Responses of solitary tract nucleus neurons to taste and mechanical stimulations of the oral cavity in decerebrate rats

Electrophysiological responses to appetitive and consummatory behavior in the rostral nucleus tractus solitarius in awake, unrestrained rats

Introduction

As the intermediate nucleus in the brainstem receiving information from the tongue and transmitting information upstream, the rostral portion of the nucleus tractus solitarius (rNTS) is most often described as a "taste relay". Although recent evidence implicates the caudal NTS in a broad neural circuit involved in regulating ingestion, there is little information about how cells in the rNTS respond when an animal is eating solid food.

Methods

Single cells in the rNTS were recorded in awake, unrestrained rats as they explored and ate solid foods (Eating paradigm) chosen to correspond to the basic taste qualities: milk chocolate for sweet, salted peanuts for salty, Granny Smith apples for sour and broccoli for bitter. A subset of cells was also recorded as the animal licked exemplars of the five basic taste qualities: sucrose, NaCl, citric acid, quinine and MSG (Lick paradigm).

Results

Most cells were excited by exploration of a food-filled well, sometimes responding prior to contact with the food. In contrast, cells that were excited by food well exploration became significantly less active while the animal was eating the food. Most cells were broadly tuned across foods, and those cells that were recorded in both the Lick and Eating paradigms showed little correspondence in their tuning across paradigms.

Discussion

The preponderance of robust responses to the appetitive versus the consummatory phase of ingestion suggests that multimodal convergence onto cells in the rNTS may be used in decision making about ingestion.

Neurobiological Mechanisms of Nicotine Reward and Aversion

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the rewarding actions of nicotine contained in tobacco that establish and maintain the smoking habit. nAChRs also regulate the aversive properties of nicotine, sensitivity to which decreases tobacco use and protects against tobacco use disorder. These opposing behavioral actions of nicotine reflect nAChR expression in brain reward and aversion circuits. nAChRs containing α4 and β2 subunits are responsible for the high-affinity nicotine binding sites in the brain and are densely expressed by reward-relevant neurons, most notably dopaminergic, GABAergic, and glutamatergic neurons in the ventral tegmental area. High-affinity nAChRs can incorporate additional subunits, including β3, α6, or α5 subunits, with the resulting nAChR subtypes playing discrete and dissociable roles in the stimulatory actions of nicotine on brain dopamine transmission. nAChRs in brain dopamine circuits also participate in aversive reactions to nicotine and the negative affective state experienced during nicotine withdrawal. nAChRs containing α3 and β4 subunits are responsible for the low-affinity nicotine binding sites in the brain and are enriched in brain sites involved in aversion, including the medial habenula, interpeduncular nucleus, and nucleus of the solitary tract, brain sites in which α5 nAChR subunits are also expressed. These aversion-related brain sites regulate nicotine avoidance behaviors, and genetic variation that modifies the function of nAChRs in these sites increases vulnerability to tobacco dependence and smoking-related diseases. Here, we review the molecular, cellular, and circuit-level mechanisms through which nicotine elicits reward and aversion and the adaptations in these processes that drive the development of nicotine dependence. SIGNIFICANCE STATEMENT: Tobacco use disorder in the form of habitual cigarette smoking or regular use of other tobacco-related products is a major cause of death and disease worldwide. This article reviews the actions of nicotine in the brain that contribute to tobacco use disorder.

Influences of ethanol and temperature on sucrose-evoked response of gustatory neurons in the hamster solitary nucleus

Taste-responsive neurons in the nucleus of the solitary tract (NST), the first gustatory nucleus, often respond to thermal or mechanical stimulation. Alcohol, not a typical taste modality, is a rewarding stimulus. In this study, we aimed to investigate the effects of ethanol (EtOH) and/or temperature as stimuli to the tongue on the activity of taste-responsive neurons in hamster NST. In the first set of experiments, we recorded the activity of 113 gustatory NST neurons in urethane-anesthetized hamsters and evaluated responses to four basic taste stimuli, 25% EtOH, and 40°C and 4°C distilled water (dH₂O). Sixty cells responded to 25% EtOH, with most of them also being sucrose sensitive. The response to 25% EtOH was significantly correlated with the sucrose-evoked response. A significant correlation was also observed between sucrose- and 40°C dH₂O- and between 25% EtOH- and 40°C dH₂O-evoked firings. In a subset of the cells, we evaluated neuronal activities in response to a series of EtOH concentrations, alone and in combination with 32 mM sucrose (EtOH/Suc) at room temperature (RT, 22°C–23°C), 40°C, and 4°C. Neuronal responses to EtOH at RT and 40°C increased as the concentrations increased. The firing rates to EtOH/Suc were greater than those to EtOH or sucrose alone. The responses were enhanced when solutions were applied at 40°C but diminished at 4°C. In summary, EtOH activates most sucrose-responsive NST gustatory cells, and the concomitant presence of sucrose or warm temperatures enhance this response. Our findings may contribute to elucidate the neural mechanisms underlying appetitive alcohol consumption.

