Distribution of tachykinin- and opioid-expressing neurons in the hamster solitary nucleus: an immuno- and in situ hybridization histochemical study

Phenotyping neurons activated in the mouse brain during restoration of salt debt

Salt overconsumption contributes to hypertension, which is a major risk factor for stroke, heart and kidney disease. Characterising neuronal pathways that may control salt consumption is therefore important for developing novel approaches for reducing salt overconsumption. Here, we identify neurons within the mouse central amygdala (CeA), lateral parabrachial nucleus (LPBN), intermediate nucleus of the solitary tract (iNTS), and caudal NTS (cNTS) that are activated and display Fos immunoreactivity in mice that have consumed salt in order to restore a salt debt, relative to salt replete and salt depleted controls. Double-label immunohistochemical studies revealed that salt restoring mice had significantly greater densities of activated enkephalin neurons within the CeA and iNTS, while statistically significant changes within the LPBN and cNTS were not observed. Furthermore, within the CeA, restoration of salt debt conferred a significant increase in the density of activated calretinin neurons, while there was no change relative to control groups in the density of activated neurons that co-expressed protein kinase C delta (PKC-δ). Taken together, these studies highlight the importance of opioid systems within the CeA and iNTS in neuronal processes associated with salt restoration, and may aid the development of future pharmacological and other strategies for reducing salt overconsumption.

The μ-opioid receptor agonist DAMGO presynaptically suppresses solitary tract-evoked input to neurons in the rostral solitary nucleus

Taste processing in the rostral nucleus of the solitary tract (rNST) is subject to modulatory influences including opioid peptides. Behavioral pharmacological studies suggest an influence of μ-opioid receptors in rNST, but the underlying mechanism is unknown. To determine the cellular site of action, we tested the effects of the μ-opioid receptor agonist DAMGO in vitro. Whole cell patch-clamp recordings were made in brain stem slices from GAD67-GFP knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous promoter for GAD67, a synthetic enzyme for GABA. Neuron counts showed that ∼36% of rNST neurons express GABA. We recorded monosynaptic solitary tract (ST)-evoked currents (jitter ≤ 300 μs) in both GAD67-EGFP-positive (GAD67+) and GAD67-EGFP-negative (GAD67-) neurons with equal frequency (25/31; 22/28), but the inputs to the GAD67+ neurons had significantly smaller paired-pulse ratios compared with GAD67- neurons. DAMGO (0.3 μM) significantly suppressed ST-evoked currents in both cell types (mean suppression = 46 ± 3.3% SE), significantly increased the paired-pulse ratio of these currents, and reduced the frequency of spontaneous miniature excitatory postsynaptic currents but did not diminish their amplitude, indicating a presynaptic site of action. Under inhibitory amino acid receptor blockade, DAMGO was significantly more suppressive in GAD67+ neurons (59% reduction) compared with GAD67- neurons (35% reduction), while the reverse was true in normal artificial cerebrospinal fluid (GAD67+: 35% reduction; GAD67-: 57% reduction). These findings suggest that DAMGO suppresses activity in rNST neurons predominantly via a presynaptic mechanism, and that this effect may interact significantly with tonic or evoked inhibitory activity.

Neural coding of taste by simultaneously recorded cells in the nucleus of the solitary tract of the rat

The nucleus of the solitary tract (NTS) receives input from taste buds on the rostral tongue from the chorda tympani (CT) nerve. How this input is processed by the NTS was the subject of the present investigation. Here we used tetrodes to record from pairs or small groups of NTS cells as they responded to taste stimuli or electrical stimulation of the CT nerve in urethane-anesthetized rats. Once a pair (or small group) of NTS cells were isolated and identified as showing an evoked response to CT nerve stimulation, taste stimuli were presented in separate trials. Tastants consisted of 0.1 M NaCl, 0.01 M HCl, 0.01 M quinine HCl, and 0.5 M sucrose. Responses to various patterns of CT stimulation were then recorded. Functional connections among simultaneously recorded NTS cells were implied from analysis of cross-correlation functions of spike trains. We identified four groups of cells, not all of which responded to taste, with staggered latencies of response to CT nerve stimulation, ranging from ∼3 to 35 ms in ∼8- to 12-ms increments. Analyses of putative functional connectivity along with latencies of CT-evoked responses suggested that CT input arrives at the NTS in pulses or waves, each of which activates recurrent excitatory connections among NTS cells. These actions may amplify the incoming signal and refine its temporal pattern.

