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

Characterization of mouse chorda tympani responses evoked by stimulation of anterior or posterior fungiform taste papillae

Different gustatory papilla types vary in their locations on the tongue. Distinctions have often made between types, but variation within fungiform papillae has seldom been explored. Here, regional differences in fungiform papillae were investigated by flowing solutions selectively over either an anterior fungiform (AF, tongue tip) or a posterior fungiform (PF, middle third) region as taste-evoked activity was measured in the chorda tympani nerve of C57BL/6J (B6) mice. Significantly larger responses were evoked by NaCl applied to the AF than PF region, and the ENaC blocker amiloride reduced the NaCl response size only for the former. Umami synergy, based on co-presenting MSG and IMP, was larger for the AF than PF region. The regions did not differ in response size to sour chemicals, but responses to l-lysine, l-arginine, sucrose, and tetrasodium pyrophosphate were larger for the AF than PF region. Thus, fungiform papillae on the tongue tip differed from those found further back in their transduction mechanisms for salty and umami compounds. Gustatory sensitivity also showed regional variation, albeit with a complex relationship to palatability and taste quality. Overall, the data support a regional organization for the mouse tongue, with different functional zones for the anterior, middle, and posterior thirds.

Separate functions for responses to oral temperature in thermo-gustatory and trigeminal neurons

Oral temperature is a component and modifier of taste perception. Both trigeminal (V) and taste-sensitive cells, including those in the nucleus of the solitary tract (NTS), can respond to oral temperature. However, functional associations in thermal sensitivity between V and gustatory neurons are poorly understood. To study this we recorded electrophysiological responses to oral stimulation with cool (9, 15, 25, 32, and 34 °C) and warm (40 and 45 °C) temperatures from medullary V (n = 45) and taste-sensitive NTS (n = 27) neurons in anesthetized mice. Results showed temperatures below 34 °C activated the majority of V neurons but only a minority of NTS units. V neurons displayed larger responses to cooling and responded to temperatures that poorly stimulated NTS cells. Multivariate analyses revealed different temperatures induced larger differences in responses across V compared with NTS neurons, indicating V pathways possess greater capacity to signal temperature. Conversely, responses to temperature in NTS units associated with gustatory tuning. Further analyses identified two types of cooling-sensitive V neurons oriented toward innocuous or noxious cooling. Multivariate analyses indicated the combined response of these cells afforded distinction among a broad range of cool temperatures, suggesting multiple types of V neurons work together to represent oral cooling.

Influence of stimulus and oral adaptation temperature on gustatory responses in central taste-sensitive neurons

The temperature of taste stimuli can modulate gustatory processing. Perceptual data indicate that the adapted temperature of oral epithelia also influences gustation, although little is known about the neural basis of this effect. Here, we electrophysiologically recorded orosensory responses (spikes) to 25°C (cool) and 35°C (warm) solutions of sucrose (0.1 and 0.3 M), NaCl (0.004, 0.1, and 0.3 M), and water from taste-sensitive neurons in the nucleus of the solitary tract in mice under varied thermal adaptation of oral epithelia. Conditions included presentation of taste stimuli isothermal to adaptation temperatures of 25°C (constant cooling) and 35°C (constant warming), delivery of 25°C stimuli following 35°C adaptation (relative cooling), and presentation of 35°C stimuli following 25°C adaptation (relative warming). Responses to sucrose in sucrose-oriented cells (n = 15) were enhanced under the constant and relative warming conditions compared with constant cooling, where contiguous cooling across adaptation and stimulus periods induced the lowest and longest latency responses to sucrose. Yet compared with constant warming, cooling sucrose following warm adaptation (relative cooling) only marginally reduced activity to 0.1 M sucrose and did not alter responses to 0.3 M sucrose. Thus, warmth adaptation counteracted the attenuation in sucrose activity associated with stimulus cooling. Analysis of sodium-oriented (n = 25) neurons revealed adaptation to cool water, and cooling taste solutions enhanced unit firing to 0.004 M (perithreshold) NaCl, whereas warmth adaptation and stimulus warming could facilitate activity to 0.3 M NaCl. The concentration dependence of this thermal effect may reflect a dual effect of temperature on the sodium reception mechanism that drives sodium-oriented cells.