Neural Coding of Food Is a Multisensory, Sensorimotor Function

This review is a curated discussion of the relationship between the gustatory system and the perception of food beginning at the earliest stage of neural processing. A brief description of the idea of taste qualities and mammalian anatomy of the taste system is presented first, followed by an overview of theories of taste coding. The case is made that food is encoded by the several senses that it stimulates beginning in the brainstem and extending throughout the entire gustatory neuraxis. In addition, the feedback from food-related movements is seamlessly melded with sensory input to create the representation of food objects in the brain.

GLP-1 acts on habenular avoidance circuits to control nicotine intake

Tobacco smokers titrate their nicotine intake to avoid its noxious effects, sensitivity to which may influence vulnerability to tobacco dependence, yet mechanisms of nicotine avoidance are poorly understood. Here we show that nicotine activates glucagon-like peptide-1 (GLP-1) neurons in the nucleus tractus solitarius (NTS). The antidiabetic drugs sitagliptin and exenatide, which inhibit GLP-1 breakdown and stimulate GLP-1 receptors, respectively, decreased nicotine intake in mice. Chemogenetic activation of GLP-1 neurons in NTS similarly decreased nicotine intake. Conversely, Glp1r knockout mice consumed greater quantities of nicotine than wild-type mice. Using optogenetic stimulation, we show that GLP-1 excites medial habenular (MHb) projections to the interpeduncular nucleus (IPN). Activation of GLP-1 receptors in the MHb-IPN circuit abolished nicotine reward and decreased nicotine intake, whereas their knockdown or pharmacological blockade increased intake. GLP-1 neurons may therefore serve as 'satiety sensors' for nicotine that stimulate habenular systems to promote nicotine avoidance before its aversive effects are encountered.

Immunocytochemical organization and sour taste activation in the rostral nucleus of the solitary tract of mice

Sensory inputs from the oropharynx terminate in both the trigeminal brainstem complex and the rostral part of the nucleus of the solitary tract (nTS). Taste information is conveyed via the facial and glossopharyngeal nerves, while general mucosal innervation is carried by the trigeminal and glossopharyngeal nerves. In contrast, the caudal nTS receives general visceral information largely from the vagus nerve. Although the caudal nTS shows clear morphological and molecularly delimited subdivisions, the rostral part does not. Thus, linking taste-induced patterns of activity to morphological subdivisions in the nTS is challenging. To test whether molecularly defined features of the rostral nTS correlate with patterns of taste-induced activity, we combined immunohistochemistry for markers of various visceral afferent and efferent systems with c-Fos-based activity maps generated by stimulation with a sour tastant, 30 mM citric acid. We further dissociated taste-related activity from activity arising from acid-sensitive general mucosal innervation by comparing acid-evoked c-Fos in wild-type and "taste blind" P2X₂ /P2X₃ double knockout (P2X-dbl KO) mice. In wild-type mice, citric acid stimulation evoked significant c-Fos activation in the central part of the rostral nTS-activity that was largely absent in the P2X-dbl KO mice. P2X-dbl KO mice, like wild-type mice, did exhibit acid-induced c-Fos activity in the dorsomedial trigeminal brainstem nucleus situated laterally adjacent to the rostral nTS. This dorsomedial nucleus also showed substantial innervation by trigeminal nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP), a marker for polymodal nociceptors, suggesting that trigeminal general mucosal innervation carries information about acids in the oral cavity. J. Comp. Neurol. 525:271-290, 2017. © 2016 Wiley Periodicals, Inc.

Salivary peptide tyrosine-tyrosine 3-36 modulates ingestive behavior without inducing taste aversion

Hormone peptide tyrosine-tyrosine (PYY) is secreted into circulation from the gut L-endocrine cells in response to food intake, thus inducing satiation during interaction with its preferred receptor, Y2R. Clinical applications of systemically administered PYY for the purpose of reducing body weight were compromised as a result of the common side effect of visceral sickness. We describe here a novel approach of elevating PYY in saliva in mice, which, although reliably inducing strong anorexic responses, does not cause aversive reactions. The augmentation of salivary PYY activated forebrain areas known to mediate feeding, hunger, and satiation while minimally affecting brainstem chemoreceptor zones triggering nausea. By comparing neuronal pathways activated by systemic versus salivary PYY, we identified a metabolic circuit associated with Y2R-positive cells in the oral cavity and extending through brainstem nuclei into hypothalamic satiety centers. The discovery of this alternative circuit that regulates ingestive behavior without inducing taste aversion may open the possibility of a therapeutic application of PYY for the treatment of obesity via direct oral application.

