Sex differences in salt preference and taste reactivity in rats

Previous studies have shown that female rats consume significantly more sodium chloride (NaCl) than do age-matched males. The gustatory contribution to this sex difference was examined in the following experiments. In Experiment 1, female rats demonstrated a higher two-bottle preference for NaCl ranging from 0.03 M to 1.0 M than did age-matched males. Next, to determine if the animal's sex modified gustatory sensitivity for NaCl, taste reactivity responses elicited by intraoral infusions (0.8 ml) of NaCl (0.03 M, 0.15 M, 0.3 M, and 1.0 M) were measured in age-matched male and female Sprague-Dawley rats. Intraoral infusions of NaCl were administered in ascending concentration order on successive days. During the intraoral infusion, the animal's oral motor taste reactivity responses were videotaped and subsequently analyzed to determine the number of ingestive and aversive responses. Intraoral infusions of 0.15 M and 0.3 M NaCl elicited reliably more ingestive responses and 1.0 M NaCl more aversive responses in females than in males. Because differences in taste reactivity were not found for all those concentrations for which female rats showed a higher preference than did males, changes in gustatory sensitivity contributes to, but does not appear to fully account for the female rats' preference for NaCl.     

Responses of the hamster chorda tympani nerve to binary component taste stimuli: evidence for peripheral gustatory mixture interactions

Studies of taste mixtures suggest that stimuli which elicit different perceptual taste qualities physiologically interact in the gustatory system and thus. are not independently processed. The present study addressed the role of the peripheral gustatory system in these physiological interactions by measuring the effects of three heterogeneous taste mixtures on responses of the chorda tympani (CT) nerve in the hamster ( Mesocricetus auratus). Binary taste stimuli were presented to the anterior tongue and multi-fiber neural responses were recorded from the whole CT. Stimuli consisted of a concentration series of quinine. HCI (QHCI: 1–30 mM), sodium chloride (NaCl: 10–250 mM). sucrose (50–500 mM) and binary combinations of the three different chemicals. Each mixture produced a unique pattern of results on CT response magnitudes measured 10 s into the response. Sucrose responses were inhibited by quinine in QHCI-sucrose mixtures. Neural activity did not increase when quinine was added to 50–250 mM NaCl in QHCINaCl mixtures. However, the neural activity elicited by sucrose-NaCl mixtures was greater than the activity elicited by either component stimulus presented alone. The results demonstrate that gustatory mixture interactions are initiated at the level of the taste bud or peripheral nerve. Mechanisms for these interactions are unknown. The results are consistent with one component stimulus modifying the interaction of the other component stimulus with its respective transduction mechanism. Alternatively, peripheral inhibitory mechanisms may come into play when appetitive and aversive stimuli are simultaneously presented to the taste receptors.     

The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats

One or two bottle preference tests, i.e., relative fluid consumption, constitute the primary methodology for determining acceptance or rejection of tastes in animals other than humans. These tests require organisms to initiate and maintain drinking behavior and, therefore, can not be applied to preparations which do not eat or drink spontaneously. The taste reactivity test, a new method for assessing responses to gustatory stimuli, circumvents this shortcoming. A 50 μl taste stimulus is injected directly into the oral cavity of a freely moving rat and the immediate response videotaped for frame by frame analysis. Each of the sapid stimuli used (4 concentrations of sucrose, NaCl, HCl, and quinine HCl) generated a stereotyped response derived from 3 lexicon of 4 mimetic (movements of lingual, masticatory, and facial musculature) and 5 body response components. Responses to taste stimuli were highly consistent within and between rats. For example, sapid sucrose, NaCl and HCl stimuli elicited a response sequence beginning with low amplitude, rhythmic mouth movements, followed by rhythmic tongue protrusions, and then lateral tongue movements. No body movements accompanied these mimetic responses. In contrast, quinine in concentrations at and above 3 × 10⁻⁵ M (1/2 log step above the absolute behabioral threshold for quinine) elicited a response pattern beginning with gaping and proceeding through as many as 5 body responses. These normative data for the intact rat can be directly compared to the taste reactivity of neurally ablated preparations which do not spontaneously feed or drink. Such comparisons can be utilized in determining the neural substrates necessary for the execution and regulation of ingestive behavior.     

