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.     

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.     

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.     

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.     

Development of taste responses in rat nucleus of solitary tract

Extracellular responses from neurons in the nucleus of the solitary tract (NST) were studied in rats aged 5 days 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 5-7 days of age and single-unit responses were recorded from 111 neurons in four other age groups of 14-20 days, 25-35 days, 50-60 days, and adult. NST neurons in rats aged 5-7 days consistently responded to relatively high concentrations (0.5 M) of NH4Cl and KCl and to citric and hydrochloric acid. However, they often did not respond to 0.5 M NaCl or to 0.1 M NH4Cl. Single NST neurons in rats aged 14 days and older characteristically responded to all 0.1 and 0.5 M salts and to both acids. At least 75% of neurons also responded to sucrose and sodium saccharin, and 46% responded to all of these stimuli and quinine hydrochloride. After 14 days, no developmental changes occurred in the number of stimuli to which neurons responded. There were substantial developmental alterations in the response magnitudes to some chemical stimuli. Average response frequencies increased after 35 days of age for 0.1 and 0.5 M NaCl, LiCl, KCl, and for sucrose and sodium saccharin. Response frequencies for NH4Cl, citric and hydrochloric acid, and quinine hydrochloride, however, did not change throughout development. The proportion of single NST 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 NST neuron responded maximally to KCl. There were also no developmental differences in response latencies in rats aged 14 days and older. Response frequencies of second-order NST neurons generally reflect changes in responses from the primary afferent, chorda tympani fibers, throughout development; however, the increases in salt response frequencies from NST neurons occur comparatively later in development. Furthermore, at all ages, the taste responses to monochloride salts include higher response frequencies and a general loss in response specificity in NST compared to chorda tympani neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of taste responses to Na+ salts by epithelial Na+ channel blockers in gerbil

The Na+ transport inhibitor amiloride blocks taste responses to NaCl by 60-70%. The purpose of the present study was to determine if greater inhibition could be achieved with three potent amiloride analogs that are specific for the epithelial Na+ channel: phenamil, 2',4'-dimethylbenzamil, and 3',4'-dichlorobenzamil. Application of phenamil (100 microM) to the anterior tongue blocked integrated responses to NaCl from the chorda tympani nerve by 98.04%, but had no significant effect on sucrose or NH4Cl. This finding suggests that the epithelial Na+ channel alone transduces the taste of NaCl in gerbil. The residual 30-40% of the response that is not blocked by amiloride can simply be explained by the fact that amiloride is less potent than phenamil. On average, 100 microM phenamil blocked responses to Na+ salts with a variety of anions by 94.2%; 100 microM 2',4'-dimethylbenzamil, by 89.83%; and 100 microM 3',4'-dichlorobenzamil, by 72.56%. Small residual responses to salts of glutamate and phosphate were not eliminated by the amiloride analogs; this suggests that other transduction mechanisms may account for a small portion of taste responses for these salts in the gerbil.     

Amiloride-insensitive units of the chorda tympani nerve are necessary for normal ammonium chloride detectability in the rat

In spite of its common use as a standard stimulus in peripheral nerve recordings, relatively little is known about the psychophysics of NH-sub-4Cl taste. Rats' detection threshold for this salt was tested under a variety of conditions, including amiloride (100 muM) treatment and bilateral chorda tympani (CT) nerve transection. Detectability was measured with a 2-lever operant discrimination procedure used previously to measure detection thresholds for NaCl and KCl. Although NH-sub-4Cl and KCl appear to share a common taste quality and transduction mechanism, the logistic function and threshold for NH-sub-4Cl were found to be more similar to those of NaCl than to those of KCl. Like that of KCl, however, the detection threshold for NH4Cl increased significantly with CT transection (0.54 log-sub-1-sub-0 units, p < .004), but not with amiloride adulteration. This finding supports the hypothesis that the CT is necessary for normal salt detection regardless of stimulus, and suggests that amiloride does not appreciably impact responses to nonsodium salts at the behavioral level.     

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.     

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.     

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.     

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.     

