Potentiation of GABAergic synaptic transmission in the rostral nucleus of the solitary tract

Enhancing GABAergic Tone in the Rostral Nucleus of the Solitary Tract Reconfigures Sensorimotor Neural Activity

Recent work has shown that most cells in the rostral, gustatory portion of the nucleus tractus solitarius (rNTS) in awake, freely licking rats show lick-related firing. However, the relationship between taste-related and lick-related activity in rNTS remains unclear. Here, we tested whether GABA-derived inhibitory activity regulates the balance of lick- and taste-driven neuronal activity. Combinatorial viral tools were used to restrict the expression of channelrhodopsin 2-enhanced yellow fluorescent protein to GAD1⁺ GABAergic neurons. Viral infusions were bilateral in rNTS. A fiber-optic fiber attached to a bundle of drivable microwires was later implanted into the rNTS. After recovery, water-deprived rats were presented with taste stimuli in an experimental chamber. Trials were five consecutive taste licks [NaCl, KCl, NH₄Cl, sucrose, monosodium glutamate/inosine-5'-monophosphate, citric acid, quinine, or artificial saliva (AS)] separated by five AS rinse licks on a variable ratio 5 schedule. Each taste lick triggered a 1 s train of laser light (25 Hz; 473 nm; 8-10 mW) in a random half of the trials. In all, 113 cells were recorded in the rNTS, 50 cells responded to one or more taste stimuli without GABA enhancement. Selective changes in response magnitude (spike count) within cells shifted across-unit patterns but preserved interstimulus relationships. Cells where enhanced GABAergic tone increased lick coherence conveyed more information distinguishing basic taste qualities and different salts than other cells. In addition, GABA activation significantly amplified the amount of information that discriminated palatable versus unpalatable tastants. By dynamically regulating lick coherence and remodeling the across-unit response patterns to taste, enhancing GABAergic tone in rNTS reconfigures the neural activity reflecting sensation and movement.

Taste-specific cell assemblies in a biologically informed model of the nucleus of the solitary tract

Although the cellular organization of many primary sensory nuclei has been well characterized, questions remain about the functional architecture of the first central relay for gustation, the rostral nucleus of the solitary tract (NTS). Here we used electrophysiological data recorded from single cells in the NTS to inform a network model of taste processing. Previous studies showed that electrical stimulation of the chorda tympani (CT) nerve initiates two types of inhibitory influences with different time courses in separate groups of NTS cells. Each type of inhibition targeted cells with distinct taste response properties. Further analyses of these data identified three NTS cell types differentiated by their latency of evoked response, time course of CT evoked inhibition, and degree of selectivity across taste qualities. Based on these results, we designed a model of the NTS consisting of discrete, reciprocally connected, stimulus-specific "cell" assemblies. Input to the network of integrate-and-fire model neurons was based on electrophysiological recordings from the CT nerve. Following successful simulation of paired-pulse CT stimulation, the network was tested for its ability to discriminate between two "taste" stimuli. Network dynamics of the model produced biologically plausible responses from each unit type and enhanced discrimination between taste qualities. We propose that an interactive network of taste quality specific cell assemblies, similar to our model, may account for the coherence in across-neuron patterns of NTS responses between similar tastants.

Properties of GABAergic neurons in the rostral solitary tract nucleus in mice

The rostral nucleus of the solitary tract (rNST) plays a pivotal role in taste processing. The rNST contains projection neurons and interneurons that differ in morphology and intrinsic membrane properties. Although characteristics of the projection neurons have been detailed, similar information is lacking on the interneurons. We determined the intrinsic properties of the rNST GABAergic interneurons using a transgenic mouse model that expresses enhanced green fluorescent protein under the control of a GAD67 promoter. Glutamic acid decarboxylase-green fluorescent protein (GAD67-GFP) neurons were distributed throughout the rNST but were concentrated in the ventral subdivision with minimal interaction with the terminal field of the afferent input. Furthermore, the density of the GAD67-GFP neurons decreased in more rostral areas of rNST. In whole cell recordings, GAD67-GFP neurons responded with either an initial burst (73%), tonic (18%), or irregular (9%) discharge pattern of action potentials (APs) in response to membrane depolarization. These three groups also differed in passive and AP characteristics. Initial burst neurons had small ovoid or fusiform cell bodies, whereas tonic firing neurons had large multipolar or fusiform cell bodies. Irregular firing neurons had larger spherical soma. Some of the initial burst and tonic firing neurons were also spontaneously active. The GAD67-GFP neurons could also be categorized in subgroups based on colocalization with somatostatin and parvalbumin immunolabeling. Initial burst neurons would transmit the early dynamic portion of the encoded sensory stimuli, whereas tonic firing neurons could respond to both dynamic and static components of the sensory input, suggesting different roles for GAD67-GFP neurons in taste processing.

