Department of Cell Biology and Anatomy, Louisiana State University Health Sciences Center, New Orleans, Louisiana 70112
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ABSTRACT |
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Lo, Fu-Sun and Reha S. Erzurumlu. Neonatal Deafferentation Does Not Alter Membrane Properties of Trigeminal Nucleus Principalis Neurons. J. Neurophysiol. 85: 1088-1096, 2001. In the brain stem trigeminal complex of rats and mice, presynaptic afferent arbors and postsynaptic target cells form discrete modules ("barrelettes"), the arrangement of which duplicates the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. Within the barrelette region of the nucleus principalis of the trigeminal nerve (PrV), neurons participating in barrelettes and those with dendritic spans covering multiple barrelettes (interbarrelette neurons) can be identified by their morphological and electrophysiological characteristics as early as postnatal day 1. Barrelette cells have focal dendritic processes, are characterized by a transient K+ conductance (IA), whereas interbarrelette cells with larger soma and extensive dendritic fields characteristically exhibit low-threshold T-type Ca2+ spikes (LTS). In this study, we surveyed membrane properties of barrelette and interbarrelette neurons during and after consolidation of barrelettes in the PrV and effects of peripheral deafferentation on these properties. During postnatal development (PND1-13), there were no changes in the resting potential, composition of active conductances and Na+ spikes of both barrelette and interbarrelette cells. The only notable changes were a decline in input resistance and a slight increase in the amplitude of LTS. The infraorbital (IO) branch of the trigeminal nerve provides the sole afferent input source to the whisker pad. IO nerve transection at birth abolishes barrelette formation as well as whisker-related neuronal patterns all the way to the neocortex. Surprisingly this procedure had no effect on membrane properties of PrV neurons. The results of the present study demonstrate that distinct membrane properties of barrelette and interbarrelette cells are maintained even in the absence of input from the whiskers during the critical period of pattern formation.
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INTRODUCTION |
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The patterned array of whisker
follicles and sinus hairs on the snout of nocturnal rodents is
replicated by the distribution of whisker-specific trigeminal axon
arbors and select sets of postsynaptic neurons in several sensory
nuclei of brain stem trigeminal complex (BSTC) (Bates and
Killackey 1985; Belford and Killackey 1979
;
Erzurumlu et al. 1980
; Ma 1993
; Ma
and Woolsey 1984
). The whisker-specific neuronal modules in the
BSTC are referred to as "barrelettes" (Ma and Woolsey
1984
). Barrelette neurons of the nucleus principalis of the
trigeminal nerve (PrV) convey this pattern to the ventroposteromedial
nucleus of the thalamus, where their synaptic partners, the
thalamocortical projection cells form the "barreloids," which in
turn relay this information to the layer IV granule cells of the
primary somatosensory cortex where the "barrels" form
(Belford and Killackey 1980
; Van der Loos
1976
; Van der Loos and Woolsey 1973
). Barrelette
formation in the BSTC begins shortly before birth and is consolidated
by postnatal day (PND) 5 (Belford and Killackey 1980
;
Chiaia et al. 1992
; Erzurumlu and Killackey
1983
; Ma 1993
; O'Leary et al.
1994
; Woolsey 1990
). Many lines of evidence
indicate that central whisker-related patterns depend on an intact
sensory periphery during a critical period in development (Bates
et al. 1982
; Belford and Killackey 1980
;
Durham and Woolsey 1984
), and more recent
studies indicate that patterning of neural elements is mediated via
N-methyl-D-aspartate (NMDA) receptors
(Iwasato et al. 1997
, 2000
; Kutsuwada et al. 1996
; Li et al. 1994
). Disruptions of the
sensory periphery before PND 3-4 irreversibly alter the development
and maintenance of whisker-specific central neural patterns (see
Erzurumlu and Killackey 1982
; Jhaveri and
Erzurumlu 1992
; O'Leary et al. 1994
;
Woolsey 1990
for reviews). In mice with targeted
mutations of the critical subunit of the NMDA receptor, these patterns
fail to develop (Iwasato et al. 1997
; Li et al.
1994
).
