Chem. Senses 24: 37-46,
1998
© Oxford University Press
NMDA and non-NMDA Receptors Mediate Responses in the Primary Gustatory Nucleus in Goldfish
Department of Cellular and Structural Biology, University of Colorado Health Sciences Center, Denver, CO 80262, USA 1 Department of Pharmacology, University of Colorado Health Sciences Center, Denver, CO 80262, USA
Correspondence to be sent to: Dr Cynthia A. Smeraski, Department of Cellular and Structural Biology, Campus Box B111, University of Colorado Health Sciences Center, 4200 E. Ninth Ave., Denver, CO 80262, USA
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Abstract |
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Primary gustatory afferents from the oropharynx of the goldfish, Carassius auratus, terminate in the vagal lobe, a laminated structure in the dorsal medulla comparable to the gustatory portion of the nucleus of the solitary tract in mammals. We utilized an in vitro brain slice preparation to test the role of different ionotropic glutamate receptor subtypes in synaptic transmission of gustatory information by recording changes in field potentials after application of various glutamate receptor antagonists. Electrical stimulation of the vagus nerve (NX) evokes two short-latency postsynaptic field potentials from sensory layers of the vagal lobe. 6,7-Dinitroquinoxaline-2,3-dione and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, two non-N-methyl-D-aspartate (NMDA) ionotropic receptor antagonists, blocked these short-latency potentials. Slower potentials that were revealed under Mg2+-free conditions, were abolished by the NMDA receptor antagonist, D(–)-2-amino-5-phosphonovaleric acid (APV). Repetitive stimulation produced short-term facilitation, which was attenuated by application of APV. These results indicate that the synaptic responses in the vagal lobe produced by stimulation of the gustatory roots of the NX involve both NMDA and non-NMDA receptors. An NMDA receptor-mediated facilitation may serve to amplify incoming bursts of primary afferent activity.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Excitatory amino acids (EAAs) are the neurotransmitters of primary afferent fibers in many sensory systems. Recent evidence from immunocytochemical and electrophysiological studies indicate that glutamate or glutamate receptors mediate neurotransmission at the primary central targets of visual (Li et al., 1996
), auditory (Hackney et al., 1996), somatosensory (Clements and Beitz, 1991), olfactory (Berkowitz et al., 1994) and
vomeronasal (Dudley and Moss, 1995) systems. Recent studies of the
gustatory system in mammals and fish indicate that glutamate receptors are involved in gustatory
transmission as well (Finger, 1994; Wang and Bradley, 1995; Li and Smith, 1997; Smeraski et al.,
1998).
Study of neurotransmitters involved at the primary gustatory nucleus in mammals, the
nucleus of the solitary tract (nTS), is complicated by the fact that the nTS not only receives taste
information from the oropharynx via facial, glossopharyngeal and vagus nerves, but also receives
general visceral sensory information from cardiovascular, respiratory and gastrointestinal
systems
via the glossopharyngeal and vagus nerves (Hamilton and Norgren, 1984;
Loewy, 1990). Although the nTS in mammals is organized into subnuclei
according to a rough somatotopic/viscerotopic mapping of afferents from oropharyngeal and
visceral regions, there is extensive rostrocaudal overlap of functional domains (Whitehead and Frank, 1983; Hamilton and Norgren, 1984; Altschuler et
al., 1989; Loewy, 1990; Takagi et al., 1995). This distribution of diverse projections
along the rostrocaudal extent of the nTS makes it difficult to study in vitro the synaptic
transmission intrinsic to only the gustatory system. Further, electrical stimulation of the solitary
tract in in vitro preparations of the mammalian nTS not only activates gustatory fibers,
but is likely to activate intranuclear, interneuronal systems as well (Brooks et al., 1992; Kawai and Senba, 1996). Conversely,
determining the relative contributions of glutamate receptor subtypes in vivo is difficult
without control over the extracellular medium and antagonist concentrations. Nonetheless, both in vitro and in vivo studies of the rostral nTS suggest that both non-NMDA
and NMDA glutamate receptor subtypes contribute to gustatory transmission (Wang
and Bradley, 1995; Li and Smith, 1997). Likewise, in caudal,
non-gustatory nTS, although fast synaptic transmission involves primarily non-NMDA receptors
(Andresen and Yang, 1990; Andresen and Kunze, 1994), a recent intra-cellular study has found that NMDA receptor-mediated currents also
contribute to synaptic transmission between visceral afferent fibers and nTS neurons (Aylwin et al., 1997).
