion channels, and transcription factors present in sensory neurons. After target validation through the construction of null mutant mice, high-throughput cell-based screens can be used to identify potential drug candidates. As a result of these approaches, a number of receptors and ion channels present in sensory neurons such as voltage-gated sodium channels [sensory neuron specific (SNS) and Na channel novel] and ATP-gated (P2X3), capsaicin-gated [vanilloid receptor 1(VR1)], and proton-gated [acid-sensing ion channel (ASIC)] channels are now under investigation as potential new analgesic drug targets.
nociception; dorsal root ganglia neurons; channels; receptors
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ARTICLE |
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GENETIC APPROACHES to the study of pain
pathways have relied upon three strategies. First, deficits in pain
perception in human or mouse strains have been mapped and the relevant
genes identified. Using this approach, mutations in TrkA have been
found to account for the human chronic insensitivity to pain with
anhydrosis (CIPA) syndrome (17). However, genetic studies in humans
have yet to identify any novel targets that may be useful for analgesic drug development. Recent studies on pain-related behavior in different mouse strains suggest that this approach may be more fruitful in the
hunt for analgesic drug targets (16). A second approach uses simple
multicellular organisms as models of humans. Mutants that show deficits
in the responses to noxious stimuli in Caenorhabditis elegans
or Drosophila melanogaster have allowed the identification of a
number of genes that play an important role in the specification of
sensory neuronal phenotype; many of these factors have mammalian homologs that also play a role in sensory neurogenesis. Once again, however, little progress in the discovery of novel analgesic targets has come from these analyses, although many useful insights into mammalian sensory neurogenesis have been obtained (reviewed in Ref. 5).
A third approach, discussed here, relies on the identification of genes
that are only expressed in damage-sensing sensory neurons (2). A high
proportion of peripheral sensory neurons are specifically involved with
the detection of tissue damage (nociception). Molecular genetics,
combined with advances in electrophysiology and cell biology, is now
enabling us to dissect the molecular mechanisms involved in this
process. Some of the genes that encode the receptor subtypes, ion
channels, and regulators of gene expression that are present in
nociceptive sensory neurons but not other cell types have been
catalogued (Ref. 2; Table 1).
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Gene deletion studies in mice can shed light on the physiological role of these various proteins. This information is potentially useful in terms of developing novel strategies for analgesic drug development and also provides insights into the function of one of the best-characterized classes of mammalian neuronal subtypes. A further approach to defining functionally significant molecules in nociception is to identify genes that are regulated in conditions of acute or chronic pain or nerve damage. The application of differential display methods and gene-chip technology to identify pain state-modulated transcripts has recently also become an area of intense research activity.
A number of recent reviews deal with the heterogeneity of sensory
neuron subtypes found in cranial and dorsal root ganglia (DRG) (5, 29).
Sensory neurons can be defined in terms of their sensory modality or
the expression of histochemical markers, functional channels,
receptors, and neuropeptides. Damage-sensing small-diameter sensory
neurons fall into two roughly equal populations with distinct trophic
factor requirements. About one-half of the neurons express the nerve
growth factor (NGF) receptors TrkA and p75, whereas a separate
population, which can be surface labeled with isolectin B4 (IB4), are
TrkA and p75 negative and have a trophic requirement for glia-derived
neurotrophic factors (GDNF). Three other GDNF-related factors have been
identified. Persephin, neurturin, and artemin as well as GDNF signal
through an unusual receptor complex comprising the tyrosine kinase
receptor c-Ret associated with transforming growth factor- (TGF-
)
receptor-like subunits that define the specificity of ligand binding.
The central terminals of these two classes of nociceptors are found in
different areas of lamina II of the spinal cord, but the functional
significance of this organizational difference is as yet unknown (29).
Chemical mediators that are known to evoke a sensation of pain include
acids, ATP, and chilli pepper extracts. Specific receptors for all
these molecules have recently been identified on sensory neurons, and
some of these receptors are highly selective in their expression
pattern. Low pH is often found in damaged tissue (26). A proton-gated
cation channel named acid-sensing ion channel (ASIC) expressed
throughout the nervous system is expressed on DRG neurons. The ASIC
channels comprise a two-transmembrane subunit that places them in the
family of amiloride-sensitive epithelial sodium channels (ENaC).
