Responses of gustatory cells in the nucleus of the solitary tract of the hamster after NaCl or amiloride adaptation.

1. The responses of single nucleus of the solitary tract (NST) neurons in the hamster were recorded to an array of Na+ and non-Na+ stimuli under each of three adaptation conditions: distilled H2O, 0.032 M NaCl, and 10 microM amiloride. Each adapting solution flowed for 60 s before delivery of one of seven test stimuli: 0.032 M NaCl, NaNO3, and Na-gluconate, 0.1 M KCl and sucrose, 1 mM HCl, and 3 mM quinine hydrochloride (QHCl). Stimuli were dissolved in distilled H2O (H2O and NaCl adaptation conditions) or 10 microM amiloride (amiloride adaptation condition). 2. Both amiloride treatment and NaCl adaptation reduced responses to the Na+ stimuli. The effects of NaCl adaptation were generally greater than those of amiloride, and the responses to the Na+ salts were reduced by NaCl adaptation in every cell that responded to NaCl, regardless of its best-stimulus classification. Amiloride treatment suppressed the responses to Na+ salts with larger anions (NaNO3 and Na-gluconate) more than the response to NaCl. 3. Unlike amiloride treatment, NaCl adaptation also reduced responses to several non-Na+ stimuli (KCl, HCl, and QHCl). This effect occurred primarily in the NaCl-best neurons that were most highly responsive to NaCl and that showed a postexcitatory suppression after NaCl. This suppression has been observed in recordings from the chorda tympani nerve in both rats and hamsters and in taste receptor cell responses recorded in situ in the rat. If it is a receptor phenomenon, these data would imply that some NaCl-sensitive receptor cells are also responsive to these non-Na+ electrolytes. 4. The effects of amiloride on the responses to Na+ stimuli were not limited to NaCl-best neurons, but occurred in sucrose-best cells as well. These results suggest that the sucrose-best cells in the NST receive converging input from sucrose- and NaCl-best chorda tympani fibers, because there is little Na+ sensitivity in the peripheral sucrose-best fibers and the amiloride sensitivity is restricted to NaCl-best chorda tympani fibers. The responses to NaCl in the few HCl- and QHCl-best NST neurons were not affected by amiloride. 5. Rinsing the tongue with amiloride for 60 s resulted in a reduction in the baseline response rate of NST cells. This effect occurred primarily in NaCl- and sucrose-best NST neurons and implies that much of the spontaneous activity in these brain stem cells arises from amiloride-sensitive channel activity in the peripheral receptor cells. 6. The results of human psychophysical studies show very different effects of NaCl adaptation and amiloride treatment. Adaptation to NaCl produces a robust and specific reduction in the saltiness of all salts. The present results show that NaCl adaptation reduces the responses of all cells sensitive to NaCl. Treatment of the human tongue with amiloride produces a proportionately smaller reduction in the response to NaCl than it does in rodents, and it appears to have no effect on saltiness. Rather, amiloride has been shown to specifically reduce the sour side taste of NaCl, Nagluconate, and LiCl. Therefore conclusions about the effects of amiloride on taste quality based on rodent electrophysiology are questionable.

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)

Responses of single hamster parabrachial neurons to binary taste mixtures of NaCl with sucrose or QHCl.