Amiloride increases sodium chloride taste detection threshold in rats

The epithelial sodium-channel blocker amiloride has been shown to inhibit sodium responses in the 7th cranial nerve of the rat. In the signal detection task used in this study, amiloride (100 microM) treatment raised the NaCl threshold by approximately 1 log10 unit. The inhibition constant for amiloride was 1 microM at 0.013 M NaCl. Because the NaCl intake of adult rats has been shown to be related to the level of dietary NaCl exposure early in development, rats were exposed by way of maternal diet to 1 of 3 diets (0.1% NaCl, n = 8; 1.0% NaCl, n = 8; 3.0% NaCl, n = 9) from conception through weaning, to determine whether this treatment affects taste sensitivity. At Postnatal Day 30, rats were placed on 1.0% NaCl chow. This treatment did not affect NaCl detection or amiloride sensitivity in adulthood. The amiloride-induced shifts in NaCl sensitivity functions imply that the transcellular sodium transduction pathway is necessary for normal NaCl detection in the rat.     

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.     

Gustatory signal processing in the glossopharyngeo-hypoglossal reflex arc of the frog

The relationship between the gustatory input and motor output in the glossopharyngeo-hypoglossal reflex was analyzed on the basis of neuronal activities in the solitary tract and hypoglossal motor nuclei of bullfrogs. Concentration-response relations for NaCl, quinine and acetic acid, obtained from the glossopharyngeal (IXth) nerve and simultaneously recorded from the hypoglossal (XIIth) nerve, were expressed relative to the response of each nerve to 1 M NaCl. Compared with a relatively small amount of the afferent input for acid, the reflex motor output was much larger in the relative value. A similarly high output relation was obtained for warmed saline but not for quinine and cooled saline. Although the responsiveness of the nucleus tractus solitarius neurons to 1 M NaCl and 1 mM quinine was not significantly different from that of the hypoglossal motoneurons, responses to 10 mM acetic acid were greater in the latter neurons than in the former by a factor of about 5.2. These phenomena were consistent with those in the peripheral nerves. The solitary tract neurons responsive to NaCl, quinine and acid showed both the phasic and tonic components of discharges. According to classification by a transiency index, the discharge mode became more phasic for the hypoglossal motoneurons responsive to NaCl and quinine, but more tonic for those responsive to acid. The above-mentioned chemoreflex is thus regulated by the intrinsic neural network which sends signals to the XIIth nerve after modifying not only the amount but also the temporal pattern of gustatory nerve signals for a particular taste.     

Electrophysiological and behavioral studies on the taste of umami substances in the rat.

Electrophysiological and behavioral experiments were performed to reveal taste properties of "umami" substances such as monosodium glutamate (MSG) and disodium inosine monophosphate (IMP) in rats. To eliminate the taste effects of Na ions contained in these umami substances, we dissolved them in 0.01 mM amiloride, which is known to block sodium responses. In the electrophysiological study, taste responses of the whole chorda tympani nerve were recorded. The magnitude of responses to MSG (or IMP) at concentrations below 0.1 M (or 0.01 M) was less than 10% of that to 0.1 M NaCl. On the other hand, the mixtures of MSG and IMP showed responses 2-7 times larger than the arithmetric sum of the responses to each component of the mixtures. A new sweet taste inhibitor (Gymnema sylvestre extract) strongly suppressed neural responses to mixtures of MSG and IMP as well as sucrose, but only weakly or negligibly to individual solutions of these umami substances. In the behavioral study, the brief exposure two-bottle preference test and conditioned taste aversion paradigm were used. MSG was most preferred at 0.3 M (preference ratio = 57%), IMP, at 0.01 M (61%), and both were less preferred or rejected at higher concentrations. In contrast, mixtures of MSG and IMP were more preferred at a broad concentration range (e.g., 82% for 0.1 M MSG + 0.01 M IMP). Aversive conditioning to umami substances was generalized to sucrose, and vice versa, but not to 0.1 M NaCl, 0.01 M HCl, and 0.1 mM quinine hydrochloride.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sodium deficient rats are unmotivated by sodium chloride solutions mixed with the sodium channel blocker amiloride.

