Influence of GABA on neurons of the gustatory zone of the rat nucleus of the solitary tract

A computational analysis of signal fidelity in the rostral nucleus of the solitary tract

Neurons in the rostral nucleus of the solitary tract (rNST) convey taste information to both local circuits and pathways destined for forebrain structures. This nucleus is more than a simple relay, however, because rNST neurons differ in response rates and tuning curves relative to primary afferent fibers. To systematically study the impact of convergence and inhibition on firing frequency and breadth of tuning (BOT) in rNST, we constructed a mathematical model of its two major cell types: projection neurons and inhibitory neurons. First, we fit a conductance-based neuronal model to data derived from whole cell patch-clamp recordings of inhibitory and noninhibitory neurons in a mouse expressing Venus under the control of the VGAT promoter. We then used in vivo chorda tympani (CT) taste responses as afferent input to modeled neurons and assessed how the degree and type of convergence influenced model cell output frequency and BOT for comparison with in vivo gustatory responses from the rNST. Finally, we assessed how presynaptic and postsynaptic inhibition impacted model cell output. The results of our simulations demonstrated 1) increasing numbers of convergent afferents (2-10) result in a proportional increase in best-stimulus firing frequency but only a modest increase in BOT, 2) convergence of afferent input selected from the same best-stimulus class of CT afferents produced a better fit to real data from the rNST compared with convergence of randomly selected afferent input, and 3) inhibition narrowed the BOT to more realistically model the in vivo rNST data. NEW & NOTEWORTHY Using a combination of in vivo and in vitro neurophysiology together with conductance-based modeling, we show how patterns of convergence and inhibition interact in the rostral (gustatory) solitary nucleus to maintain signal fidelity. Although increasing convergence led to a systematic increase in firing frequency, tuning specificity was maintained with a pattern of afferent inputs sharing the best-stimulus compared with random inputs. Tonic inhibition further enhanced response fidelity.

Inhibitory modulation of optogenetically identified neuron subtypes in the rostral solitary nucleus

Inhibition is presumed to play an important role in gustatory processing in the rostral nucleus of the solitary tract (rNST). One source of inhibition, GABA, is abundant within the nucleus and comes both from local, intrasolitary sources and from outside the nucleus. In addition to the receptor-mediated effects of GABA on rNST neurons, the hyperpolarization-sensitive currents, Ih and IA, have the potential to further modulate afferent signals. To elucidate the effects of GABAergic modulation on solitary tract (ST)-evoked responses in phenotypically defined rNST neurons and to define the presence of IA and Ih in the same cells, we combined in vitro recording and optogenetics in a transgenic mouse model. This mouse expresses channelrhodopsin 2 (ChR2) in GAD65-expressing GABAergic neurons throughout the rNST. GABA positive (GABA+) neurons differed from GABA negative (GABA-) neurons in their response to membrane depolarization and ST stimulation. GABA+ neurons had lower thresholds to direct membrane depolarization compared with GABA- neurons, but GABA- neurons responded more faithfully to ST stimulation. Both IA and Ih were present in subsets of GABA+ and GABA- neurons. Interestingly, GABA+ neurons with Ih were more responsive to afferent stimulation than inhibitory neurons devoid of these currents, whereas GABA- neurons with IA were more subject to inhibitory modulation. These results suggest that the voltage-gated channels underlying IA and Ih play an important role in modulating rNST output through a circuit of feedforward inhibition.

Neural dynamics in response to binary taste mixtures

Taste stimuli encountered in the natural environment are usually combinations of multiple tastants. Although a great deal is known about how neurons in the taste system respond to single taste stimuli in isolation, less is known about how the brain deals with such mixture stimuli. Here, we probe the responses of single neurons in primary gustatory cortex (GC) of awake rats to an array of taste stimuli including 100% citric acid (100 mM), 100% sodium chloride (100 mM), 100% sucrose (100 mM), and a range of binary mixtures (90/10, 70/30, 50/50, 30/70, and 10/90%). We tested for the presence of three different hypothetical response patterns: 1) responses varying monotonically as a function of concentration of sucrose (or acid) in the mixture (the "monotonic" pattern); 2) responses increasing or decreasing as a function of degree of mixture of the stimulus (the "mixture" pattern); and 3) responses that change abruptly from being similar to one pure taste to being similar the other (the "categorical" pattern). Our results demonstrate the presence of both monotonic and mixture patterns within responses of GC neurons. Specifically, further analysis (that included the presentation of 50 mM sucrose and citric acid) made it clear that mixture suppression reliably precedes a palatability-related pattern. The temporal dynamics of the emergence of the palatability-related pattern parallel the temporal dynamics of the emergence of preference behavior for the same mixtures as measured by a brief access test. We saw no evidence of categorical coding.

Excitatory and inhibitory synaptic function in the rostral nucleus of the solitary tract in embryonic rat

The embryonic development of synapses in the rostral nucleus of the solitary tract (rNST) was investigated in rat to determine when synapses begin to function. Using a brain slice preparation we studied appearance of synaptic receptors on second order rNST neurons and investigated the development of postsynaptic responses elicited by afferent nerve stimulation. Prenatal excitatory and inhibitory synaptic responses were recorded as early as E14. Glutamatergic and GABAergic postsynaptic responses were detected as early as E16. Both NMDA and AMPA receptors contributed to glutamatergic postsynaptic responses. GABAergic postsynaptic responses resulted primarily from activation of GABA(A) receptors. However, functional GABA(C) receptors were also demonstrated. A glycinergic postsynaptic response was not found although functional glycine receptors were demonstrated at E16. Solitary tract (ST) stimulation-evoked EPSCs, first detected at E16, were eliminated by glutamate receptor antagonists. ST-evoked IPSPs, also detected at E16, were eliminated by GABA(A) receptor antagonist. Thus, considerable prenatal development of rNST synaptic connections occurs and this will ensure postnatal function of central taste processing circuits.