Interestingly, amiloride has recently been shown to have analgesic
effects in a variety of animal pain models (14). The ASIC family
includes the mammalian degenerin (MDEG) channels, renamed ASIC2 and
-2
, and a related clone expressed in DRG sensory neurons named
DRASIC or ASIC3 (27). Interestingly, all the ASIC clones have highly
conserved extracellular cysteine residues, suggesting that the overall
topology of the extracellular domain is similar within this particular
class of receptor. ASIC3/DRASIC is found in large-diameter sensory
neurons that may be mechanoreceptive. Two splice variants
of an additional channel called MDEG homologs have been identified.
MDEG1 is a functional proton-gated ion channel, but MDEG2 is a splice
variant that does not function except as a heteromultimer with ASIC or
DRASIC. MDEG1 and MDEG2 are also known as ASIC2
and ASIC2
,
respectively. Only ASIC2
is found in sensory neurons. A splice
variant of ASIC1 with a unique 5'-region and the same COOH
terminal and 3'-untranslated region as ASIC-
named ASIC-
has also been recently identified by homology cloning. ASIC-
has a
unique 172-amino acid NH2-terminal region with a putative
first transmembrane domain that is homologous to DRASIC channel
sequence (27, 29). A single ASIC-
3.2-kb transcript is expressed in
sensory neurons and other tissues. A similar-size transcript is
expressed by ASIC-
in DRG alone and is present only in a subset of
small-diameter sensory neurons. The functional properties of ASIC-
are subtly different from those of other members of this family. Unlike
ASIC-
, both DRASIC and ASIC-
show fast and slow components of
proton-evoked channel opening. However, the slow response of ASIC-
is less marked than with DRASIC and is unaffected by the application of
amiloride, which blocks the fast component in both channels but
potentiates the slow component in DRASIC. Interestingly, in sensory
neurons in culture, proton-evoked ion fluxes are calcium permeable,
which implies that either the capsaicin-gated cation channel or a novel combination of ASIC subunits mediates proton-evoked fluxes.
Identification of sensory neuron-specific (SNS) channels that alone or
in combination account for a long-lasting component of proton-evoked
cation flux provides another potential new route to the development of
anti-inflammatory analgesic drugs.
The injection of ATP into human blister bases can evoke pain consistent
with a direct action of ATP on nociceptive sensory neurons (27). These
effects are probably mediated by the P2X class of ATP-gated ion
channels that have been shown to be expressed by DRG neurons. Such
receptors comprise a family of glycosylated proteins of apparent
molecular mass of ~60 kDa, which have intracellular NH2-
and COOH-terminal domains, two membrane-spanning hydrophobic domains,
the second of which lines the channel pore, and a large extracellular
domain. The subunits are encoded by different genes, although alternatively spliced transcripts of particular subtypes also
occur. The channels are 35-50% identical at the amino acid level
and are permeable by both divalent cations. Despite apparent structural
similarity to the ASIC family, P2X receptors have no sequence identity
with these channels and have evolved a similar topological organization
by convergent evolution. Seven different P2X receptors have been cloned
and expressed in Xenopus oocytes, and their distribution has
been analyzed by in situ hybridization. Six P2X-subtype mRNA
transcripts are expressed in sensory neurons of the dorsal root,
nodose, and trigeminal ganglia. One subtype, P2X3, is expressed
selectively in small-diameter damage-sensing sensory neurons (10).
