Development of taste responses in the rat parabrachial nucleus

Extracellular responses from neurons in the parabrachial nuclei (PBN) were studied in rats 4 days old to adulthood during chemical stimulation of the tongue with monochloride salts, citric and hydrochloric acids, sucrose, sodium saccharin, and quinine hydrochloride. Multiunit taste responses were recorded in rats at 4-7 days of age and single-unit responses were recorded from 121 neurons in four other age groups of 14-20 days, 25-35 days, 50-60 days, and adults. PBN neurons in rats 4-7 days old consistently responded to 0.1 M solutions of NH4Cl and NaCl, to 0.5 M solutions of NH4Cl, NaCl, and KCl, and to 1.0 M sucrose, 0.1 M sodium saccharin, 0.1 M citric acid, and 0.1 N HCl. They often did not respond, however, to 0.1 M KCl and 0.01 M quinine hydrochloride. Single PBN neurons in rats 14 days old and older characteristically responded to all stimuli, which consisted of 0.1 and 0.5 M salts, acids, sucrose, sodium saccharin, and quinine hydrochloride. Thus no developmental differences occurred in the number of stimuli to which neurons responded after rats were 14 days old. With the exception of responses to hydrochloric acid, there were significant increases in response frequencies to all stimuli after 14 days of age. Average response frequencies to NH4Cl and citric acid increased after 20 days of age and those to NaCl, LiCl, KCl, sucrose, sodium saccharin, and quinine hydrochloride increased after 35 days of age. Average response frequencies for hydrochloric acid did not alter after 14 days of age. The proportion of single PBN neurons that responded maximally to specific monochloride salts did not change during development. Most single neurons in all age groups responded equally well to NH4Cl, NaCl, and LiCl. No PBN neuron responded maximally to KCl. Developmental differences in response frequencies of third-order gustatory neurons in the PBN generally reflect developmental response changes in first-order neurons of the chorda tympani nerve and second-order neurons of the solitary nucleus. However, unique developmental changes are evident in the PBN. Thus the ontogenetic changes that occur in PBN responses likely relate to modifications of lower-order peripheral and central nervous system afferents and peripheral receptor sensitivities.     

Lack of amiloride sensitivity in SHR and WKY glossopharyngeal taste responses to NaCl.

To explore possible functional strain differences in taste receptors located on the posterior tongue, we recorded electrophysiological taste responses from the glossopharyngeal nerve of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Multifiber responses to a concentration series (0.5 M to 2.0 M) of NaCl, KCl and NH4Cl were recorded before and after lingual application of the epithelial sodium transport blocker, amiloride. Responses to a concentration series (0.0025 M to 0.1 M) of quinine hydrochloride were also recorded. When expressed relative to the 0.5-M NH4Cl response, responses to the monochloride salts were equivalent between SHR and WKY. Surprisingly, NaCl responses were not suppressed by the sodium transport blocker, amiloride. This is in direct contrast to the dramatic suppression observed in the chorda tympani. Also, relative responses to quinine were greater in the glossopharyngeal nerve of SHR than WKY. These results indicate that taste receptors innervated by the glossopharyngeal nerve lack amiloride sensitivity and that posterior taste receptor function to monochloride salts is equivalent between SHR and WKY.     