Previous morphological studies showed that the barrelette
region of the PrV contains three types of cells: barrelette,
interbarrelette, and GABAergic cells (Ginestal and Matute
1993; Ma 1991
, 1993
). Barrelette cells have
dendritic trees confined to a single barrelette (Arends and
Jacquin 1993
) and project to the thalamus (Erzurumlu et
al. 1980
). Interbarrelette cells have dendritic trees spanning several barrelettes and contribute internuclear connections within the
BSTC (Jacquin et al. 1990
; Nasution and Shigenaga
1987
). GABAergic cells participate in local inhibitory
circuits. Recently we reported that in the PrV, barrelette, and
interbarrelette neurons can be distinguished by their
electrophysiological properties and morphological features (Lo
et al. 1999
). By using whole cell patch recording, immuno-histochemistry, and intracellular biocytin staining techniques, we showed that barrelette cells receive excitatory inputs from one
whisker and a strong lateral inhibition from neighboring whiskers. Interbarrelette cells receive excitatory inputs from multiple sources
and a weak lateral inhibition. In both types of cells, the excitation
is mediated by both NMDA and non-NMDA receptors, whereas the inhibition
is exclusively mediated by GABAA receptors. Specific electrophysiological properties of barrelette neurons undoubtedly play a role in transfer of "pattern"-related
information from incoming trigeminal afferents to barrelette cells or
conveying this information to upstream trigeminal centers such as the
dorsal thalamus (Erzurumlu and Guido 1996
). In
the first part of the present study, we investigated developmental
changes in membrane properties of neurons in the barrelette region of
PrV of neonatal rats (PND1-13). As early as at PND 1, PrV neurons can
be electrophysiologically identified as barrelette and interbarrelette
cells. The barrelette cell is characterized by a prominent transient
K+ conductance
(IA), whereas the interbarrelette
cell, by a low-threshold T-type Ca2+ spike (LTS).
During postnatal development from PND 1 to 13, there are no changes in
resting potential, composition of active conductances, and
Na+ spike of both barrelette and interbarrelette
cells except a decline of input resistance and a slight increase in
amplitude of the LTS. These observations suggest that membrane
properties of PrV neurons are well developed at birth.
The infraorbital (IO) branch of the trigeminal nerve carries all the
information from the whiskers and sinus hairs to the BSTC and if
sectioned or partially damaged during the first few days after birth,
related central patterns are completely or partially abolished in a
predictable fashion (e.g., see Bates and Killackey 1985;
Belford and Killackey 1979
, 1980
; Durham and
Woolsey 1984
; Yip et al. 1987
). In the second
part of the present study, we examined whether peripheral
deafferentation affects membrane properties of PrV neurons in the
barrelette region. Surprisingly, the membrane properties of PrV neurons
after denervation were indistinguishable from those with an intact IO
nerve up to a week after nerve transection.
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METHODS |
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Brain slice preparation
Sprague-Dawley rat pups ranging in age from PND 1 to 13 were deeply anesthetized with fluothane (Halothane) and then killed by decapitation. The brain was removed and immersed in cold (4°C), sucrose-based artificial cerebrospinal fluid [ACSF, containing (in mM): 234 sucrose, 2.5 KCl, 1.25 NaH2PO4, 10 MgSO4, 24 NaHCO3, 11 glucose, and 0.5 CaCl2 bubbled with 95% O2-5% CO2 (pH = 7.4)]. The brain stem was embedded in 2% agar and cut into 500-µm-thick transverse sections with a vibratome (Electron Microscopy Sciences). Slices containing the PrV were placed in a submerged-type recording chamber (Fine Science Tools) and continuously perfused (2 ml/min) with normal ACSF [containing (in mM) 124 NaCl, 2.5 KCl, 1.25 NaH2PO4, 2 MgSO4, 26 NaHCO3, 10 glucose, and 2 CaCl2, pH = 7.4] at room temperature.