T1r3 taste receptor involvement in gustatory neural responses to ethanol and oral ethanol preference

Elevated alcohol consumption is associated with enhanced preference for sweet substances across species and may be mediated by oral alcohol-induced activation of neurobiological substrates for sweet taste. Here, we directly examined the contribution of the T1r3 receptor protein, important for sweet taste detection in mammals, to ethanol intake and preference and the neural processing of ethanol taste by measuring behavioral and central neurophysiological responses to oral alcohol in T1r3 receptor-deficient mice and their C57BL/6J background strain. T1r3 knockout and wild-type mice were tested in behavioral preference assays for long-term voluntary intake of a broad concentration range of ethanol, sucrose, and quinine. For neurophysiological experiments, separate groups of mice of each genotype were anesthetized, and taste responses to ethanol and stimuli of different taste qualities were electrophysiologically recorded from gustatory neurons in the nucleus of the solitary tract. Mice lacking the T1r3 receptor were behaviorally indifferent to alcohol (i.e., ∼50% preference values) at concentrations typically preferred by wild-type mice (5-15%). Central neural taste responses to ethanol in T1r3-deficient mice were significantly lower compared with C57BL/6J controls, a strain for which oral ethanol stimulation produced a concentration-dependent activation of sweet-responsive NTS gustatory neurons. An attenuated difference in ethanol preference between knockouts and controls at concentrations >15% indicated that other sensory and/or postingestive effects of ethanol compete with sweet taste input at high concentrations. As expected, T1r3 knockouts exhibited strongly suppressed behavioral and neural taste responses to sweeteners but did not differ from wild-type mice in responses to prototypic salt, acid, or bitter stimuli. These data implicate the T1r3 receptor in the sensory detection and transduction of ethanol taste.

Modulation of human cortical swallowing motor pathways after pleasant and aversive taste stimuli

Human swallowing involves the integration of sensorimotor information with complexities such as taste; however, the interaction between the taste of food and its effects on swallowing control remains unknown. We assessed the effects of pleasant (sweet) and aversive (bitter) tastes on human cortical swallowing motor pathway excitability. Healthy adult male volunteers underwent a transcranial magnetic stimulation (TMS) mapping study (n = 9, mean age: 34 yr) to assess corticobulbar excitability before and up to 60 min after 10-min liquid infusions either 1) as swallowing tasks or 2) delivered directly into the stomach. Infusions were composed of sterile water (neutral), 10% glucose (sweet), and 0.5 mM quinine hydrochloride (bitter). The order of delivery was randomized, and each infusion was given on separate days. Pharyngeal motor-evoked potentials (PMEPs) were recorded from an intraluminal catheter as a measure of corticobulbar excitability and compared using repeated-measures and one-way ANOVA. After the swallowing task (water, glucose, or quinine), repeated-measures ANOVA revealed a significant time interaction across tastants (P </= 0.01). One-way ANOVA for each taste showed changes in PMEP amplitudes for both quinine (P </= 0.001) and glucose (P </= 0.009) solutions but not for water (P = 0.1). Subsequent t-tests showed that glucose and quinine reduced PMEPs by 47% (SD 34) and 37% (SD 54), respectively, at 30 min (P </= 0.03). No changes were observed after the infusion of any solution directly into the stomach (P = 0.51). In conclusion, cortical swallowing pathways are similarly modulated by both sweet and bitter tasting stimuli. Changes likely reflect a close interaction between taste and swallowing activity mediated in the central nervous system.

Taste recognition: food for thought

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for an animal's survival. Although olfaction and vision contribute to food detection, the gustatory system acts as a final checkpoint control for food acceptance or rejection behavior. Recent studies with model organisms such as mice and Drosophila have identified candidate taste receptors and examined the logic of taste coding in the periphery. Despite differences in terms of gustatory anatomy and taste-receptor families, these gustatory systems share a basic organization that is different from other sensory systems. This review will summarize our current understanding of taste recognition in mammals and Drosophila, highlighting similarities and raising several as yet unanswered questions.

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 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.

Two loci of the insular cortex project to the taste zone of the nucleus of the solitary tract in rats

Distribution of insular cortical neurons projecting to the taste zone of the solitary tract nucleus (NTS) was examined histologically in rats. Injection of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the taste responsive regions in the NTS resulted in labeling of cells in layer V within almost entire extent of the rostrocaudal axis of the granular and dysgranular areas of the insular cortex (IC) bilaterally with a clear contralateral dominance. The density of the cells was highest in the taste area of the IC [11] and second highest in the IC area around the bregma level, showing a bimodal distribution. After WGA-HRP injection into the IC taste area or the caudal IC, dense or sparse anterograde labeling was seen in the rostral NTS, respectively. The results indicate that not only the IC taste area but also the caudal IC exerts control influences directly upon the NTS taste zone.