Sweet-bitter and umami-bitter taste interactions in single parabrachial neurons in C57BL/6J mice

We investigated sweet-bitter and umami-bitter mixture taste interactions by presenting sucrose or umami stimuli mixed with quinine hydrochloride (QHCl) while recording single-unit activity of neurons in the parabrachial nucleus (PbN) of urethane-anesthetized C57BL/6J mice. A total of 70 taste-responsive neurons were classified according to which stimulus evoked the greatest net response (36 sucrose-best, 19 NaCl-best, 6 citric acid-best, and 9 QHCl-best). Although no neurons responded best to monopotassium glutamate (MPG) or inosine 5'-monophosphate (IMP), the combination of these two stimuli evoked a synergistic response (i.e., response > 120% of the sum of the component responses) in all sucrose-best and some NaCl-best neurons (n = 43). Adding QHCl to sucrose or MPG + IMP resulted in suppression of the response (responses to mixture < responses to the more effective component) in 41 of 43 synergistic neurons. Neurons showing QHCl suppression were classified into two types: an "MS1" type (n = 27) with suppressed responses both to sucrose and MPG + IMP and an "MS2" type (n = 14) that showed suppressed responses only to sucrose. No neuron displayed suppressed responses to MPG or IMP alone. The suppression ratio (1 - mixture response/sucrose or MPG + IMP response) of sucrose and MPG + IMP in MS1 neurons had a weak positive correlation (r = 0.36). The pattern of reconstructed recording sites of neuron types suggested chemotopic organization in the PbN. Although a peripheral basis for QHCl suppression has been demonstrated, our results suggest that convergence in the PbN plays a role in shaping responses to taste mixtures.

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.

Characteristics of rostral solitary tract nucleus neurons with identified afferent connections that project to the parabrachial nucleus in rats

Afferent information derived from oral chemoreceptors is transmitted to second-order neurons in the rostral solitary tract nucleus (rNST) and then relayed to other CNS locations responsible for complex sensory and motor behaviors. Here we investigate the characteristics of rNST neurons sending information rostrally to the parabrachial nucleus (PBN). Afferent connections to these rNST-PBN projection neurons were identified by anterograde labeling of the chorda tympani (CT), glossopharyngeal (IX), and lingual (LV) nerves. We used voltage- and current-clamp recordings in brain slices to characterize the expression of both the transient A-type potassium current, IKA and the hyperpolarization-activated inward current, Ih, important determinants of neuronal repetitive discharge characteristics. The majority of rNST-PBN neurons express IKA, and these IKA-expressing neurons predominate in CT and IX terminal fields but were expressed in approximately half of the neurons in the LV field. rNST-PBN neurons expressing Ih were evenly distributed among CT, IX and LV terminal fields. However, expression patterns of IKA and Ih differed among CT, IX, and LV fields. IKA-expressing neurons frequently coexpress Ih in CT and IX terminal fields, whereas neurons in LV terminal field often express only Ih. After GABAA receptor block all rNST-PBN neurons responded to afferent stimulation with all-or-none excitatory synaptic responses. rNST-PBN neurons had either multipolar or elongate morphologies and were distributed throughout the rNST, but multipolar neurons were more often encountered in CT and IX terminal fields. No correlation was found between the biophysical and morphological characteristics of the rNST-PBN projection neurons in each terminal field.

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.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Local circuit input to the medullary reticular formation from the rostral nucleus of the solitary tract

The intermediate reticular formation (IRt) subjacent to the rostral (gustatory) nucleus of the solitary tract (rNST) receives projections from the rNST and appears essential to the expression of taste-elicited ingestion and rejection responses. We used whole cell patch-clamp recording and calcium imaging to characterize responses from an identified population of prehypoglossal neurons in the IRt to electrical stimulation of the rNST in a neonatal rat pup slice preparation. The calcium imaging studies indicated that IRt neurons could be activated by rNST stimulation and that many neurons were under tonic inhibition. Whole cell patch-clamp recording revealed mono- and polysynaptic projections from the rNST to identified prehypoglossal neurons. The projection was primarily excitatory and glutamatergic; however, there were some inhibitory GABAergic projections, and many neurons received excitatory and inhibitory inputs. There was also evidence of disinhibition. Overall, bath application of GABA(A) antagonists increased the amplitude of excitatory currents, and, in several neurons, stimulation of the rNST systematically decreased inhibitory currents. We have hypothesized that the transition from licks to gapes by natural stimuli, such as quinine monohydrochloride, could occur via such disinhibition. We present an updated dynamic model that summarizes the complex synaptic interface between the rNST and the IRt and demonstrates how inhibition could contribute to the transition from ingestion to rejection.

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Cholinergic modulation of neurons in the gustatory region of the nucleus of the solitary tract

The rostral portion of the nucleus of the solitary tract (rNST) is an obligatory relay for gustatory afferent input on its way to the forebrain. Previous studies have demonstrated excitation of rNTS neurons by glutamate and substance P and inhibition by gamma-aminobutyric acid (GABA) and met-enkephalin (ENK). Despite the existence of cholinergic neurons and putative terminals within the rNTS, there are no data on the effects of acetylcholine (ACh) on rNTS processing. Here, we use patch-clamp recording of rNTS neurons in vitro to examine ACh-mediated responses and voltage-gated conductances in these cells. Results revealed (1) intrinsic voltage-gated inhibition via activation of voltage-gated potassium A-channels (I(A)), found almost exclusively in the medial rNTS, and hyperpolarization-activated potassium/sodium channels (I(h)), found more frequently in the lateral rNST; and (2) ligand-gated inhibition via activation of muscarinic m2 ACh receptors (mAChRs) linked to inward rectifier potassium channels (K(ir)) evenly distributed throughout the rNTS, a mechanism dependent on cholinergic inputs. Muscarinic responses were blocked by AFDX-116, a selective m2 mAChR antagonist, and by BaCl2, an antagonist of K(ir) channels. In addition, many rNTS neurons exhibited excitation via alpha7 and non-alpha7 nicotinic AChRs. Non-alpha7 nAChRs, blocked by 10 microM mecamylamine, occurred more frequently in the lateral rNTS. In contrast, alpha7 nAChRs, blocked by 20 nM methyllcaconitine, were evenly distributed across the nucleus. As previously reported for voltage-activated conductances, none of these currents was related to neuronal morphology. These voltage- and ligand-dependent inhibitory mechanisms would be expected to contribute to the modulation of gustatory processing through the NST.