Indirect evidence for heteromultimeric channels in sensory neurons has
been provided by coexpression studies of distinct P2X subunits in
Xenopus oocytes. Some nodose ganglion cells desensitize slowly
in response to ,
-methylene-ATP. However, the oocyte-expressed
P2X3 receptor desensitizes rapidly. If P2X2 (first identified in PC12
cells) is coexpressed with P2X3 in oocytes, then a slowly desensitizing
form of the receptor is formed (23). Structural studies using
cross-linking reagents suggest that monomeric receptors may exist as
trimers, unlike other known classes of ion channels. Recent evidence
suggests that P2X3 is expressed predominantly by c-Ret-positive
GDNF-sensitive sensory neurons that comprise a subset of the nociceptor
population (29). Evidence of a role for P2X3 in nociception comes from in vivo studies after subplantar injection of ATP and the ultrapotent congener
,
-methylene-ATP, which shows some selectivity for P2X3 in rat paws. Hindpaw lifting and licking occurred in animals injected subplantar with
,
-methylene-ATP (6). The hypothesis that P2X3
receptors have a specialized role in nociception is supported by the
observation that P2X3 immunostaining and ATP-induced inward current
indeed appear in fluorescence-traced nociceptors of rat tooth pulp. In
the absence of specific inhibitors for P2X subtypes, the generation of
null mutants for P2X receptors is a route to defining their function.
P2X1, -2, -3, and -4 null mutants are now available, and an analysis of
their responses to noxious stimuli is underway. Preliminary studies
suggest that all the ATP-evoked fast densensitizing responses of
primary sensory neurons from DRG in culture are lost in mouse P2X3 null
mutant mice.
Chilli pepper extracts containing capsaicin cause pain, and recently a sensory neuron-specific receptor [vanilloid receptor 1 (VR1)] activated both by capsaicin and noxious heat (46°C) was cloned (8, 9). With the use of a rat DRG cDNA library in a shuttle vector, pools of clones that conferred a capsaicin response were isolated and individual clones that conferred capsaicin sensitivity were characterized. Increases in intracellular calcium in HEK-293 cells transfected with defined pools of cDNA clones were measured using the calcium-sensitive fura 2. The functional receptor resembles members of the "TRP dye" family of proteins in terms of topological organization. TRP proteins are six-transmembrane monomers first identified in Drosophila in the transient receptor potential (TRP) mutants, which show deficits in photoreception. VR1 has the characteristic NH2-terminal ankyrin repeats, and considerable sequence similarity is also apparent in, but is limited to, the sixth transmembrane domain, its flanking sequences, and the loop between transmembrane domains 5 and 6 believed to form part of the pore region. Six mammalian trp genes have been cloned and sequenced, either through initial searches of expressed sequence tag databases using the Drosophila trp amino acid sequence as the query sequence or through degenerate RT-PCR using primers designed from the highly conserved regions (transmembrane domains 5 and 6) of Drosophila trp and trpl: trp1, -3, -4, and -6 have been sequenced in their full length, and partial sequences have also been reported for trp2 and -5, the former of which may be a pseudogene. Functional expression of the available full-length clones in mammalian cells enhances capacitative calcium entry (CCE), whereas expression of trp cDNA fragments in an antisense orientation interfered with endogenous CCE providing a direct connection between trp(s) and store-operated calcium entry.
The capsaicin receptor VR1 probably exists in a multimeric form (Hill coefficient of 2 for capsaicin) as a cation-selective ion channel with a preference for calcium but does not appear to play any role in CCE. Whether VR1 can heteromultimerize with other TRPs or further VR1-like receptors is as yet unknown, but there is certainly strong evidence for an additional VR1-like receptor present in mast cells, and a VR1-like protein that is gated by insulin-like growth factor has recently been identified. There appear to be at least four distinct VR receptors based on analysis of sequence databases, although only information on VR1 and VR-like (VR-L) receptors is available in the literature (29). The existence of other VR1-like receptors and the large number of TRP family members raises the possibility that members of this channel family may be gated by ligands other than noxious heat. It is already clear that trp(s) play a critical role in phototransduction in Drosophila, and evidence that bradykinin, a hyperalgesic mediator, can gate trp3 via activation of Gq heterotrimeric G proteins has been obtained in a heterologous expression system. Interestingly, brain-derived neurotrophic factor (BDNF) has also been shown to be capable of activating TRP3, suggesting a role for these channels in mediating neurotrophin signaling (22). Homology cloning and expression studies of VR1 in combination with other trp channels and their regulation by G protein-coupled receptors should provide interesting evidence not only on mechanisms of nociception but also for the normal gating mechanisms of trp channels. Interestingly, VRs are also gated by low pH, often found at sites of tissue injury. An inward current is evoked by bradykinin, prostaglandin E2, and 5-hydroxytryptamine (5-HT) in sensory neurons at low pH but not at physiological pH (26), and the current is blocked by 10 µM Capsazepine. It is thus possible that the VR/ion channel is responsible for the inward current.