Taste-responsive neurons of the glossopharyngeal nerve of the rat

1. Taste sensibilities of neurons in mammalian glossopharyngeal nerves have been inadequately studied, although they innervate the majority of taste buds and may provide unique taste information. 2. Extracellular responses of glossopharyngeal neural units to taste stimuli infused into foliate or vallate papillae were recorded in anesthetized rats. A 0.3-ml/min infusion of stimuli into papillae resulted in short-latency, 5-s nerve-impulse rates that approached 10 times the response rates observed using less invasive means of stimulation. 3. Sucrose, Na saccharin, NaCl, NH4Cl, KCl, HCl, citric acid, acetic acid, MgSO4, and quinine.HCl were effective stimuli for glossopharyngeal neurons at concentrations that have behavioral significance. 4. Response spectra for individual neural units with either foliate or vallate receptive fields fell into three clusters. Forty-six percent were A units that responded most strongly to acids and chloride salts, NH4Cl being the most effective; neither quinine nor sucrose was effective. Twenty-three percent were S units that responded to sugars and saccharin; quinine, salts, and acids were not effective. Thirty-one percent were Q units that responded to quinine; neither NaCl, HCl, nor sucrose was effective stimulus for these fragile units. 5. Glossopharyngeal A neural units were more sensitive to 1 mM HCl than were electrolyte-sensitive H units of the chorda tympani, although both respond generally to salts and acids. Units relatively specific for sodium salts (N units), which are common in the chorda tympani nerve, were not found in the glossopharyngeal nerve, which explains losses in sodium-specific behavior after cutting only the chorda tympani nerve. 6. Q units were the only glossopharyngeal neural units that responded significantly to quinine, and units with similar response spectra do not occur in the chorda tympani nerve. Q units probably mediate aversive reflexes to quinine that are eliminated by cutting only the glossopharyngeal nerve. Glossopharyngeal S neural units were more sensitive to sucrose and are more common than their counterparts in the chorda tympani, although it is not known how they might compare with sugar-sensitive units in the greater superficial petrosal nerve. 7. These data strongly suggest that posterior taste bud fields innervated by the glossopharyngeal nerve are specialized for functions different from those of anterior taste bud fields innervated by the facial nerve.     

Neural representation of the taste of NaCl and KCl in gustatory neurons of the hamster solitary nucleus.

NaCl and KCl are monovalent salts that can be discriminated behaviorally by hamsters on the basis of their tastes. We examined the effects of the passive Na+ channel blocker amiloride on responses to both of these salts in 34 taste-responsive neurons of the nucleus of the solitary tract (NST) in the hamster. The effects of amiloride were assessed with two different, commonly employed stimulus protocols. Additionally, concentration-response functions for each salt were measured in 37 neurons. Cells were characterized by their best response to (in M) 0. 03 NaCl, 0.1 sucrose, 0.003 HCl, 0.001 quinine hydrochloride, and 0. 1 KCl. In neurons classified as NaCl-best, amiloride reversibly blocked responses to both NaCl and KCl. In neurons classified as HCl-best, amiloride had no effect on either stimulus. In sucrose-best neurons, amiloride blocked the response to NaCl but not KCl. These results support the hypothesis that both salts are transduced by at least two different receptor mechanisms. In the NST, information arising from these different inputs is maintained in discrete populations of neurons. In addition to differences in amiloride sensitivity, the cell types also differed in their responses to the salts across concentration. At midrange salt concentrations, NaCl-best neurons were far more responsive to NaCl than KCl, whereas HCl- and sucrose-best neurons responded equivalently to the two salts at all concentrations. Because NaCl- and HCl-best cells cannot by themselves distinguish NaCl from KCl, it is the relative activity across these cell types that comprises the code for taste discrimination.     

Alterations of salt taste perception in the developing rat

Behavioral correlates of changing neurophysiological taste sensitivities during development were assessed with a conditioned taste aversion procedure. Young rats (age 25-30 days) avoided 0.1M monochloride salts and 1.0M sucrose reliably less than adults (age 90-105 days), but the two groups did not differ when the conditioned stimulus (CS) was 0.1M citric acid. Analyses of generalization gradients revealed that young rats were unable to discriminate among the tastes of NaCl, NH4Cl, and KCl, whereas adults readily made such discriminations. Both age groups had similar generalization gradients when the CS was 1.0M sucrose or 0.1M citric acid. These data indicate that quantitative and qualitative aspects of salt taste perception alter with age. Furthermore, the behavioral changes noted in the present study correspond closely with previous findings from developmental studies of neurophysiological taste responses.     

