| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
The Journal of Neurophysiology Vol. 79 No. 2 February 1998, pp. 595-604
Copyright ©1998 by the American Physiological Society
1 Department of Biologic and Materials Sciences, School of Dentistry; and 2 Department of Physiology, Medical School, University of Michigan, Ann Arbor, Michigan 48109
 |
ABSTRACT |
|---|
 Abstract
 Introduction
Methods
Results
Discussion
References
|
|---|
Grabauskas, Gintautas and Robert M. Bradley. Tetanic stimulation induces short-term potentiation of inhibitory synaptic activity in the rostral nucleus of the solitary tract. J. Neurophysiol. 79: 595-604, 1998. Whole cell recordings from neurons in the rostral nucleus of the solitary tract (rNST) were made to explore the effect of high-frequency tetanic stimulation on inhibitory postsynaptic potentials (IPSPs). IPSPs were elicited in the rNST by local electrical stimulation after pharmacological blockade of excitatory synaptic transmission. Tetanic stimulation at frequencies of 10-30 Hz resulted in sustained hyperpolarizing IPSPs that had a mean amplitude of 
68 mV. The hyperpolarization resulted in a decrease in neuronal input resistance and was blocked by the 
-aminobutyric acid-A (GABAA) antagonist bicuculline. For most of the neurons (n = 87/102), tetanic stimulation resulted in a maximum hyperpolarization immediately after initiation of the tetanic stimulation, but for some neurons the maximum was achieved after three or more consecutive shock stimuli in the tetanic train of stimuli. When the extracellular Ca2+ concentration was reduced, the maximum IPSP amplitude was reached after several consecutive shock stimuli in the tetanic train for all neurons. Tetanic stimulation at frequencies of 30 Hz and higher resulted in IPSPs that were not sustained but decayed to a more positive level of hyperpolarization. In some neurons the decay was sufficient to become depolarizing and resulted in a biphasic IPSP. It was possible to evoke this biphasic IPSP in all the neurons tested if the cells were hyperpolarized to 75 to 85 mV. The ionic mechanism of the depolarizing IPSPs was examined and was found to be due to an elevation of the extracellular K+ concentration and accumulation of intracellular Cl
. Tetanic stimulation increased the mean 80-ms decay time constant of a single shock-evoked IPSP up to 8 s. The length of the IPSP decay time constant was dependent on the duration and frequency of the tetanic stimulation as well as the extracellular Ca2+ concentration. Afferent sensory input to the rNST consists of trains of relatively high-frequency spike discharges similar to the tetanic stimulation frequencies used to elicit the IPSPs in the brain slices. Thus the short-term changes in inhibitory synaptic activity in the slice preparation probably occur in vivo and may play a key role in taste processing by facilitating synaptic integration.
The rostral nucleus of the solitary tract (rNST) is the site of the first central synapse of afferent fibers innervating taste buds and somatosensory receptors of the oral cavity (Bradley et al. 1985 Slice preparation
Male and female rats, 3-4 wk old, were used. Although these are young rats, we recently have demonstrated that the intrinsic properties of rNST neurons have reached mature values by this age (Bao et al. 1995
Drug application
Drugs were applied to the slice by switching the normal physiological saline to one containing known concentrations of the drug. Drugs used in these experiments were bicuculline methiodide (BMI) obtained from Sigma, and 2-hydroxysaclofen (OH-saclofen) supplied by Research Biochemicals International. The concentrations of the different drugs used were those we have used in our previous studies of rNST neurons and were 400 µM for OH-saclofen and 20 or 100 µM for BMI (Wang and Bradley 1993 Stimulation procedure
Postsynaptic potentials (PSPs) were elicited by delivery of stimuli (0.1 ms duration) via a bipolar stimulating electrode consisting of tightly twisted pairs of 70-µm-diam, Teflon-insulated, platinum wires (200 µm overall diameter, including insulation, 140 µm diameter metal stimulating surface) placed under direct visual control in the region of the solitary tract in rostral (2.5-3.0 mm rostral to the obex) or intermediate (~1.5 mm rostral to the obex) portions of rNST. The intensity of the stimulus was adjusted to evoke IPSPs and ranged from 0.1 to 3.0 mA. The distance between the stimulating electrode and recording site was between 0.5 and 1.0 mm. In some experiments single shock stimuli were used, and in other experiments tetanic trains of stimuli lasting between 50 ms and 5 s at frequencies ranging from 10 to 100 Hz were used.