Distinct regional distributions of NK1 and NK3 neurokinin receptor immunoreactivity in rat brainstem gustatory centers

Tachykinins and their receptors are present in gustatory centers, but little is known about tachykinin function in gustation. In this study, immunohistochemical localization of substance P and two centrally prevalent neurokinin receptors, NK1 and NK3, was carried out in the rostral nucleus of the solitary tract and the caudal parabrachial nucleus to evaluate regional receptor/ligand correspondences. All three proteins showed regional variations in labeling density that correlated with distinct sites in gustatory centers. In the rostral nucleus of the solitary tract, the relative densities of substance P and NK1 receptors varied in parallel across subnuclei, with both being moderate to dense in the dorsocentral, chemoresponsive zone. NK3 receptors had a distinct distribution in the caudal half of this zone, suggesting a unique role in processing taste input from the posterior tongue. In the caudal parabrachial nucleus, substance P and NK1 receptor immunoreactivities were dense in the pontine taste area, while NK3 receptor labeling was sparse. The external medial subnucleus had substantial NK3 receptor and substance P labeling, but little NK1 receptor immunoreactivity. These findings suggest that distinct tachykinin ligand/neurokinin receptor combinations may be important in local processing of information within brainstem gustatory centers.

Distribution of immunoreactive GABA and glutamate receptors in the gustatory portion of the nucleus of the solitary tract in rat

The distribution of glutamate (GLU) and gamma-aminobutyric acid (GABA) receptors within the gustatory portion of the rat nucleus of the solitary tract (gNST) was investigated using immunohistochemical, histological and neural tract tracing techniques. Numerous somata throughout the gNST were immunoreactive for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors, while few were labeled for kainate receptors. AMPA and NMDA receptors were particularly abundant in the rostral central (RC) subdivision of the gNST, which receives most of the primary afferent input from the oral cavity and contains most of the gNST neurons that project to the parabrachial nuclei (PBN). This finding supports electrophysiological evidence that AMPA and NMDA receptors are involved in responses to orosensory input and indicates that their action may influence ascending taste signals as well. Compared to the ionotropic GLU receptors, few cell bodies were immunoreactive for metabotropic GLU receptors. Somata immunoreactive for GABA(A) and GABA(B) receptors were located throughout the nucleus. The densest neuropil labeling was for GABA(A) receptors in the ventral (V) subnucleus, the gNST subdivision that sends output to brainstem oromotor centers. The distributions of immunolabeling for GLU and GABA receptors imply that different functional roles may exist for specific receptors within this nucleus.

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.

Opioid modulation of taste responses in the nucleus of the solitary tract

Gustatory processing within the medulla is modulated by a number of physiologic and experiential factors. Several neurotransmitters, including excitatory amino acids, GABA, and substance P, are involved in synaptic processing within the rostral portion of the nucleus of the solitary tract (NST). Endogenous opiates have been implicated in the regulation of feeding behavior and in taste palatability and gustatory responses in the parabrachial nuclei are reduced by systemic morphine. In the present experiments, extracellular recording of neuronal activity within the NST in response to taste input was combined with local microinjection of met-enkephalin (Met-ENK) and naltrexone (NLTX) to determine the effect of these agents on gustatory activity. The anterior tongue was stimulated with anodal current pulses to determine the time course of drug action (n=85 cells) and with prototypical taste stimuli (0.032 M sucrose, NaCl, and quinine hydrochloride, and 0.0032 M citric acid) to investigate the effects of these opioid compounds on taste-evoked responses (n=80 cells). Among these 165 taste-responsive neurons in the NST, the activity of 39 (23.6%) was suppressed by Met-ENK. These effects were dose-dependent and blockable by NLTX, which alone was without effect, suggesting that opiates do not maintain a tonic inhibitory influence. Immunohistochemical experiments demonstrated both micro - and delta-opioid receptors within the gustatory portion of the NST; previous studies had shown numerous fiber terminals containing Met-ENK. These data suggest that endogenous opiates play an inhibitory role in gustatory processing within the medulla.

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.

Taste responses of neurons in the hamster solitary nucleus are modulated by the central nucleus of the amygdala

Previous studies have shown a modulatory influence of forebrain gustatory areas, such as the gustatory cortex and lateral hypothalamus, on the activity of taste-responsive cells in the nucleus of the solitary tract (NST). The central nucleus of the amygdala (CeA), which receives gustatory afferent information, also exerts descending control over taste neurons in the parabrachial nuclei (PbN) of the pons. The present studies were designed to investigate the role of descending amgydaloid projections to the NST in the modulation of gustatory activity. Extracellular action potentials were recorded from 109 taste-responsive cells in the NST of urethan-anesthetized hamsters and analyzed for a change in excitability following electrical and chemical stimulation of the CeA. Electrical stimulation of the CeA orthodromically modulated 36 of 109 (33.0%) taste-responsive NST cells. An excitatory response was observed in 33 (30.28%) cells. An initial decrease in excitability to electrical stimulation of the CeA, suggestive of postsynaptic inhibition, was observed in three (2.75%) NST taste cells. NST cells modulated by the CeA were significantly less responsive to taste stimuli than cells that were not. Many of these cells were under the modulatory influence of the contralateral CeA (28/36 = 77.8%) as well as the ipsilateral (22/36 = 61.1%); 14 (38.9%) were excited bilaterally. Latencies for excitation were longer after ipsilateral than after contralateral CeA stimulation. Microinjection of DL-homocysteic acid (DLH) into the CeA mimicked the effect of electrical stimulation on each of the nine cells tested: DLH excited eight and inhibited one of these electrically activated NST cells. Application of subthreshold electrical stimulation to the CeA during taste trials increased the taste responses of every CeA-responsive NST cell (n = 7) tested with this protocol. These effects would enhance taste discriminability by increasing the signal-to-noise ratio of taste-evoked activity.