Studies of the mammalian gustatory system have focused on rostral regions of the nTS that receive input from facial and glossopharyngeal nerves. In contrast, studies of synaptic transmission that focus on more caudal regions of the nTS involve vagal input associated with general visceral systems (e.g. cardiovascular). Thus, the relative contributions of glutamate receptor subtypes to gustatory transmission of the vagus nerve remains unclear.
The gustatory system in goldfish, unlike mammalian systems, has a clear segregation of
gustatory nuclei from general visceral sensory nuclei, as well as a clear separation of vagal from
facial or glossopharyngeal gustatory nuclei. Vagal gustatory afferents project topographically (Morita and Finger, 1985) to a specialized vagal lobe used in food
selection and retention (Sibbing et al., 1986). The separation of
gustatory from general visceral sensory input and the overall organization of the vagal lobe (Morita and Finger, 1985) make it an ideal structure in which to examine
the physiological and pharmacological properties of a primary vagal gustatory nucleus.
Extracellular recordings from an in vitro slice preparation (Finger
and Dunwiddie, 1992) revealed two short-latency negative-going postsynaptic
potentials evoked from sensory layers of the vagal lobe following electrical stimulation of
incoming vagal gustatory root fibers. The elimination of these responses by kynurenic acid, a
non-specific glutamate antagonist, suggests that EAA receptors may mediate synaptic
transmission in this primary vagal gustatory nucleus (Finger, 1994; Smeraski et al., 1998). Here we report the respective
contributions of NMDA and non-NMDA receptors on responses evoked by stimulation of the
vagal primary gustatory fibers.
Portions of the data reported below have been presented in abstract form (Smeraski et al., 1996a,b; Finger et
al., 1997).
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Materials and methods |
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Slice preparation
For the initial experiments we prepared goldfish (10–20 cm, Carassius auratus) for in vitro physiological recordings as described previously (Finger
and Dunwiddie, 1992). The brains were removed and blocked in the transverse plane
following transcardial perfusion with cold, oxygenated artificial cerebrospinal fluid (aCSF, see
below). Subsequent slicing (300–800 µm thick) on a Vibratome involved
mounting the vagal lobe on a platform with acrylate tissue glue (Vetbound, 3M, St Paul, MN)
and embedding the lobe in 2%agar. Latter preparations of the vagal lobe eliminated prior
perfusion with aCSF and the use of a Vibratome. Instead, after rapid removal of the brain from
the skull, the brain was placed on an operating platform made of Sylgard (Dow Corning,
Midland, MI).The vagal lobe then was cut n the transverse plane with a scalpel and the slices
(500 µm–1 mm thick) covered with oxygenated aCSF.
Extracellular recordings
Vagal lobe slices were placed in a recording chamber and superfused continuously with fresh
oxygenated aCSF (20–25°) at a flow rate of 2 ml/min. Teflon-coated nichrome
bipolar electrodes connected to a Medical Systems stimulus isolation unit were used to stimulate
the afferent gustatory fibers (NX, vagus nerve, Figure 1). Generally,
single pulses (0.2 ms in duration at 20 V) or a pair of pulses (60 ms apart) were applied every
5–15 s while searching for responses from local regions (within layers VI–VIII) of
the sensory layers of the vagal lobe. The paired-pulse stimulation paradigm has the advantage of
permitting assessment of both the initial response (following the first stimulus of a pair), as well
as any short-term facilitation/ inhibition of the second response (Finger and
Dunwiddie, 1992). The recording electrode (2M NaCl-filled glass micropipette) was
placed at a site along the cut surface of the vagal lobe that exhibited the maximum amplitude of
the evoked postsynaptic population responses at a given stimulus strength (see Results; Finger and Dunwiddie, 1992). After the optimal recording site was
determined, stimulus strengths were reduced to evoke approximately one half to two thirds the
maximal amplitude response. Stimulation rates for paired-pulse paradigms (with
inter-pulse-intervals [PIs]of 60 ms) were every 15, 30 or 60 s. For repeated
stimulation, trains of five pulses with IPIs of 15 ms were applied once every 60 or 90 s for
5–6 min, followed by a period of no stimulation for 5–10 min.