A comparison of voluntary salt-intake behavior in Nax-gene deficient and wild-type mice with reference to peripheral taste inputs

The Na(x) channel, a subfamily of voltage-gated sodium channels, is thought to be a specific sodium receptor in the central nervous system. Our previous study revealed that Na(x)-gene-deficient mice consumed excessive amounts of NaCl even under water-deprived conditions. In the present study, to investigate whether the peripheral taste inputs are involved in the abnormal intake of salt in Na(x)-deficient mice (homo), voluntary intake of various taste solutions in homo and wild-type mice (wild) was examined under non-deprived conditions. Homo showed a higher preference for 0.15 M NaCl solution than wild. Preference ratios for other basic tastants were identical between groups. Transection of the chorda tympani (CT) or the glossopharyngeal (GP) nerve had little effect on salt-intake behavior in homo and wild. Although combined transection of the superior laryngeal (SL) and GP nerves decreased NaCl intake in homo but not in wild, there were no differences in preference ratios for NaCl in homo before and after SL+GP transection. On the other hand, preference ratios for NaCl in wild tended to increase after combined SL and GP transection. Consequently, preference ratios for NaCl after SL+GP transection were no different between homo and wild. While electrophysiological responses of the CT and the GP to various taste solutions were indistinguishable between homo and wild, those of the SL to NaCl in homo were smaller than those in wild only at lower concentrations (0.01 and 0.03 M). Thus, chemosensory inputs from the oro-pharyngeal regions had little effect on abnormal salt intake in homo, if any. From these results, it is suggested that the higher preference for NaCl in homo is mainly due to the lack of Na(x) channels in the central nervous system.     

Response properties of fibers in the hamster superior laryngeal nerve

The purpose of the present investigation was to record electrophysiological responses from single fibers in the hamster superior laryngeal nerve (SLN) that were responsive to chemical stimulation of the larynx. Twenty chemical solutions, commonly used in studies of mammalian gustatory physiology, were applied to taste buds on and around the epiglottis. These stimuli were dissolved in physiological saline. Responses were the number of impulses elicited over a 15-s period following stimulus onset, above or below the background activity elicited by a previous rinse with saline. Unlike fibers in the hamster chorda tympani or glossopharyngeal nerves, SLN units were not easily classifiable into response types. Excitatory stimuli were primarily acids and bitter-tasting stimuli, with the order of their effectiveness being urea tartaric acid > HCl > KCl > citric acid > caffeine > quinine hydrochloride > acetic acid. The sweet-tasting stimuli and most salts other than KCl were primarily inhibitory, with the order of inhibitory effectiveness being CaCl₂ > sucrose > fructose > LiCl > NaNO₃ > Li₂SO₄ > NaCl. A hierarchical cluster analysis of fibers yielded no distinct clusters, yet differing sensitivities across the fibers were suggested. SLN fibers are highly responsive to sour and bitter stimuli, although they are not sensitive to fine differences in taste quality, as are fibers in other gustatory nerves.     

Cardiovascular responses to gustatory and mechanical stimulation of the nasopharynx in rats

The effects of natural (mechanical and gustatory) stimulation of the nasopharynx or electrical stimulation of the pharyngeal branch of the glossopharyngeal (PH-IXth) nerve on the changes in heart rate (HR) and arterial blood pressure (BP) were investigated in paralyzed and anesthetized rats. Afferent responses in the PH-IXth nerve were also investigated. Electrical stimulation of the PH-IXth nerve elicited a tachycardia and an increase in BP. Among the gustatory (1.0 M NaCl, 0.03 M HCl, 0.03 M QHCl, 1.0 M sucrose, H₂O, and 0.9% NaCl) and mechanical stimuli applied to the nasopharynx, 1.0 M sucrose and 0.9% NaCl were ineffective in changing HR and BP; the rest of the stimuli were strongly effective as was the case with electrical stimulation of the PH-IXth nerve. Responses were evoked in the PH-IXth nerve by nasopharyngeal stimulation with the stimuli which were effective in producing cardiovascular responses. On the other hand, 1.0 M sucrose and 0.9% NaCl, which were ineffective stimuli for cardiovascular responses, did not produce any response in the PH-IXth nerve. There was a high correlation between the magnitude of the responses in the PH-IXth nerve and those of the cardiovascular system. These results indicate that gustatory and mechanical information carried in the PH-IXth nerve innervating the nasopharynx plays an important role in cardiovascular regulation as well as the sense of taste.     