Anion size modulates salt taste in rats

The purpose of this study was to investigate the influence of anion size and the contribution of the epithelial sodium channel (ENaC) and the transient receptor potential vanilloid-1 (TRPV1) channel on sodium-taste responses in rat chorda tympani (CT) neurons. We recorded multiunit responses from the severed CT nerve and single-cell responses from intact, narrowly tuned and broadly tuned, salt-sensitive neurons in the geniculate ganglion simultaneously with stimulus-evoked summated potentials to signal when the stimulus contacted the lingual epithelium. Artificial saliva served as the rinse and solvent for all stimuli (0.3 M NH(4)Cl, 0.5 M sucrose, 0.03-0.5 M NaCl, 0.01 M citric acid, 0.02 M quinine hydrochloride, 0.1 M KCl, and 0.03-0.5 M Na-gluconate). We used the pharmacological antagonist benzamil to assess NaCl responses mediated by ENaC, and SB-366791 and cetylpyridinium chloride to assess responses mediated by TRPV1. CT nerve responses were greater to NaCl than Na-gluconate at each concentration; this was attributed mostly to broadly tuned, acid-generalist neurons that responded with higher frequency and shorter latency to NaCl than Na-gluconate. In contrast, narrowly tuned NaCl-specialist neurons responded more similarly to the two salts, but with subtle differences in temporal pattern. Benzamil reduced CT nerve and single-cell responses only of narrowly tuned neurons to NaCl. Surprisingly, SB-366791 and cetylpyridinium chloride were without effect on CT nerve or single-cell NaCl responses. Collectively, our data demonstrate the critical role that apical ENaCs in fungiform papillae play in processing information about sodium by peripheral gustatory neurons; the role of TRPV1 channels is an enigma.     

Gustatory neuron types in rat geniculate ganglion

We used extracellular single-cell recording procedures to characterize the chemical and thermal sensitivity of the rat geniculate ganglion to lingual stimulation, and to examine the effects of specific ion transport antagonists on salt transduction mechanisms. Hierarchical cluster analysis of the responses from 73 single neurons to 3 salts (0.075 and 0.3 M NaCl, KCl, and NH(4) Cl), 0.5 M sucrose, 0.01 M HCl, and 0.02 M quinine HCl (QHCl) indicated 3 main groups that responded best to either sucrose, HCl, or NaCl. Eight narrowly tuned neurons were deemed sucrose-specialists and 33 broadly tuned neurons as HCl-generalists. The NaCl group contained three identifiable subclusters: 18 NaCl-specialists, 11 NaCl-generalists, and 3 QHCl-generalists. Sucrose- and NaCl-specialists responded specifically to sucrose and NaCl, respectively. All generalist neurons responded to salt, acid, and alkaloid stimuli to varying degree and order depending on neuron type. Response order was NaCl > HCl = QHCl > sucrose in NaCl-generalists, HCl > NaCl > QHCl > sucrose in HCl-generalists, and QHCl = NaCl = HCl > sucrose in QHCl-generalists. NaCl-specialists responded robustly to low and high NaCl concentrations, but weakly, if at all, to high KCl and NH(4) Cl concentrations after prolonged stimulation. HCl-generalist neurons responded to all three salts, but at twice the rate to NH(4) Cl than to NaCl and KCl. NaCl- and QHCl-generalists responded equally to the three salts. Amiloride and 5-(N,N-dimethyl)-amiloride (DMA), antagonists of Na(+) channels and Na(+)/H(+) exchangers, respectively, inhibited the responses to 0.075 M NaCl only in NaCl-specialist neurons. The K(+) channel antagonist, 4-aminopyridine (4-AP), was without a suppressive effect on salt responses, but, when applied alone in solution, it evoked a response in many HCl-generalists and one QHCl-generalist neuron so tested. Of the 39 neurons tested for their sensitivity to temperature, 23 responded to cooling and chemical stimulation, and 20 of these neurons were HCl-generalists. Moreover, the responses to the four standard stimuli were reduced progressively at lower temperatures in HCl- and QHCl-generalist neurons, but not in NaCl-specialists. Thus sodium channels and Na(+)/H(+) exchangers appear to be expressed exclusively on the membranes of receptor cells that synapse with NaCl-specialist neurons. In addition, cooling sensitivity and taste-temperature interactions appear to be prominent features of broadly tuned neuron groups, particularly HCl-generalists. Taken all together, it appears that lingual taste cells make specific connections with afferent fibers that allow gustatory stimuli to be parceled into different input pathways. In general, these neurons are organized physiologically into specialist and generalist types. The sucrose- and NaCl-specialists alone can provide sufficient information to distinguish sucrose and NaCl from other stimuli, respectively.     