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Pharmacology
Known concentrations of antagonists were tested by using calibrated syringe pumps to inject antagonists into the flow of aCSF (2 ml/min) that entered the recording chamber. Different concentrations of antagonists were obtained by adjusting the speed of the syringe pump. The stock solutions of antagonists were 100–1000 times the final chamber concentration, so injection of drugs did not significantly affect the flow rate or pH. All drug concentrations listed below refer to final bath concentrations. Antagonists were applied for 15–30 min, to allow for bath equilibration and until the evoked waveform stabilized. The interval between application of different drugs was at least 20–30 min, to allow the evoked responses to approach initial control levels or stabilize after washout of the drug. However, in cases when a combination of drugs was used, each drug was added to the bathing medium sequentially, so that effects of the first drug alone were observed. In some cases the order of application of each drug in subsequent trials was then reversed. Antagonists at concentrations selective for non-NMDA receptors were 6,7-dinitroquinoxaline-2,3-dione (DNQX; 5 or 10 µM) and 6-nitro-7-sulphamoyl-benzo[f]quinoxaline-2,3-dione (NBQX or its disodium salt; 1 or 5 µM). D(–)-2-amino-5-phosphonovaleric acid (APV; 50 µM, dissolved in water) was applied to block NMDA receptors. The antagonists DNQX, NBQX and NBQX disodium salt were obtained from Tocris Cookson, Inc. (Ballwin, MO), APV from Sigma (St Louis, MO). DNQX and NBQX were first dissolved in a small quantity (~200 µl) of dimethyl sulfoxide (DMSO; Sigma), then diluted with water so that the final concentration of DMSO in the bathing medium was not greater than 0.05% The lipophilic antagonists DNQX and NBQX do not typically wash out quickly from tissue. In our goldfish slices, both of the drugs required long (~ 2 h) washout times for recovery, in those cases when recovery occurred. These long times may be due to a variety of technical factors, but are most likely related to the thicker slices used in the present studies.
aCSF medium
Standard aCSF contained (in mM) 131 NaCl, 20 NaHCO3, 2 KCl, 1.25 KH2PO4 monobasic, 2 MgSO4, 2.5 CaCl2, 10 dextrose,
pH 7.0–7.2 after oxygenation (Mathieson and Maler, 1988). In
addition to standard aCSF, Mg2+-free aCSF (MgSO4 omitted and
CaCl2 increased to 4.5 mM) was also used as a bathing medium to reveal any
responses blocked by the presence of Mg2+. Also, Ca2+-free aCSF
bathing medium was made similar to the standard aCSF, but contained 2.5 mM MgCl2 and no CaCl2.
Data analysis
Electrophysiological data were amplified using a high gain differential AC amplifier, digitized with an A/D converter (RC Electronics) and subsequently analyzed by a PC based software developed in the laboratory (of T.V.D.). Evoked field potentials from single slices were averaged over several minutes (5–36 sweeps) of stable responding before, during and after application of antagonists or change in bathing medium (aCSF). Changes in amplitude of the postsynaptic responses under antagonistic conditions were characterized as a percent of the control (without antagonists) and exemplified changes in synaptic efficacy. A change of 20%from control was used as a conservative cutoff to indicate a substantial change in response amplitude was due to antagonist application.
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Results |
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Non-NMDA receptor antagonists: DNQX and NBQX
As has been previously reported, electrical stimulation of vagal afferents
evokes a complex response; the two principal negative-going components (N2 and N3, Figure
2A) are synaptically mediated (Finger and Dunwiddie, 1992). Both 2 and N3 peak responses were abolished following application of a selective
non-NMDA receptor antagonist, DNQX (10 µM, n = 7 slices). In four of
the seven slices, the amplitudes of both N2 and N3 population responses approached control
levels within 30–100 min of washout of the antagonist (Figure 2).
In the remaining three slices, only the N2 peak showed minor recovery to control levels
(
40%of control amplitude) even after 40–75 min of washout of the
antagonist.