Hypothalamic and amygdalar neuronal responses to various tastant solutions during ingestive behavior in rats

The forebrain, including the amygdala (AM) and hypothalamus, may be a higher brain center that modulates the activity of a brainstem neural system that influences ingestive behavior via descending projections. In this study, to elucidate the characteristics of sensory information processing in the forebrain in relation to this putative connection, we recorded neuronal activity in the AM and hypothalamus [lateral hypothalamic area (LHA), medial hypothalamic area (MHA)] of rats during discrimination of conditioned sensory stimuli and the ingestion of various tastant solutions. Of 420 responsive AM neurons identified, 24 were taste responsive and located mainly in the central nucleus of the AM. Multivariate analyses of these taste neurons suggested that in the AM, taste quality is processed on the basis of palatability. In the hypothalamus, of 282 LHA and MHA neurons recorded, 144 responded to one or more conditioned auditory stimuli and/or licking of one or more solutions. Stress, which is known to influence feeding behavior, increased the mean spontaneous activity of LHA neurons but decreased the mean spontaneous neuronal activity of MHA neurons. This pattern of changes in spontaneous neuronal activity correlated with alterations in feeding behavior during stress. Furthermore, the activity of both AM and LHA neurons was modulated flexibly during conditioned associative learning. Together, the data suggest that the activity of the AM and hypothalamic neurons is altered when animals must modulate ingestive behavior by learning a new stimulus associated with food and by being exposed to stress, suggesting that these forebrain areas are important modulators of the activity of a basic neural system in the brainstem that influences ingestive behavior.

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.

Fos-like immunoreactivity in the brain stem following oral quinine stimulation in decerebrate rats

The present study compared the distribution of Fos-like immunoreactivity (FLI) following intraoral stimulation with quinine monohydrochloride (QHCl) in awake intact rats to the pattern obtained in chronic supracollicular decerebrate (CD) rats. Because the behavioral rejection response to QHCl is evident in the CD rat, it was hypothesized that the pattern of FLI in the lower brain stem should be similar in both groups. Overall, the distribution of FLI in the brain stem was quite similar in both intact and CD groups, and QHCl stimulation increased FLI in the rostral (gustatory) nucleus of the solitary tract, the parabrachial nucleus (PBN), and the lateral reticular formation (RF) compared with an unstimulated control group. The CD group differed from the intact group, however, with a trend toward less FLI in the RF and a shift in the pattern of label away from the external subdivision of the PBN. CD rats also had increased FLI in the caudal nucleus of the solitary tract, with or without intraoral infusions. The distribution of QHCl-induced FLI in the brain stem of intact rats thus indicates both local sensorimotor processing as well as the influence of forebrain structures.

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.

Activation of neurons in rat trigeminal subnucleus caudalis by different irritant chemicals applied to oral or ocular mucosa

To investigate the role of trigeminal subnucleus caudalis in neural mechanisms of irritation, we recorded single-unit responses to application of a variety of irritant chemicals to the tongue or ocular mucosa in thiopental-anesthetized rats. Recordings were made from wide dynamic range (WDR) and nociceptive-specific units in superficial layers of the dorsomedial caudalis (0-3 mm caudal to obex) responsive to mechanical stimulation and noxious heating of the ipsilateral tongue ("tongue" units) and from WDR units in ventrolateral caudalis (0-2 caudal to obex) responsive to mechanical and noxious thermal stimulation of cornea-conjunctiva and frequently also surrounding skin ("cornea-conjunctival" units). The following chemicals were delivered topically (0.1 ml) onto the dorsal anterior tongue or instilled into the ipsilateral eye: capsaicin (0.001-1% = 3.3 x 10(-2) to 3.3 x 10(-5) M), ethanol (15-80%), histamine (0.01-10% = 9 x 10(-1) to 9 x 10(-4) M), mustard oil (allyl-isothiocyanate, 4-100% = 4 x 10(-1) to 10 M), NaCl (0.5-5 M), nicotine (0.01-10% = 6 x 10(-1) to 6 x 10(-4) M), acidified phosphate buffer (pH 1-6), piperine (0.01-1% = 3.5 x 10(-2) to 3.5 x 10(-4) M), serotonin (5-HT; 0.3-3% = 1.4 x 10(-1) to 1.4 x 10(-2) M), and carbonated water. The dose-response relationship and possible tachyphylaxis were tested for each chemical. Of 32 tongue units, 31 responded to one or more, and frequently all, chemicals tested. The population responded to 75.3% of the various chemicals tested (</=10 per unit). The incidence of responses was independent of the order of chemicals tested, except for capsaicin, which reduced subsequent responses. Responses to histamine, nicotine, 5-HT, and ethanol had a more rapid onset and shorter duration compared with capsaicin, acid, and mustard oil. Responses to all chemicals increased in a dose-related manner. Successive responses to repeated application decreased significantly for nicotine, 5-HT, capsaicin, and piperine. Spontaneous firing increased significantly 5-10 min after initial application of capsaicin. Of 31 corneal-conjunctival units, 29 responded to one or more chemicals, and the population responded to 65% of all chemicals tested. Responses increased in a dose-related manner for all chemicals, and successive responses decreased significantly for histamine, nicotine, ethanol, acid, and capsaicin. Responses of tongue units to histamine and nicotine were reduced significantly by ceterizine (H1 antagonist) and mecamylamine, respectively. Mecamylamine also significantly reduced responses of corneal-conjunctival units to nicotine. Different classes of irritant chemicals contacting the oral or ocular mucosa can activate individual sensory neurons in caudalis, presumably via independent peripheral transduction mechanisms. Multireceptive units with input from the tongue or cornea-conjunctiva exhibited a similar spectrum of excitability to different irritant chemicals. Such neurons would not be capable of discriminating among different chemically evoked irritant sensations but could contribute to a common chemical sense.