Perceptual characteristics of the amiloride-suppressed sodium chloride taste response in the rat

The contribution of amiloride-sensitive membrane components to the perception of NaCl taste was assessed by using a conditioned taste aversion procedure. Eight independent groups of adult rats were conditioned to avoid either 0.1M NaCl, 0.5M NaCl; 0.1M NH4Cl, or 1.0M sucrose while their tongues were exposed either to water or to the sodium transport blocker amiloride hydrochloride. In contrast to rats exposed to water during conditioning, rats exposed to amiloride were unable to acquire a conditioned taste aversion to 0.1M NaCl. Differences in the acquisition of taste aversions between the amiloride- and nonamiloride-treated groups were not apparent when the conditioned stimulus (CS) was 0.5M NaCl, 0.1M NH4Cl, or 1.0M sucrose. Although the magnitude of the 0.5M NaCl aversion was similar between amiloride- and non-amiloride-treated rats, the perceptual characteristics of the CS differed between groups. Analyses of stimulus generalization gradients revealed that amiloride-treated rats generally avoided all monochloride salts after conditioning to 0.5M NaCl but not nonsodium salts or nonsalt stimuli. In contrast, rats not treated with amiloride only generalized the 0.5M NaCl aversion to sodium salts. No differences in generalization gradients occurred between groups when the CS was 0.1M NH4Cl or 1.0M sucrose. These findings suggest that the "salty" taste of NaCl is primarily related to the amiloride-sensitive portion of the functional taste response in rats. Conversely, the portion of the NaCl response insensitive to amiloride appears to have "sour-salty" perceptual characteristics and does not appear to be perceived as being salty.     

Behavioral and neural gustatory responses in rabbit

To provide more information on a potentially valuable preparation for studies in taste and appetite, we have examined the taste preferences (and aversions) and chorda tympani sensitivity of the rabbit. Adult male New Zealand rabbits were given a two-bottle preference test between water and various molar concentrations of NaCl, KCl, sucrose, sodium saccharin, quinine hydrochloride and HCl. The rabbits exhibited the expected preferences for sucrose and aversions for quinine and HCl. Unexpectedly, however, the rabbits exhibited only a mild preference for NaCl, a stronger preference for KCl, and an aversion to sodium saccharin. Multiunit discharges of the chorda tympani nerve to the same taste stimuli indicated that the anterior tongue receptors are acutely sensitive to KCl, NaCl and quinine, but not to sucrose, HCl and saccharin. The chorda tympani was more responsive to KCl than to NaCl. Dilute concentrations of both NaCl and sodium saccharin elicited a two-component response consisting of an immediate excitatory phase followed by a tonic inhibitory phase. This complex response pattern of the whole nerve to NaCl and sodium saccharin is discussed in relation to the impulse frequencies in hypothesized water-sensitive and salt-sensitive fibers. Both the behavioral and neural data are discussed in relation to similar data obtained in rat and hamster.     

The role of transient receptor potential vanilloid-1 on neural responses to acids by the chorda tympani,glossopharyngeal and superior laryngeal nerves in mice

The transient receptor potential vanilloid-1 (TRPV1) receptor acts as a polymodal nociceptor activated by capsaicin, heat, and acid. TRPV1, which is expressed in sensory neurons innervating the oral cavity, is associated with an oral burning sensation in response to spicy food containing capsaicin. However, little is known about the involvement of TRPV1 in responses to acid stimuli in either the gustatory system or the general somatosensory innervation of the oropharynx. To test this possibility, we recorded electrophysiological responses to several acids (acetic acid, citric acid and HCl) and other taste stimuli from the mouse chorda tympani, glossopharyngeal and superior laryngeal nerves, and compared potential effects of iodo-resiniferatoxin (I-RTX), a potent TRPV1 antagonist, on chemical responses of the three nerves. The results indicated that in the chorda tympani nerve, I-RTX (1–100 nM) did not affect responses to acids, sucrose and quinine HCl, but reduced responses to NaCl (I-RTX at concentrations of 10 and 100 nM) and KCl and NH₄Cl (100 nM). In contrast, in the glossopharyngeal nerve, I-RTX significantly suppressed responses to all acids and salts, but not to sucrose and quinine HCl. Responses to acetic acid were suppressed by I-RTX even at 0.1 nM concentration. The superior laryngeal nerve responded in a concentration-dependent manner to acetic acid, citric acid, HCl, KCl, NH₄Cl and monosodium l-glutamate. The responses to acetic acid, but not to the other stimuli, were significantly inhibited by I-RTX. These results suggested that TRPV1 may be involved in the mechanism for responses to acids presented to the posterior oral cavity and larynx. This high degree of responsiveness to acetic acid may account for the oral burning sensation, known as a flavor characteristic of vinegar.     