The effects of glossopharyngeal and chorda tympani nerve cuts on the ingestion and rejection of sapid stimuli: an electromyographic analysis in the rat

The present study tested the effects of bilateral section of either the chorda tympani or glossopharyngeal nerves on the production of oro-pharyngeal electromyographic (EMG) responses to intra-oral sapid stimulation. The responses of adult rats fitted with intra-oral cannulas and fine-wire electrodes in the anterior digastric (jaw opening) and thyropharyngeus (swallowing) muscles were examined following direct oral stimulation with water and 5 concentrations of sucrose, NaCl, and quinine monohydrochloride (QHCl). One group of rats was tested both before and after bilateral removal of the chorda tympani. A second group of rats was tested subsequent to bilateral removal of the glossopharyngeal nerves. A normal EMG response pattern to suprathreshold QHCl consisted of several intra-oral licks followed by a series of large amplitude mouth openings (gapes). In addition, there was a longer latency to the first swallow following QHCl stimulation compared to water stimulation. Cutting either nerve affected this rejection response to QHCl, but produced little change in the ingestive response to the other stimuli. Following chorda tympani nerve cuts, rats showed an increased latency to the first gape and a small reduction in the number of gapes across the 5 concentrations of QHCl (16%). In contrast, bilateral section of the glossopharyngeal nerves produced a much larger reduction in the number of gapes (54%), but had no effect on the latency to the first gape. In addition, the latency to swallow suprathreshold QHCl was shorter following glossopharyngeal nerve cuts. These observations suggest that gustatory receptors on the anterior tongue, innervated by the chorda tympani, initiate a rejection response, but that receptors on the posterior tongue, innervated by the glossopharyngeal nerve, are necessary for a sustained rejection sequence.     

Comparing immune activation (lipopolysaccharide) and toxin (lithium chloride)-induced gustatory conditioning: lipopolysaccharide produces conditioned taste avoidance but not aversion

Feeding and drinking typically involve both appetitive and consummatory behaviors. Appetitive behaviors include those behaviors produced by an animal prior to the actual consumption, such as approach movements, whereas consummatory behaviors (such as licking and chewing) are involved in the actual consumption of food. The present research compared the gustatory conditioning effects of bacterial lipopolysaccharide (LPS) and lithium chloride (LiCl) in two different paradigms, conditioned taste avoidance and conditioned taste aversion which differentially affect the appetitive and consummatory components of feeding. Male rats were implanted with intraoral cannulae and habituated to a water deprivation schedule and afterwards received two conditioning days (Days 1 and 4). Each conditioning day consisted of 1 h access to a novel sucrose solution (0.3 M) immediately followed by a systemic injection of LPS (200 microg/kg), LiCl (0.15 M, 3 meq) or NaCl vehicle. Conditioned taste aversion was assessed using the taste reactivity test on Day 7, where orofacial and somatic responses were videotaped and analyzed during 3 brief (1 min) exposures to the sucrose solution. Conditioned taste avoidance was assessed on Days 8 and 9 using a two-bottle preference test (sucrose versus water). Animals conditioned with LiCl displayed typical aversive-like responses in the taste reactivity paradigm evidenced by significant reductions in positive ingestive responses (P<0.05) and an increase in active aversive responses (P<0.05) relative to controls. Furthermore, LiCl treatment resulted in conditioned avoidance of sucrose in the two-bottle preference test characterized by a decreased sucrose preference (P<0.05). Conditioning with LPS produced a reduced sucrose preference (P<0.05) relative to controls, comparable to the avoidance seen in LiCl-treated rats. In contrast, conditioning with LPS resulted in similar positive ingestive responses to intraorally infused sucrose as seen in controls. The present results demonstrate that LPS treatment produces conditioned avoidance but not aversion and suggest that LPS can selectively condition the appetitive aspects of feeding whereas the consummatory behaviors remain unaffected.     