Data analysis
Recordings were made using an Axoclamp 2B amplifier (Axon Instruments) in current-clamp mode. Bridge balance was carefully monitored throughout the experiments and adjusted when necessary. The junction potential due to K gluconate (10 mV) was subtracted from the recorded membrane voltages (Standen and Stanfield 1992 The results are based on recordings from 102 neurons in 88 slices taken from 63 rats. The basic characteristics of all the neurons were measured in control physiological saline. Resting membrane potentials were between
Tetanic stimulation at frequencies mimicking the normal input to rNST neurons resulted in marked short-term changes in the IPSPs recorded from these cells. The tetanically induced IPSPs were blocked in the presence of the GABAA antagonist bicuculline and were therefore mediated by GABA. The time course of the tetanically induced IPSPs was increased when compared with single shock-induced IPSPs, and the length of the decay time constant of the IPSPs was dependent on both the duration and the frequency of the tetanic stimulation as well as the concentration of extracellular Ca2+. In addition, tetanic stimulation could also elicit a biphasic response consisting of an initial hyperpolarizing potential followed by a late depolarizing potential sufficient to initiate action potentials. Similar biphasic inhibitory responses have previously been reported to occur in the hippocampus and are reported here for the first time in rNST. Thus high-frequency afferent input to the rNST is capable of modifying the inhibitory synaptic activity that would consequently influence the processing of sensory information. With the use of a totally different experimental approach, tetanic stimulation has recently been shown to have a marked long-term effect on inhibitory synaptic activity in the caudal, nongustatory, NST as well (Glaum and Brooks 1996 Ionic mechanisms of the biphasic IPSPs
At tetanic frequencies above 30 Hz, the amplitude of the hyperpolarizing IPSPs were not maintained and for several cells became biphasic with an initial hyperpolarization followed by a slow depolarization. These biphasic IPSPs in response to high-frequency (100-200 Hz) stimulation or by application of a high concentration of GABA have been reported to occur in the hippocampus by a number of investigators but either do not occur in other brain areas or have not been systematically investigated (see Lambert and Grover 1995 Functional significance
It is becoming clear that afferent information arriving at the rNST is not only transmitted rostrally but results in the initiation of inhibitory activity in GABAergic second-order neurons. The results of the present study indicate that afferent input at frequencies reported in the literature would result in complex inhibitory activity. In fact the tetanic stimulation we employed results in short-term plastic changes in the inhibitory synapses of the rNST. Moreover, the short-term changes in the IPSPs are influences by the frequency of the tetanic stimulation. Thus the frequency of the afferent input to the rNST would potentially result in different inhibitory activity. In addition, as discussed above, the high-frequency afferent input is capable of suppressing spontaneous activity resulting in modulation of any chronic inhibitory activity present in rNST.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
; Hamilton and Norgren 1984; Hanamori and Smith 1989). These afferent fibers, traveling in the intermediate (VII) and glossopharyngeal (IX) nerves, form the solitary tract (ST) and make excitatory synapses with rNST neurons (Grabauskas and Bradley 1996; Wang and Bradley 1995). Recently, we examined the characteristics of the postsynaptic potentials at this synapse using intracellular recording in a horizontal brain slice preparation of the rat rNST. The postsynaptic potentials elicited by electrical stimulation of the ST have a complex waveform, resulting from a mixture of both excitatory and inhibitory postsynaptic potentials (Grabauskas and Bradley 1996). Thus both excitation and inhibition are involved in synaptic transmission at the first central synapse in the taste pathway. Although the role of excitation at this synapse seems clear, the role of inhibition is less obvious because most taste stimuli excite second-order neurons and only a few examples of inhibition by gustatory stimuli have been reported (Travers and Smith 1979). However, there is now physiological, pharmacological, and anatomic evidence indicating that inhibition mediated by -aminobutyric acid (GABA) has a major influence on synaptic processing in the rNST. For example, investigators using immunocytochemical techniques have revealed that a major population of rNST neurons are GABAergic (Davis 1993; Lasiter and Kachele 1988), and in electrophysiological studies in vitro rNST, neurons have been shown to respond to GABA (Liu et al. 1993; Wang and Bradley 1993, 1995). The present series of experiments was designed to explore in more detail the mechanisms of inhibition in rNST.