Different spatial expressions of c-Fos in the nucleus of the solitary tract following taste stimulation with sodium, potassium, and ammonium ions in rats

Cation-specific epithelial receptors on the tongue have been well demonstrated. However, active regions along the nucleus of the solitary tract (NST) for cations Na(+), K(+), NH4(+) are still unclear, even though the best responses of NST neurons to taste stimuli vary depending on the cell. In the present study, the spatial distribution patterns of cation-specific active regions in the NST are investigated. The tongues of urethane-anesthetized Sprague-Dawley rats (n = 25) were stimulated with artificial saliva (control), 0.5 M NaCl, 1.0 M NaCl, 0.5 M KCl, and 0.3 M NH(4) Cl. Then, the three-dimensional positions of c-Fos-like-immunoreactive (cFLI) cells in the NST were generated. The spatial distributions of cFLI cells in the NST were compared among five taste stimulations. cFLI cells were observed throughout the NST, irrespective of the stimulus; however, the intermediate-medial central regions of the NST had higher numbers of cFLI cells than the other regions in all taste stimulations. Analysis of images revealed that the activated regions in the NST differed significantly depending on the cations. The intermediate-dorsal-central region and the caudal-ventral region were activated by a 0.5 M concentration of sodium, the rostral-ventral region and the intermediate-dorsal/ventral region were activated by a 1.0 M concentration of sodium, the intermediate-dorsal/ventral region was activated by potassium ions, and the rostral-ventral region and the intermediate-ventral central region were activated by ammonium ions. These results suggest that the responses of NST cells to cation salt ions are regulated differentially.

Topographical representations of taste response characteristics in the rostral nucleus of the solitary tract in the rat

The rostral nucleus of the solitary tract (rNST) is the first-order taste relay in rats. This study constructed topographical distributions of taste response characteristics (best-stimulus, response magnitude, and taste-tuning) from spike discharges of single neurons in the rNST. The rNST is divided into four subregions along the rostrocaudal (RC) axis, which include r1-r4. We explored single-neuron activity in r1-r3, using multibarreled glass microelectrodes. NaCl (N)-best neurons were localized to the rostral half of r1-r3, while HCl (H)-best and sucrose (S)-best neurons showed a tendency toward more caudal locations. NaCl and HCl (NH)-best neurons were distributed across r2-r3. The mean RC values and Mahalanobis distance indicated a significant difference between the distributions of N-best and NH-best neurons in which N-best neurons were located more rostrally. The region of large responses to NaCl (net response >5 spikes/s) overlapped with the distribution of N-best neurons. The region of large responses to HCl extended widely over r1-r3. The region of large responses to sucrose was in the medial part of r2. The excitatory region (>1 spike/s) for quinine overlapped with that for HCl. Neurons with sharp to moderate tuning were located primarily in r1-r2, while those with broad tuning were located in r2-r3. The robust responses to NaCl in r1-r2 primarily contributed to sharp to moderate taste-tuning. Neurons in r3 tended to have broad tuning, apparently due to small responses to both NaCl and HCl. Therefore, the rNST is spatially organized by neurons with distinct taste response characteristics, suggesting that these neurons serve different functional roles.

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.

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.