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.

Redistribution and increased specificity of GABA(B) receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of gamma-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA(B) receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA(B) receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA(B) receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.

Changes in GABA-immunoreactivity during development of the rostral subdivision of the nucleus of the solitary tract

GABA plays an important role in the processing of gustatory information in the rostral nucleus of the solitary tract. The following study used post-embedment immunohistochemistry in the rat brainstem to localize GABA at both the light and electron microscopic levels to characterize the developmental distribution of GABA and synaptogenesis of GABA-immunoreactive terminals in the rostral nucleus of the solitary tract. During the first postnatal week, GABA is present in the rostral nucleus of the solitary tract, but less of it is synaptic than any time later in development. Of the few synaptic terminals present at postnatal day 1, less than 20% are GABA-immunoreactive. This proportion more than doubles to reach adult levels by postnatal day 10. By weaning (postnatal day 20), GABA-immunoreactive cells are found in nearly the same density as in the adult. Development continues after weaning and is characterized by a disproportionate loss of non-GABA-containing cells. Finally, one previously identified subtype of GABA-immunoreactive terminal matures very late during the postweaning phase of development. The study provides the first analysis of the development of GABA-related circuitry in the rostral nucleus of the solitary tract using anatomical methods. These data provide the background with which to view the emerging physiology of developing taste neurons.

Biophysical properties and responses to neurotransmitters of petrosal and geniculate ganglion neurons innervating the tongue

The properties of afferent sensory neurons supplying taste receptors on the tongue were examined in vitro. Neurons in the geniculate (GG) and petrosal ganglia (PG) supplying the tongue were fluorescently labeled, acutely dissociated, and then analyzed using patch-clamp recording. Measurement of the dissociated neurons revealed that PG neurons were significantly larger than GG neurons. The active and passive membrane properties of these ganglion neurons were examined and compared with each other. There were significant differences between the properties of neurons in the PG and GG ganglia. The mean membrane time constant, spike threshold, action potential half-width, and action potential decay time of GG neurons was significantly less than those of PG neurons. Neurons in the PG had action potentials that had a fast rise and fall time (sharp action potentials) as well as action potentials with a deflection or hump on the falling phase (humped action potentials), whereas action potentials of GG neurons were all sharp. There were also significant differences in the response of PG and GG neurons to the application of acetylcholine (ACh), serotonin (5HT), substance P (SP), and GABA. Whereas PG neurons responded to ACh, 5HT, SP, and GABA, GG neurons only responded to SP and GABA. In addition, the properties of GG neurons were more homogeneous than those of the PG because all the GG neurons had sharp spikes and when responses to neurotransmitters occurred, either all or most of the neurons responded. These differences between neurons of the GG and PG may relate to the type of receptor innervated. PG ganglion neurons innervate a number of receptor types on the posterior tongue and have more heterogeneous properties, while GG neurons predominantly innervate taste buds and have more homogeneous properties.

The neural code for taste in the brain stem: response profiles

In the study of the neural code for taste, two theories have dominated the literature: the across neuron pattern (ANP), and the labeled line theories. Both of these theories are based on the observations that taste cells are multisensitive across a variety of different taste stimuli. Given a fixed array of taste stimuli, a cell's particular set of sensitivities defines its response profile. The characteristics of response profiles are the basis of both major theories of coding. In reviewing the literature, it is apparent that response profiles are an expression of a complex interplay of excitatory and inhibitory inputs that derive from cells with a wide variety of sensitivity patterns. These observations suggest that, in the absence of inhibition, taste cells might be potentially responsive to all taste stimuli. Several studies also suggest that response profiles can be influenced by the taste context, defined as the taste stimulus presented just before or simultaneously with another, under which they are recorded. A theory, called dynamic coding, was proposed to account for context dependency of taste response profiles. In this theory, those cells that are unaffected by taste context would provide the signal, i.e., the information-containing portion of the ANP, and those cells whose responses are context dependent would provide noise, i.e., less stimulus specific information. When singular taste stimuli are presented, noise cells would provide amplification of the signal, and when complex mixtures are presented, the responses of the noise cells would be suppressed (depending on the particular combination of tastants), and the ratio of signal to noise would be enhanced.

GABA-mediated corticofugal inhibition of taste-responsive neurons in the nucleus of the solitary tract

The nucleus of the solitary tract (NST) receives descending connections from several forebrain targets of the gustatory system, including the insular cortex. Many taste-responsive cells in the NST are inhibited by gamma-aminobutyric acid (GABA). In the present study, we investigated the effects of cortical stimulation on the activity of gustatory neurons in the NST. Multibarrel glass micropipettes were used to record the activity of NST neurons extracellularly and to apply the GABA(A) antagonist bicuculline methiodide (BICM) into the vicinity of the cell. Taste stimuli were 0.032 M sucrose (S), 0.032 M NaCl (N), 0.00032 M citric acid (H), and 0.032 M quinine hydrochloride (Q), presented to the anterior tongue. Each of 50 NST cells was classified as S-, N-, H-, or Q-best on the basis of its response to chemical stimulation of the tongue. The ipsilateral insular cortex was stimulated both electrically (0.5 mA, 100 Hz, 0.2 ms) and chemically (10 mM DL-homocysteic acid, DLH), while the spontaneous activity of each NST cell was recorded. The baseline activity of 34% of the cells (n=17) was modulated by cortical stimulation: eight cells were inhibited and nine were excited. BICM microinjected into the NST blocked the cortical-induced inhibition but had no effect on the excitatory response. Although the excitatory effects were distributed across S-, N-, and H-best neurons, the inhibitory effects of cortical stimulation were significantly more common in N-best cells. These data suggest that corticofugal input to the NST may differentially inhibit gustatory afferent activity through GABAergic mechanisms.