Two types of inhibitory influences target different groups of taste-responsive cells in the nucleus of the solitary tract of the rat

Electrical stimulation of the chorda tympani nerve (CT; innervating taste buds on the rostral tongue) is known to initiate recurrent inhibition in cells in the nucleus of the solitary tract (NTS, the first central relay in the gustatory system). Here, we explored the relationship between inhibitory circuits and the breadth of tuning of taste-responsive NTS neurons. Initially, NTS cells with evoked responses to electrical stimulation of the CT (0.1 ms pulses; 1 Hz) were tested with each of four tastants (0.1 M NaCl, 0.01 M HCl, 0.01 M quinine and 0.5 M sucrose) in separate trials. Next, the CT was electrically stimulated using a paired-pulse (10-2000 ms interpulse interval; blocks of 100 trials) paradigm. Forty-five (30 taste-responsive) of 51 cells with CT-evoked responses (36 taste-responsive) were tested with paired pulses. The majority (34; 75.6%) showed paired-pulse attenuation, defined as fewer evoked spikes in response to the second (test) pulse compared with the first (conditioning) pulse. A bimodal distribution of the peak of paired-pulse attenuation was found with modes at 10 ms and 50 ms in separate groups of cells. Cells with early peak attenuation showed short CT-evoked response latencies and large responses to relatively few taste stimuli. Conversely, cells with late peak attenuation showed long CT-evoked response latencies and small taste responses with less selectivity. Results suggest that the breadth of tuning of an NTS cell may result from the combination of the sensitivities of peripheral nerve inputs and the recurrent influences generated by the circuitry of the NTS.

Synaptic responses of neurons controlling the parotid and von Ebner salivary glands in rats to stimulation of the solitary nucleus and tract

Salivary secretion results from reflex stimulation of autonomic neurons via afferent sensory information relayed to neurons in the rostral nucleus of the solitary tract (rNST), which synapse with autonomic neurons of the salivatory nuclei. We investigated the synaptic properties of the afferent sensory connection to neurons in the inferior salivatory nucleus (ISN) controlling the parotid and von Ebner salivary glands. Mean synaptic latency recorded from parotid gland neurons was significantly shorter than von Ebner gland neurons. Superfusion of GABA and glycine resulted in a concentration-dependent membrane hyperpolarization. Use of glutamate receptor antagonists indicated that both AMPA and N-methyl-D-aspartate (NMDA) receptors are involved in the evoked excitatory postsynaptic potentials (EPSPs). Inhibitory postsynaptic potential (IPSP) amplitude increased with higher intensity ST stimulation. Addition of the glycine antagonist strychnine did not affect the amplitude of the IPSPs significantly. The GABA(A) receptor antagonist, bicuculline (BMI) or mixture of strychnine and BMI abolished the IPSPs in all neurons. IPSP latency was longer than EPSP latency, suggesting that more than one synapse is involved in the inhibitory pathway. Results show that ISN neurons receive both excitatory and inhibitory afferent input mediated by glutamate and GABA respectively. The ISN neuron response to glycine probably derives from descending connections. Difference in the synaptic characteristics of ISN neurons controlling the parotid and von Ebner glands may relate to the different function of these two glands.