1. Although human psychophysical responses to taste mixtures have been investigated extensively, there have been few reports on the neurophysiological coding of taste mixtures in the mammalian gustatory system. In recent studies we have investigated the responses of single third-order neurons in the hamster parabrachial nucleus (PbN) to anterior tongue stimulation with binary mixtures of heterogenous taste stimuli including sucrose+QHCl, sucrose+citric acid, and NaCl+citric acid. Some of these stimulus combinations evoked mixture suppression, or response frequencies that were less than that evoked by the more effective component (MEC) presented alone, which is analogous to the mixture suppression reported in human psychophysical studies of similar taste mixtures. In the current report we extend our investigation to include NaCl+QHCl and NaCl+sucrose mixtures. 2. The action potentials of single PbN neurons were recorded extracellularly. Four concentrations of each stimulus were employed: NaCl and sucrose at 0.001, 0.01, 0.1, and 1.0 M; QHCl at 0.00032, 0.0032, 0.032, and 0.1 M. All stimuli were tested alone and in mixture; the NaCl+sucrose and NaCl+QHCl mixtures were formed by pairing the four concentrations of each stimulus with the strongest concentration of the other stimulus. 3. For both NaCl+sucrose and NaCl+QHCl mixtures, the response frequencies evoked by the mixtures did not differ from those evoked by the MEC presented alone, whether averaged across all neurons or across subgroups of NaCl- or sucrose-best cells. Furthermore, the across-neuron patterns (ANPs) of mixture responses were similar to those of the MECs.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of single hamster parabrachial neurons to binary taste mixtures of citric acid with sucrose or NaCl.

1. Although taste experience generally arises from a mixture of gustatory stimuli, most neurophysiological studies of the mammalian central gustatory system have focused on responses to single chemical stimuli. Recently, in a study of single third-order neurons in the hamster parabrachial nucleus (PbN), we reported that mixture suppression occurs in the responses to binary mixtures of sucrose and QHCl presented to the anterior tongue. Mixture suppression was reflected both in reduced response frequencies and in an altered pattern of responses across neurons. In the current report we extend our investigation of CNS neuron responses to binary mixtures of heterogeneous stimuli to include sucrose+citric acid mixtures and NaCl+citric acid mixtures. The response to each mixture was compared with the response to the more effective component (MEC) presented alone, and those that differed by more than a selected criterion (based on response variability) were identified. 2. For all mixture responses recorded, 29% (79/256) involved mixture suppression (mixture response < MEC response), only 6% (18/276) were greater than the response to MEC, and 65% (179/276) did not differ from the response to the MEC. 3. In Experiments 1 and 2, neurons were tested with four concentrations of sucrose or citric acid each presented alone and in binary mixtures with a single strong concentration of the other stimulus. Sucrose suppression (mixture response < sucrose response) occurred in 24% of mixture responses and was exhibited almost exclusively by sucrose-best neurons, primarily to the mixtures that contained the stronger sucrose and citric acid concentrations. Sucrose suppression involved a 40% reduction of mixture response frequencies compared with responses to the sucrose component alone. 4. In Experiments 3 and 4, neurons were tested with four concentrations of NaCl or citric acid each presented alone and in binary mixtures with a single strong concentration of the other stimulus. NaCl suppression (mixture response < NaCl response) occurred in 21% of mixture responses and was displayed by both sucrose-best and NaCl-best neurons. NaCl suppression involved a 28% reduction in mixture response frequencies compared with responses to the NaCl component alone. In all experiments citric acid suppression (mixture response < citric acid response) was observed in only 6% of mixture responses and was relatively small in magnitude. 5. The across-neuron patterns (ANPs) of taste responses, which are correlated with behavioral measures of taste similarity, were compared for mixtures and components.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of single hamster parabrachial neurons to binary taste mixtures: mutual suppression between sucrose and QHCl.