The contribution of ligand-gated channels to the formation of mechanosensitive ion channel complexes should also be considered. C fibers in isolation can be activated mechanically, suggesting that receptors are present on the sensory neurons themselves (29). Of the mechanosensitive channels defined by genetic studies of C. elegans and D. melanogaster, all seem to fall into the class of two transmembrane receptors exemplified by the ENaC/degenerin family (see, e.g., Ref. 1). Expression cloning of rat mechanoreceptors in Xenopus oocytes suggested that ATP released through mechanical distortion may activate the P2Y1 receptor present on large-diameter sensory neurons, whereas more active mechanical distortion causes activation of the P2X3 receptor (24). A variety of mechanisms for mechanosensation may thus exist. Two novel Drosophila DEG/ENaC proteins, Pickpocket (PPK) and Ripped Pocket (RPK), appear to be ion channel subunits (1). Expression of RPK generated multimeric sodium channels that were dominantly activated by a mutation associated with neurodegeneration. Amiloride and gadolinium, which block mechanosensation in vivo, inhibited RPK channels. Although PPK did not form channels on its own, it associated with and reduced the current generated by a related human brain sodium channel. The vertebrate homologs of these channels are thus candidates for mechanoreception. Studies in C. elegans suggest that trp channels may also play a role in mechanosensory transduction (29).
Apart from channels activated by ligands released from damaged tissue,
or noxious mechanical and thermal stimuli, there are also voltage-gated
sodium channels that are characteristically expressed only by
damage-sensing neurons (3, 18, 28). Sodium channels (VGSC) present in
both the peripheral and central nervous systems contain a large
membrane-spanning -subunit of 260 kDa, which has four repeated
domains of six transmembrane segments. In addition, there are
associated regulatory subunits, a
1-subunit of 36 kDa
and a covalently associated
2-subunit of 33 kDa. There is indirect evidence that a number of different
2-subunits exist. The
-subunit mRNAs can direct the
translation of functional channels. However, the accessory
1- and
2-subunits enhance functional channel expression in Xenopus oocytes and regulate the kinetic properties of expressed channels. The functional voltage-gated sodium
channel
-subunit SNS (3), also homology cloned and named PN3, is
particularly interesting because it corresponds to an unusual type of
sodium channel present in small-diameter sensory neurons that is
resistant to the puffer fish poison TTX. Indirect evidence suggests
that the TTX-resistant (TTXr) sodium channel plays a unique role
in the transmission of nociceptive information to the spinal
cord. Thus bradykinin-dependent release of
calcitonin gene-related peptide (CGRP), as well as the depolarization of dorsal horn cells elicited through C-fiber activation, was apparently insensitive to peripherally applied TTX (29).
The sodium channel transcripts present in dorsal root ganglia have been explored by Northern blots and in situ hybridization. The neuronal forms type I and II are present, whereas the embryonic type III reappears after axotomy (28). Both SNS and the TTX-sensitive (TTXs) channel PN1 are present at high levels in peripheral neurons. PN1, type I, NaCh6, and type II TTXs transcripts occur in descending order of abundance. The atypical sodium channel NaG is expressed predominantly by Schwann cells but is also found in sensory neurons. Only the small-diameter sensory neuron-specific SNS and NaN subunits are exclusively present in small-diameter sensory neurons, however. NaN or SNS2 was identified in small-diameter sensory neurons recently (3, 12, 28). The channel contains the appropriate sodium selectivity filters and voltage sensor motifs, although it has three fewer positively charged residues in its S4 domains than SNS. Very interestingly, the channel expresses a serine residue at the same position of the TTX binding site as SNS, suggesting that the channel is likely to be TTX resistant. There is evidence from studies of SNS knockout mice that NaN encodes a persistent sodium current, which is very slowly inactivating around the resting membrane potential, where this residual TTXr sodium current is detectable.