Rats were made sodium deficient by furosemide injection and then offered 20 min of access to 0.05 M NaCl mixed with the sodium channel blocker amiloride. Compared with a sodium deficient control group that was also offered 0.05 M NaCl, these rats drank very little. A subsequent test conducted in the same manner with 20 min of access to 0.3 M NaCl mixed with amiloride produced similar results. It is concluded that amiloride blocks the neural information required for generating the attractive taste of NaCl to the sodium deficient rat.     

Mechanism of action of some bitter-tasting compounds on frog taste cells.

Effects of some bitter-tasting compounds on frog taste receptors were examined by recording glossopharyngeal nerve responses. The order of effectiveness of the compounds was quinine greater than brucine greater than formanilide greater than caffeine greater than urea. When the effects of quinine, brucine and caffeine on electrical responses in taste cells were examined, they all produced a depolarization associated with an increased input resistance. The action of the three compounds on taste receptors therefore, operates with a similar mechanism. The electrical responses in cells, produced by quinine, progressed slowly with time. Such effects with quinine are similar to those with procaine. After adaptation to quinine, the nerve responses to various chemical stimuli were gradually reduced in magnitude, while the electrical responses in taste cells during stimulation by chemicals became smaller. The mechanism of the effects of bitter stimuli are discussed in light of recent findings on the interaction of bitter stimuli with lipid monolayers and the extraction of lipid from bovine taste papillae by bitter stimuli.     

Sense of taste in a new world monkey,the common marmoset: recordings from the chorda tympani and glossopharyngeal nerves

Whole nerve, as well as single fiber, responses in the chorda tympani proper (CT) and glossopharyngeal (NG) nerves of common marmosets were recorded during taste stimulation with three salts, four acids, six bitter compounds and more than 30 sweeteners. We recorded responses of 49 CT and 41 NG taste fibers. The hierarchical cluster analysis distinguished three major clusters in both CT and NG: S, Q, and H. The S(CT) fibers, 38% of all CT fibers, responded only to sweeteners. The S(CT) fibers did not respond during stimulation with salts, acids, and bitter compounds but exhibited OFF responses after citric and ascorbic acids, quinine hydrochloride (QHCl), and salts (in 80% of S(CT) fibers). S(NG) fibers, 50% of all NG fibers, also responded to sweeteners but not to stimuli of other taste qualities (except for citric acid, which stimulated 70% of the S(NG) fibers). Some sweeteners, including natural (the sweet proteins brazzein, monellin) and artificial [cyclamate, neohesperidin dihydrochalcone (NHDHC), N-3,5-dichlorophenyl-N'-(S)-alpha-methylbenzylguanidineacetate (DMGA), N-4-cyanophenylcarbamoyl-(R,S)-3-amino-3-(3,4-methylenedioxyphenyl) propionic acid (CAMPA)] did not elicit responses in the S fibers. In general, the response profiles of the S(CT) and S(NG) clusters were very similar, the correlation coefficient between the responses to sweeteners in these clusters was 0.94. Both the Q(CT) and the Q(NG) fibers (40 and 46% of all fibers) were predominantly responsive to bitter compounds, although their responses to the same set of bitter compounds were quite different. Sweeteners with sweet/bitter taste for humans also stimulated the Q clusters. The H clusters (22 and 3% of all fibers) were predominantly responsive to acids and did not respond to stimuli of other taste qualities. However, bitter stimuli, mainly QHCl, inhibited activity in 70% of H(CT) fibers. Among a total of 90 fibers from both nerves there was only 1 NaCl-best fiber in CT. We found, however, that 35% of the CT fibers reacted to salts with inhibition of activity during stimulation, followed by an OFF response. This OFF response was diminished or eliminated by amiloride. These characteristics indicate that amiloride-sensitive sodium channels are involved in salt transduction in marmosets. In the two NG fibers responding to NaCl, we recorded neither suppression by amiloride nor OFF responses. Comparison of marmoset data with those of other nonhuman primates studied, rhesus and chimpanzee, demonstrates phylogenetic trends in the organization of taste system. This can help to uncover pathways of primate evolution.     

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.