Learning-related long-term potentiation of inhibitory synapses in the cerebellar cortex

Despite the widespread distribution of inhibitory synapses throughout the central nervous system, plasticity of inhibitory synapses related to associative learning has never been reported. In the cerebellum, the neural correlate of fear memory is provided by a long-term potentiation (LTP) of the excitatory synapse between the parallel fibers (PFs) and the Purkinje cell (PC). In this article, we provide evidence that inhibitory synapses in the cerebellar cortex also are affected by fear conditioning. Whole-cell patch-clamp recordings of spontaneous and miniature GABAergic events onto the PC show that the frequency but not the amplitude of these events is significantly greater up to 24 h after the conditioning. Adequate levels of excitation and inhibition are required to maintain the temporal fidelity of a neuronal network. Such fidelity can be evaluated by determining the time window for multiple input coincidence detection. We found that, after fear learning, PCs are able to integrate excitatory inputs with greater probability within short delays, but the width of the whole window is unchanged. Therefore, excitatory LTP provides a more effective detection, and inhibitory potentiation serves to maintain the time resolution of the system.

LTP of GABAergic synapses in the ventral tegmental area and beyond

One of the mechanisms by which the experience-dependent reorganization of neural circuitry can occur is through changes in synaptic strength. Almost every excitatory synapse in the mammalian brain exhibits LTP (long-term potentiation) or LTD (long-term depression), two cellular mechanisms of synaptic plasticity. However, LTP and LTD have been reported much more rarely at fast inhibitory GABA(A) receptor synapses. Our recent study suggests that in vivo morphine initiates a long-lasting alteration of GABAergic synapses in the ventral tegmental area (VTA) by blocking the mechanisms required for LTP of GABAergic synapses. Here we put this work into the context of other examples of synaptic plasticity at GABAergic synapses.

Membrane and synaptic properties of nucleus tractus solitarius neurons projecting to the caudal ventrolateral medulla

Projections from the nucleus of tractus solitarius (NTS) to the caudal ventrolateral medulla (CVLM) are important in mediating autonomic reflexes. However, little is known about the cellular properties of the CVLM-projecting NTS neurons. In this study, the CVLM-projecting NTS neurons were retrogradely labeled by fluorescent microspheres injected into the CVLM. Whole cell voltage- and current-clamp recordings were performed on labeled NTS neurons in coronal brainstem slices. Compared with unlabeled neurons, the labeled NTS neurons had more depolarized resting membrane potentials, larger input resistance, and higher firing activity in response to depolarizing currents. Bath application of an ionotropic glutamate receptor antagonist kynurenic acid and a non-NMDA receptor antagonist CNQX significantly decreased the firing activity in the majority of labeled NTS neurons. In contrast, an NMDA receptor antagonist AP5 failed to alter the firing activity in labeled neurons tested. While the glycine receptor antagonist strychnine had no effect on the firing activity, blockade of GABA(A)receptors with bicuculline significantly increased the firing rate in the majority of labeled NTS neurons. Furthermore, CNQX blocked the majority of spontaneous excitatory postsynaptic currents (EPSCs) and evoked EPSCs elicited by stimulation of the tractus solitarius. The residual spontaneous and evoked EPSCs were abolished by the nicotinic receptor antagonist mecamylamine and the purinergic P2X receptor antagonist iso-PPADS. Finally, while bicuculline completely blocked the miniature inhibitory postsynaptic currents (IPSCs), the spontaneous and evoked IPSCs were abolished by a combination of bicuculline and strychnine in labeled NTS neurons. Collectively, these data suggest that the CVLM-projecting neurons are a population of neurons with distinctive membrane properties.

Differential induction of long term synaptic plasticity in inhibitory synapses of the hippocampus

Long term synaptic plasticity has been more extensively studied in excitatory synapses, but it is also a property of inhibitory synapses. Many inhibitory synapses target hippocampal pyramidal neurons of the CA1 region. They originate from several interneuron classes that subdivide the surface area that they target on the pyramidal cell. Thus, many interneurons preferentially innervate the perisomatic area and axon hillock of the pyramidal cells while others preferentially target dendritic branches and spines. Methods to preferentially activate dendritic or somatic inhibitory synapses onto pyramidal neurons have been devised. By using these methods, the present work demonstrates that a stimulation pattern that induces long term potentiation (LTP) in excitatory synapses of the Schaffer collaterals is also capable of inducing distinct types of long term plastic changes in different classes of inhibitory synapses: Induction of long term depression (LTD) was seen in dendritic inhibitory synapses whereas LTP was observed in somatic inhibitory synapses. These findings suggest that inhibitory synapses arising from different interneuron classes may respond to the same stimulus according to their specific plastic potential enabling a spatial combinatorial pattern of inhibitory effects onto the pyramidal cell.