The gustatory sense in foetal sheep during the last third of gestation

1. Histological studies revealed that presumptive taste buds are present in the foetal sheep tongue at 50 days. By 100 days the taste buds appear morphologically mature.2. To determine if foetal taste buds are functional, electrophysiological recordings were made of activity in the chorda tympani nerve of sheep foetuses. Single and multi-fibre preparations were studied in foetuses aged 100 days to term.3. Responses were recorded to lingual stimulation with salts, acids, glycerol, glycine, sodium saccharin, quinine HCl and amniotic fluid. Responses of the foetal chorda tympani to lingual stimulation with a series of monochloride salts and with increasing concentrations of one salt were similar to responses recorded in lambs and adults.4. The peripheral gustatory system of the foetal sheep is functional for at least the last third of gestation. Foetal taste experiences may influence the formation of adult taste preferences or may aid the foetus in monitoring its environment.     

Peripheral taste responses in genetically hypertensive rats

In two-bottle preference-aversion tests, the spontaneously hypertensive rat (SHR) tolerates higher concentrations of NaCl than the normotensive Wistar-Kyoto (WKY). In contrast, the inbred Dahl salt-sensitive (S/JR) and inbred Dahl salt-resistant (R/JR) rat show similar preferences for NaCl. In order to determine if taste receptor function was also altered between the hypertensive rat and its normotensive control, we recorded electrophysiological taste responses from the chorda tympani (CT) nerve in SHR, WKY, S/JR and R/JR rats. Responses to a concentration series (0.05 M to 0.5 M) of NaCl, NaAcetate, KCl, NH4Cl and CaCl2 were recorded before and after lingual application of amiloride hydrochloride, an epithelial sodium transport blocker. When expressed relative to the 0.5 M NH4Cl response, responses to the majority of stimuli were equivalent between the SHR and WKY. By comparison, relative responses to NaCl were greater in the R/JR than S/JR; however, the magnitude of amiloride suppression was equivalent between these two strains. Relative responses to the majority of the remaining salts did not differ between the S/JR and R/JR. These results suggest that taste receptor function may be equivalent between the hypertensive rat and its normotensive control.     

Chorda tympani responses in two inbred strains of mice with different taste preferences.

Behavioral studies suggest that there are significant differences in the taste systems of the inbred mouse (Mus musculus) strains: C57BL/6J (B6) and DBA/2J (D2). In an attempt to understand the biological basis of the behavioral differences, we recorded whole-nerve chorda tympani responses to taste solutions and compared the results to intake of similar solutions in nondeprived mice. Stimuli included a test series composed of 0.1 M sodium chloride, 0.3 M sucrose, 10 mM sodium saccharin, 3 mM hydrochloric acid, and 3 mM quinine hydrochloride, as well as concentration series for the same substances. Neural activity of the chorda tympani that was evoked by sucrose, saccharin, or NaCl was greater in B6 than D2 mice; and neural threshold for sucrose was lower in B6 mice, but neural thresholds for HCl and quinine were lower in D2 mice. B6 mice drank more sucrose and saccharin but less quinine than D2 mice; thus, sucrose and saccharin preference were positively correlated, but NaCl and quinine aversiveness were negatively correlated with the chorda tympani results. Nonetheless, genes involved in the structuring of taste receptors and/or the chordae tympani, which transduce taste stimuli having diverse perceptual qualities, differ for the two mouse strains.     