Effect of salivation on neural taste responses in freely moving rats: analyses of salivary secretion and taste responses of the chorda tympani nerve

The outer surface of the mammalian taste receptor cell is usually covered with saliva, which may affect the initial process of gustation. To ascertain the interaction between salivation and gustation, salivary secretion from the submandibular and parotid glands and taste responses of the chorda tympani nerve were analyzed in the rat, during grooming, eating, and licking of the four standard taste stimuli (sucrose, NaCl, HCl, and quinine hydrochloride). Regions of the tongue surface bathed by saliva secreted from the each gland were examined, and it was found that: (1) Rats frequently groomed, and the anterior part of the tongue, innervated by the chorda tympani nerve, was usually covered with a mixture of submandibular saliva and substances on the body surface. (2) Licking of acceptable sucrose and NaCl solutions elicited initial phasic and long-lasting tonic taste responses, and did not evoked saliva enough to wash away the stimuli from the oral cavity. Licking of rejectable quinine evoked only a small phasic taste response and was followed by taste rejection behavior, accompanied by maximum salivation which could wash out the stimuli. (3) When taste responses were compared under awake and anesthetized (the tongue adapted to water) condition, sucrose response was larger, while responses to other taste stimuli were smaller under the awake condition. Rise time of the phasic NaCl response was longer under the awake condition. These taste response alterations may reflect the effects of prolonged adaptation of the tongue to the mixture of submandibular saliva and body surface substances, and flow rate of licked taste stimuli on the tongue surface.     

Regulation of bitter taste responses by tumor necrosis factor

Inflammatory cytokines are important regulators of metabolism and food intake. Over production of inflammatory cytokines during bacterial and viral infections leads to anorexia and reduced food intake. However, it remains unclear whether any inflammatory cytokines are involved in the regulation of taste reception, the sensory mechanism governing food intake. Previously, we showed that tumor necrosis factor (TNF), a potent proinflammatory cytokine, is preferentially expressed in a subset of taste bud cells. The level of TNF in taste cells can be further induced by inflammatory stimuli. To investigate whether TNF plays a role in regulating taste responses, in this study, we performed taste behavioral tests and gustatory nerve recordings in TNF knockout mice. Behavioral tests showed that TNF-deficient mice are significantly less sensitive to the bitter compound quinine than wild-type mice, while their responses to sweet, umami, salty, and sour compounds are comparable to those of wild-type controls. Furthermore, nerve recording experiments showed that the chorda tympani nerve in TNF knockout mice is much less responsive to bitter compounds than that in wild-type mice. Chorda tympani nerve responses to sweet, umami, salty, and sour compounds are similar between TNF knockout and wild-type mice, consistent with the results from behavioral tests. We further showed that taste bud cells express the two known TNF receptors TNFR1 and TNFR2 and, therefore, are potential targets of TNF. Together, our results suggest that TNF signaling preferentially modulates bitter taste responses. This mechanism may contribute to taste dysfunction, particularly taste distortion, associated with infections and some chronic inflammatory diseases.     

The effects of lipopolysaccharide and lithium chloride on the ingestion of a bitter-sweet taste: comparing intake and palatability

Activation of the immune system with lipopolysaccharide (LPS) has been shown to result in decreased consumption of normally preferred substances while at the same time not affecting palatability. The present study examined the effects LPS administration on both intake and palatability of a relatively unpalatable bitter-sweet taste. Bitter is thought to signal a danger cue to an animal representing a potential toxin-containing food. Using a one-bottle consumption test, voluntary intake of a sucrose-quinine (0.15 M sucrose + 0.00015 M quinine; S-Q) solution was assessed in rats on two conditioning days (days 1 and 4) after a systemic injection with LPS, LiCl, or NaCl. On the test day (day 7), rats were given 1h access to the same solution in the absence of any injection. In a separate experiment, rats fitted with intraoral cannulae received similar testing schedules, however, the solution was delivered intraorally, activating only the consummatory responses of the animal. During conditioning, rats received 5 brief (1 min) intraoral infusions of the taste across a 1h period following injections of LPS, LiCl or NaCl. Individual taste reactivity responses were recorded and analyzed. Both LPS and LiCl resulted in decreased consumption of the unpalatable taste relative to controls on the test day, suggesting typical conditioned taste avoidance. When the consummatory responses were examined, LPS-treatment produced an increase in active oral rejection relative to NaCl- and LiCl-treated groups on both conditioning days. The present study demonstrates that although both LPS- and LiCl-treatment result in similar conditioned avoidance using an intake measure, they do not elicit similar patterns of taste reactivity responding to intraoral infusions of the bitter-sweet taste. Furthermore, the present results suggest that immune activation with LPS-treatment results in increased rejection of a mildly aversive stimulus and supports the hypothesis that reorganization of behavioral priorities occurs during bacteria-induced sickness.     