1. Although taste experience typically arises from a mixture of gustatory stimuli, nearly all previous neurophysiological studies of the mammalian central gustatory system have focused on responses to single chemical stimuli. To begin to systematically examine CNS responses to taste mixtures, we recorded the extracellular activity of single third-order neurons in the hamster PbN to anterior tongue stimulation with binary mixtures of sucrose and QHCl. In experiment 1, neurons were tested with four concentrations of sucrose (0.001, 0.01, and 1.0 M) presented alone and mixed with 0.1 M QHCl. In experiment 2, neurons were tested with four concentrations of QHCl (0.00032, 0.0032, 0.032, and 0.1 M) presented alone and mixed with 1.0 M sucrose. 2. The response to each binary mixture was compared with the response to the more effective component (MEC) presented alone, and those that differed by more than a selected criterion (based on response variability) were identified. Of all mixture responses, 37% (59/158) involved mixture suppression (mixture response < MEC response), only 4% (6/158) were greater than the MEC, and 59% (94/158) were classified as not different than the response to the MEC. Most neurons that displayed mixture suppression did so at several mixture concentrations. 3. Sucrose suppression (mixture response < sucrose response) was prevalent among neurons most responsive to sucrose and for the mixtures that contained the stronger sucrose concentrations. Among neurons that displayed sucrose suppression, the magnitude of suppression was significantly correlated with sucrose response magnitude but not with QHCl response magnitude. These and other factors suggest that a neuron's capacity to display sucrose suppression to sucrose+QHCl mixtures is related to its sucrose sensitivity. 4. QHCl suppression (mixture response < QHCl response) was less prevalent than sucrose suppression, and the neurons that displayed QHCl suppression were almost exclusively a subset of those that displayed sucrose suppression to the same or different mixtures. This finding and the observation that one-third of all mixture responses involved mutual suppression (response to the mixture less than that to either component alone), suggest an association between the factors underlying sucrose suppression and QHCl suppression. 5. The across-neuron patterns (ANPs) of taste responses, which are thought to represent taste quality, were compared for mixtures and components. In general, the ANP for each mixture was similar to (significantly correlated with) the ANP of the more stimulatory component. However, for the mixture that evoked the greatest sucrose suppression, the mixture ANP was more similar to the ANP of the less stimulatory component.(ABSTRACT TRUNCATED AT 400 WORDS)

Enduring alterations in neurophysiological taste responses after early dietary sodium deprivation.

1. Neurophysiological taste responses from neurons in the nucleus of the solitary tract (NST) were studied in four groups of rats during chemical stimulation of the tongue with sodium and non-sodium salts, citric acid, and sucrose. The four groups of rats consisted of those fed a NaCl-deficient diet (0.03% NaCl) from day 3 postconception to at least day 50 postnatal (deprived rats), rats initially fed the NaCl-deficient diet during development and then placed on a NaCl-replete diet at adulthood for > or = 5 wk (control-deprived rats), and rats always fed the NaCl-replete diet (control rats). 2. Compared with controls, dietary NaCl deprivation instituted early in development resulted in highly attenuated average response frequencies to sodium salts (as much as 50%) but not to nonsodium salts and nonsalt stimuli. Concomitantly, most NST neurons in deprived rats responded "best" to NH4Cl and few responded best to NaCl. This is in contrast to that observed in controls, where the same proportion of neurons responded best to NaCl and best to NH4Cl. 3. Taste responses in recovered rats exhibited a hyperresponsiveness to many sodium salts compared with controls. That is, sodium salts elicited average response frequencies significantly greater (as much as 100%) than that obtained in controls. The proportions of neurons responding best to NaCl or to NH4Cl were opposite to that in deprived rats. In recovered rats, the proportion of neurons that responded best to NaCl was much greater than that which responded best to NH4Cl. 4. Rats deprived of dietary NaCl only as adults responded like controls. Therefore the environmental manipulations must occur during early periods of development. 5. These findings show that early dietary manipulations of sodium and subsequent replacement of dietary sodium have neurophysiological effects relatively selective for sodium-elicited taste responses. Furthermore, because recordings in recovered rats were obtained > or = 5 wk after feeding the NaCl-replete diet, it appears as though early NaCl deprivation permanently alters the functional organization of the NTS. Although it is likely that alterations in peripheral neural activity play a role in the functional development of NTS neurons responsive to taste stimuli, other non-activity-related factors may also be important.

Convergence in mammalian nucleus of solitary tract during development and functional differentiation of salt taste circuits.