Electrophysiological methods
Recordings began 1 h after incubation in normal ACSF. Whole
cell patch micropipettes were pulled horizontally in two stages from
borosilicate glass (WPI, K150F-4) with a P-87 puller (Sutter Instrument). The patch electrodes were backfilled with a
potassium-based solution [containing (in mM) 140 K-Gluconate, 10 HEPES, 1.1 EGTA-Na, 0.1 CaCl2, 2 MgCl2, 2 ATP-Na, and 0.2 GTP-Na, pH = 7.25]
with a tip resistance of 7-10 M
. Neurons in the ventral part of the PrV (barrelette region) were blindly patched with the techniques described by Blanton et al. (1989)
and Ferster
and Jagadeesh (1992)
. In brief, patch-electrode resistance was
monitored in Bridge Mode of Axoclamp 2B amplifier by measuring the
voltage drop induced by a current pulse (
100 pA, 200 ms). An increase
in resistance of 20-50 M
was taken as a sign that the tip of the
electrode contacted the surface of a neuron. A steady negative pressure was applied with a 5-ml syringe to form a gigaohm seal. Then a brief
suction was used to break into the neuronal soma. The formation of
whole cell configuration was indicated by a sudden drop in seal
resistance and a DC drop of >55 mV. After "break-in," the serial
resistance was completely compensated with bridge balance, and junction
potential (Neher 1992
) was not corrected. We only collected data from cells with resting membrane potential negative to
55 mV and input resistance >200 M
with an Instrutek VR10B interface unit and stored on a Macintosh Power PC (9500/132) with Pulses (HEKA) software program.
Input resistance of recorded neurons was determined by the voltage drop
induced by a small negative current pulses (10 pA, 200 ms) at resting
potential. Different DC pulse protocols were used to induce active
conductances of trigeminal neurons. The identification of various
active conductances was based on their voltage dependency and
pharmacology. For example, the transient K+
conductance (IA) was specifically
blocked by 1 mM 4-aminopyradine (4-AP), the hyperpolarization-activated
H conductance was blocked by 1 mM CsCl2, and the
low-threshold Ca2+ T-type conductance was blocked
by 200 µM NiCl2. To compare voltage-dependent conductances among different cells, the membrane potential was held at
60 mV except in cases where indicated.
For biocytin-labeling experiments, we filled the patch electrodes with 1% biocytin dissolved in potassium-based solution. Once membrane properties and synaptic responses were characterized, the cells were filled intracellularly with biocytin by passing AC pulses (±1 nA, 60 ms for each cycle, 100 cycles) through the biocytin-filled recording electrode.
IO nerve transection
In a separate set of experiments, PND 0 pups were anesthetized, and a unilateral incision was made caudal to the whisker pad. The IO nerve can be visualized under the skin and connective tissues as it emerges from the infraorbital foramen. The nerve was then cut with sterile microscissors just between the eye and the caudal edge of the whisker pad. The wound was sutured, and the pup allowed to recover from anesthesia. The lesions were made unilaterally, therefore allowing for direct comparisons in brain stem slices between the lesioned and unoperated sides. Once the pups recovered from anesthesia, they were returned to the home cage with their mother. At PND 4-8, these pups were killed by an overdose of Fluothane, and the brain stems were dissected out after decapitation. The brain stems were rinsed in ACSF with sucrose and sectioned into slices as described in the preceding text for recordings. In each case, the lesioned and normal sides were marked. Whole cell recordings were then performed in the ventral (barrelette) part of the PrV.
Histological methods
One hour after biocytin injection, the slice was fixed by 4% paraformaldehyde in 0.1 M phosphate buffer for 48 h. The fixed slice was transferred into phosphate-buffered saline (PBS) at 4°C and then incubated in 10% methanol +3% H2O2 overnight. After several rinses in PBS, the slice was reacted with avidin-biotin complex (ABC Elite kit, Vector Laboratories) overnight at 4°C (1:100 in PBS with 1.8% NaCl and 0.5% Triton X-100). Next day, the slice was rinsed again in PBS and 0.1 M acetate buffer (pH = 6.0) and incubated in glucose oxidase-nickel ammonium sulfate and diaminobenzidine until the labeled cells could be visualized. The slice was rinsed in acetate buffer and PBS, mounted on to a slide, dehydrated, and coverslipped. Labeled cells were drawn with a drawing tube attached to a Nikon Labophot microscope.