; Frank et al. 1988; Ogawa et al. 1968, 1973). In addition, there is considerable convergence at the synapse between the afferent input and the second-order rNST neurons, so that response frequencies are 4.3 times higher than responses recorded in primary afferent taste fibers (Doetsch and Erickson 1970; Vogt and Mistretta 1990). Thus the second-order neuron in the taste pathway would normally receive a high frequency of input when taste stimuli are flowed over the tongue.
).
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
). The preparation of horizontal rNST brain slices has already been described in detail (Bradley and Sweazey 1992; Grabauskas and Bradley 1996). Briefly, rats were anesthetized with pentobarbital sodium (50 mg/kg) and decapitated. The whole brain, including the cervical spinal cord, was rapidly removed and placed in ice-cold physiological saline containing (in mM) 124 NaCl, 2.5 KCl, 2.5 CaCl2, 1.3 MgSO4, 26 NaHCO3, 1.25 KH2PO4, and 25 glucose, gassed with 95% O2-5% CO2 to give a pH of 7.3. The brain was transected at the level of the pons and just below the obex and the cerebellum removed. Horizontal 300-µm slices containing the whole NST were cut on a Vibratome and placed in a holding chamber. After at least 1 h recovery, the slice containing the NST was transferred to the recording chamber (volume of 1 ml), where it was submerged and held in place by a net and continuously superfused (2 ml/min) with physiological saline at room temperature. Because the PSPs elicited by electrical stimulation are mixed excitatory and inhibitory potentials once the basic properties of a rNST neuron was established, we performed all further experiments in the presence of 50 µM D-2-amino-5-phosphonovalerate (APV) and 20 µM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to block the excitatory component of the PSPs. The resulting IPSPs were evoked by direct electrical stimulation of rNST inhibitory interneurons (Glaum and Brooks 1996; Grabauskas and Bradley 1996). In 12 experiments we used a N-2-hydroxyethylpiperazine-N
-2-ethanesulfonic acid (HEPES)-buffered solution to elucidate the role of HCO3 ions in the inhibitory synaptic transmission. In these experiments with HCO3-free physiological saline, the NaHCO3 was replaced by 10 mM HEPES, and 16 mM Na2C4H4O4 (sodium succinate) and gassed with O2. The pH was adjusted with NaOH to 7.4. To ensure that the HCO3/CO2-buffered solution was completely exchanged with the HCO3-free (HEPES buffered) perfusing solution, in four experiments the slices were first incubated in the HCO3-free solution and then switched to the HCO3/CO2-buffered perfusing solution. In experiments examining the effects of different concentrations of extracellular Ca2+ and K+ ions on the IPSPs, the ion concentration in the perfusing solution was reduced by isomolar substitution of NaCl. Even though the volume of the slice chamber (1 ml) was small enough to allow for rapid exchange of contents, we waited for >5 min before making further recordings to allow the cell to stabilize after perfusing solutions were changed.
View larger version (23K):
[in a new window]
FIG. 1.