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind μ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A μ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that μ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.

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.

Taste-evoked Fos expression in nitrergic neurons in the nucleus of the solitary tract and reticular formation of the rat

The current investigation used double labeling for NADPHd and Fos-like immunoreactivity to define the relationship between nitric oxide synthase-containing neural elements and taste-activated neurons in the nucleus of the solitary tract (NST) and subjacent reticular formation (RF). Stimulation of awake rats with citric acid and quinine resulted in significant increases in the numbers of double-labeled neurons in both the NST and RF, suggesting that some medullary gustatory neurons utilize nitric oxide (NO) as a transmitter. Overall, double-labeled neurons were most numerous in the caudal reaches of the gustatory zone of the NST, where taste neurons receive inputs from the IXth nerve, suggesting a preferential role for NO neurons in processing gustatory inputs from the posterior oral cavity. However, double-labeled neurons also exhibited a preferential distribution depending on the gustatory stimulus. In the NST, double-labeled neurons were most numerous in the rostral central subnucleus after either stimulus but had a medial bias after quinine stimulation. In the RF, after citric acid stimulation, there was a cluster of double-labeled neurons with distinctive large soma in the parvicellular division of the lateral RF, subjacent to the rostral tip of NST. In contrast, in response to quinine, there was a cluster of double-labeled neurons with much smaller soma in the intermediate zone of the medial RF, a few hundred micrometers caudal to the citric acid cluster. These differential distributions of double-labeled neurons in the NST and RF suggest a role for NO in stimulus-specific gustatory autonomic and oromotor reflex circuits.

Nicotine suppression of gustatory responses of neurons in the nucleus of the solitary tract

This study investigated effects of nicotine applied to the tongue surface on responses of gustatory neurons in the nucleus of the solitary tract (NTS) in rats. In pentobarbital-anesthetized rats, single-unit recordings were made from NTS units responsive to one or more tastants (sucrose, NaCl, citric acid, monosodium glutamate, quinine). Application of nicotine (0.87, 8.7, or 600 mM) excited gustatory NTS units and significantly attenuated NTS unit responses to their preferred tastant in a dose-dependent manner. The depressant effect of nicotine was equivalent regardless of which tastant best excited the NTS unit. Nicotinic excitation of NTS units and depression of their tastant-evoked responses were both significantly attenuated by the nicotinic antagonist mecamylamine, which itself did not excite NTS units. In rats with bilateral trigeminal ganglionectomy, nicotine still excited nearly all NTS units but no longer depressed tastant-evoked responses. Nicotine did not elicit plasma extravasation when applied to the tongue. The results indicate that nicotine directly excites NTS units by gustatory nerves and inhibits their tastant-evoked responses by a nicotinic acetylcholine receptor-mediated excitation of trigeminal afferents that inhibit NTS units centrally.

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.

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.

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.

Suppression of central taste transmission by oral capsaicin

Because intraoral capsaicin is reported to reduce the perceived intensity of certain taste qualities, we investigated whether it affects the central processing of gustatory information. The responses of gustatory neurons in the nucleus tractus solitarius (NTS) to tastant stimuli were recorded before and after lingual application of capsaicin in anesthetized rats. Thirty-four NTS units were characterized as responding best to sucrose (0.3 m), NaCl (0.1 m), citric acid (0.03 m), monosodium glutamate (0.2 m), or quinine (0.001 m). During lingual application of 330 microm capsaicin for 7 min, the firing rate increased for five units and decreased for four units; the remainder were unaffected. Immediately after capsaicin, responses to each tastant were in nearly all cases depressed (mean, 61.5% of control), followed by recovery in most cases. NTS tastant-evoked unit responses were unaffected by lingual application of vehicle (5% ethanol). Capsaicin elicited an equivalent reduction (to 64.5%) in tastant-evoked responses of nine additional NTS units recorded in rats with bilateral trigeminal ganglionectomy, arguing against a trigeminally mediated central effect. Furthermore, capsaicin elicited a puncate pattern of plasma extravasation in the tongue that matched the distribution of fungiform papillae. These results support a peripheral site of capsaicin suppression of taste possibly via direct or indirect effects on taste transduction or taste receptor cell excitability. The depressant effect of capsaicin on gustatory transmission might underlie its ability to reduce the perceived intensity of some taste qualities.