For denervation experiments, cytochrome oxidase histochemistry
(Wong-Riley and Welt 1980) was used as an anatomical
marker for barrelettes at PND5. Briefly, the rat pups were overdosed with pentobarbital sodium and perfused transcardially with PBS followed
by 4% paraformaldehyde in 0.1 M phosphate buffer (pH = 7.4).
After overnight fixation at 4°C, the brains were transferred to 30%
sucrose in phosphate buffer at 4°C for cryoprotection. The brain stem
was sectioned frozen in the coronal plane at a thickness of 75 µm.
Sections were incubated in a solution of 90 ml PBS, 40 mg cytochrome C,
10 mg diaminobenzidine and 4 g sucrose at 37°C until they turned
golden brown. At this point, the reaction was stopped by several rinses
in PBS, mounted on to glass slides, and coverslipped with a
glycerol-based mounting medium. Photomicrographs from these sections
were taken using Coolsnap computerized imaging system. The figures were
made with the use of ADOBE Photoshop 4 program. No alterations were
made in the images except for adjustments of brightness and contrast.
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RESULTS |
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Physiological identification of barrelette and interbarrelette cells
We recorded voltage responses to intracellular current pulses from
77 neurons in the barrelette region of PrV of normal neonatal rats (PDN
1-13) and 15 neurons from denervated rats. Barrelette neurons showed a
transient K+ conductance
(IA) and a hyperpolarization-acitvated
mixed cation conductance (IH). These
cells were further identified as barrelette neurons by intracellular
biocytin labeling (Fig. 1A)
(also see Lo et al. 1999). Membrane hyperpolarization
from resting potential resulted in a "depolarizing sag" (Fig. 1,
B and C, indicated by H) that was blocked by
Cs2+ (see also Lo et al. 1999
). On
the cessation of current pulses, the repolarization of membrane
potential was delayed by a hyperpolarizing tail (indicated by A in Fig.
1B). Depolarization from a hyperpolarizing state exhibited a
hyperpolarizing notch (indicated by A in Fig. 1C) that
delayed the generation of Na+ spikes. The notch
was blocked by application of 4-AP (Lo et al. 1999
).
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Interbarrelette neurons possess a prominent LTS mediated by T-type
Ca2+ channels and an H conductance. Intracellular
biocytin labeling shows that these cells have long dendritic trees that
span multiple barrelette territories (Fig. 1D) (also see
Arends and Jacquin 1993; Lo et al. 1999
).
In these neurons, membrane hyperpolarization also caused a depolarizing
sag (Fig. 1, E and F, indicated by H).
Repolarization (Fig. 1E) or depolarization (Fig.
1F) from hyperpolarizing state induced a triangular LTS
(Fig. 1, E and F, indicated by T) that proved to
be mediated by T-type Ca2+ channels (Lo et
al. 1999
). We next surveyed developmental changes in membrane
properties of barrelette and interbarrelette cells and followed
peripheral denervation as described in the following text.
Resting membrane potential of barrelette and interbarrelette neurons during postnatal development
Scatter plots of resting membrane potentials of both barrelette
and interbarrelette neurons (Fig.
2A) exhibited little changes between PND 1 and 13. The values for barrelette (, n = 49) and interbarrelette cells (
, n = 28)
intermingled at all postnatal ages. We arbitrarily divided animals into
three age groups: PND 1-3 (sensitive period, during which
whisker-specific patterns can be altered by whisker lesions or IO nerve
damage), PND 4-6 (consolidation period, during which whisker-specific
patterns are consolidated and are no longer subject to structural
changes following whisker lesions or IO nerve damage), and PND 7-13
(mature, adult-like period). The averaged resting potentials of
barrelette cells were
60.0 ± 1.20 mV (n = 10)
at PND 1-3,
63.1 ± 1.16 mV (n = 20) at PND
4-6, and
61.3 ± 0.48 mV (n = 19) at PND 7-13. There was no significant difference (P > 0.05) in the
averaged resting potentials among the three different age groups (Fig. 2B). The averaged resting potentials of interbarrelette
cells were
61.2 ± 1.29 mV (n = 7) at PND 1-3,
60.1 ± 1.04 mV (n = 12) at PND 4-6, and
60.3 ± 1.13 mV (n = 9) at PND 7-13. There was
no difference in resting membrane potential (P > 0.05)
during postnatal development either. Furthermore there was no
difference (P > 0.05) between barrelette and
interbarrelette cells at each age group examined. Therefore resting
membrane potential for both barrelette and interbarrelette cells at
birth are similar to those seen 2 wk later when the pups have already
begun whisking behavior.