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Another non-NMDA competitive antagonist, NBQX, has been reported to be more effective than DNQX in blocking AMPA responses (Yu and Miller, 1995
). At 5 µM concentrations, NBQX blocked both N2 and N3 responses (n
=4 slices, not shown). Recovery of the evoked responses following removal of NBQX
(60–140 min) from the bathing medium was minimal in three slices; neither response (N2
or N3) recovered in the fourth slice. Thus the two non-NMDA antagonists DNQX and NBQX
are effective at blocking the fast synaptic responses to gustatory nerve stimulation. Mg2+-free aCSF and NMDA receptor antagonist: APV
To test whether NMDA receptors are involved in primary afferent
activation of sensory neurons in the vagal lobe, the brain slices were superfused with Mg2+-free aCSF. The removal of Mg2+ from the
medium eliminates the voltage-dependent Mg2+-block of NMDA
receptors and can reveal slower wave components characteristic of NMDA receptor-mediated
responses (McBain and Mayer, 1994). Responses evoked in Mg2+-free medium are illustrated in Figures 3 and 4. Following vagal
nerve stimulation in Mg2+-free medium, an
additional negative-going potential with longer peak latency (~10–15 ms) and duration
(20–40 ms) typically was uncovered. The amplitudes of N2 and N3 population responses
to the first stimulus pulse of a pair also were generally enhanced, which is likely attributable to
increased transmitter release in the Mg2+-free high-Ca2+
buffer. These changes in the response waveform under Mg2+-free conditions
suggest that NMDA receptors are present but make little contribution to the synaptic currents
evoked by single pulse stimulation in standard aCSF conditions.
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Addition of APV, a potent competitive NMDA receptor antagonist, blocked the slower component evoked under Mg2+-free conditions. The recovery phase of the evoked responses (following the first stimulus pulse) returned to baseline sooner (horizontal dotted line, Figures 3
C and 4C), as in control
conditions. Application of APV in Mg2+-free medium also reversibly
decreased the amplitude of the N2 response by >20%in 2/6 slices. The amplitude of the
N3 component decreased 
20%in 5/6 slices in the presence of APV. Recovery of N3
following washout of APV occurred in five of the six slices. These results indicate that the late
potential that appears under Mg2+-free conditions is mediated by NMDA
receptors, and that an NMDA receptor-mediated component (i.e. APV-sensitive) may contribute
to the field potential that underlies the N2 and N3 peaks.
In standard aCSF, a second stimulus pulse at a 60 ms interpulse interval, reliably
evoked responses with both the N2 and N3 components. In contrast, in Mg2+-free
medium, the responses (both fast N2 and N3 peaks and slow components) following
the second stimulus pulse were abolished or greatly diminished (Figures 3B,E and 4B,D). In the presence of APV, the evoked
responses following the second stimulus pulse (arrow in Figures 3C
and 4C) partially return. Generally, APV added to Mg2+-free bathing medium converted the evoked response to waveforms that resemble those
under control conditions (in aCSF containing Mg2+).
Effects of NMDA and non-NMDA antagonist `cocktails' in Mg2+-free aCSF
To gain a better understanding of the relative contributions of NMDA
and non-NMDA receptors in evoked responses, we sequentially added antagonists to the
Mg2+-free medium. Figures 3–5 present postsynaptic responses in Mg2+-free
medium with combinations of
NMDA and non-NMDA receptor
antagonists (APV, DNQX and NBQX). Following the elimination of the NMDA
receptor-mediated components by application of APV, addition of a second antagonist, DNQX
(Figure 3D), to the bathing medium eliminated the remaining N2 and N3
fast components of the evoked waveform in one slice. Three additional slices were tested with
the order of the antagonists reversed: first DNQX, then DNQX + APV (Figure 4). Application of the non-NMDA antagonist, DNQX alone (5 or 10 µM) to
the Mg2+-free medium abolished or greatly attenuated the fast components
(i.e. the N2 and N3 potentials) in each of three slices. Application of DNQX did not affect the
slower, long lasting component of the waveform (Figure 4E). A complete
blockade of the evoked responses (N2, N3 and the late component) was achieved in each of the
three slices by the combination of DNQX and the NMDA antagonist, APV (Figure 4F). Although slower components recovered following washout of the
antagonists, only a partial re- covery of the N2 and N3 peaks were observed in two of the three
slices initially treated with DNQX. Similar results (elimination of only the fast peak components
of the aveform) were obtained using 1 <"Symbol"><"Times NR MT"> NBQX
(Figure 5), and with 5 <"Symbol"><"Times NR MT"> NBQX in three other slices.