Activity-dependent bidirectional modification of inhibitory synaptic transmission in rat subthalamic neurons

Rebound burst activity can be generated in neurons in the subthalamic nucleus (STN) by strong GABAergic inhibitory inputs from the globus pallidus externa (GPe) that is reciprocally connected with the STN. It has been proposed that the rebound burst activity in STN neurons is a key event for generating synchronized rhythmic burst activity in the GPe-STN loop, which may be relevant to the resting tremor in Parkinson's disease. Here we report that rebound burst firing of STN neurons induces long-lasting bidirectional modifications of GABAergic synaptic transmission in STN neurons themselves. Using the gramicidin perforated-patch clamp technique in the brain slice preparation, we recorded IPSPs from STN neurons during electrical stimulation of the internal capsule. Rebound spikes triggered by hyperpolarizing current pulses were used to induce modification of inhibitory synaptic transmission. We found that long-lasting potentiation of IPSPs could be induced in the neurons exhibiting three or more rebound spikes that had interspike intervals shorter than half of those during base spontaneous activity, whereas long-lasting depression or no change of IPSP amplitude was likely to be observed in neurons that had no rebound burst or two rebound spikes within a burst. The potentiation or depression of IPSPs was associated with a negative or positive shift of reversal potential of IPSPs (E(IPSP)). The modifications of IPSPs were dependent on activation of postsynaptic voltage-gated calcium channels. This study is the first demonstration that activity-dependent bidirectional modifications of inhibitory synaptic transmission are attributable to bidirectional shifts of E(IPSP).

Benzodiazepine involvement in LTP of the GABA-ergic IPSC in rat hippocampal CA1 neurons

Benzodiazepine binding sites are present on gamma-aminobutyric acid (GABA) receptors in hippocampal neurons. Diazepam is known to potentiate the amplitude and prolong the decay of GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs). In this study, benzodiazepine involvement in long-term potentiation (LTP) of the IPSC was examined. Whole-cell recordings of IPSCs were made from rat hippocampal CA1 neurons in a slice preparation. LTP was induced by a tetanic stimulation in the stratum radiatum (2 trains of 100 Hz for 1 s, 20 s inter-train interval) while pharmacologically blocking ionotropic glutamate receptors. During LTP, the amplitude of the IPSCs was potentiated in the majority of neurons with the IPSC decay and shape unaffected. Diazepam (5 microM) potentiated the IPSC amplitude and prolonged the decay when applied before, but not during, LTP. In neurons in which LTP could not be induced by a tetanic stimulation, diazepam did not increase the amplitude of the pre-tetanic IPSC. Flumazenil, at a concentration (10 microM) that blocked the enhancement of the IPSC by applied diazepam, had no effect on the IPSC amplitude when applied before LTP induction but significantly decreased the IPSC when applied during LTP maintenance. The antagonist, when applied during the tetanic stimulation, did not block LTP, suggesting that benzodiazepine receptors do not participate in LTP induction. These results indicate that the maintenance of LTP of the IPSC involves (a) the release of endogenous benzodiazepine agonist(s) and/or (b) the participation of benzodiazepine binding sites on subsynaptic GABA(A) receptors.