Modulation of parabrachial taste neurons by electrical and chemical stimulation of the lateral hypothalamus and amygdala

The lateral hypothalamus (LH) and the central nucleus of the amygdala (CeA) exert an influence on ingestive behavior and are reciprocally connected to gustatory and viscerosensory areas, including the nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN). We investigated the effects of LH and CeA stimulation on the activity of 101 taste-responsive neurons in the hamster PbN. Eighty three of these neurons were antidromically activated by stimulation of these sites; 57 were antidromically driven by both. Of these 83 neurons, 21 were also orthodromically activated--8 by the CeA and 3 by the LH. Additional neurons were excited (n = 5) or inhibited (n = 8) by these forebrain nuclei but not antidromically activated. Taste stimuli were: 0.032 M sucrose, 0.032 M sodium chloride (NaCl), 0.032 M quinine hydrochloride (QHCl), and 0.0032 M citric acid. Among the 34 orthodromically activated neurons, more sucrose-best neurons were excited than inhibited, whereas the opposite occurred for citric-acid- and QHCl-best cells. Neurons inhibited by the forebrain responded significantly more strongly to citric acid and QHCl than cells excited by these sites. The effects of electrical stimulation were mimicked by microinjection of DL-homocysteic acid, indicating that cells at these forebrain sites were responsible for these effects. These data demonstrate that many individual PbN gustatory neurons project to both the LH and CeA and that these areas modulate the gustatory activity of a subset of PbN neurons. This neural substrate is likely involved in the modulation of taste activity by physiological and experiential factors.     

Gustatory responses of neurons in the nucleus of the solitary tract of behaving rats.

1. The activity of 117 single neurons was recorded in the rostral nucleus of the solitary tract (NST) and tested with each of four standard chemical stimuli [sucrose, NaCl, citric acid, and quinine HCl (QHCl)] and distilled water in awake, behaving rats. In 101 of these neurons, at least one sapid stimulus elicited a significant taste response. The mean spontaneous rate of the taste neurons was 4.1 +/- 5.8 (SD) spike/s. The mean response magnitudes were as follows: sucrose, 10.6 +/- 11.7; NaCl, 8.6 +/- 14.6; citric acid, 6.2 +/- 7.8; and QHCl, 2.4 +/- 6.6 spikes/s. 2. On the basis of their largest response, 42 taste neurons were classified as sucrose-best, 25 as NaCl-best, 30 as citric acid-best, and 4 as QHCl-best. The mean spontaneous rates for these categories were 4.9 +/- 6.2 for sucrose-best cells, 5.8 +/- 7.4 for NaCl-best, 1.6 +/- 2.0 for citric acid-best, and 5.8 +/- 6.0 spikes/s for QHCl-best. The spontaneous rate of the citric acid-best neurons was significantly lower than that of the other categories. 3. At the standard concentrations, 45 taste cells (44.6%) responded significantly to only one of the gustatory stimuli. Of the 30 acid-best neurons, 23 (76.7%) responded only to citric acid. For sucrose-best cells, specific sensitivity was less common (18/42, 42.9%), and for NaCl-best neurons, it was relatively uncommon (3/25, 12%). One of the 4 QHCl-best neurons was specific. In a concentration series, more than one-half of the 19 specific neurons tested responded to only one chemical at any strength. 4. The mean entropy for the excitatory responses of all gustatory neurons was 0.60. Citric acid-best cells showed the least breadth of responsiveness (0.49), sucrose-best cells were somewhat broader (0.56), but NaCl-best and QHCl-best cells were considerably less selective (0.77 and 0.79, respectively). Inhibition was observed infrequently and never reached the criterion for significance. 5. In the hierarchical cluster analysis, the four largest clusters segregated neurons primarily by best-stimulus category. The major exception to this was a group of sucrose-best neurons that also responded to NaCl and were grouped with the NaCl-best neurons. In a two-dimensional space, the specific taste neurons, those that responded to only one of the four standard sapid stimuli, remained in well-separated groups. These specific groups, however, were joined in a ring-like formation by other neurons that responded to more than one of the sapid stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neural representation of bitter taste in the nucleus of the solitary tract