To determine the type and extent of neural rearrangements that are made during functional differentiation of circuits for salt taste processing, we determined receptive field size and salt response characteristics of second-order taste cells in 3 age groups of sheep. Neurophysiological recordings were made from single cells in the nucleus of the solitary tract (NST) in fetal, perinatal, and postnatal sheep. Responses to NH4Cl, NaCl, and KCl were measured, and location and number of fungiform papillae in the receptive field were determined by stimulating individual papillae with anodal electrical current. The data are compared with previous, parallel measures from chorda tympani nerve afferent taste fibers to permit conclusions about convergence or divergence onto second-order cells. Receptive field size of second-order taste neurons increases during development, in contrast to the decrease in field size observed previously for chorda tympani nerve fibers during the same period. Furthermore, receptive fields of second-order cells are significantly larger than those of first-order fibers at perinatal and lamb ages, but not fetal. Thus, there is convergence of first-order taste afferents onto brain-stem neurons, and the convergence increases remarkably between fetal and perinatal periods. Associated with the increase in convergence are increased salt response frequencies relative to afferent fibers for NaCl in perinatal animals and lambs, and for KCl in lambs. The increase in frequencies occurs before NST neurons are functionally mature, as indicated by the rapid response adaptation of many cells in young animals. Convergence in NST during development apparently functions to maximize gain for processing neural responses to NaCl. In the periphery, response frequencies to NaCl are very low in fetuses, and increase progressively during development. In the NST, NaCl response frequencies are high even in fetuses, and remain high. The process of convergence onto second-order cells is accomplished with maintenance of order in afferent projections because receptive fields of NST neurons are composed of fungiform papillae that are clustered together, not dispersed over the tongue. Our quantification of taste receptive field size at 2 neural levels provides strong evidence for increasing convergence in the NST during development. Altering patterns of afferent neural input and geometry of second-order neurons may have a role in establishing convergence. The convergence has an apparently special function: to increase gain for NaCl taste sensation. Therefore, neural rearrangements during differentiation of salt taste pathways result in specific functional outcomes.

Neonatal hyperthyroidism in the rat: thyroxine accelerates the development of unconditioned but not learned responses to tastes.

The effects of neonatal hyperthyroidism induced by thyroxine injection on the development of unconditioned and learned behaviors mediated by the gustatory system of the rat were investigated. The development of unconditioned ingestive responses evoked by 10% sucrose and 0.1% HCl taste solutions was advanced several days by thyroxine treatment. However, the emergence of taste aversion learning involving 10% sucrose and LiCl injection was not advanced and may have been slightly delayed. Thus, the ontogenesis of unconditioned and learned behaviors mediated by the gustatory system was not influenced uniformly by neonatal thyroxine treatment.

Ontogenesis of learning: IV. Dissociation of memory and perceptual-altering processes mediating taste neophobia in the rat.

The rat's neophobic response to tastes can be conceptualized as depending on two processes: memory processes that store a representation of an experienced taste so that it can be recognized as familiar on subsequent encounters, and perceptual-altering processes that respond differentially to novel versus familiar tastes. We investigated the ontogeny of these processes by studying the preweanling rat's behavioral reaction to a 10% sucrose solution. Experiments I and II suggest that the memory processes mature earlier (by 7-8 days of age) than the perceptual-altering processes (about 11 days of age). Experiment III suggests that pups do not acquire an aversion to sucrose paired with illness until they are 12-days-old, implying a close correspondence between the maturation of processes mediating neophobia and taste aversion learning. These findings are consistent with our previous work (Vogt & Rudy, 1984), and our view that ontogenetic dissociations in taste-guided behavior reflect caudal-to-rostral maturation in the ascending gustatory system.

Ontogenesis of learning: I. Variation in the rat's reflexive and learned responses to gustatory stimulation.

These experiments indicate that by Day 15 after birth, the processes that mediate a number of taste-controlled behaviors in the rat are functional. These include the sensory processes necessary to detect and respond reflexively to sucrose, the event-learning processes that reduce the rat's neophobic reaction to sucrose, and the integrative-learning processes that enable it to learn an aversion to sucrose when paired with lithium toxicosis, even when these events are separated by 1 hr. These capacities, however, did not emerge simultaneously. Those necessary to detect and respond reflexively to sucrose emerged prior to those that contribute to the learned control of taste-guided behaviors. It is argued that these age-related dissociations in behavioral capacities reflect a caudal-to-rostral maturational sequence of components of the ascending gustatory system that are thought to underlie these capacities.