Application of both NBQX and APV eliminated fast and late components of the response (Figure
5, bottom trace). The N2 and N3 peak responses partially recovered in one slice but did not
recover in two slices during NBQX and APV washout. NMDA receptor-mediated slower
components did return upon washout of antagonists (not shown).
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NMDA receptors, APV and short-term synaptic plasticity
The application of APV had only slight effects on the response amplitudes of N2 and N3 in
slices superfused with standard aCSF medium: N2 decreased by 20%in only 1/4
slices, and N3 decreased by 20%in 3/4 slices tested in APV. Figure 6 illustrates the effects of APV on responses evoked following the first stimulus pulse
in the slice that exhibited an APV-sensitive component at the N2 peak. The amplitudes of N2 and
N3 in Figure 6 were reduced by ~27 and 40% respectively. The
apparent effect of APV on the amplitude of N3 was proportionally more than its effect on N2
peaks in each of the slices tested. Following washout of APV, the amplitudes returned to control
levels.
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Under control conditions, the amplitudes of the synaptic potentials following the second pulse of a pair were sometimes facilitated relative to the responses to the first pulse, although in other cases, both N2 and N3 were depressed, or one was attenuated and the other facilitated (see Figures 2
A and 3A). Synaptic facilitation and
fatigue in the vagal lobe have been described previously by Finger and Dunwiddie (1992), and were found to be influenced by repetition rate and number of pulses in the
stimulus train. To further characterize this short-term synaptic plasticity, stimulus trains of five
pulses with IPIs of 15 ms were applied to the vagal afferents to induce synaptic facilitation.
Typically under repetitive stimulation, N2 responses were either unaffected or fatigued, while N3
responses were facilitated following the second, third and fourth stimulus pulse (i.e. within
45–60 ms following the first stimulus). The response following the fifth pulse, occurring
>60 ms after the first stimulus pulse, was markedly reduced.
Figure 7 illustrates the effects of APV on facilitation evoked by
repetitive stimulation. Comparable results were obtained from four additional slices that
exhibited facilitation. APV has its greatest effect on the magnitude of N3 following the second,
third or fourth pulse of a stimulus train. APV attenuated the facilitation of the N3 responses and
also slightly diminished the amplitude of each of the N2 responses. As in paired-pulse
paradigms, APV had lesser effects on N2 than N3. These results implicate an NMDA
receptor-mediated mechanism in the short-term facilitation associated with gustatory nerve
stimulation.
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Discussion |
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The goal of this study was to characterize the neuro-transmitters and receptors utilized by primary gustatory afferents of the vagus nerve. We find that both non-NMDA (kainate/AMPA) and NMDA receptor subtypes of the glutamate family of receptors are involved in this system.
Non-NMDA and NMDA antagonists mediate fast synaptic transmission
Fast glutamatergic synaptic transmission is generally mediated by non-NMDA receptors
whereas potentials that have slow onset (~10–20 ms to peak) with long durations are
typically mediated by NMDA receptors (McBain and Mayer, 1994). The
elimination of the fast postsynaptic response peaks N2 and N3 following the administration of
either DNQX and NBQX establish that AMPA/kainate receptors with properties similar to their
mammalian counterparts play an essential role in mediating gustatory transmission. The slight
decrease in the amplitudes of N2 and N3 responses in some slices following the addition of PV
to the standard aCSF bathing medium (Figure 6) suggests that
NMDA-type receptors also may play a role in fast synaptic transmission (within 2–5 ms).
Further, under Mg2+-free conditions, in the presence of DNQX or NBQX,
some short-latency APV-sensitive activity remains. NMDA receptors, although typically
attributed with slow kinetic properties, also mediate fast potentials at auditory nerve synapses in
goldfish (Wolszon et al., 1997) and vestibular synapses in frogs
(Straka and Dieringer, 1996) and rats (Kinney et al.,
1994).