Learning-based recovery from perceptual impairment in salt discrimination after permanently altered peripheral gustatory input

Rats lacking input to the chorda tympani (CT) nerve, a facial nerve branch innervating anterior tongue taste buds, show robust impairments in salt discrimination demonstrating its necessity. We tested the sufficiency of the CT for salt taste discrimination and whether the remaining input provided by the greater superficial petrosal (GSP) nerve, a facial nerve branch innervating palatal taste buds, or by the glossopharyngeal (GL) nerve, innervating posterior tongue taste buds, could support performance after extended postsurgical testing. Rats presurgically trained and tested in a two-response operant task to discriminate NaCl from KCl were subjected to sham surgery or transection of the CT (CTx), GL (GLx), or GSP (GSPx), alone or in combination. While initially reduced postsurgically, performance by rats with an intact GSP after CTx + GLx increased to normal over 6 wk of testing. Rats with CTx + GSPx consistently performed near chance levels. In contrast, rats with GSPx + GLx were behaviorally normal. A subset of rats subjected to sham surgery and exposed to lower concentrations during postsurgical testing emulating decreased stimulus intensity after neurotomy showed no significant impairment. These results demonstrate that CTx changes the perceptual nature of NaCl and/or KCl, leading to severe initial postsurgical impairments in discriminability, but a "new" discrimination can be relearned based on the input of the GSP. Despite losing ∼75% of their taste buds, rats are unaffected after GSPx + GLx, demonstrating that the CT is not only necessary, but also sufficient, for maintaining salt taste discrimination, notwithstanding the unlikely contribution of the small percentage of taste receptors innervated by the superior laryngeal nerve.

Comparison of somatostatin and corticotrophin-releasing hormone immunoreactivity in forebrain neurons projecting to taste-responsive and non-responsive regions of the parabrachial nucleus in rat

Several forebrain areas have been shown to project to the parabrachial nucleus (PBN) and exert inhibitory and excitatory influences on taste processing. The neurochemicals by which descending forebrain inputs modulate neural taste-evoked responses remain to be established. This study investigated the existence of somatostatin (SS) and corticotrophin-releasing factor (CRF) in forebrain neurons that project to caudal regions of the PBN responsive to chemical stimulation of the anterior tongue as well as more rostral unresponsive regions. Retrograde tracer was iontophoretically or pressure ejected from glass micropipettes, and 7 days later the animals were euthanized for subsequent immunohistochemical processing for co-localization of tracer with SS and CRF in tissue sections containing the lateral hypothalamus (LH), central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (BNST), and insular cortex (IC). In each forebrain site, robust labeling of cells with distinguishable nuclei and short processes was observed for SS and CRF. The results indicate that CRF neurons in each forebrain site send projections throughout the rostral caudal extent of the PBN with a greater percentage terminating in regions rostral to the anterior tongue-responsive area. For SS, the percentage of double-labeled neurons was more forebrain site specific in that only BNST and CeA exhibited significant numbers of double-labeled neurons. Few retrogradely labeled cells in LH co-expressed SS, while no double-labeled cells were observed in IC. Again, tracer injections into rostral PBN resulted in a greater percentage of double-labeled neurons in BNST and CeA compared to caudal injections. The present results suggest that some sources of descending forebrain input might utilize somatostatin and/or CRF to exert a broad influence on sensory information processing in the PBN.

A study of the science of taste: On the origins and influence of the core ideas

Our understanding of the sense of taste is largely based on research designed and interpreted in terms of the traditional four “basic” tastes: sweet, sour, salty, and bitter, and now a few more. This concept of basic tastes has no rational definition to test, and thus it has not been tested. As a demonstration, a preliminary attempt to test one common but arbitrary psychophysical definition of basic tastes is included in this article; that the basic tastes are unique in being able to account for other tastes. This definition was falsified in that other stimuli do about as well as the basic words and stimuli. To the extent that this finding might show analogies with other studies of receptor, neural, and psychophysical phenomena, the validity of the century-long literature of the science of taste based on a few “basics” is called into question. The possible origins, meaning, and influence of this concept are discussed. Tests of the model with control studies are suggested in all areas of taste related to basic tastes. As a stronger alternative to the basic tradition, the advantages of the across-fiber pattern model are discussed; it is based on a rational data-based hypothesis, and has survived attempts at falsification. Such “population coding” has found broad acceptance in many neural systems.