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.

Gustatory and multimodal neuronal responses in the amygdala during licking and discrimination of sensory stimuli in awake rats

The amygdala (AM) receives information from various sensory modalities via the neocortex and directly from the thalamus and brain stem and plays an important role in ingestive behaviors. In the present study, neuronal activity was recorded in the AM and amygdalostriatal transition area of rats during discrimination of conditioned sensory stimuli and ingestion of sapid solutions. Of the 420 responsive neurons, 227 responded exclusively to one sensory modality, 120 responded to two or more modalities, and the remaining 73 could not be classified. Among the responsive neurons, 108 responded to oral-sensory stimulation (oral-sensory neurons). In detailed analyses of 84 of these oral-sensory neurons, 24 were classified as taste responsive and were located mainly in the central nucleus of the AM. The other 60 oral-sensory neurons were classified as nontaste oral-sensory neurons and were distributed widely throughout the AM. Both the taste and nontaste oral-sensory neurons also responded to other sensory stimuli. Of the 24 taste neurons, 21 were tested at least with four standard taste solutions. On the basis of the magnitudes of their responses to these sapid stimuli, the taste neurons were classified as follows: seven sucrose-best, four NaCl-best, three citric acid-best, and six quinine HCl-best. The remaining cell responded significantly only to lysine HCl and monosodium glutamate. Multivariate analyses of these 21 taste neurons suggested that, in the AM, taste quality was processed based on palatability. Taken with previous lesion studies, the present results suggest that the AM plays a role in the evaluation of taste palatability and in the association of taste stimuli with other sensory stimuli.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. I. Masticatory muscle motor systems

Oromotor behavior results from the complex interaction between jaw, facial, and lingual muscles. The experiments in this and subsequent papers identify the sources of multisynaptic input to the trigeminal, facial, and hypoglossal motor nuclei. In the current experiments, pseudorabies virus (PRV-Ba) was injected into the jaw-opening (anterior digastric and mylohyoid) and jaw-closing muscles (masseter, medial pterygoid, and temporalis) in bilaterally sympathectomized rats. Injection volumes ranged from 2 to 21 microl with average titers of 2.8 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were consistent between each of the individual muscles and muscle groups. A predictable myotopic labeling pattern was produced in the trigeminal motor nucleus (Mo 5). Transneuronally labeled neurons occurred in regions known to project directly to Mo 5 motoneurons including the principal trigeminal sensory and supratrigeminal areas, Kölliker-Fuse region, nucleus subcoeruleus, and the parvicellular reticular formation. Maximum survival times revealed polysynaptic connections from the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial pontine reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, the results define the structure of the multisynaptic brainstem neural circuits controlling mandibular movement in the rat.

Anterior and posterior oral cavity responsive neurons are differentially distributed among parabrachial subnuclei in rat

The responses of single parabrachial nucleus (PBN) neurons were recorded extracellularly to characterize their sensitivity to stimulation of individual gustatory receptor subpopulations (G neurons, n = 75) or mechanical stimulation of defined oral regions (M neurons, n = 54) then localized to morphologically defined PBN subdivisions. Convergence from separate oral regions onto single neurons occurred frequently for both G and M neurons, but converging influences were more potent when they arose from nearby locations confined to the anterior (AO) or posterior oral cavity (PO). A greater number of G neurons responded optimally to stimulation of AO than to PO receptor subpopulations, and these AO-best G neurons had higher spontaneous and evoked response rates but were less likely to receive convergent input than PO-best G neurons. In contrast, proportions, response rates, and convergence patterns of AO- and PO-best M neurons were more comparable. The differential sensitivity of taste receptor subpopulations was reflected in PBN responses. AO stimulation with NaCl elicited larger responses than PO stimulation; the converse was true for QHCl stimulation. Within the AO, NaCl elicited a larger response when applied to the anterior tongue than to the nasoincisor duct. Hierarchical cluster analysis of chemosensitive response profiles suggested two groups of PBN G neurons. One group was composed of neurons optimally responsive to NaCl (N cluster); the other to HCl (H cluster). Most N- and H-cluster neurons were AO-best. Although they were more heterogenous, all but one of the remaining G neurons were unique in responding best or second-best to quinine and so were designated as quinine sensitive (Q+). Twice as many Q+ neurons were PO- compared with AO-best. M neurons were scattered across PBN subdivisions, but G neurons were concentrated in two pairs of subdivisions. The central medial and ventral lateral subdivisions contained both G and M neurons but were dominated by AO-best N-cluster G neurons. The distribution of G neurons in these subdivisions appeared similar to distributions in most previous studies of PBN gustatory neurons. In contrast to earlier studies, however, the external medial and external lateral-inner subdivisions also contained G neurons, intermingled with a comparable population of M neurons. Unlike cells in the central medial and ventral lateral subnuclei, nearly every neuron in the external subnuclei was PO best, and only one was an N-cluster cell. In conclusion, the present study supports a functional distinction between sensory input from the AO and PO at the pontine level, which may represent an organizing principle throughout the gustatory neuraxis. Furthermore, two morphologically distinct pontine regions containing orosensory neurons are described.