Time course of GABA in the synaptic clefts of inhibitory synapses in the rostral nucleus of the solitary tract

Concentration and time course of neurotransmitter in the synaptic cleft determines the amplitude and the duration of the resulting postsynaptic current. However, technical limitations involved in monitoring the time course of neurotransmitter concentration in the extra-cellular space have prevented direct evaluation of factors that influence neurotransmitter level in the cleft. Tetanic stimulation results in saturation of postsynaptic GABA(A) receptors in the rostral nucleus of the solitary tract (rNST) and GABA diffusion defines the decay time course of the inhibitory potentials or currents (IPSP/Cs). By applying a GABA concentration-response curve to these data it is possible to calculate the GABA concentration transient in the clefts of rNST inhibitory synapses. The analysis indicates that tetanic stimulation produces a GABA concentration that exceeds the concentration of neurotransmitter required to activate all postsynaptic GABA(A) receptors, resulting in short-term modification of the IPSP/Cs decay time. Moreover, the results also demonstrate that the rate of diffusion of GABA from the synaptic cleft is defined by two exponentials. A mathematical model of this process has been developed that supports these conclusions.

GABAB receptor- and metabotropic glutamate receptor-dependent cooperative long-term potentiation of rat hippocampal GABAA synaptic transmission

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABAB receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca2+ rise by postsynaptic injection of GDP-beta-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca2+-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABABR, mGluR, G-proteins and intracellular Ca2+, which lead to a sustained potentiation of GABAA synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.

Suppression of limbic motor seizures by electrical stimulation in thalamic reticular nucleus

Kindling is a model of temporal lobe epilepsy in which repeated electrical stimulations in limbic areas lead to progressive increase of seizure susceptibility, culminating in generalized convulsions and the establishment of a permanent epileptic syndrome. We studied here the effect of stimulations in the thalamic reticular nucleus (TRN) on the development of seizures and hippocampal hyperexcitability in kindling elicited from the ventral hippocampus in rats. Animals given 12 kindling stimulations per day with 30-min intervals for 4 consecutive days developed generalized convulsions on day 4. Stimulations in TRN delivered simultaneously with those in the hippocampus induced marked suppression of seizure generalization. Similarly, the number of generalized seizures and the duration of behavioral convulsions were reduced when rats subjected to 40 kindling stimulations with 5-min intervals during about 3 h were costimulated in the TRN. The anticonvulsant effect of TRN costimulation was detected also when rats were test-stimulated in the hippocampus at 24 h and 2 and 4 weeks after the initial 40 hippocampal stimulations. Our data provide the first evidence that TRN stimulations can act to suppress limbic motor seizures in hippocampal kindling and suggest a new approach for seizure control in temporal lobe epilepsy.

Early GABA(A) receptor clustering during the development of the rostral nucleus of the solitary tract

While there is an abundance of gamma-aminobutyric acid (GABA) in the gustatory zone of the nucleus of the solitary tract of the perinatal rat, we know that GABAergic synapse formation is not complete until well after birth. Our recent results have shown that GABA(B) receptors are present at birth in the cells of the nucleus; however, they do not redistribute and cluster at synaptic sites until after PND10. The present study examined the time course of appearance and redistribution of GABA(A) receptors in the nucleus. GABA(A) receptors were also present at birth. However, in comparison to GABA(B) receptors, GABA(A) receptors underwent an earlier translocation to synaptic sites. Extrasynaptic label, for example, of GABA(A) receptors was non-existent compared to GABA(B) receptors at PND10 and well-defined clusters of GABA(A) receptors could be seen as early as PND1. We propose that while GABA(A), receptors may play an early neurotransmitter role at the synapse, GABA(B) receptors may play a non-transmitter neurotrophic role.

Frequency-dependent properties of inhibitory synapses in the rostral nucleus of the solitary tract