Based on the molecular findings that many bitter taste receptors (T2Rs) are expressed within the same receptor cells, it has been proposed that bitter taste is encoded by the activation of discrete neural elements. Here we examined how a variety of bitter stimuli are represented by neural activity in central gustatory neurons. Taste responses (spikes/s) evoked by bathing the tongue and palate with intensity-matched concentrations (in M) of 2 sugars (0.32 sucrose and 0.5 D-fructose), ethanol (40%), 4 salts (0.01 NaCl, 0.008 NaNO(3), 0.01 MgCl(2), and 0.05 KCl), 2 acids (0.003 HCl and 0.005 citric acid), and 10 bitter ligands (0.007 quinine-HCl, 0.015 denatonium benzoate, 0.003 l-cysteine, 0.001 nicotine, 0.005 strychnine-HCl, 0.04 tetraethylammonium chloride, 0.03 atropine-SO(4), 0.005 brucine-SO(4), 0.03 papaverine-HCl, and 0.009 sparteine) were recorded from 51 neurons in the nucleus of the solitary tract of anesthetized rats. Cluster analysis was used to categorize neurons into types based on responses to sucrose, NaCl, HCl, and quinine-HCl. Three groupings emerged: type S (responded optimally to sweets), type N (sodium-optimal), and type H/Q (responded robustly to bitters, acids, and salts). Multivariate analyses revealed that across-neuron patterns of response among bitter stimuli were strongly correlated. However, neural type H/Q, which was most responsive to bitter tastants, was not differentially sensitive to bitter stimuli and Na(+) salts, which rats perceive as distinct. Thus central neurons most responsive to bitter substances receive significant input from receptors that mediate other tastes, indicating that bitter stimuli are not represented by activity in specifically tuned neurons.     

Responses to basic taste qualities in rats selectively bred for high versus low saccharin intake

Rats selectively bred for relatively high (HiS) and relatively low (LoS) saccharin intake were offered sweet (sucrose), bitter (quinine, sucrose octaacetate), salty (sodium chloride), starchy (Polycose((R))), and sour (citric acid) solutions at several concentrations; sucrose/quinine, and sucrose/citric acid mixtures were also tested. Compared to HiS rats, LoS rats displayed weaker preferences for and lower consumption of sweet, salty, and starchy solutions. HiS and LoS rats did not differ in responses to simple bitter or sour solutions or to adulteration of sucrose with citric acid. However, quinine adulteration reduced sucrose preference more among LoS rats. Thus, selection on a saccharin intake phenotype has yielded line differences on all hedonically positive tastants and, probably as a consequence of that difference, greater finickiness specifically towards bittersweet solutions in the low saccharin-consuming line. Additional work can clarify the psychobiological mechanisms for the phenotypic difference and, potentially, the reasons for its relationship to measures of emotionality.     

Temperature modulates taste responsiveness and stimulates gustatory neurons in the rat geniculate ganglion

In humans, temperature influences taste intensity and quality perception, and thermal stimulation itself may elicit taste sensations. However, peripheral coding mechanisms of taste have generally been examined independently of the influence of temperature. In anesthetized rats, we characterized the single-cell responses of geniculate ganglion neurons to 0.5 M sucrose, 0.1 M NaCl, 0.01 M citric acid, and 0.02 M quinine hydrochloride at a steady, baseline temperature (adapted) of 10, 25, and 40 degrees C; gradual cooling and warming (1 degrees C/s change in water temperature >5 s) from an adapted tongue temperature of 25 degrees C; gradual cooling from an adapted temperature of 40 degrees C; and gradual warming from an adapted temperature of 10 degrees C. Hierarchical cluster analysis of the taste responses at 25 degrees C divided 50 neurons into two major categories of narrowly tuned (Sucrose-specialists, NaCl-specialists) and broadly tuned (NaCl-generalists(I), NaCl- generalists(II), Acid-generalists, and QHCl-generalists) groups. NaCl specialists were excited by cooling from 25 to 10 degrees C and inhibited by warming from 10 to 25 degrees C. Acid-generalists were excited by cooling from 40 to 25 degrees C but not from 25 to 10 degrees C. In general, the taste responses of broadly tuned neurons decreased systematically to all stimuli with decreasing adapted temperatures. The response selectivity of Sucrose-specialists for sucrose and NaCl-specialists for NaCl was unaffected by adapted temperature. However, Sucrose-specialists were unresponsive to sucrose at 10 degrees C, whereas NaCl-specialists responded equally to NaCl at all adapted temperatures. In conclusion, we have shown that temperature modulates taste responsiveness and is itself a stimulus for activation in specific types of peripheral gustatory neurons.     