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

Whole-cell recordings were made from neurons in the rostral nucleus of the solitary tract in horizontal brainstem slices. Monosynaptic GABAA receptor-mediated inhibitory postsynaptic potentials were evoked by single stimulus shocks or by high-frequency tetanic stimulation in the presence of glutamate receptor blockers. While single stimulus-evoked inhibitory postsynaptic potentials had variable amplitudes, tetanic stimulation-induced, hyperpolarizing postsynaptic potentials were of a more constant amplitude. Furthermore, tetanic stimulation resulted in potentiation of the amplitude of single stimulus shock-evoked inhibitory postsynaptic potentials. Of 55 neurons that were tested, potentiation lasted over 30 min for 11, 10-30 min for 13, less than 10 min for 23 and no potentiation occurred in eight. Tetanic stimulation did not result in potentiation of the tetanic stimulus-evoked hyperpolarizing postsynaptic potentials. Both the single stimulus shock- and tetanic stimulus-evoked potentials had similar inhibition concentration-response curves to the GABAA antagonist, bicuculline methiodide (EC50 = 0.75 and 0.83, respectively), indicating that they were mediated by the same postsynaptic receptors. By comparing the effect of bicuculline methiodide on the amplitude of the single stimulus shock-evoked inhibitory postsynaptic potentials and the tetanic stimulus-evoked hyperpolarizing potentials, we concluded that a single stimulus shock does not activate all postsynaptic GABAA receptors. However, tetanic stimulation results in activation of all postsynaptic GABAA receptors and induces long-lasting changes in the presynaptic GABAergic neuron. These long-lasting changes of the presynaptic neuron facilitate the release of GABA during single stimulus shock and, as a consequence, more postsynaptic receptors are activated during single stimulus shock-evoked synaptic transmission. This conclusion is supported by the results of experiments in which the extracellular Ca2+ concentration was manipulated to change the amount of neurotransmitter released from the presynaptic GABAergic terminals. The single stimulus shock-evoked inhibitory postsynaptic potentials were sensitive to the extracellular Ca2+ concentration, whereas tetanic stimulus-evoked inhibitory post-synaptic potentials were essentially insensitive to extracellular Ca2+ concentration. The relationship between the single stimulus shock-evoked inhibitory postsynaptic potential amplitude and extracellular Ca2+ concentration indicates that, in control physiological saline containing 2.5 mM Ca2+, a single stimulus shock activates less than half the postsynaptic GABA receptors. The phenomenon of long-lasting potentiation of inhibitory transmission within the rostral nucleus of the solitary tract may be important in the processing of gustatory information and play a role in taste-guided behaviors.

Structure and function of gustatory neurons in the nucleus of the solitary tract. IV. The morphology and synaptology of GABA-immunoreactive terminals

In the visual, auditory and somatosensory systems, insight into the synaptic arrangements of specific types of neurons has proven useful in understanding how sensory processing within that system occurs. The neurotransmitter GABA is present in the nucleus of the solitary tract and based on the fact that the vast majority of cells respond to GABA, its agonists and antagonists, and that over 45% of synaptic terminals in the rostral subdivision of the nucleus of the solitary tract are GABA-immunoreactive, GABA is thought to play an important role in gustatory processing. The following study was carried out to establish the distribution of GABA-immunoreactive terminals within the nucleus of the solitary tract. Specifically, the distribution on to physiologically-identified gustatory neurons was determined using post-embedding electron immuno-histochemistry. GABA-immunoreactive terminals synapse with gustatory neuronal somata and all portions of their dendrites, but non-GABAergic terminals synapse only with distal dendrites of the gustatory cells and on to correspondingly small unidentified dendritic profiles in the neuropil. There is a differential distribution of two subtypes of GABA-immunoreactive terminals on to proximal and distal portions of the gustatory neurons as well. Finally, a model for the synaptic arrangements involving gustatory and GABAergic neurons is proposed.

Excitatory and inhibitory modulation of taste responses in the hamster brainstem

The rostral portion of the nucleus of the solitary tract (NST) contains second-order gustatory neurons, sends projections to the parabrachial complex and brainstem reticular formation, and receives descending projections from several nuclei of the ascending gustatory pathway. Electrophysiological responses of NST neurons can be modulated by several factors, including blood glucose and insulin levels and taste aversion conditioning. We are using extracellular electrophysiological recording in vivo, combined with local microinjection of neurotransmitter agonists and antagonists, to study the mechanisms by which taste responses of cells in the hamster NST can be modulated. Afferent fibers of the chorda tympani (CT) nerve make excitatory synaptic contact with NST neurons; this excitation is probably mediated by the excitatory amino acid glutamate. Microinjection of kynurenic acid, a nonspecific glutamate receptor antagonist, into the NST completely and reversibly blocks afferent input from the CT nerve, produced by either anodal electrical or chemical stimulation of the anterior tongue. The non-NMDA ((RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) also completely blocks gustatory input to these cells, whereas the N-methyl-D-aspartate (NMDA) antagonist DL-2-amino-5-phosphonovalerate (APV) produces only a small effect. There are many gamma-aminobutyric acid (GABA)-containing neurons within the NST and taste-responsive NST cells are maintained under a tonic GABAergic inhibition. Microinjection of the GABAA receptor antagonist bicuculline methiodide increases the taste responsiveness of NST neurons, whereas application of GABA inhibits taste responses in these cells. Preliminary data show that GABAergic inhibition can be produced by stimulation of the gustatory cortex. There are both intrinsic substance P (SP)-containing neurons and extrinsic SP-immunoreactive fibers in the rostral NST. Microinjection of SP into the NST enhances the responses of many NST cells to gustatory stimulation; NaCl-best neurons are preferentially excited by SP.