Taste and acceptance of pyrophosphates by rats and mice

The palatability and taste quality of pyrophosphates were evaluated in a series of behavioral and electrophysiological experiments. In two-bottle choice tests with water, rats strongly preferred some concentrations of Na3HP2O7 and Na4P2O7, moderately preferred some concentrations of K4P2O7 and Fe4(P2O7)3, and were indifferent to or avoided all concentrations of Ca2P2O7 and Na2H2P2O7. The contribution of sodium to the preference for sodium pyrophosphates was ascertained: 1) Rats with a choice between Na4P2O7 and NaCl preferred 1 mM Na4P2O7 to 4 mM NaCl but preferred 40 or 150 mM NaCl to 10 mM Na4P2O7, 2) blocking salt taste transduction by mixing Na4P2O7 with amiloride reduced preferences but did not eliminate them, and 3) three mouse strains (FVB/J, C57BL/6J, and CBA/J) known to differ in sodium preference had the same rank order of preferences for Na3HP2O7 and NaCl, but peak preferences were higher for Na3HP2O7 than for NaCl. The taste qualities of pyrophosphates were determined by measuring taste-evoked responses of neurons in the nucleus of the solitary tract of rats. Across-neuron patterns of activity for sodium pyrophosphates were similar to that of NaCl but the pattern of Na3HP2O7 plus amiloride was unique from those of sweet, salty, sour, bitter, and umami stimuli. Taken together, the results indicate that the high palatability of some concentrations of Na3HP2O7 and Na4P2O7 is due partially to their salty taste, but there must also be another cause, which may include a novel orosensory component distinct from the five major taste qualities.

Psychophysical investigations of cetylpyridinium chloride in rats: its inherent taste and modifying effects on salt taste

Salts are transduced by at least 2 mechanisms: (a) antagonized by amiloride and (b) antagonized by cetylpyridinium chloride (CPC). The authors report on 4 behavioral experiments in rats that characterize the orosensory properties of CPC itself as well as its effect in suppressing the intensity of NaCl and KCl taste. Experiments 1 and 2 indicated that CPC has a quinine-like taste quality. Experiments 3 and 4 demonstrated that the recognition of KCl, but not NaCl, is modestly reduced by mixture with CPC. However, control experiments call into question the mechanism of the salt suppression of CPC, because both CPC-salt and quinine-salt mixtures had similar effects. The relevance of these studies for understanding salt and bitter taste coding is discussed.

Differential gurmarin suppression of sweet taste responses in rat solitary nucleus neurons

We examined the effect of the sweet transduction blocker gurmarin on taste responses recorded from neurons in the rat solitary nucleus (NST) to determine how gurmarin sensitivity is distributed across neuronal type. Initially, responses evoked by washing the anterior tongue and palate with 0.5 M sucrose, 0.1 M NaCl, 0.01 M HCl, and 0.01 M quinine-HCl were recorded from 35 neurons. For some cells, responses to a sucrose concentration series (0.01-1.0 M) or an array of sweet-tasting compounds were also measured. Gurmarin (10 microg/ml, 2-4 ml) was then applied to the tongue and palate. Stimuli were reapplied after 10-15 min. Neurons were segregated into groups based on similarities among their initial response profiles using hierarchical cluster analysis (HCA). Results indicated that sucrose responses recorded from neurons representative of each HCA-defined class were suppressed by gurmarin. However, a disproportionate percentage of cells in each group displayed sucrose responses that were substantially attenuated after gurmarin treatment. Postgurmarin sucrose responses recorded from neurons that composed 57% of class S, 40% of class N, and 33% of class H were suppressed by >or=50% relative to control. On average, attenuation was statistically significant only in class S and N neurons. Although the magnitude of gurmarin-induced response suppression did not differ across sucrose concentration, responses to different sweet-tasting compounds were differentially affected. Responses to NaCl, HCl, or quinine were not suppressed by gurmarin. Results suggest that information from gurmarin-sensitive and -insensitive receptor processes converges onto single NST neurons.