Neural coding of aversive and appetitive gustatory stimuli: interactions in the hamster brain stem

There is increasing evidence, both electrophysiological and behavioral, that bitter and sweet stimuli drive parallel pathways in the gustatory brainstem. Here we report two lines of investigation that suggest significant interactions among these parallel systems. First, responses recorded from single cells in the hamster's parabrachial nuclei (PbN) show that quinine hydrochloride (QHCl) produces a substantial suppression (> 40%) of the responses of PbN cells to sucrose. Sucrose stimulation has a reciprocal suppressive effect on the response to QHCl. These results imply that aversive and appetitive stimuli produce mutual inhibition in the gustatory system; studies of the chorda tympani nerve response suggest that this inhibition likely arises within the brainstem. A second line of investigation, using both an in vitro brainstem slice preparation and in vivo pharmacological manipulations of cells in the hamster NST, has demonstrated an inhibitory network within the rostral NST that plays a role in the modulation of taste activity. Patch-clamp and extracellular recording studies in vitro show that cells within the rostral central subdivision of the NST are inhibited by gamma-aminobutyric acid (GABA); this mediation is largely through the GABAA receptor subtype. Here we show that responses to taste stimulation recorded extracellularly from NST cells in vivo can be inhibited by local microinjections of GABA; this inhibition is blocked by the GABAA receptor antagonist bicuculline methiodide. Responses to sucrose are significantly more inhibited than those to NaCl or KCl. These combined lines of evidence show that appetitive and aversive stimuli activate mutually inhibitory systems within the brainstem and suggest that the basis for this interaction is a GABAergic inhibitory network within the NST.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of fungiform papillae, taste buds, and their innervation in the hamster

Fungiform taste buds in mature hamsters are less subject to neurotrophic influences than those of other species. This study evaluates taste-bud neurotrophism during development in hamsters by examining the relation between growing nerves and differentiating fungiform papillae. Chorda tympani (CT) or lingual (trigeminal) nerve (LN) fibers were labelled with Lucifer Yellow as they grew into (CT fibers) or around (LN fibers) developing taste buds. Developing fungiform papillae and taste pores were counted with the aid of a topical tongue stain. The tongue forms on embryonic days (E) 10.5-11 and contains deeply placed CT and LN fibers but no papillae. By E12, the tongue epithelium develops scattered elevations. These "eminences" selectively become innervated by LN fibers that grow to the epithelium earlier and in larger numbers than CT fibers. Definitive fungiform papillae form rapidly during E13-14 and become heavily innervated by LN fibers. Intraepithelial CT fibers, rare at E13, invariably innervate fungiform papillae containing nascent taste buds at E14. During E14-15 (birth = E15-16), most papillae contain taste buds with pores, extensive perigemmal LN innervation, and extensive intragemmal CT innervation. At birth, numbers of fungiform papillae and taste pores are adultlike. The results show that fungiform eminences begin forming in the absence of innervation. The subsequent differentiation of definitive fungiform papillae and their innervation by LN fibers occur synchronously, prior to the differentiation of taste buds and their CT innervation. The hamster is precocious (e.g., compared to rat) in terms of LN development and the structural maturity of the anterior tongue at birth.

Gustatory and tactile stimulation of the posterior tongue activate overlapping but distinctive regions within the nucleus of the solitary tract

Both the gustatory and somatosensory systems provide necessary sensory input for the initiation and control of oromotor behaviors. Behavioral studies indicate that somatosensory input from the posterior tongue (PT) is important in initiating swallowing, whereas PT taste input is particularly important in gustatory rejection reflexes. However, there have been few studies of the central representation of PT gustatory or tactile responses. In the present study, electrophysiological multi-unit recording techniques were used to map the location of PT-mediated taste and tactile responses in the nucleus of the solitary tract (NST) of the rat. A stimulation technique that allows taste stimuli to be introduced directly and specifically into the papillae trenches was used to optimally activate PT taste receptors located within the circumvallate (CV) and foliate (FOL) papillae. The results demonstrated that non-PT responsive sites dominated the rostral half of the rostral division of NST (rNST), while PT-responsive sites dominated the caudal half. Some PT-responsive sites extended into the caudal NST. Both gustatory and tactile stimuli were effective at 28% of PT-responsive locations (taste-tactile sites), whereas at the remaining locations, only tactile stimulation was effective (tactile-only sites). Although these two types of PT-responsive sites exhibited some anatomical overlap, their distributions were distinctive, with taste-tactile sites restricted medially and the laterally located tactile-only sites offset caudally. On the other hand, responses arising from stimulation of the CV and FOL exhibited no anatomical organization, i.e., responses to stimulation of both papillae were coexistensive. On average, of the four tastants used (0.01 M Na saccharin, 0.3 M NaCl, 0.01 M quinine hydrochloride, 0.03 M HCl), HCl was the most effective stimulus for both the CV and FOL. The present results delimit the regions of the NST that provide a substrate for the gustatory and somatosensory limbs of PT-mediated oromotor reflexes.