The voltage-sensitive Mg2+-block, characteristic of NMDA
receptors, can be removed following depolarization of the neurons (presumably) by glutamate
activating AMPA/kainate receptors. This suggests that those slices that had responses with an
APV-sensitive component may have been depolarized sufficiently through non-NMDA receptors
to briefly relieve the Mg2+-block of the NMDA receptor. Since glutamate or
another EAA is likely released at this synapse, NMDA receptor-mediated responses then could
be evoked. Because non-NMDA receptors exhibit rapid decay times (Edmonds et
al., 1995), the release from Mg2+-block would be only
transient. That APV only affected the responses in some slices may reflect differences in the
relative amount of depolarization of the postsynaptic neurons from slice to slice (due to
differences in stimulus strengths), or possibly a heterogeneous distribution of NMDA receptors.
The complete elimination of the peaks under AMPA/ kainate antagonist conditions is not inconsistent with the observation that NMDA receptor-mediated (APV-sensitive) components partially underlie the N2 and N3 peaks. The Mg2+-block in standard aCSF would have prevented activation of NMDA receptors, and because non-NMDA receptors were blocked by selective antagonists, sufficient depolarization of the cells to remove the Mg2+-block would not occur. This is also consistent with the hypothesis that NMDA and non-NMDA receptors can be colocalized on the same target cells.
Preliminary results from radioligand binding studies for NMDA and AMPA in tissue
sections of the vagal lobe impart further evidence of the presence of these glutamate receptor
subtypes (Smeraski et al., 1996b). Recent studies showing
uptake of cobalt in cells activated by EAAs also indicate the presence of glutamate receptors in
the vagal lobe (Smeraski et al., 1997). Our results extend to the
vagal taste system the findings observed in the facial and glosso-pharyngeal taste systems in the
rostral nTS of mammals (Wang and Bradley, 1995; Li and
Smith, 1997): that non-NMDA and NMDA receptor subtypes are intrinsic to
synaptic transmission and that an EAA, like glutamate, is the neurotransmitter of the primary
gustatory afferents. Furthermore, caudal portions of the mammalian nTS that receive vagal input
from viscera (cardiovascular, pulmonary, gastrointestinal) also were found to contain
non-NMDA and NMDA receptors (Reis et al., 1981; Miller and Felder, 1988; Andresen and Yang, 1990; Andresen and Kunze, 1994; Zhang and Mifflin,
1995; Andresen and Mendelowitz, 1996; Aylwin et al., 1997). In both rostral (gustatory) and more caudal (general visceral)
portions of the nTS, non-NMDA receptors contribute significantly to the fast component of
primary afferent transmission, as in our vagal lobe preparations.
Slower NMDA receptor-mediated potential uncovered in Mg2+-free aCSF
Removing the Mg2+ from the bathing medium of the slice revealed an NMDA receptor-mediated (APV-sensitive) synaptic potential with a slow rise time and prolonged duration. In addition, the enhancement of the amplitudes of N2 nd N3 responses under Mg2+-free conditions was attenuated by APV.
As in standard aCSF, addition of non-NMDA antagonists to the Mg2+-free aCSF eliminates the fast components of the evoked response, but does not affect the slower potential observed in Mg2+-free aCSF. This slower potential was APV-sensitive and thus mediated by NMDA receptors. In Mg2+-free aCSF with DNQX/NBQX, the onset of the NMDA receptor-mediated field potential occurs with the same latency as the peak of the N2 response. However, because the kinetics of NMDA receptor activation are relatively slow, the earliest peak of the NMDA receptor-mediated component lags the N2 peak by ~ 1 ms. This explains why APV had little effect on the N2 peak compared with N3. Because of the longer latency of onset and slower kinetics of the NMDA receptor-mediated component, its contribution is greater to the N3 response than to the N2 peak (both in standard physiological concentrations of Mg2+ and Mg2+-free conditions).
An unexpected effect of Mg2+-free bathing medium was the
elimination of both the rapid and slow components of the response following the second stimulus
pulse in the paired-pulse paradigm. The finding that APV partially rescues the response
following the second pulse indicates that NMDA receptors are involved in the loss of the second
evoked response. Activation of NMDA receptors, and the increase in intracellular Ca2+, may play a role in the subsequent inactivation of the synaptic response. One
characteristic of NMDA receptors is that they `run down' or inactivate with
increases in intracellular Ca2+ (Rosenmund and Westbrook, 1993; McBain and Mayer, 1994; Rosenmund et al., 1995). If
this occurs in the receptors underlying the N2 and N3 responses, it
could explain the interaction between the NMDA responses and the amplitude of the synaptic
response to a second stimulus pulse. An intracellular rise of Ca2+ ions might
indirectly inactivate the non-NMDA receptors (e.g. via dephosphorylation) and block the fast
components of the response (Lisman, 1989). An alternative possibility is
that the site of interaction is presynaptic, i.e. that activation of presynaptic NMDA receptors can
inhibit the subsequent release of transmitter to a second stimulus. The inactivation of the second
response is temporary: both slow and fast components return within 15 s. It is possible that the
inactivation of the second response by increases in intracellular Ca2+ may be a
protective mechanism to limit activation and Ca2+ influx into the postsynaptic
cell.