A: recording of a sustained hyperpolarizing inhibitory postsynaptic potential (IPSP) in a rostral nucleus of the solitary tract (rNST) neuron in response to tetanic stimulation in the presence of 20 µM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 µM D-2-amino-5-phosphonovalerate (APV; top trace). The sustained hyperpoliarization amplitude was 71 mV. After the tetanic stimulation was terminated, the cell membrane potential returned to its resting level with a prolonged time constant. Application of 100 µM bicuculline methiodide (BMI) completely blocked the tetanic stimulation evoked IPSP (bottom trace). B: change in input resistance during tetanic stimulation. During tetanic stimulation the input resistance of the recorded cell decreased by 50%. After the tetanic stimulation was stopped, the input resistance of the recorded cell returned to control levels with a time course similar to the decay time of IPSP. Input resistance was measured by applying 100-pA, 100-ms current steps. The amplitudes of negative potentials deflections correspond to the input resistance changes of the recorded cell. Horizontal bar indicates duration of tetanic stimulation at 50 Hz.
View larger version (19K):
[in a new window]
FIG. 2.
A: time course of the development of the tetanically evoked IPSPs. For some rNST neurons the maximum IPSP amplitude was achieved after several consecutive stimuli of the tetanic stimulus train (left), whereas for other neurons the maximum IPSP amplitude was achieved at the beginning of the tetanic stimulation (right). B: influence of extracellular Ca+ concentration on the development of the tetanically evoked IPSPs. Although the extracellular Ca+ concentration had no effect on the final amplitude of the tetanically evoked IPSPs, it did influence the time course of the development of the IPSPs. When the extracellular Ca2+ was 2.5 mM, the maximum IPSP amplitude was reached after the 1st shock of the tetanic stimulation. When the extracellular Ca2+ was 0.3 mM, the maximum IPSP amplitude was reached after several shocks (11) of the tetanic stimulation. Both intracellular recordings obtained at the same stimulus strength.
-aminoethyl ether)-N,N,N,N-tetraacetic acid, 1 MgCl2, 1 CaCl2, 2 ATP, and 0.2 GTP. Pipette solutions were adjusted to a pH 7.2-7.3 with KOH and had an osmolarity of 275-292 mosM. Electrode resistance was between 5 and 8 M
. Electrodes were positioned under visual control over the medial rNST in an area extending from 1.5 to 2.5 mm rostral to the obex.
View larger version (13K):
[in a new window]
FIG. 3.
Influence of tetanic stimulus frequency on the characteristics of the IPSP. A: tetanic stimulation at 30 Hz resulted in sustained hyperpolarization of the neuron (top trace). At higher stimulus frequencies the amplitude of IPSP was not sustained and decayed to a more positive level of hyperpolarization before termination of the tetanic stimulation (bottom trace). B: During a 3-s tetanic stimulation at 30 Hz, the resulting membrane hyperpolarization slowly decayed but did not reach the resting membrane potential before the termination of the tetanic stimulus. At higher frequencies (50 and 70 Hz) the membrane hyperpolarization decayed back to the resting membrane potential and became depolarizing initiating action potentials before termination of the tetanic stimulus. The time course of the decay of the IPSP was faster as the tetanic stimulus frequency increased. Horizontal bar indicates duration of tetanic stimulation.
, 1995).
). Recordings were considered acceptable if the resting membrane potential was greater than 50 mV, action-potential amplitude was >50 mV, and neuron input resistance was >300 M. The time constants of the IPSPs were measured by fitting a single-exponential function.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
50 and 68 mV (56 ± 0.8 mV, mean ± SE). Overshooting action-potential amplitudes ranged from 52 to 95 mV (65 ± 1.65 mV) with a mean duration measured at half-amplitude of 2.5 ± 0.18 ms. Input resistance, determined from the steady-state portion of the response to a 100 ms, 100 pA hyperpolarizing current pulse varied between 330 and 800 M(463 ± 25 M). Thirty percent of the recorded cells were spontaneously active, generating action potentials at a frequency 0.5-7 Hz. Application of the glutamate receptor antagonists CNQX and APV resulted in a hyperpolarization from their resting membrane potential by 1-5 mV (2.2 ± 0.2 mV) indicating a small tonic glutaminergic input to the neurons.