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

The role of gamma-aminobutyric acid (GABA) as an inhibitory neurotransmitter in the rostral, gustatory zone of the nucleus of the solitary tract (rNST) was examined using whole cell recordings in brain slices of the adult rat medulla. Superfusion of GABA resulted in a concentration-dependent reduction in input resistance in 68% of the neurons in rNST. The change in input resistance was often accompanied by membrane hyperpolarization. The effect of GABA was a direct action on the postsynaptic membrane since it could be elicited when synaptic transmission was blocked by tetrodotoxin or in a low Ca2+ and high Mg2+ perfusing solution. The mean reversal potential of the GABA effect was about -60 mV, determined by applying GABA at different holding potentials, or from the intersection of current-voltage curves measured in control saline and saline containing GABA. When neurons were separated into groups based on intrinsic membrane properties, some neurons in each group responded to GABA. Superfusion of the slices with either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, caused a decrease in input resistance accompanied by membrane hyperpolarization. The GABAA antagonist bicuculline either totally or partially blocked the neuronal response to GABA and blocked the response to muscimol but did not antagonize responses to baclofen. Superfusion of the GABAB antagonist phaclofen depressed the membrane responses to GABA. The use of the GABAA and GABAB agonists and antagonists demonstrates that some neurons in rNST have both GABAA and GABAB receptors. Since most rNST neurons studied respond to GABA, inhibition probably plays a major role in sensory processing by the rNST.

Distribution of synapses on identified cell types in a gustatory subdivision of the nucleus of the solitary tract

Two morphological types of neurons in the rostral nucleus of the solitary tract (NST) in the hamster send axons to the parabrachial nucleus (PBN). Elongate cells have oval cell bodies and 2 mediolaterally oriented primary dendrites. Large stellate cells have polygonal cell bodies and 3-5 radiating primary dendrites. Both cell types are located in the rostral central subdivision of the NST, surrounded by primary afferent axons from the oral cavity. This study uses electron microscopy to evaluate the synaptic inputs to horseradish peroxidase (HRP)-labelled elongate and stellate PBN projection cells. Three types of axon terminals provide most of the synapses on the labelled cells. Primary-like terminals contain large, clear, round vesicles and engage in asymmetrical synaptic junctions; they resemble gustatory (facial) afferent terminals identified previously (Whitehead, J. Comp. Neurol. 244:72, 1986). Axon terminals containing small, pleomorphic vesicles (SP terminals), form symmetrical junctions, and resemble Golgi II endings. Terminals containing medium-sized pleomorphic vesicles (MP terminals) form asymmetrical junctions. These types of axon terminals distribute differentially on the labelled cells. Primary-like inputs are largely restricted to distal dendrites and their spines. SP terminals provide more synaptic coverage than primary-like or MP terminals; for both cell types the SP inputs are concentrated proximally on dendrites and cell bodies. The data suggest that elongate and large stellate cells function as second-order projection neurons in the ascending taste system. The density, spatial distribution, and timing of activation of the various types of synapses could relate to the electrophysiological response properties of the projection neurons.

Corticofugal influence on taste responses in the parabrachial pons of the rat

Although the anatomy of centrifugal input to gustatory neural structures has been described, little is known of the physiological mechanisms that convey this influence or of their functional significance. As a first step in the investigation of these issues, the effect of a reversible lesion in the gustatory neocortex (GN) on the neural code for taste in the parabrachial nucleus of the pons (PbN) was studied in rats. Electrophysiological responses to taste stimuli bathed over the tongue were recorded from single units in the PbN before, after and following recovery from an infusion of procaine-HCl into the GN. Test stimuli consisted of sapid solutions of NaCl (0.1 M), HCl (0.01 M), sucrose (0.5 M), Na-saccharin (0.004 M) and quinine-HCl (0.01 M). Infusions of procaine into the GN were correlated with both specific and nonspecific effects on the responsivity to gustatory stimuli in the PbN. Specific effects included: (1) changes in the magnitude of response to some tastants, but not others, in a given PbN unit, (2) changes in the across unit patterns produced by sweet stimuli and (3) the appearance of OFF responses in a subset of PbN units. Nonspecific effects were evidenced by changes in the spontaneous rates of activity and by enhancement or suppression of responses across all the tastants tested in a subset of PbN units. Comparison of these results with reports on the effects of decerebration suggests that some of these effects may be accounted for by interruption of the descending input from the GN to the PbN. In addition, the stimulus-specific effects that were noted following procaine infusion into the GN provide support for the suggestion that the GN specifically modifies the electrophysiological patterns that are evoked by salient taste stimuli.