Neural circuits for taste. Excitation, inhibition, and synaptic plasticity in the rostral gustatory zone of the nucleus of the solitary tract

The rostral nucleus of the solitary tract (rNST) plays a key role in modulating, organizing and distributing the sensory information arriving at the central nervous system from gustatory receptors. However, except for some anatomical studies of rNST synapses, the neural circuits responsible for this first stage in synaptic processing of taste information are largely unknown. Over the past few years we have used an in vitro brain slice preparation of the rNST to study synaptic processing, and it has become apparent that the rNST is a very complex neural relay. Synaptic potentials recorded in rNST neurons resulting from stimulation of afferent taste fibers are a composite of excitatory and inhibitory post synaptic potentials. Pure excitatory postsynaptic potentials (EPSP) can be isolated by using gamma-aminobutyric acid type A (GABAA) receptor blockers to eliminate the inhibitory postsynaptic potentials (IPSP). Application of glutamate ionotropic receptor blockers effectively eliminates all postsynaptic activity, indicating that glutamate is the transmitter at the first central synapse in the taste pathway. Stimulation of the afferent taste fibers originating from the anterior (chorda tympani) and posterior (glossopharyngeal) tongue results in a postsynaptic potential that is a complex sum of the two individual potentials. Thus, rNST neurons receive convergent synaptic input from the anterior and posterior tongue. The IPSP component of the synaptic potentials in rNST results from stimulation of interneurons. If these IPSPs are initiated by tetanic stimulation they undergo both short-term and long-term changes. Short-term changes result in the development of biphasic depolarizing IPSPs, and long-term changes result in potentiation of the IPSPs that can last over an hr in some neurons. This remarkable synaptic plasticity may be involved in the mechanism of learned taste behaviors. Synaptic transmission in rNST consists of excitation combined with inhibition. The inhibition does not simply depress excitation but probably serves many roles such as shaping and limiting excitation, coordinating the timing of synaptic events and participating in synaptic plasticity. Knowledge of these synaptic mechanisms is essential to understanding how the rNST processes taste information.

Ionic mechanism of GABAA biphasic synaptic potentials in gustatory nucleus of the solitary tract

Gamma-aminobutyric acid (GABA) is the principal neurotransmitter of synaptic inhibition in the gustatory nucleus of the solitary tract (rNST). High-frequency activation of GABA neurons in the rNST results in biphasic inhibitory postsynaptic potentials (IPSPs) that are initially hyperpolarizing but then became depolarizing. Our results indicate that high-frequency stimulation evokes redistribution of Cl- and K+ ions that shifts IPSP reversal potential in a more positive direction, which produces a biphasic or depolarizing IPSP.

Effects of GABA on neurons of the gustatory and visceral zones of the parabrachial nucleus in rats

Response characteristics of neurons in the gustatory and visceral zone of the parabrachial nucleus (PBN) to gamma-aminobutyric acid (GABA) were examined using whole cell recordings in brain slices of the rat. Based on the recording site, neurons were divided into three groups: neurons in the dorsolateral quadrant of the PBN (DL-neurons), neurons in the dorsomedial quadrant of the PBN (DM-neurons) and neurons in the ventromedial quadrant of the PBN (VM-neurons). Recordings were made from 44 DL-, 43 DM-, 39 VM-neurons. Superfusion of GABA resulted in a concentration-dependent reduction in input resistance in 67.5% of the neurons in the PBN (73.1% of the DL-, 62.5% of the DM-, 66.7% of the VM-neurons). No obvious difference of the concentration-response curve was found among three groups. The mean reversal potential of the GABA effect was about -74 mV and no significant differences were observed among three groups of neurons. The GABA response was partly or completely blocked by the GABAA antagonist bicuculline in all neurons tested. Superfusion of the GABAA agonist muscimol resulted in a decrease of the input resistance in all neurons tested. It was concluded that GABA functions as an inhibitory neurotransmitter in both gustatory and visceral part of the PBN, mediated in part, by GABAA receptors.

Structure and function of gustatory neurons in the nucleus of the solitary tract. III. Classification of terminals using cluster analysis

In sensory systems, insight into synaptic arrangements on cells of known physiological response properties has helped our understanding of the structural basis for these properties. To carry out these types of studies, however, synaptic types in the region of interest must be defined. Unfortunately, defining synaptic types in the brainstem has proved to be a challenging enterprise. Our study was done to classify synapses in the gustatory part of the nucleus solitarius using objective quantitative criteria and a cluster analysis procedure. Cluster analysis allows classification of a population of objects, such as synaptic terminals, into groups that exhibit similar characteristics. Six terminal types were identified using cluster analysis and subsequent analyses of variance and post hoc tests. Unlike classification schemes used for the cerebral cortex, where synaptic apposition density thickness and shape of vesicles is useful (Gray's Type I and II synapses), the concentration of vesicles in a terminal was a more useful measurement with which to classify terminals in the nucleus solitarius. To validate that vesicle density (vesicles/microm2) is a useful defining characteristic to classify terminals in the nucleus solitarius, terminals of a known type were used. GABAergic terminals were identified using postembedding immunohistochemical techniques, and their vesicle density was determined. GABAergic terminals fall into the range of two of the terminal types defined by the cluster analysis and, based on vesicle density, two types of GABAergic terminals were identified. We conclude that vesicle density is a helpful means to identify synapses in this brainstem nucleus.