68 ± 5 mV (n = 102). Although 20 µM BMI was sufficient to block single shock-evoked IPSPs, it was not sufficient to block tetanic stimulation-evoked IPSPs, which required 100 µM BMI for effective elimination (Fig. 1A). Tetanic stimulation also resulted in a 10-60% decrease in cell input resistance measured by injecting 100-pA, 100-ms, 2-Hz pulses during the tetanic stimulation (Fig. 1B).
View larger version (23K):
[in a new window]
FIG. 4.
A: influence of membrane potential on the characteristics of the tetanically evoked IPSP in an extracellular solution buffered with HCO 3/CO2 (left) and an extracellular solution buffered with N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid (HEPES; right). In the bicarbonate buffer at the neuron's resting membrane potential (58 mV), the tetanically evoked IPSP decayed back toward the resting membrane potential before the termination of the tetanic stimulation. When the cell was hyperpolarized to 79 mV, the IPSP response had a biphasic character. Hyperpolarization of the neuron to 91 mV resulted in a purely depolarizing IPSP. Similar results were obtained in the HEPES-buffered extracellular perfusate. In this perfusate the responses to tetanic stimulation were of a smaller IPSP amplitude. Horizontal bar indicates a 5-s tetanic stimulation at 50 Hz. B: plots of the IPSP amplitude at the initiation and termination of the tetanic stimulation when the cell was exposed to a HCO3/CO2 (
, initiation; 
, termination) and a HEPES (
, initiation; 
, termination) buffered extracellular solution at different membrane potentials. The reversal potential of the IPSP in either the HCO3/CO2 or HEPES buffer was the same. C: exposure of the neuron to a HCO3-free solution (right) reduced the amplitude of the single shock-evoked (
) IPSP when compared with the single shock IPSP recorded in HCO3-containing solution (left). The reduction of the IPSP amplitude was not accompanied by a change of the IPSP reversal potential. D: relationship between the IPSP amplitudes and the neuron membrane potential. The reversal potential of the single shock-evoked IPSP was 89 mV in both the HCO3-free solution and in HCO3-containing extracellular solution.
). We examined the role of Ca2+ in the synaptic facilitation resulting from tetanic stimulation by reducing the extracellular Ca2+ concentration from 2.5 to 0.3 mM (n = 12). The amplitude of the IPSP was reduced by the lowered external Ca2+ concentration, and the facilitation was then observed in all the tested neurons. However, the 0.3-mM external Ca2+ did not influence the maximum amplitude of the hyperpolarization (Fig. 2B). Thus the reduced external Ca2+ concentration influences the rate of synaptic facilitation, but not the sum of the IPSPs resulting from tetanic stimulation.
View larger version (22K):
[in a new window]
FIG. 5.
Effect of different external concentrations of K+ on the characteristics of the single shock and tetanically evoked IPSP. A: at higher concentrations of the external K+, the single shock-evoked ( ) IPSP reversal potential was shifted to a more positive value. The IPSP reversal potential was 95 mV for 1 mM, 89 mV for 2.5 mM, and 72 mV for 10 mM external concentrations of K+. Membrane potential was manipulated by current injection into the neuron. B: relationship between the amplitude of the single shock-evoked IPSPs and the neuron's membrane potential when exposed to different extracellular K+ concentrations. C: effect of different external concentrations of K+ on the characteristics of a tetanically evoked IPSP recorded at different membrane potentials. In the presence of 2.5 mM external K+, the IPSP amplitude was reduced from its initial amplitude by 12-15 mV at different membrane potentials (left). In presence of 10 mM external K+, the reduction of the IPSP from its initial amplitude was 3-8 mV. In addition, the depolarizing IPSP occurred at a more positive membrane potential when the neuron was exposed to a 10-mM external K+. Horizontal bar indicates a 5-s tetanic stimulation at 50 Hz. D: plots of the IPSP amplitude at the initiation and termination of the tetanic stimulation when the neuron was exposed to 2.5 mM (, initiation; , termination) and 10 mM (, initiation; , termination) external K+.