Gold et al. (15) and England et al. (13) demonstrated the functional modulation of TTX-insensitive (TTXi) VGSC activity in sensory neurons by inflammatory mediators that are known to lower pain thresholds. Prostaglandin E2, adenosine, and serotonin all increased the magnitude of sodium current, shifted its conductance-voltage relation in a hyperpolarizing direction, and increased the rates of activation and inactivation of sodium channels in small-diameter sensory neurons in culture. Such data suggest that TTXi sodium currents play an important role in regulating pain thresholds.
Although SNS does not appear to be completely dependent on NGF for expression, there is an upregulation of both the transcript and the protein, together with the appearance of an unusual, and apparently nonfunctional, splice variant on addition of NGF to sensory neurons in culture. In addition, a soluble factor released by Schwann cells that may well be NGF seems to be involved in the upregulation of SNS expression (28). Given the restricted expression of trkA on the neuronal subpopulation that is principally concerned with nociception, NGF regulation of sodium channels, particularly SNS, may play a role in regulating inflammatory pain thresholds. Interestingly, IB4-positive GDNF-sensitive small-diameter sensory neurons also express SNS, consistent with the observation that other factors, such as GDNF, regulate SNS expression in this class of nociceptor. In contrast to the upregulation of TTXr currents and SNS transcripts with inflammatory mediators, a variety of manipulations that lead to neuropathic pain (ligature-induced and diabetic neuropathies) lead to a downregulation of SNS expression. This suggests that SNS may play a more significant role in inflammatory than neuropathic pain states. However, there is increasingly strong evidence that the type III embryonic form of sodium channel is upregulated in neuropathic pain states. The rapid repriming nature of this channel makes it a very exciting analgesic drug target for analgesic therapies targeting damaged peripheral nerves (28).
The antihyperalgesic agent BW2040W92 (29) appears to be a use-dependent blocker of TTXr activity in sensory neurons, and some of its antihyperalgesic actions may be ascribed to SNS block, although the compound also acts on TTXs channels. These observations provide further indirect support for a role for SNS in setting pain thresholds. Because the unique low-affinity TTX binding site in the SNS channel atrium has been partially defined, substituted guanidines specifically directed against SNS could provide an additional approach to selective channel block. A direct approach to determine the functional significance of SNS is to ablate the expression of the channel in a null mutant mouse and measure the behavioral and electrophysiological consequences. Studies of such null mutant mice demonstrate that all TTXr activity found in sensory neurons apart from a persistent sodium current probably corresponding to NaN is encoded by SNS (4). The behavioral correlates of a loss of SNS expression are of major interest in terms of nociceptive processing. Behavioral analysis depends on careful manipulation of genetic backgrounds by repeated back-crosses to produce congenic strains, as well as the assessment of any compensatory developmental changes that may occur in sns null mutants to obtain meaningful data. The loss of SNS seems to lead to a compensatory upregulation of expression of the TTXs channel PN1, with lower thresholds of electrical excitation of C fibers in null mutants. There are, nonetheless, major deficits in pain pathways in these null mutants, in particular in response to noxious mechanical stimulation, emphasizing the important role of SNS in nociception (4). Interestingly, low-dose systemic lidocaine dramatically enhances the analgesic phenotype of the mutant mouse at concentrations that do not affect wild-type mouse behavior. As TTXs currents are more sensitive to lidocaine than TTXr currents, this effect suggests that the analgesic phenotype of the SNS null mutant would be much more dramatic but for the compensatory upregulation of TTXs channels in these animals. The generation of NaN knockouts and the analysis of the phenotype of the combined SNS/NaN knockouts will be particularly illuminating in assessing the significance of TTXr currents for nociception and pain processing.