Tonic GABA(A) receptor-mediated neurotransmission in the dorsal vagal complex regulates intestinal motility in rats

Vagal motor outflow from the dorsal vagal complex is important in the regulation of intestinal motility. The aim of our study was to test the hypothesis that within the dorsal vagal complex, tonic GABA(A)-receptor mediated neurotransmission modulates intestinal motility. The GABA(A) receptor antagonist, bicuculline (methiodide), was microinjected into the dorsal vagal complex, and the effects on small intestinal and colonic motility were investigated. Rats were anesthetized and the mean arterial pressure and heart rate were monitored. Jejunal and colonic motility were measured manometrically, and motility indices were calculated manually. Bicuculline at concentrations of 0.25 or 0.5 mM in 30 nl was microinjected bilaterally into the dorsal vagal complex through stereotaxically placed micropipettes. The injection sites were confirmed histologically using the dye Alcian Blue. Bicuculline (0.5 mM) inhibited spontaneous jejunal motility by 76.3%, colonic motility by 51.7%, mean arterial pressure by 23.3% and heart rate by 27.6%. The lower concentration of bicuculline (0.25 mM) showed no inhibitory effects on intestinal motility but decreased mean arterial blood pressure by 24.1% and heart rate by 13.6%. Bilateral cervical vagotomy attenuated the bicuculline (0.5 mM)-induced inhibition of spontaneous jejunal motility, whereas the bicuculline effect on colonic motility was unaffected. The results of this study show that GABA(A) receptor-mediated neurotransmission in the dorsal vagal complex is involved in autonomic integration of motility of the small intestine and colon. Furthermore, our results indicate that the dorsal vagal complex regulation of jejunal motility involves vagal outflow, whereas vagal pathways do not participate in the bicuculline-induced inhibition of colonic motility.

Synaptic relationships between the chorda tympani and tyrosine hydroxylase-immunoreactive dendritic processes in the gustatory zone of the nucleus of the solitary tract in the hamster

The toxic lectin ricin was applied to the hamster chorda tympani (CT), producing anterograde degeneration of its terminal boutons within the gustatory zone of the nucleus of the solitary tract (NST). Immunocytochemistry was subsequently performed with antiserum against tyrosine hydroxylase (TH), and the synaptic relationships between degenerating CT terminal boutons and either TH-immunoreactive or unlabeled dendritic processes were examined at the electron microscopic level. Degenerating CT terminal boutons formed asymmetric axodendritic synapses and contained small, clear, spherical synaptic vesicles that were densely packed and evenly distributed throughout the ending, with no accumulation at the active synaptic. The degenerating CT terminated on the dendrites of TH-immunoreactive neurons in 36% (35/97) of the cases. The most frequent termination pattern involved the CT and two or three other inputs in synaptic contact with a single immunoreactive dendrite, resulting in a glomerular-like structure that was enclosed by glial processes. In 64% (62/97) of the cases, the degenerating CT was in synaptic contact with unlabeled dendrites, often forming a calyx-like synaptic profile that surrounded much of the perimeter of a single unlabeled dendrite. These results indicate that the TH-immunoreactive neurons of the gustatory NST receive direct input from the CT and taste receptors of the anterior tongue and that the termination patterns of the CT vary with its target neuron in the gustatory NST. The glomerular-like structure that characterizes many of the terminations of the CT provides an opportunity for the convergence of several functionally distinct inputs (both gustatory and somatosensory) onto putative dopaminergic neurons that may shape their responsiveness to the stimulation of the oral cavity.

Neurons in the vagal complex of the rat respond to mechanical and chemical stimulation of the GI tract

Perfusing the duodenum with acid solutions dramatically reduces gastric motility and acid secretion. We propose that the presence of acid in the proximal small intestine initiates a vagovagal reflex that excites inhibitory neurons in the nucleus of the solitary tract (NST) and reduces the activity of the neurons in the dorsal motor nucleus of the vagus nerve (DMNV). However, results from several investigations suggest that the relevant circuit may not be as simple as we had believed. The present study was designed to address this dilemma by employing intracellular and extracellular recording and intracellular labeling techniques to provide direct information on the activity of neurons in the NST and DMNV during and after intestinal exposure to acid solutions. The results obtained prove that NST and DMNV neurons respond to HCl in the duodenum. In some instances, these neurons were very stimulus specific, although the majority of the cells in our sample (47% of NST neurons and 86% of DMNV neurons) also responded to distension of the stomach and/or duodenum. It is important to note, however, that many of the more broadly responsive neurons in the dorsal vagal complex were able to distinguish between mechanical and chemical stimulation of the gastrointestinal (GI) tract. Most of the NST neurons that responded to duodenal perfusion with HCl were excited by this stimulus. Conversely, activity of most of the DMNV neurons decreased after the onset of the HCl stimulus. These findings verify the existence of a vagovagal reflex pathway initiated by duodenal perfusion with acid. Presumably, this reflex would decrease gastric motility and acid secretion, reducing the amount of acid that enters the duodenum and ultimately protecting the intestinal mucosa.

Tetanic stimulation induces short-term potentiation of inhibitory synaptic activity in the rostral nucleus of the solitary tract

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 gamma-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.

Immunohistochemical localization of GABAA receptors in comparison with GABA-immunoreactive structures in the nucleus tractus solitarii of the rat

The localization of GABAA receptors was studied by immunohistochemistry in the nucleus tractus solitarii of the rat using a monoclonal antibody (bd17) against the beta-subunit. The pattern of distribution was compared with that of GABA-immunoreactive axons and nerve terminals. Positive staining for GABAA receptors was confined to regions near the surface of neuronal somata and their processes. The highest density of positive staining for GABAA receptors was seen in the central part of the rostral nucleus tractus solitarii where GABA-positive terminals were also rather dense. At both intermediate and caudal levels of the nucleus tractus solitarii, a moderate density of positive staining for GABAA receptors was located in the ventrolateral part, including the ventrolateral subnucleus. In these regions, the density of GABA-positive terminals was low. In the medial nucleus tractus solitarii, including the medial subnucleus, very little or no positive staining for GABAA receptors was detected, although many GABA-positive terminals were observed. The results suggest that the central part of the rostral nucleus tractus solitarii is controlled by the GABAergic system via GABAA receptors, but in the medial subnucleus of the nucleus tractus solitarii the GABA neurons appear to act via receptors that are not detectable by the antibody used.