75 to 85 mV. At levels of hyperpolarization more negative than 85 mV, the tetanic response consisted of a purely depolarizing IPSP (Fig. 4A, left). When the IPSP amplitude at the beginning () and end () of the tetanic stimulation was plotted at different membrane potentials, the beginning IPSP amplitude reversed at 85 mV, and the final IPSP amplitude was 2-25 mV more positive than the initial IPSP amplitude (Fig. 4B). Thus the characteristic of the tetanically induced IPSPs were dependent on the neuron's membrane potential.
concluded that HCO3 ions are responsible for the generation of the depolarizing phase of GABAA-mediated IPSPs in hippocampal neurons. We therefore examined the role of HCO3 ions in the biphasicdepolarizing IPSPs in rNST neurons. Superfusion of the slices with a HCO3-free solution resulted in 10-50% reduction of the single shock evoked IPSP amplitude (Fig. 4C, right) compared with neurons in bicarbonate superfusate (Fig. 4C, left), but had no effect on the IPSP reversal potential (Fig. 4D). When the amplitude of the initial () and final () IPSP resulting from tetanic stimulation was measured in HCO3-free superfusate, the initial amplitude was reduced by 10-35%, but there was no consistent effect on either the time course, amplitude of the decay phase, or the reversal potential of the IPSP (n = 12; Fig. 4A, right, and Fig. 4B).
View larger version (32K):
[in a new window]
FIG. 6.
Influence of tetanic stimulus frequency and duration on the decay time of the IPSP in control and low Ca2+ concentration perfusing solution. A: the duration of the decay time of the IPSP increases with increasing frequency of the tetanic stimulation. Arrows indicate the single shock-evoked IPSP before and after tetanic stimulation. Bar indicates tetanic stimulus duration. B: relationship between the frequency of the tetanic stimulus and the decay time constant of the IPSP. All measurements are made with a 1-s duration tetanic stimulus. The decay time of the IPSP also depends on Ca2+ concentration in the perfusing solution. With low (0.3 mM) extracellular Ca2+, the decay time constants are shorter than decay time constants measured in control extracellular solutions. Values are means ± SE. C: the duration of the decay time of the IPSP increases with increasing duration of the tetanic stimulation at a constant frequency. In spontaneously active neurons, higher values of the decay time constant of the IPSP also result in an increasing delay of the occurrence of the 1st spike after termination of the tetanic stimulus. Bar indicates the stimulus duration at 30 Hz frequency. D: relationship between duration of the tetanic stimulus and the length of the decay time constant of the IPSP. All measurements were obtained at 30 Hz. The decay time of the IPSP is also depended on Ca2+ concentration in the perfusing solution. A lower extracellular Ca2+ concentration results in shorter decay time constants than those measured in control extracellular solutions. Values are means ± SE. E: the decay time constant of the tetanically evoked IPSP depends on the extracellular Ca2+ concentration. For spontaneously active neurons, it also delays the occurrence of the 1st spike after termination of the tetanic stimulus (left). However, the decay time as well as the occurrence of the 1st spike is markedly shorter when the extracellular Ca2+ concentration is lowered (right). Horizontal bars indicate 1 s tetanic stimulation at different frequencies.
; Thompson and Gähwiler 1989). We examined the effect of different extracellular K+ concentrations (1, 2.5, and 10 mM) on the IPSP amplitude and reversal potential during single shock and 1-5 s duration tetanic stimulation (n = 10). Single shock stimulation at various membrane holding potentials revealed that the external K+ concentration influenced the reversal potential of the IPSP. The reversal potential of single shock-evoked IPSPs was 93 ± 1.1 mV (n = 8) for 1 mM external K+ concentration, 87 ± 0.6 mV (n = 25) for 2.5 mM external K+ concentration, and 71 ± 1.3 mV (n = 10) for 10 mM external K+ concentration. These results indicate that elevation of the external K+ concentration shifted the reversal potential of the IPSPs in a positive direction (Fig. 5, A and B). However, application of an external solution containing 1 mM K+ resulted in a 0.5- to 2.0-mV membrane hyperpolarization, and superfusion with 10 mM K+ resulted in membrane depolarization by 6-9 mV when compared with the resting membrane potential recorded in the control superfusate (2.5 mM K+). Tetanic stimulation in the presence of 10 mM external K+ revealed that the difference between the initial and final IPSP amplitude ( and ) was 45-90% smaller (n = 15) when compared with the difference between the initial and final IPSP amplitude ( and ) in control superfusate (2.5 mM K+; Fig. 5C). In addition, for the 10-mM external K+ solution, the biphasic pattern of response occurred at more positive membrane potentials than in control superfusate (55 to 70 mV,n = 8, Fig. 5, C and D).