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Decrease in input resistance during postnatal development
Scatter plotting of input resistances of barrelette and
interbarrelette neurons (Fig. 2C) suggested a decrease in
input resistance during postnatal development. For barrelette cells,
the mean input resistance at PND 1-3 (742.9 ± 86.7 M) was
similar to that at PND 4-6 (749.8 ± 47.9 M
, P > 0.05). However, the mean input resistance at PND 7-13 (519.0 ± 43.8 M
) was significantly lower than that at PND 1-3
(P < 0.02, Fig. 2D,**). The mean input
resistance of interbarrelette cells was 659.6 ± 111.9 M
at PND
1-3, then it declined to 488.0 ± 55.4 M
at PND 4-6 and
finally to 346.1 ± 37.5 M
at PND 7-13, which was
significantly smaller than seen at PND 1-3 (P < 0.02, Fig. 2D, **). The decline of input resistance for
interbarrelette cells appeared earlier than for barrelette cells,
suggesting that interbarrelette cells may develop faster than
barrelette cells. At PND 1-3, there was no difference in averaged
input resistance between barrelette cells and interbarrelette cells
(P > 0.05). However, at PND 4-6 and 7-13, the input
resistance of interbarrelette cells was significantly smaller
(P < 0.02) than that of barelette cells (Fig.
2D, ##). This also suggests that postnatal growth of
interbarrelette cells is more prominent than barrelette cells.
Development of active conductances
During postnatal development, active conductances of barrelette cells (n = 49) did not change noticeably. At PND 1-3, barrelette cells possessed A and H conductances and Na+ spikes (Fig. 3A) with similar characteristics observed at PND 4-6 (Fig. 3C) and at PND 7-13 (Fig. 3E), suggesting that active conductances of barrelette cells are fully developed at birth. In interbarrelette cells, the LTS was already present at PND 1-3 (Fig. 3B). However, the amplitude of LTS slightly increased during development (Fig. 3, D and F) as shown by the scatter plot of LTS amplitudes (Fig. 4A). The mean amplitude of LTS at PND 7-13 (15.2 ± 1.4 mV) was ~48% larger (P < 0.05) than that at PND 1-3 (10.7 ± 1.1 mV, Fig. 4B). In addition, the duration of LTS appeared to get shorter in older PrV neurons (Fig. 3, B, D, and F). Since we did not apply K+ channel blockers in the present study, we could not tell if the shortening was caused by a change in kinetics of Ca2+ channels or by an increase in K+ conductance. Nevertheless the number of Na+ spikes that superimposed on the LTS decreased during development (Fig. 4, C and D).
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Denervation abolishes patterning of PrV neurons into barrelettes but does not affect their membrane properties
Cytochrome oxidase (CO) or succinic dehydrogenase (SDH)
histochemistry are reliable markers that reveal whisker-specific
patterns formed by presynaptic terminals and their postsynaptic targets at various stations along the trigeminal neuraxis (Belford and Killackey 1980; Yip et al. 1987
). Following IO
nerve transection at birth, CO staining showed absence of
whisker-specific patterns in the BSTC on the denervated side by PND5
(Fig. 5). We recorded 15 neurons from the
ventral part (equivalent barrelette region of normal animals) of the
denervated PrV at PND 4-8. All recorded neurons could also be
classified as "barrelette" or "interbarrelette" neurons by
their electrophysiological properties. The resting membrane potentials
and input resistances of "barrelette" cells were plotted together
with barrelette cells of normal rats, and there were no differences
between the two groups (Fig. 6,
A and C). Similarly "interbarrelette" cells
on the denervated side had resting potentials and input resistances
indistinguishable from the undamaged side (Fig. 6, B and
D). In summary, early peripheral denervation, which alters
the organization of barrelettes and consequently barreloids and
barrels, did not change the resting potential and developmental
decrease in input resistance of neurons in PrV. Active conductances of
PrV neurons of denervated rats were similar to those of normal rats. As
shown in Fig. 7, "barrelette" cells
exhibited prominent A and H conductances (Fig. 7, A and B), whereas "interbarrelette" cells showed T and H
conductances (Fig. 7, C and D).