Acid and salt responses in mouse taste cells

Acid and salt responses of taste cells induced by natural stimulation have not been investigated with exception of early studies with conventional microelectrode method, due to the toxicity of high concentration of salt or low pH of acid stimuli applied to isolated taste cells. This indicates that the application of rapid and localized stimulation to the apical membrane of taste cells is necessary for recording of natural responses to salt or acid stimuli using patch clamp technique. Recently we have developed a procedure to accomplish the quasi-natural condition including rapid, localized stimuli to the apical receptive membrane and the maintenance of taste bud polarity. In this review, we present our recent results obtained under quasi-natural condition using patch clamp techniques, comparing with the previously proposed hypothesis. One of our major finding is the fact that the acid-induced responses of taste cells in the mouse fungiform papillae are never suppressed by amiloride but an apical proton-gated conductance and a basolateral Cl(-) conductance possibly contribute to sour transduction. On the other hand, salt-induced responses are suppressed by amiloride, although the salt-induced responses recorded from a single cell involve both amiloride-sensitive and -insensitive components. Furthermore, the amiloride-insensitive component of salt responses possibly consists of multiple subcomponents including an apical sodium-gated nonselective cation conductance and a basolateral Cl(-) conductance. Recent reports also support the hypothesis that both acid and salt responses require specific receptor mechanisms of inorganic cations such as H(+) and Na(+) at the apical receptive membrane.

Neural representation of salts in the rat solitary nucleus: brain stem correlates of taste discrimination

One mechanism of salt taste transduction by gustatory receptor cells involves the influx of cations through epithelial sodium channels that can be blocked by oral application of amiloride. A second mechanism is less clearly defined but seems to depend on electroneutral diffusion of the salt through the tight junctions between receptor cells; this paracellular pathway is insensitive to amiloride. Because the first mechanism is more sensitive to sodium salts and the second to nonsodium salts, these peripheral events could underlie the ability of rats to discriminate sodium from nonsodium salts on the basis of taste. Behavioral experiments indicate that amiloride, at concentrations that are tasteless to rats, impairs a rat's ability to discriminate NaCl from KCl and may do so by making both salts taste like KCl. In the present study, we examined the neural representation of NaCl and KCl (0.05-0.2 M), and mixtures of these salts with amiloride (0, 3, and 30 microM), to explore the neural correlates of this behavioral result. NaCl and KCl were represented by distinct patterns of activity in the nucleus of the solitary tract. Amiloride, in a concentration-dependent manner, changed the pattern for NaCl to one more characteristic of KCl, primarily by reducing activity in neurons responding best to NaCl and sucrose. The effect of amiloride concentration on the response to 0.1 M NaCl in NaCl-best neurons was virtually identical to its effect on behavioral discrimination performance. Modeling the effects of blocking the amiloride-insensitive pathway also resulted in highly similar patterns of activity for NaCl and KCl. These results suggest that activity in both the amiloride-sensitive and -insensitive pathways is required for the behavioral discrimination between NaCl and KCl. In the context of published behavioral data, the present results suggest that amiloride-sensitive activity alone is not sufficient to impart a unique signal for the taste of sodium salts.

Neuronal cell types and taste quality coding

Over the past 25 years, there have been two opposing views of how taste information is represented in the activity of gustatory neurons. One view, the across-fiber pattern (AFP) theory, postulates that taste quality is represented by the pattern of activity across the afferent population. Stimuli with similar tastes produce similar patterns of activity. The other view is that activity in a few distinct neuron types codes taste quality in a "labeled-line" fashion. Neurons responding best to sucrose, for example, would represent "sweetness," and those responding best to NaCl would code "saltiness." Some of these neuron types appear to have a biological significance, such as the NaCl-best cells, which receive input about sodium stimuli exclusively from an amiloride-sensitive epithelial ion channel. However, the relatively broad tuning of these neurons makes it unlikely that they are capable of unambiguously coding information about taste quality. Rather, these neuron types play a critical role in establishing unique AFPs that distinguish among taste stimuli. The relative activity across these cell types represent taste quality, much like the patterns of activity across broadly tuned photoreceptors code information about stimulus wavelength.