.
have suggested that GABA is capable of acting on presynaptic GABAB receptors to decrease its own evoked release. It is possible that this presynaptically controlled reduction in GABA release may be responsible for the decay of the hyperpolarization during tetanic stimulation. We therefore used the GABAB receptor antagonist OH-saclofen to block the GABAB receptors. Application of 400 µM OH-saclofen (n = 8) did not prevent the decay of the hyperpolarization during the tetanic stimulation, indicating that presynaptic GABAB receptors do not play a role in the decay process in rNST (Fig. 7).
View larger version (19K):
[in a new window]
FIG. 7.
Two superimposed traces of a tetanically evoked IPSP in the same neuron in control superfusate containing 20 µM CNQX and 50 µM APV ( 
) and a solution containing the -aminobutyric acid-B (GABAB) antagonist 400 µM 2-hydroxysaclofen (OH-saclofen; ···). No significant difference was observed in the time course of IPSP after application of the GABAB antagonist. Bar indicates duration of tetanic stimulation at 50 Hz.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
).
), no functional significance has been attached to this activity. It is possible that spontaneous activity may serve to release a chronic level of inhibitory neurotransmitter, and the prolonged decay phase that follows tetanic stimulation, by suppressing the spontaneous activity, may serve to eliminate the chronic inhibition and therefore indicate the presence of afferent information. Thus the prolonged posttetanic decay phase of the IPSP may be significant in the transmission of sensory information.
).
). Although this could explain the initial decrease in the hyperpolarization, it cannot explain the depolarizing component of the IPSPs. The second mechanism proposed for the decreased IPSP amplitude is a presynaptic one resulting from autoinhibition of GABA release due to activation of presynaptic GABAB receptors (Davies et al. 1990; Deisz and Prince 1989). However, experiments in which we blocked the GABAB receptors using OH-saclofen had no effect on the IPSPs, indicating that a presynaptic mechanism was probably not involved in the rNST neurons.
; Wong and Watkins 1982). However, this process is rather slow and develops over tens of seconds. Also, it has been suggested that the reduction of the hyperpolarizing IPSPs that follows tetanic stimulation could be partly due to intracellular Cl accumulation resulting from passive flow of Cl ions through GABA-activated Cl channels (McCarren and Alger 1985), which has been shown to significantly decrease the IPSP driving force (Thompson and Gähwiler 1989). These results explain the reduction of IPSP amplitude but cannot explain the biphasic response or the depolarizing IPSP when the neuron membrane potential was held more positive than the IPSP reversal potential.
have proposed that Cl ions are responsible for the hyperpolarizing phase and HCO3 ions are responsible for the depolarizing phase of the biphasic IPSPs in hippocampal neurons. However, in our experiments exposure of the neurons to a HCO3-free solution had no effect on the time course and amplitude of the IPSP decay phase, and we therefore concluded that the depolarizing phase of the IPSPs was not mediated by HCO3 ions. Grover et al. (1993) also examined the influence of HCO3 on depolarizing IPSPs in hippocampal pyramidal neurons and concluded that superfusion with HCO3 did not produce shifts in the IPSP reversal potential but did reduce their amplitudes. They accounted for these effects by changes in extracellular acidification due to the lower buffering capacity of the HEPES buffer when compared with the HCO3/CO2-buffered solution. Consequently, replacement of the HCO3/CO2-bufferedextracellular solution with HEPES buffer results in nonspecific changes rather than the involvement of the HCO3 in the generation of biphasic and depolarizing responses.