Short-term synaptic plasticity involves NMDA receptors
The facilitation of the N3 potential following repetitive stimulation is APV-sensitive and therefore mediated by NMDA receptors. The repeated stimuli likely removed the voltage sensitive Mg2+-block, allowing activation of the NMDA receptor as described in the experiments above. One possible mechanism for this interaction would be postsynaptic, i.e. that the Ca2+ influx associated with the activation of the NMDA receptor induces a transient facilitation of the AMPA/kainate component of the response. Alternatively, it is certainly possible that an NMDA receptor-mediated Ca2+ influx into the presynaptic nerve terminal enhances transmitter release during successive responses in a stimulus train. However, note that the amplitude of the N2 response during repetitive stimulation did not increase.
The rapid attenuation of the N3 facilitation following 60 ms of repetitive stimulation
under control conditions (Figure 7, fifth pulse) is reminiscent of the
inactivation of the responses following the second stimulus in the paired-pulse experiments with
IPIs of 60 ms. The time frames of either attenuated facilitation or inactivation overlap, possibly
indicating similar control mechanisms. The mechanism underlying this effect could be either
presynaptic (Zucker, 1989) or postsynaptic (Lisman, 1989; Rosenmund and Westbrook, 1993; Rosenmund et al., 1995; Wang and Kelley, 1996), as proposed
earlier.
In any case, the NMDA receptor-mediated short-term plasticity of the response implies
that the temporal characteristics of incoming gustatory signals are crucial in determining the
efficacy of transmission of gustatory information. Thus, a burst of initial activity from primary
afferents should be transmitted much more effectively than a like number of temporally spaced
impulses. Primary gustatory afferents typically show a transient rise in activity followed by a
slower declining plateau of activity (Konishi and Zotterman, 1961; Smith and Bealer, 1975). The NMDA receptor-mediated short-term
facilitation would serve to amplify the second-order response to the incoming burst. Further, the
subsequent inhibition of responses after the first 50–60 ms may provide a clear time
marker for stimulus onset.
Alternatively, short-term plasticity may function to integrate inputs to different portions
of the dendritic tree of second-order neurons. In the vagal lobe, most cells have radially oriented
dendrites which span layers (Morita et al., 1983
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Accepted November 2, 1998
This article has been cited by other articles:
) receiving
inputs from different oral surfaces: branchial inputs end more superficially in the lobe than do
palatal gustatory inputs (Morita and Finger, 1985). The co-ordinated
activation of the two vagal lobe layers would be expected to produce facilitated responses
relative to the activity in only one of the afferent systems. Thus, a food particle trapped between
the palatal organ and branchial surface, as occurs during feeding (Sibbing et al., 1986), woul
Acknowledgments
This work was supported by National Institutes of Health Grant DC00147 (T.E.F.),
Institutional NRSA Training Grant, Basic Neuroscience (C.A.S.) and the Veterans
Administration Medical Research Service (T.V.D.). We thank Bärbel Bötger for
assistance in preparation of the slices.
References
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Abstract
Introduction
Materials and methods
Results
Discussion
References
Altschuler, S., Boa, X., Beiger, D., Hopkins, D. and Miselis, R. (1989) Viscerotopic representation of the upper alimentary tract in the rat: sensory ganglia
and nuclei of the solitary and spinal trigeminal tracts.J. Comp. Neurol., 283, 248–68.[Web of Science][Medline]
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E. Harvey-Girard and R. J. Dunn
Excitatory Amino Acid Receptors of the Electrosensory System: The NR1/NR2B N-Methyl-D-Aspartate Receptor
J Neurophysiol,
February 1, 2003;
89(2):
822 - 832.
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