Hypoglossal motor nerve activity elicited by taste and thermal stimuli applied to the tongue in rats

Single unit activity of hypoglossal motor nerve fibers which innervate the tongue muscles was recorded in lightly anesthetized non-decerebrate and acute decerebrate rats. The pattern of responses to taste and thermal stimuli applied to the tongue surface was classified into 4 types. The type 1 response is characterized by short-lasting rhythmic burst discharges, the type 2 consists of both the rhythmic burst and tonic discharges, the type 3 is long-lasting tonic discharges and the type 4 shows short-lasting burst or short-lasting tonic discharges. In non-decerebrate rats, most of the fibers (93%) showed no or a few spontaneous firings. Sucrose and NaCl were the most effective stimulants, and 70–80% of the fibers showed the type 1 response to these stimuli. Calculating the correlations between response patterns of the fiber to a pair of the stimuli, sucrose and NaCl, and HCl and quinine produced a similar response profile, respectively. In decerebrate rats, however, about 21% of fibers showed a highly regular spontaneous firing (about 30 Hz). Rhythmic burst responses (types 1 and 2) were not induced, and thermal (especially cold) stimulation elicited much better responses than the taste stimuli. HCl and quinine, but not sucrose and NaCl, produced a similar response profile. These characteristic properties of the response in acute decerebrate rats may in part be attributed to inactivation of a ‘rhythmic center’ in the brain stem.     

Effects of electrical stimulation of the glossopharyngeal nerve on cells in the nucleus of the solitary tract of the rat

Electrophysiological responses to electrical stimulation of the lingual branch of the glossopharyngeal (GP) nerve (which innervates taste buds on the caudal 1/3 of the tongue) were recorded from single cells in the rostral nucleus of the solitary tract (NTS) of anesthetized rats. Electrical stimulation was delivered as single pulses (n=55), paired-pulses (n=15) and tetanic trains (n=11). NTS cells with GP-evoked responses were also tested for responsivity to taste stimuli (0.1 M NaCl, 0.5 M sucrose, 0.01 M HCl and 0.01 M quinine HCl). Fifty-five neurons were studied: 49 cells showed GP-evoked (mean latency+/-SEM=18.0+/-1.32 ms); seven of these were taste-responsive. Spontaneous rate of these cells was low (mean+/-SEM=1.4+/-0.3 spikes per second; median=0.21 spikes per second) and many cells showed no spontaneous activity. Paired-pulse stimulation of the GP nerve in 13 rats produced both paired-pulse suppression (n=11) and paired-pulse enhancement (n=4); tetanic stimulation (25 Hz, 1.0 s) produced sustained (>20 s) increases or decreases in firing rate in 7 of 11 cells tested. Histological data suggested that GP-evoked responses recorded in the most rostral NTS were likely the result of polysynaptic connections. Cells with GP-evoked responses formed a heterogeneous group in terms of their response properties and differed from cells with evoked responses to chorda tympani (CT; which innervates taste buds on the rostral 1/3 of the tongue) nerve stimulation. These differences may reflect the respective functional specializations of the GP and CT nerves.     

Glossopharyngeal nerve transection eliminates quinine-stimulated fos-like immunoreactivity in the nucleus of the solitary tract: implications for a functional topography of gustatory nerve input in rats.

The relationship between specific gustatory nerve activity and central patterns of taste-evoked neuronal activation is poorly understood. To address this issue within the first central synaptic relay in the gustatory system, we examined the distribution of neurons in the nucleus of the solitary tract (NST) activated by the intraoral infusion of quinine using Fos immunohistochemistry in rats with bilateral transection of the chorda tympani (CTX), bilateral transection of the glossopharyngeal nerve (GLX), or combined neurotomy (DBLX). Compared with nonstimulated and water-stimulated controls, quinine evoked significantly more Fos-like-immunoreactive (FLI) neurons across the rostrocaudal extent of the gustatory NST (gNST), especially within its dorsomedial portion (subfield 5). Although the somatosensory aspects of fluid stimulation contributed to the observed increase in FLI neurons, the elevated number and spatial distribution of FLI neurons in response to quinine were remarkably distinguishable from those in response to water. GLX and DBLX produced a dramatic attenuation of quinine-evoked FLI neurons and a shift in their spatial distribution such that their number and pattern were indiscernable from those observed in water-stimulated controls. Although CTX had no effect on the number of quinine-evoked FLI neurons within subfield 5 at intermediate levels of the gNST, it produced intermediate effects elsewhere; yet, the spatial distribution of the quinine-evoked FLI neurons was not altered by CTX. These findings suggest that the GL provides input to all FLI neurons responsive to quinine, however, some degree of convergence with CT input apparently occurs in this subpopulation of neurons. Although the role of these FLI neurons in taste-guided behavioral responses to quinine remains speculative, their possible function in oromotor reflex control is considered.     