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DISCUSSION |
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Using whole cell recording and biocytin-labeling techniques, we have studied membrane properties of neurons in the barrelette region of PrV in neonatal rats (PND 1-13). Morphologically identified barrelette cells are characterized by a transient K+ conductance (IA), whereas interbarrelette cells by a low-threshold Ca2+ conductance (IT). During postnatal development, the resting potential, composition of active conductances, and Na+ spike remain unchanged. The main changes are a decrease in input resistance and a slight increase in amplitude of LTS. Denervation by cutting infraorbital nerve at birth abolishes barrelettes in PrV but does not affect passive membrane properties, composition of active conductances, and Na+ spikes. Therefore membrane properties of PrV neurons are well developed at birth and are not regulated by afferent input from the whiskers during early postnatal days.
Functional significance of active conductances of barrelette and interbarrelette cells
From PND 1 to 13, all barrelette cells show a transient K+ conductance (IA), whereas all interbarrelette cells show a low-threshold Ca2+ conductance (IT). Therefore the two conductances may be considered as physiological signatures of the two types of cells. Because we have not recorded from GABAergic neurons in PrV, we do not know if they have class-specific conductances.
Our preliminary voltage-clamp results from barrelette cells demonstrate
that A-type K+ channels are largely inactivated
at resting potential (data not shown). Thus
IA may not contribute much to the
resting potential of barrelette cells. The function of
IA is likely to modulate the
inhibitory postsynaptic potential (IPSP) in barrelette cells. Trigeminal inputs induce a long-lasting IPSP in barrelette cells even
at PND 1 (Erzurumlu and Lo 1999; Lo et al.
1999
). The hyperpolarization caused by the IPSP may
deinactivate A-type channels. Therefore when the IPSP is repolarized,
A-type channels are activated so that the IPSP is prolonged and the
rebound spike at the end of the IPSP is delayed. Meanwhile the IPSP may
also activate H conductance, which curtails the IPSP and enhances the
rebound excitation. Thus the IPSP in barrelette cells could be
dynamically controlled by these two conductances. The IPSP in
barrelette cells is mediated by both feedforward and lateral inhibitory
circuits (Lo et al. 1999
). The lateral inhibition from
surrounding whiskers sharpens the receptive field, whereas the
feedforward inhibition curtails postsynaptic excitation. Thus
barrelette cells in PrV transmit peripheral vibrissal signals to the
thalamus with a sharpened receptive field and phasic temporal
properties. Another function of the IPSP is to counteract NMDA
receptor-mediated excitatory postsynaptic potential (EPSP) (Lo
et al. 1999
; Ramoa and McCormick 1994b
). A
change in the IPSP may secondarily regulate Ca2+
influx through NMDA receptors. The activity of NMDA receptors plays a
critical role in whisker-specific pattern formation in the rodent BSTC
(Iwasato et al. 1997
; Kutsuwada et al.
1996
; Li et al. 1994
). Present results suggest
that activity of NMDA receptors may also be regulated by
IA and
IH through their control on the IPSP.
Interbarrelette cells receive excitatory inputs from multiple sources
and probably represent the activation level of PrV (Lo et al.
1999). Since interbarrelette cells connect to other subnuclei of the BSTC (Jacquin et al. 1990
; Nasution and
Shigenaga 1987
), their function is probably to coordinate the
activity of different brain stem trigeminal subnuclei. Interbarrelette
cells have both IT and
IH. These conductances are
attributable to the generation of rhythmic bursting activity
(Luthi and McCormick 1998
; Soltesz et al.
1991
; Steriade et al. 1993
). Such activity has
been observed on "bursting neurons" of gerbil PrV, which are
similar to our interbarrelette cells (Sandler et al.