; Heinemann and Lux 1977; Malenka and Kocsis 1982). For example, it has been demonstrated that stimulation of hippocampal slices at frequencies of 2-30 Hz results in an increase in extracellular K+ concentration from 5 to 12 mM (Benninger et al. 1980). The mechanism that may contribute to the increase in external K+ caused by activation of GABAA receptors includes an outward counter/cotransport of K+ with Cl/HCO3 (Kaila 1994). Redistribution of K+ and Cl ions leads to a reduction in the IPSP driving force and changes in reversal potential (Barker and Ransom 1978; Thompson 1994). The contribution of elevated extracellular K+ in depolarizing IPSPs has also been studied by Wong and Watkins (1982), who demonstrated that an increase in extracellular K could increase the amplitude of the depolarizing GABA response. We also examined the influence of extracellular K+ concentration on the IPSPs recorded from rNST neurons. Elevation of the extracellular K+ concentration suppressed the reduction of the IPSP amplitude during tetanic stimulation and changed the IPSP reversal potential. We have therefore concluded that tetanic stimulation of GABAergic neurons in the rNST results in an elevation of extracellular K+ concentration and accumulation of intracellular Cl, which changes the IPSP reversal potential. This redistribution of Cl and K+ produces a decay of the IPSP amplitude and as a consequence results in biphasic or depolarizing IPSPs.
). It is possible therefore that these short-term changes in inhibitory synaptic activity are a key element in the organization of complex oral reflex activity.
| |
ACKNOWLEDGEMENTS |
|---|
This work was supported by National Institute of Deafness and Other Communication Disorders Grant DC-00288 to R. M. Bradley.
| |
FOOTNOTES |
|---|
Address for reprint requests: R. M. Bradley, Dept. of Biologic and Materials Sciences, Rm. 6228, School of Dentistry, University of Michigan, Ann Arbor, MI 48109-1078.
Received 3 April 1997; accepted in final form 26 September 1997 .
| |
REFERENCES |
|---|
|
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
in hippocampal CA3 neurons.
J. Neurophysiol.
61: 512-523, 1989.This article has been cited by other articles:
|
|
 |
|
  R. M. Bradley, H. Fukami, and T. Suwabe Neurobiology of the Gustatory-Salivary Reflex Chem Senses, January 1, 2005; 30(suppl_1): i70 - i71. [Full Text] [PDF]
|
|
|
|
 |
|
 P. M. Di Lorenzo, C. H. Lemon, and C. G. Reich Dynamic Coding of Taste Stimuli in the Brainstem: Effects of Brief Pulses of Taste Stimuli on Subsequent Taste Responses J. Neurosci., October 1, 2003; 23(26): 8893 - 8902. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Grabauskas and R. M. Bradley Frequency-Dependent Properties of Inhibitory Synapses in the Rostral Nucleus of the Solitary Tract J Neurophysiol, January 1, 2003; 89(1): 199 - 211. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. H. Lemon and P. M. Di Lorenzo Effects of Electrical Stimulation of the Chorda Tympani Nerve on Taste Responses in the Nucleus of the Solitary Tract J Neurophysiol, November 1, 2002; 88(5): 2477 - 2489. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Grabauskas and R. M. Bradley Postnatal Development of Inhibitory Synaptic Transmission in the Rostral Nucleus of the Solitary Tract J Neurophysiol, May 1, 2001; 85(5): 2203 - 2212. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Koga and R. M. Bradley Biophysical Properties and Responses to Neurotransmitters of Petrosal and Geniculate Ganglion Neurons Innervating the Tongue J Neurophysiol, September 1, 2000; 84(3): 1404 - 1413. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C.-Y. Chen, J. M. Horowitz, and A. C. Bonham A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS Am J Physiol Heart Circ Physiol, October 1, 1999; 277(4): H1350 - H1360. [Abstract] [Full Text] [PDF]
|
|
|
|