Taste function in methadone-maintained opioid-dependent men

It has been shown repeatedly that opioid dependence is associated with increased consumption of refined sugars. It is possible that this association results from altered taste reactivity in opioid-dependent subjects. Thus, in the present study, we compared taste responses to sweet, bitter, sour and salty solutions in methadone-maintained opioid-dependent men and healthy control subjects. The two groups did not differ in terms of rated intensity or pleasantness of sucrose (1-30%), quinine (0.001-0.005%), citric acid (0.02-0.1%) and sodium chloride (0.18-0.9%) solutions. Proportions of 'sweet-likers', i.e. subjects rating a 30% sucrose (0.88 M) solution as the most pleasant, were also similar in both groups. In line with the previous findings, the methadone-maintained subjects reported adding more table sugar to caffeinated beverages. The results of the present study suggest that changes in taste reactivity may not be responsible for altered dietary choices in opioid addicts.

Reduced aversion to oral capsaicin following neurotoxic destruction of superficial medullary neurons expressing NK-1 receptors

Aversion to capsaicin (0.1-10 ppm) was assessed using a two-bottle paired preference paradigm, before and after intracisternal injection of substance P conjugated to saporin (SP-SAP) to ablate neurons in superficial medullary and cervical dorsal horn that express NK-1 receptors. Before SP-SAP, there was a concentration-dependent decrease in consumption of capsaicin with a threshold of 0.1-0.3 ppm. Following SP-SAP, significantly more capsaicin solution was consumed at 1- and 10-ppm concentrations. These results support a role for substance P in the mediation of high, but not low, levels of capsaicin-induced oral irritation.

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

Neurokinin-1 receptor immunoreactivity in the nucleus of the solitary tract in the hamster

Substance P (SP) modulates the activity of taste-responsive neurons in the gustatory zone of the nucleus of the solitary tract (NST) in the hamster. The distribution of the neurokinin-1 (NK1) receptor (i.e. the SP receptor) was mapped and compared with the distribution of SP immunoreactivity to identify the sites of ligand-receptor interactions. NK1-immunoreactive puncta and somata were located mostly in the rostral lateral, upper half of the rostral central and medial NST subnuclei. These subnuclei also contained intense SP-immunoreactive puncta, and are known to receive substantial inputs via gustatory and somatosensory afferent fibers. The ventral subnucleus, which is involved in visceromotor reflexes accompanying ingestion, contained little NK1 or lighter SP-immunoreactivity. These findings suggest that SP modulates taste activity destined for the ascending gustatory pathway at the level of the first central synapse in the gustatory pathway.

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.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.

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.

Differential response to kappa-opioidergic agents in dietary fat selection between Osborne-Mendel and S5B/P1 rats

We have investigated the central effect of a kappa-opioid agonist and an antagonist on the macronutrient preference in two strains of rat, the Osborne Mendel (OM) and S5B/P1 rats, that have different susceptibility to obesity and differential preference for dietary fat intake. OM rats prefer diets high in fat and are sensitive to diet-induced obesity, whereas S5B/P1 prefer a low fat diet and are resistant to high-fat diet-induced obesity. Rats adapted to a two-choice high fat (HF)/low fat (LF) diet were food deprived (20 h) and then infused into the third cerebroventricle with 10 micrograms nor-binaltorphimine (nor-BNI), a selective kappa-antagonist. Nor-BNI preferentially suppressed HF intake, but not LF intake in OM rats, whereas it affected neither diet in S5B rats. Infusion of U50488, a selective kappa-agonist (33 nmol), into the third cerebroventricle in sated rats, potently stimulated the intake of HF only in the OM rats, whereas it induced a significant but moderate stimulation of intake of both HF and LF diets in the S5B/P1 rats. Total energy intake following U50488 was not significantly different between the two strains. These findings suggest that the enhanced sensitivity of the OM rats to kappa-opioid stimulation for dietary fat may contribute to their preference for dietary fat and possibly their increased susceptibility for obesity.