Effect of diazepam binding inhibitor (DBI) on the fluid intake, preference and the taste reactivity in mice.

We have reported that a diazepam binding inhibitor (DBI)-like peptide is released by the aversive quinine stimuli 'Chem. Senses 25 (2000) 739'. To determine the effect of DBI on the fluid intake, we injected a DBI peptide fragment into the fourth ventricle in mice. DBI suppressed the intake of 5% sucrose, water and 0.9 mM quinine-HCl and the preference for 0.05% saccharin. Administration (i.p.) of flumazenil, a benzodiazepine receptor antagonist, 20 min before the injection of DBI (i.c.v.) antagonized the suppressive effect of DBI on the intake and the preference for saccharin. We also studied the dose dependency of the effect of DBI on the intake of 5% sucrose. Injection of DBI in excess of 3 microg suppressed the intake of 5% sucrose in mice. Furthermore, injection of DBI (i.c.v.) increased the aversive response to 0.9% NaCl in the taste reactivity in mice. These results suggest that DBI affect the preference to food.     

Gustatory preference-aversion thresholds are increased by ibotenic acid lesion of the lateral hypothalamus in the rat

The main purpose of this study was to quantitate possible changes in the rewarding and aversive values of certain gustatory stimuli produced by bilateral ibotenic acid lesions of the lateral hypothalamus in the rat. Non-operated rats served as controls. Thirteen days after the operation, rats were placed on a water-deprivation schedule during 5 consecutive days. Rats were then given the choice of one of 5 concentrations of saccharin solution, using a two-bottle procedure. Fluid intake across concentrations generated a preference-aversion curve. The same type of procedure was used to obtain the aversion curve for increasing concentrations of quinine solution. The lesioned rats as well as the control animals showed a clear preference-aversion response to saccharin solutions and an aversive response to quinine solutions. However, the highest preference score of the lesioned rats was obtained with a saccharin concentration 3 times higher than the concentration preferred by the control rats. Moreover, unlike control rats operated animals did not show aversion to the highest concentrations of saccharin solutions. Finally in the lesioned rats the aversion threshold to quinine solutions was obtained with concentration 5 times higher than the concentration inducing aversion in the control rats. At the end of these experiments, rats used as controls were submitted, in turn, to bilateral lesion of the lateral hypothalamus. The change in the preference-aversion threshold of these rats in the saccharin choice procedure was the same as that observed with naive rats. Taken together, these results suggest that in the normal rat the palatability of certain gustatory stimuli is modulated by the intrinsic neurons of the lateral hypothalamus.     

Effects of gustatory nerve transection and regeneration on quinine-stimulated Fos-like immunoreactivity in the parabrachial nucleus of the rat

The distribution of quinine-stimulated Fos-like immunoreactivity (FLI) in several subdivisions of the parabrachial nucleus (PBN) known to be responsive to gustatory stimulation was examined in rats in which the chorda tympani nerve (CT) and/or glossopharyngeal nerve (GL) was transected (Experiment 1) and in rats in which the GL was transected with regeneration promoted or prevented (Experiment 2). We confirmed previous findings in the literature by demonstrating that rats intraorally infused with 3 mM quinine showed a robust population of FLI in the waist area and the external lateral (EL) and external medial (EM) subdivisions of the PBN (Yamamoto et al. [1994] Physiol Behav 56:1197-1202; Travers et al., [ 1999] Am J Physiol 277:R384-R394). In the waist area, only GL transection significantly decreased the number of FLI-neurons elicited by intraoral infusion of quinine compared with water-stimulated controls. In the external subdivisions neither neurotomy affected the number of FLI-neurons. The effect of GL transection in the waist area was enduring for rats in which the GL did not regenerate (up to 94 days), but regeneration of the GL after 52 days restored quinine-stimulated FLI to control values. In these same GL-transected animals, there were parallel decreases in the number of gapes elicited by intraoral quinine stimulation that recovered, but only subsequent to regeneration of the GL. These data provide support for the role of the waist area in the brainstem processing that underlies oromotor rejection behaviors and also help substantiate the hypothesis that the CT and GL are relatively specialized with regard to function. Moreover, when the quinine-induced pattern of neural activity in the second central gustatory relay, as assessed by FLI, is substantially altered by the loss of peripheral gustatory input from the GL, it can be restored upon regeneration of the nerve.     