A number of receptors characterized at the molecular level, such as
prostanoid and serotonin receptors, appear to play a functional role in
nociception, as well as being broadly expressed elsewhere. The
prostacyclin receptor seems to be particularly important in the
induction of hyperalgesia, as the null mutant does not exhibit acetic
acid-induced writhing behavior. Binding and pharmacological studies
have also identified a number of 5-HT receptor subtypes on sensory
neurons. 5-HT1A, 5-HT1D, 5-HT2, and
5-HT3 are all known to be present. The 5-HT3
receptor (previously classified as a -5 nicotinic receptor subunit)
is the only directly gated cation channel present and is found on
~40% of DRG neurons grown in tissue culture. It is known to form a
heteromeric cation channel. Unmyelinated C fibers seem to respond
particularly well to 5-HT. Apart from these direct actions mediated
through the 5-HT3 receptor, the actions of other chemical
or mechanical activators of sensory neurons are enhanced by 5-HT acting
through other receptor subtypes. Glutamate receptors, nicotinic and
muscarinic receptors, and bradykinin B2 receptors also
occur on nociceptive sensory neurons (29). Interestingly, an
epibatidine-related compound (ABT-594) has potent analgesic activity
and may act on nociceptor presynaptic nicotinic receptors in the spinal
cord. It would therefore be very valuable if tissue-specific knockouts
could be generated for these globally expressed receptors to dissect
their role in nociception and pain induction. One way to do this is to
generate mice that express the bacterial recombinase Cre in DRG neurons
alone. The site-specific DNA recombinase Cre mediates the recombination
of two repeated target sites (lox-P) to a single lox-P site, with
concomitant excision of the DNA segment flanked by the lox-P sites.
Gene excision can be achieved by mating two different animals, one
carrying a target gene flanked by lox-P sites and one carrying a Cre
transgene (21). Mice expressing a functional Cre recombinase under the control of a number of DRG-specific promoters have been constructed and
should prove useful for tissue-specific ablation of floxed genes in DRG
neurons in the near future.
The exploitation of transgenic knockout mice to provide target
validation for potential new programs of analgesic drug discovery has
accelerated over the past few years. In particular, light on the second
messenger-mediated events that involve isozymes of various
tissue-specific protein kinases has identified protein kinase C
(PKC)- as a potentially important contributor to mechanisms of
peripheral hyperalgesia (18a), whereas PKC-
seems to play a specific
role in the mechanism of neuropathic pain development (29). In terms of
classical mediators implicated in pain pathways, there have been some
surprises with the demonstration of the effect of neurokinin
antagonists in humans, in whom they seem to exert profound
antidepressant activity (20) compared with the apparent primary role of
the neurokinin system in pain perception in mice (7, 11, 30). Knockouts
of opioid peptides and their cognate receptors have tended to support
our pharmacologically derived preconceptions about the role of these
receptor systems as important components of stress-induced analgesia
systems (19).
In summary, the application of molecular genetics to the study of pain mechanisms is providing useful new information. In particular, the identification of molecular targets for analgesic drug development has benefited from these studies. A better understanding of the regulatory sequences that specify sensory neuron-specific gene expression may also be useful in designing screens for modulators that alter protein expression rather than function. Such sequences will allow tissue-specific gene ablation using the Cre-lox system that will enable us to examine the contribution of broadly expressed genes to nociceptive pathways. Despite the difficulties in interpreting the phenotypic consequences of gene deletion in mouse mutant models, validation of a number of new potential drug targets has been possible. The efforts of the pharmaceutical industry will determine how quickly this information is translated into useful therapeutic agents.
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FOOTNOTES |
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* Second in a series of invited articles on Pathobiology of Visceral Pain: Molecular Mechanisms and Therapeutic Implications.
Address for reprint requests and other correspondence: J. N. Wood, Dept. of Biology, University College, London WC1E 6BT, UK (E-mail: J.Wood{at}ucl.ac.uk).
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REFERENCES |
---|
![]() ![]() ![]() ![]() |
---|
1.
Adams, CM,
Anderson MG,
Motto DG,
Price MP,
Johnson WA,
and
Welsh MJ.