Lateral ventricular injections of the NK3 agonist senktide affect salt taste-elicited responses

Central injections of the selective tachykinin NK3 receptor agonist senktide (SENK) suppresses salt appetite. Also, following SENK, intraoral infusions of hypertonic NaCl elicit fewer ingestive taste reactivity responses and more aversive responses than following intraventricular injections of isotonic saline. This pattern of taste reactivity results suggest that SENK affects the oral stimulating properties of salt taste. Before accepting this interpretation, however, alternative explanations need to be examined. The following experiments evaluated whether the effects of intraventricular SENK injection on taste reactivity could be due to: 1) a general oral motor impairment that reduces ingestive responding to tastes (Experiment 1) or; 2) SENK having aversive consequences (Experiment 2). In Experiment 1, the effects of intraventricular injections of SENK (200 ng) on taste reactivity responses to 0.5 M NaCl and 0.1 M sucrose were measured in sodium deficient rats. Intraoral infusions of 0.5 M NaCl elicited fewer ingestive taste reactivity responses following SENK than injections of isotonic saline in sodium deficient rats. Sucrose continued to elicit the same high number of ingestive taste reactivity responses following intraventricular injections of isotonic saline and SENK. Thus, SENK did not cause a general decrease in ingestive responding. A conditioned taste aversion test was employed in Experiment 2 to determine if SENK had aversive consequences. Rats were given 30 min access to alanine (0.3 M) and were then administered either lithium chloride (LiCl) or intraventricular injections of SENK (200 ng) on three consecutive days. Rats avoided alanine that was paired with LiCl, but those rats that had alanine paired with SENK showed no avoidance of the taste even after three pairings. These results replicate findings that intraventricular injections of the NK3 agonist SENK decreases the palatability of NaCl (as measured by taste reactivity) and suggest that its effect on NaCl-elicited taste reactivity is not due to the treatment causing a motor impairment or malaise.

Substance P modulates taste responses in the nucleus of the solitary tract of the hamster

The effects of substance (SP) microinjections on the electrophysiological response of gustatory neurons within the nucleus of the solitary tract (NST) were examined in hamsters following either anodal electrical or NaCl stimulation of the anterior tongue. For both types of stimulation, SP produced excitatory and suppressive effects on the activity of gustatory NST neurons, with excitatory effects being more common. In response to repetitive anodal stimulation of the tongue, the modulatory effect of SP lasted 30-400 s. In the presence of SP, the firing rate of 48% of the neurons was increased and that of 9% was decreased following NaCl stimulation. This dual action of SP could be due to direct excitation of teste-responsive neurons and to excitation of inhibitory local circuit neurons which, in turn, decrease the responsiveness of gustatory neurons.

Tachykinin NK3 receptor agonist blocks sodium deficiency-induced shift in taste reactivity

Intracerebroventricular (i.c.v.) injections of tachykinin NK3 receptor agonists suppress NaCl intake by sodium deficient rats. The brief exposure, taste reactivity test was used to examine the effect of tachykinins on the immediate, oral reinforcing properties of NaCl (0.5 M) in sodium replete and sodium deficient male rats. In sodium replete rats, intraoral infusions of NaCl elicited a mixed response comprised of a similar number of ingestive and aversive responses following i.c.v. injections of saline and succinyl-[Asp6,N-Me-Phe8]substance P, (6-11), (SENK), a NK3 receptor agonist. When sodium deficient, saline injected rats showed a reliable increase in ingestive and a decrease in aversive taste reactivity responses. Lateral i.c.v. injections of SENK blocked the shift in taste reactivity responses by sodium deficient rats, indicating that concentrated NaCl retained its aversive taste property.