Taste-responsive neurons and their locations in the solitary nucleus of the hamster

The solitary nucleus (nucleus tractus solitarii), the first central relay for taste in mammals, was studied anatomically and physiologically in the golden hamster (Mesocricetus auratus). Activity of neurons to anterior tongue stimulation with sucrose, NaCl and KCl were extracellularly recorded. Electrolytic lesions or horseradish peroxidase deposits allowed subsequent localization of recording sites. Anterior tongue taste-responsive sites were restricted to a very small part of the rostral pole of the solitary nucleus, which is about 3% of the entire nucleus. Sites were confined to the rostral-central and rostral-lateral subdivisions of Whitehead, which contain a number of morphological cell types. Some chemotopic organization was seen with multi-unit recordings, with NaCl-selective sites concentrated rostrally and sucrose- and KCl-selective sites concentrated caudally. Sites with broad sensitivity were distributed throughout the gustatory region. Single neural units showing inhibition to taste stimuli, units highly reactive to all three stimuli, and units with high spontaneous rates were seen in the solitary nucleus, as well as units that responded very selectively and had low spontaneous rates. Single units with similar response profiles to sucrose, NaCl and KCl were not segregated to separate restricted locations within the taste-reactive region; their distributions overlapped. In the hamster, neurons in the anterior tongue taste region of the solitary nucleus process taste quality information in diverse ways. Highly reactive non-specific neurons, neurons that show inhibition, and neurons with high spontaneous rates are more frequently observed in the solitary nucleus than in the afferent input fibers of the chorda tympani nerve. The small region of the rostral pole enclosing taste-responsive neurons is complexly organized in relation to taste quality and contains a number of morphological cell types whose functional role in taste is not yet known.

Thermal sensitivity of neurons in a rostral part of the rat solitary tract nucleus

While stimulating the entire oral cavity of anesthetized rats, we recorded 3 types of neurons in the solitary tract nucleus; taste, mechanoreceptive and cold neurons. Most of the taste neurons were sensitive to thermal as well as to mechanical stimulations. Taste neurons predominantly sensitive to sucrose responded to warming and those most excited by NaCl or HCl were sensitive to cooling, and significant correlations were found between sucrose and warming and between NaCl and cooling. Most of the cold-sensitive taste neurons had receptive fields (RFs) at the anterior tongue and warm-sensitive taste neurons had whole or part of the RFs at the nasoincisor duct. About half the number of mechanoreceptive neurons were sensitive to cooling, producing phasic responses. RFs of some thermosensitive mechanoreceptive neurons and cold neurons were located. Warm-sensitive mechanoreceptive neurons or warm neurons were not evident. Therefore, interaction between thermal and taste sensations in the oral cavity probably takes place in the solitary tract nucleus, as well as in the chorda tympani.

Taste responses in the nucleus tractus solitarius of the chronic decerebrate rat

The ingestive behavior of decerebrate rats has been studied for some time, yet little is known of its neural substrates. While taste fibers in rats proceed from hindbrain to thalamus and ventral forebrain, these regions return centrifugal fibers to the hindbrain by which lower-order taste activity may be influenced. We examined the functional characteristics of taste neurons in the nucleus tractus solitarii (NTS) of chronic decerebrate rats in which this reciprocal communication was disrupted and compared them with those of intact controls. Nine Wistar rats were decerebrated at the supracollicular level. After a minimum of one week recovery, they were immobilized with Flaxedil, anesthetized locally and prepared for recording. The responses of 50 taste cells were isolated bilaterally from the NTS of these animals, while the activity of 50 additional neurons was recorded from 12 intact rats under the same conditions. Taste stimuli included 7 Na-Li salts, 3 sugars, HCl and citric acids, quinine HCl and NaSaccharin. Mean spontaneous activity in decerebrates was 6.5 spikes/s, 36.0% lower than the level in intact animals. Mean evoked activity was reduced by 32.6%. Analyses of the effects of stimulus quality, intensity and time course of the responses all indicated that the decrease in activity was attributable to the inability of taste cells in decerebrate rats to respond to demands for high discharge rates. This deficit could be responsible for the failure of these animals to develop conditioned taste aversions. Neurons from decerebrate preparations did, however, retain the broad sensitivity across stimuli that characterized taste cells in intact preparations. It was also typical that most neuron response profiles from decerebrates could be grouped into 3 loose clusters with peak sensitivities to acid-salt, salt or sugar. An analysis of similarities among stimulus activity profiles indicated that Na-Li salts, sugars and an acid-quinine complex represented 3 groups of stimulus quality; in intact animals, the primary distinction was between sweet and non-sweet stimuli. Moreover, the response to sodium saccharin lost its bitter component in decerebrates. These findings were in general agreement with those derived from acute decerebrate rats.