Ripped pocket and pickpocket, novel drosophila DEG/ENaC subunits expressed in early development and in mechanosensory neurons.
Cell Biol Int
140:
143-152,
1998.
2.
Akopian, AN,
and
Wood JN.
Peripheral nervous system-specific genes identified by subtractive cDNA cloning.
J Biol Chem
270:
21264-21270,
1995
3.
Akopian, AN,
Sivilotti L,
and
Wood JN.
A tetrodotoxin-resistant sodium channel expressed by C-fibre associated sensory neurons.
Nature
379:
257-262,
1996[ISI][Medline].
4.
Akopian, AN,
Souslova V,
England S,
Okuse K,
Ogata N,
Smith UA,
Kerr BJ,
McMahon S,
Boyce S,
Hill R,
Stanfa L,
Dickenson A,
and
Wood JN.
The tetrodotoxin-resistant sodium channel SNS plays a specialised role in pain pathways.
Nature Neurosci
2:
541-548,
1999[ISI][Medline].
5.
Anderson, DJ.
Lineages and transcription factors in the specification of vertebrate primary sensory neurons.
Curr Opin Neurobiol
9:
517-524,
1999[ISI][Medline].
6.
Bland-Ward, PA,
and
Humphrey P.
Acute nociception mediated by hindpaw P2X receptor activation in the rat.
Br J Pharmacol
122:
365-371,
1997[Abstract].
7.
Cao, YQ,
Mantyh PW,
Carlson EJ,
Gillespie AM,
Epstein CJ,
and
Basbaum AI.
Primary afferent tachykinins are required to experience moderate to intense pain.
Nature
392:
390-394,
1998[ISI][Medline].
8.
Caterina, MJ,
Schumacher MA,
Tominaga M,
Rosen TA,
Line JD,
and
Julius D.
The capsaicin receptor: a heat-activated ion channel in the pain pathway.
Nature
389:
816-824,
1997[ISI][Medline].
9.
Cesare, P,
and
McNaughton P.
A novel heat-activated current in nociceptive neurons and its sensitization by bradykinin.
Proc Natl Acad Sci USA
24:
15435-15439,
1996.
10.
Chen, CC,
Akopian AN,
Sivilotti L,
Colquhoun D,
Burnstock G,
and
Wood JN.
A P2X purinoceptor expressed by a subset of sensory neurons.
Nature
377:
428-431,
1995[ISI][Medline].
11.
De Felipe, C,
Herrero JF,
O'Brien JA,
Palmer JA,
Doyle CA,
Smith AJ,
Laird JM,
Belmonte C,
Cervero F,
and
Hunt SP.
Altered nociception, analgesia and aggression in mice lacking the receptor for substance P.
Nature
392:
394-397,
1998[ISI][Medline].
12.
Dib-Hajj, SD,
Tyrrell L,
Black JA,
and
Waxman SG.
NaN, a novel voltage-gated Na channel, is expressed preferentially in peripheral sensory neurons and down-regulated after axotomy.
Proc Natl Acad Sci USA
95:
8963-8968,
1998
13.
England, S,
Bevan S,
and
Docherty RJ.
PGE2 modulates the tetrodotoxin-resistant sodium current in neonatal rat DRG neurones via the cAMP-protein kinase A cascade.
J Physiol (Lond)
495:
429-440,
1996[Abstract].
14.
Ferreira, J,
Santos AR,
and
Calixto JB.
Antinociception produced by systemic, spinal and supraspinal administration of amiloride in mice.
Life Sci
65:
1059-1066,
1999[ISI][Medline].
15.
Gold, MS,
Reichling DB,
Shuster MJ,
and
Levine JD.
Hyperalgesic agents increase a tetrodotoxin-resistant Na current in nociceptors.
Proc Natl Acad Sci USA
93:
1108-1112,
1996
16.
Hain, HS,
Belknap JK,
and
Mogil JS.
Pharmacogenetic evidence for the involvement of 5-hydroxytryptamine (serotonin)-1B receptors in the mediation of morphine antinociceptive sensitivity.