Altered taste responses in adult NST after neonatal chorda tympani denervation

Anatomic and behavioral changes have been observed in the taste system after peripheral deafferentation, but their physiological consequences remain unknown. Interestingly, a recent behavioral study suggested that peripheral denervation could induce central plasticity. After neonatal chorda tympani (CT) transection, adult rats demonstrated a marked preference for a normally avoided salt, NH(4)Cl. In the present study, taste responses were recorded from the nucleus of the solitary tract (NST) in similarly CT-denervated rats to investigate a physiological basis for this behavioral phenomenon. We hypothesized that alterations in functional connectivity of remaining afferent nerves might underlie the behavioral change. Specifically, if NST neurons formerly activated by sodium-selective CT fibers were instead driven by more broadly tuned glossopharyngeal (GL) afferents, neural coding of salt responses would be altered. Such a change should be accompanied by a shift in orotopic representation and increased NH(4)Cl responses. This hypothesis was not supported. After CT denervation, orotopy was unaltered, NH(4)Cl responsiveness declined, and no other changes occurred that could simply explain the behavioral effects. Indeed, the most pronounced consequence of CT denervation was a 68% reduction in NaCl responses, supporting previous evidence for a critical role of this nerve in coding sodium salts. In addition, we found "reorganizational" changes similar to, albeit smaller than, those observed in other sensory systems after deafferentation. There was a trend for increased responses elicited by stimulation of receptor subpopulations innervated by the GL and greater superficial petrosal nerves. In addition, the spontaneous rate of nasoincisor duct-responsive cells increased significantly. This effect on spontaneous rate is opposite to that produced by CT anesthesia, suggesting that acute versus chronic denervation may affect central taste neurons differently. In conclusion, the taste system at the medullary level seems more resistant to large-scale plasticity than other sensory systems, but nevertheless reacts to lost afferent input. Because the most robust plastic changes have been documented at cortical levels in other sensory pathways, the substrate for the behavioral effect of neonatal CT transection may be located more centrally in the gustatory system.     

Taste responses of neurons of the hamster solitary nucleus are enhanced by lateral hypothalamic stimulation

Gustatory responses in the brain stem are modifiable by several physiological factors, including blood insulin and glucose, intraduodenal lipids, gastric distension, and learning, although the neural substrates for these modulatory effects are not known. Stimulation of the lateral hypothalamus (LH) produces increases in food intake and alterations in taste preference behavior, whereas damage to this area has opposite effects. In the present study, we investigated the effects of LH stimulation on the neural activity of taste-responsive cells in the nucleus of the solitary tract (NST) of the hamster. Bipolar stimulating electrodes were bilaterally implanted in the LH, and the responses of 99 neurons in the NST, which were first characterized for their taste sensitivities, were tested for their response to both ipsilateral and contralateral LH stimulation. Half of the taste-responsive cells in the NST (49/99) were modulated by LH stimulation. Contralateral stimulation was more often effective (41 cells) than ipsilateral (13 cells) and always excitatory; 10 cells were excited bilaterally. Six cells were inhibited by ipsilateral stimulation. A subset of these cells (n = 13) was examined for the effects of microinjection of DL-homocysteic acid (DLH), a glutamate receptor agonist, into the LH. The effects of electrical stimulation were completely mimicked by DLH, indicating that cell somata in and around the LH are responsible for these effects. Other cells (n = 14) were tested for the effects of electrical stimulation of the LH on the responses to stimulation of the tongue with 0.032 M sucrose, NaCl, and quinine hydrochloride, and 0.0032 M citric acid. Responses to taste stimuli were more than doubled by the excitatory influence of the LH. These data show that the LH, in addition to its role in feeding and metabolism, exerts descending control over the processing of gustatory information through the brain stem.