To explore the parameters that define the characteristics of either inhibitory postsynaptic potentials (IPSP) or currents (IPSC) in the gustatory nucleus of the solitary tract (rNST), whole cell patch-clamp recordings were made in horizontal brain stem slices of newborn rats. Neurons were labeled with biocytin to confirm both their location and morphology. IPSPs or IPSCs were evoked by delivering either single, paired-pulse, or tetanic stimulus shocks (0.1-ms duration) via a bipolar stimulating electrode placed on the rNST. Pure IPSP/IPSCs were isolated by the use of glutamate receptor antagonists. For 83% of the single-stimulus-evoked IPSCs, the decay time course was fitted with two exponentials having average time constants of 38 and 181 ms, respectively, while the remainder could be fitted with one exponential of 59 ms. Paired-pulse stimulation resulted in summation of the amplitude of the conditioning and test-stimulus-evoked IPSCs. The decay time course of the test-stimulus-evoked IPSC was slower when compared to the decay time of the conditioning stimulus IPSC. Repeated stimulation resulted in an increase in the decay time of the IPSP/Cs where each consecutive stimulus contributed to prolongation of the decay time constant. Most of the IPSP/Cs resulting from a 1-s >/= 30-Hz tetanic stimulus exhibited an S-shaped decay time course where the amplitude of the IPSP/Cs after termination of the stimulus was initially sustained before starting to decay back to the resting membrane potential. Elevation of extracellular Ca(2+) concentration 10 mM resulted in an increase in the amplitude and decay time of single-stimulus shock-evoked IPSP/Cs. The benzodiazepine GABA(A) receptor modulator diazepam increased the decay time of single-stimulus shock-evoked IPSCs. However, application of diazepam did not affect the decay time of tetanic-stimulation-evoked IPSP/Cs. These results suggest that the decay time of single-stimulus-evoked IPSCs is defined either by receptor kinetics or neurotransmitter clearance from the synaptic cleft or both, while the decay time course of the tetanic stimulus evoked IPSP/Cs is defined by neurotransmitter diffusion from the synaptic cleft. During repetitive stimulation, neurotransmitter accumulates in the synaptic cleft prolonging the decay time constant of the IPSCs. High-frequency stimulation elevates the GABA concentration in the synaptic cleft, which then oversaturates the postsynaptic receptors, and, as a consequence, after termination of the tetanic stimulus, the amplitude of IPSP/Cs is sustained resulting in an S shaped decay time course. This activity-dependent plasticity at GABAergic synapses in the rNST is potentially important in the encoding of taste responses because the dynamic range of stimulus frequencies that result in synaptic plasticity (0-70 Hz) corresponds to the breadth of frequencies that travels via afferent gustatory nerve fibers in response to taste stimuli.

Long-term plasticity at GABAergic and glycinergic synapses: mechanisms and functional significance

Activity-dependent long-term changes in synaptic efficacy are thought to be important in learning, memory formation, neuronal development and pathological states of neuronal excitability in the CNS. For the past two decades, numerous studies have investigated long-term changes in synaptic efficacy at excitatory glutamatergic synapses. Although inhibitory synapses are essential for proper functioning of the neuronal network, attention has focused only recently on describing and characterizing plasticity at these types of synapse. Not surprisingly, different forms of plasticity at GABAergic, and the closely related glycinergic, synapses have been reported in several regions of the brain. Here we review these different forms of plasticity and focus on their possible roles in developing and adult neuronal networks.

Effects of electrical stimulation of the chorda tympani nerve on taste responses in the nucleus of the solitary tract

Despite evidence for an abundance of inhibitory synaptic processes within the taste-responsive portion of the brain stem, little is known about how these processes are activated or modulated. In this context, this study tested the hypothesis that activation of the chorda tympani nerve (CT) invokes inhibition that influences gustatory neural information processing in the rostral nucleus of the solitary tract (NTS). Stimulating electrodes were implanted in the middle ear of urethane-anesthetized rats to enable the passage of current across the CT. Electrophysiological responses to sucrose, NaCl, HCl, and quinine were recorded from single NTS neurons both individually and immediately following tetanic electrical stimulation of the CT. Additionally, NTS field responses to paired pulse stimulation of the CT were recorded. Electrical pulses delivered to the CT were found to produce a compound action potential with four components. Taste-responsive units in the NTS showed tetanus-evoked responses that varied in latency and strength. Those cells that showed strong, short latency responses to CT stimulation showed large magnitude responses to NaCl and were relatively narrowly tuned. Units with longer latencies generally responded more broadly to taste stimuli and with lower response magnitudes. Following tetanus, taste responses in 20 (43%) of the 46 units were reversibly altered in a stimulus-selective manner. Taste responses in 18 units were both enhanced and attenuated following tetanic stimulation, although attenuation was much more common. Additionally, tetanus was found to affect the temporal organization of spikes within taste responses to one stimulus in seven units (15%), four of which also showed changes in response magnitude to a different stimulus following tetanus. The influence of tetanus on taste responses was shown to be reliable and repeatable in neurons from which stimulus trials were recorded more than once. Across all units, responses to quinine were most dramatically and frequently attenuated following tetanus, while those to NaCl were least susceptible to change. NTS field responses evoked by paired pulse stimulation of the CT suggested that the initial pulse evoked an inhibitory influence in the NTS that decayed and returned to baseline by 2 s. These data are consistent with the idea that afferent input to the NTS normally activates inhibitory synaptic activity. As with other sensory systems, such inhibition may serve to facilitate contrast in the neural representation of different stimulus qualities.