Acute effects of corticosterone on LiCl-induced rapid gustatory conditioning in rats: a taste reactivity analysis

Previous research has shown that acute corticosterone treatment can have rapid effects on learning and memory. Using the taste reactivity test (TRT), the present study examined the effect of acute administration of corticosterone on sucrose palatability and the development of LiCl-induced rapid gustatory conditioning. On each of two conditioning days rats were injected with either a low dose of lithium chloride (LiCl; 0.75 mEq, i.p.) or saline (NaCl; 0.9%, i.p.) and 10 min later, received a second injection of either corticosterone (5 mg/kg, i.p.) or cyclodextrin vehicle. Rats were then placed in the TRT chamber, where 1 min intraoral infusions of sucrose (0.3 M) were delivered every 10 min. Taste reactivity responses were videotaped and later analyzed for frequency of occurrence. Rats treated with both LiCl and corticosterone showed enhanced aversive responding and reduced ingestive responding relative to control rats treated with LiCl and vehicle. The implication that corticosterone may have a rapid enhancing effect on gustatory conditioning is discussed.     

The neural code for taste in the nucleus of the solitary tract of the rat: effects of adaptation

Adaptation of the tongue to NaCl, HCl, quinine or sucrose was used as a tool to study the stability and organization of response profiles in the nucleus of the solitary tract (NTS). Taste responses in the NTS were recorded in anesthetized rats before and after adaptation of the tongue to NaCl, HCl, sucrose or quinine. Results showed that the magnitude of response to test stimuli following adaptation was a function of the context, i.e., adaptation condition, in which the stimuli were presented. Over half of all taste responses were either attenuated or enhanced following the adaptation procedure: NaCl adaptation produced the most widespread, non-stimulus-selective cross-adaptation and sucrose adaptation produced the least frequent cross-adaptation and the most frequent enhancement of taste responses. Adaptation to quinine cross-adapted to sucrose and adaptation to HCl cross-adapted to quinine in over half of the units tested. The adaptation procedure sometimes unmasked taste responses where none were present beforehand and sometimes altered taste responses to test stimuli even though the adapting stimulus did not itself produce a response. These effects demonstrated a form of context-dependency of taste responsiveness in the NTS and further suggest a broad potentiality in the sensitivity of NTS units across taste stimuli. Across unit patterns of response remained distinct from each other under all adaptation conditions. Discriminability of these patterns may provide a neurophysiological basis for residual psychophysical abilities following adaptation.     

Sodium concentration coding gives way to evaluative coding in cortex and amygdala

Typically, stimulus batteries used to characterize sensory neural coding span physical parameter spaces (e.g., concentration: from low to high). For awake animals, however, psychological variables (e.g., pleasantness/palatability) with complicated relationships to the physical often dominate neural responses. Here we pit physical and psychological axes against one another, presenting awake rats with a stimulus set including 4 NaCl concentrations (0.01, 0.1, 0.3, and 1.0 m) plus palatable (0.3 m sucrose) and aversive (0.001 m quinine) benchmarks, while recording the activity of neurons in two sites vital for NaCl taste processing, gustatory cortex (GC) and central amygdala (CeA). Since NaCl palatability (i.e., preference) follows a non-monotonic, "inverted-U-shaped" curve while concentration increases monotonically, this stimulus battery allowed us to test whether GC and CeA responses better reflect external or internal variables. As predicted, GC single-neuron and population responses reflected both parameters in separate response epochs: sodium concentration-related information appeared with the earliest taste-specific responses, giving way to palatability-related information, in an overlapping subset of neurons, several hundred milliseconds later. CeA single-neuron and population responses, meanwhile, contained only a brief period of concentration specificity, occurring just before palatability-related information emerged (simultaneously with, or slightly later than, in GC). Thus, cortex and amygdala both prominently reflect NaCl palatability late in their responses; CeA neurons largely respond to either palatable or aversive stimuli, while GC responses tend to reflect the entire palatability spectrum in a graded fashion.