1998
). Therefore the bursting activity of interbarrelette cells
may enhance coding efficiency and reliability for trigeminal
information as also observed in the lateral geniculate nucleus
(Reinagel et al. 1999
).
Membrane properties of neurons during development
Postnatal development of membrane properties has been studied in a
variety of brain structures, such as the neocortex (Annis et al.
1993; Hugenard et al. 1988
; Kang et al.
1996
; Kasper et al. 1994
; McCormick and
Prince 1987
; Zhou and Hablitz 1996
), hippocampus (Spigelman et al. 1992
), striatum (Cepeda et al.
1991
), thalamic nuclei (MacLeod et al. 1997
;
Perez Velazquez and Carlen 1996
; Pirchio et al.
1997
; Remoa and McCormick 1994a
;
Tennigkeit et al. 1998
; Warren and Jones
1997
), cranial nerve nuclei (Bao et al. 1995
;
Berger et al. 1995
, 1996
; Kandler and Friauf
1995
; Nunez-Abades et al. 1993
; Tsuzuki
et al. 1995
; Viana et al. 1994
; Vincent
and Tell 1999
), cerebellum (Groul et al. 1992
;
Molnar 1999
), spinal cord (Cameron et al.
1991
; Gao and Ziskind-Conhaim 1998
;
Martin-Caraballo and Greer 1999
), peripheral ganglia
(Donnelly 1999
), and retina (Rothe et al.
1999
; Wang et al. 1997
). Generally, a decline of input resistance has been observed in most of the tested structures, and this may reflect the growth of neurons after birth. An increase (more negative) in resting membrane potential is also observed in most
forebrain structures and cerebellar Purkinje cells. This is probably
caused by an increase of Na+,
K+ pump density (Molnar et al.
1999
).
Most tested cranial nerve nuclei neurons do not exhibit developmental
changes in resting potential and Na+ spike
amplitude, but they do show a decrease in input resistance and
Na+ spike duration. The PrV cells we recorded
from also fall into this category, except they demonstrate an unchanged
Na+ spike duration. Interestingly, within the
BSTC, neurons of the subnucleus interpolaris of the spinal trigeminal
nucleus (SPI), show a dramatic increase in Na+
spike amplitude (Guido et al. 1998). In contrast to PrV,
SPI exhibits an up-regulation of H conductance and a late emergence of
A-type K+ conductance (Guido et al.
1998
). These differences between PrV and SPI neurons may be
related to their different morphologies and projection patterns. In
both the present study and the former study, development of
electrophysiological properties of PrV and SPI neurons were studied
after birth. It would be interesting to see if there are any changes
that occur during late embryonic development during which
whisker-specific patterning within the BSTC begins to form
(Chiaia et al. 1992
).
Development of membrane properties independent of afferent input
Denervation by cutting the infraorbital nerve at birth abolishes
barrelettes in PrV, but membrane properties of PrV neurons are not
different from normal animals, suggesting that the development of
membrane properties in PrV does not depend on normal synaptic input.
This result agrees with the observation on somatosensory cortex in
which the development of Na+ spike waveform is
independent of synaptic afferents from other structures (Annis
et al. 1993). Thus it is still an open question whether
membrane properties contribute to pattern formation and its maintenance
in the rodent somatosensory pathway. In future studies, it would be of
interest to see whether membrane properties of brain stem trigeminal
neurons are altered in mice with targeted deletion of the NR1 subunit
of the NMDA receptors (Li et al. 1994
) or in mice with
transgenically lowered levels of NMDA receptor function (Iwasato
et al. 1997
), as both phenotypes lacking barrelettes in the PrV.
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ACKNOWLEDGMENTS |
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We thank A. Haeberle for help with histology and preparation of figures.
This project was supported by National Institute of Neurological Disorders and Stroke Grant NS-37070 to R. S. Erzurumlu.
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FOOTNOTES |
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Address for reprint requests: F.-S. Lo, Dept. of Cell Biology and Anatomy, Louisiana State University Medical Center, 1901 Perdido St., New Orleans, LA 70112 (E-mail: flo{at}lsuhsc.edu).
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REFERENCES |
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