GABAergic modulation of primary gustatory afferent synaptic efficacy

Modulation of synaptic transmission at the primary sensory afferent synapse is well documented for the somatosensory and olfactory systems. The present study was undertaken to test whether GABA impacts on transmission of gustatory information at the primary afferent synapse. In goldfish, the vagal gustatory input terminates in a laminated structure, the vagal lobes, whose sensory layers are homologous to the mammalian nucleus of the solitary tract. We relied on immunoreactivity for the GABA-transporter, GAT-1, to determine the distribution of GABAergic synapses in the vagal lobe. Immunocytochemistry showed dense, punctate GAT-1 immunoreactivity coincident with the layers of termination of primary afferent fibers. The laminar nature and polarized dendritic structure of the vagal lobe make it amenable to an in vitro slice preparation to study early synaptic events in the transmission of gustatory input. Electrical stimulation of the gustatory nerves in vitro produces synaptic field potentials (fEPSPs) predominantly mediated by ionotropic glutamate receptors. Bath application of either the GABA(A) receptor agonist muscimol or the GABA(B) receptor agonist baclofen caused a nearly complete suppression of the primary fEPSP. Coapplication of the appropriate GABA(A) or GABA(B) receptor antagonist bicuculline or CGP-55845 significantly reversed the effects of the agonists. These data indicate that GABAergic terminals situated in proximity to primary gustatory afferent terminals can modulate primary afferent input via both GABA(A) and GABA(B) receptors. The mechanism of action of GABA(B) receptors suggests a presynaptic locus of action for that receptor.

Postnatal development of inhibitory synaptic transmission in the rostral nucleus of the solitary tract

To explore the postnatal development of inhibitory synaptic activity in the rostral (gustatory) nucleus of the solitary tract (rNST), whole cell and gramicidin perforated patch-clamp recordings were made in five age groups of rats [postnatal day 0-7 (P0-7), P8-14, P15-21, P22-30, and P >55]. The passive membrane properties of the developing rNST neurons as well as the electrophysiological and pharmacological characteristics of single and tetanic stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were studied in brain slices under glutamate receptor blockade. During the first postnatal weeks, significant changes in resting membrane potential, spontaneous activity, input resistance, and neuron membrane time constant of the rNST neurons occurred. Although all the IPSPs recorded were hyperpolarizing, the rise and decay time constants of the single stimulus shock-evoked IPSPs decreased, and the inhibition response-concentration function to the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline methiodide (BMI) shifted to the left during development. In P0-7 and P8-14, but not in older animals, the IPSPs had a BMI-insensitive component that was sensitive to block by picrotoxin, suggesting a transient expression of GABA(C) receptors. Tetanic stimulation resulted in both short- and long-term changes of inhibitory synaptic transmission in the rNST. For P0-7 and P8-14 animals tetanic stimulation resulted in a sustained hyperpolarization that was maintained for some time after termination of the tetanic stimulation. In contrast, tetanic stimulation of neurons in P15-21 and older animals resulted in hyperpolarization that was not sustained but decayed back to a more positive level with an exponential time course. Tetanic stimulation resulted in potentiation of single stimulus shock-evoked IPSPs in ~50% of neurons in all age groups. These developmental changes in inhibitory synaptic transmission in the rNST may play an important role in shaping synaptic activity in early development of the rat gustatory system during a time of maturation of taste preferences and aversions.