Diazepam-sensitive GABA(A) receptors in the NTS participate in cardiovascular control

The present study employed neuropharmacological and receptor binding protocols to determine if diazepam-sensitive (DS) gamma-aminobutyric acid-A (GABA(A)) receptors in the nucleus tractus solitarius (NTS) participate in autonomic regulation of cardiovascular function. The first set of protocols was designed to determine if GABA(A) receptors in the NTS were functionally modulated by the benzodiazepine agonist, diazepam. Mean arterial pressure and heart rate responses to microinjection of GABAergic substances into the NTS were examined in urethane-anesthetized rats. Microinjection of the GABA(A) agonist isoguvacine into the NTS increased mean arterial pressure and heart rate, and these effects were blocked by the GABA(A) receptor antagonist, bicuculline. Preadministration of diazepam into the NTS potentiated the pressor actions of isoguvacine and had variable effects on heart rate changes. Flumazenil, a benzodiazepine antagonist, blocked the diazepam-induced potentiation of the pressor response to isoguvacine. The second protocol employed receptor autoradiography to examine the presence of DS and diazepam-insensitive (DI) GABA(A) receptors in the NTS. Autoradiography confirmed that DS GABA(A) receptors were present in the NTS; however, no measurable levels of DI GABA(A) receptors were detected. We conclude that GABA(A)-mediated integration of central autonomic control in the NTS is mediated solely by DS GABA(A) receptors.

Transfer of information about taste from the nucleus of the solitary tract to the parabrachial nucleus of the pons

In the study of the neural code for gustation, the relative sensitivity of a cell to a variety of taste stimuli is defined as its response profile. To study the construction of response profiles from incoming signals, electrophysiological responses to NaCl, HCI, quinine-HCl, sucrose and Na saccharin were recorded simultaneously in pairs of single cells: one in the nucleus of the solitary tract (NTS) and the other in the parabrachial nucleus of the pons (PbN), respectively the first and second synapses in the central pathway for gustation. Of 37 units recorded in the NTS and 32 in the PbN, 12 (32%) pairs showed evidence of functional connectivity. Although PbN responses were significantly larger than those in the NTS in general, no amplification of NTS activity was apparent among those units that were functionally connected. Analysis of NTS-PbN connectivity patterns suggests that PbN units receive input from NTS units with response profiles that are both similar and different from their own pattern of sensitivities. Further analysis suggests that the stimulus-selectivity of the response profile of a PbN unit may be determined by stimulus-selective input from NTS cells that show similar response profiles. However, input from NTS cells with response profiles different from their own appears to be non-stimulus-selective. Analysis of the organization of response profiles in the two structures suggests that the cells in both the NTS and PbN cannot be easily distinguished by their patterns of sensitivity to taste stimuli.

Distribution of GABA and glycine in the lamb nucleus of the solitary tract

The inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine are involved in several nucleus of the solitary tract (NST)-mediated functions. The distribution of these amino acids in the NST of the lamb, a species frequently used in investigations of NST-mediated behaviors, has not been described. Therefore, this study was designed to investigate the distribution of GABA and glycine in the lamb NST using immunohistochemistry. Both GABA and glycine immunoreactive cells and puncta were unevenly distributed in the lamb NST. The highest density of GABA immunoreactive cells was found in the intermediate zone of the NST, medial to the solitary tract (ST). High to moderate levels of puncta labeling were observed throughout the NST, particularly around the ST in intermediate and caudal zones. Moderate to low levels of glycine immunoreactivity were observed, with most glycine immunoreactive cells and puncta found in the caudal two-thirds of the NST in the medial, ventrolateral and dorsomedial NST. Only a few glycine immunoreactive cells and puncta were found in the rostral zone of the NST. The widespread distribution of GABA and glycine immunoreactivity in intermediate and caudal zones of the NST suggests that these inhibitory amino acids play an important role in modulating NST-mediated functions like swallowing, respiration and cardiovascular regulation in the lamb. The much higher density of GABA immunoreactivity compared to glycine immunoreactivity in the rostral zone of the NST suggests that GABA, but not glycine, is an important neurotransmitter in the processing of taste information by the lamb NST.

Localization of GABAA alpha 1 mRNA subunit in the brainstem nuclei controlling esophageal peristalsis

The nucleus of the solitary tract, the site of esophageal premotor neurons (PMN), is tonically inhibited by GABAergic neurons via the GABAA receptor. We investigated the expression of GABAA alpha 1 subunit mRNA within esophageal PMNs of the NTS utilizing transynaptic tracing with pseudorabies virus and nonisotopic in-situ hybridization. Double-labeling studies revealed that the majority of PRV-immunoreactive cells also expressed GABAA alpha 1 mRNA. The expression of GABAA subunits supports a role for GABA in the brainstem circuit controlling esophageal peristalsis.

Brainstem mediates diazepam enhancement of palatability and feeding: microinjections into fourth ventricle versus lateral ventricle

The hypothesis that benzodiazepine-induced hyperphagia is due to a specific enhancement of the palatability of foods has been supported by previous 'taste reactivity' studies of affective (hedonic and aversive) reactions to taste palatability. Diazepam and chlordiazepoxide enhance hedonic reactions of rats (rhythmic tongue protrusions, etc.) to sweet tastes in a receptor-specific fashion. A role for brainstem circuits has been indicated by a previous demonstration of the persistence of the taste reactivity enhancement by diazepam after midbrain decerebration. The present study examined whether benzodiazepine brainstem receptors are the chief substrates for palatability enhancement even in intact brains. We compared the effectiveness of benzodiazepine microinjections to elicit feeding and enhance hedonic reactions when delivered into either the lateral ventricle (forebrain) or the fourth ventricle (brainstem) of rats. The results show diazepam is reliably more effective at eliciting feeding and enhancing positive hedonic reactions to oral sucrose when microinjections are made in the fourth ventricle than in the lateral ventricle. We conclude that brainstem neural systems containing benzodiazepine-GABA receptors are likely to be the chief substrates for benzodiazepine-induced palatability enhancement.