J Pharmacol Exp Ther
291:
444-449,
1999
17.
Indo, Y,
Tsuruta M,
Hayashida Y,
Karim MA,
Ohta K,
Kawano T,
Mitsubuchi H,
Tonoki H,
Awaya Y,
and
Matsuda I.
Mutations in the TRKA/NGF receptor gene in patients with congenital insensitivity to pain with anhidrosis.
Nat Genet
13:
485-488,
1996[ISI][Medline].
18.
Khasar, SG,
Gold MS,
and
Levine JD.
A tetrodotoxin-resistant sodium current mediates inflammatory pain in the rat.
Neurosci Lett
256:
17-20,
1998[ISI][Medline].
18a.
Khasar, SG,
Lin Y-H,
Martin A,
Dadgar J,
McMahon T,
Wang D,
Hundle B,
Aley KO,
Isenberg W,
McCarter G,
Green PG,
Hodge CW,
Levine JD,
and
Messing RO.
A novel nociceptor signaling pathway revealed in protein kinase C-epsilon mutant mice.
Neuron
24:
253-260,
1999[ISI][Medline].
19.
Kieffer, BL.
Opioids: first lessons from knockout mice.
Trends Pharmacol Sci
20:
19-26,
1999[ISI][Medline].
20.
Kramer, MS,
Cutler N,
Feighner J,
Shrivastava R,
Carman J,
Sramek JJ,
Reines SA,
Liu G,
Snavely D,
Wyatt-Knowles E,
Hale JJ,
Mills SG,
MacCoss M,
Swain CJ,
Harrison T,
Hill RG,
Hefti F,
Scolnick EM,
Cascieri MA,
Chicchi GG,
Sadowski S,
Williams AR,
Hewson L,
Smith D,
and
Rupniak NM.
Distinct mechanism for antidepressant activity by blockade of central substance P receptors.
Science
281:
1640-1645,
1998
21.
Kuhn, R,
Schwenk F,
Aguet M,
and
Rajewsky K.
Inducible gene targeting in mice.
Science
269:
1427-1429,
1995[ISI][Medline].
22.
Li, H-S,
Xu X-ZS,
and
Montell C.
Activation of a TRPC3-dependent cation current through the neurotrophin BDNF.
Neuron
24:
261-273,
1999[ISI][Medline].
23.
Lewis, C,
Neidhart S,
Holy C,
North RA,
Buell G,
and
Surprenant A.
Heteropolymerization of P2X receptor subunits can account for ATP-induced current in sensory neurons.
Nature
377:
432-435,
1995[ISI][Medline].
24.
Nakamura, F,
and
Strittmatter SM.
P2Y1 purinergic receptors in sensory neurons: contribution to touch-induced impulse generation.
Proc Natl Acad Sci USA
93:
10465-10470,
1996
26.
Vyklicky, L,
Knotkova-Urbancova H,
Vitaskova Z,
Vlachova V,
Kress M,
and
Reeh PW.
Inflammatory mediators at acidic pH activate capsaicin receptors in cultured sensory neurons from newborn.
J Neurophysiol
79:
670-676,
1998
27.
Waldmann, R,
and
Lazdunski M.
H(+)-gated cation channels: neuronal acid sensors in the NaC/DEG family of ion channels.
Curr Opin Neurobiol
8:
418-424,
1998[ISI][Medline].
28.
Waxman, SG.
The molecular pathophysiology of pain: abnormal expression of sodium channel genes and its contributions to hyperexcitability of primary sensory neurons.
Pain
6:
S133-S140,
1999.
29.
Wood, JN,
and
Perl ER.
Pain.
Curr Opin Genet Dev
9:
328-332,
1999[ISI][Medline].
30.
Zimmer, A,
Zimmer AM,
Baffi J,
Usdin T,
Reynolds K,
Konig M,
Palkovits M,
and
Mezey E.
Hypoalgesia in mice with a targeted deletion of the tachykinin 1 gene.
Proc Natl Acad Sci USA
3:
2630-2635,
1998.