Excitation and adaptation in the detection of hydrogen ions by taste receptor cells: a role for cAMP and Ca(2+).

The role of intracellular cAMP and Ca(2+) in the excitation and adaptation of taste responses by HCl was investigated by direct measurement of intracellular pH (pH(i)) in polarized taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the tongue with HCl concentrations between 1 and 30 mM caused a dose-dependent increase in CT responses that were insensitive to voltage clamp of the lingual receptive field and to amiloride. At a fixed HCl concentration (20 mM) topical lingual application of 8-chlorophenylthio(CPT)-cAMP increased the magnitude of HCl-induced CT response by twofold under zero current clamp. The magnitude of the CT response increased further at -60 mV and decreased at +60 mV lingual voltage clamp but remained amiloride insensitive. In untreated polarized TRCs, apical stimulation with HCl concentrations between 1 and 30 mM HCl induced sustained decreases in TRC pH(i). The magnitude of pH(i) decrease increased with increasing HCl concentration. Following treatment of the basolateral membrane with 8-CPT-cAMP the decrease in pH(i) due to apical 1 mM HCl application was significantly increased. Treatment with cAMP alone decreased resting TRC pH(i) and inhibited the recovery of pH(i) from a basolateral NH4Cl pulse by 46%. Topical lingual application of ionomycin, a Ca(2+) ionophore, did not affect the initial CT response to 20 mM HCl +10 mM CaCl2, but the response declined rapidly to 50% of its initial level within 2 min. In polarized TRCs, basolateral exposure to ionomycin increased TRC pH(i) and activated pH(i) recovery from NH4Cl pulse by 388%. Apical HCl stimulation induced a transient decrease in resting TRC pH(i) followed by spontaneous recovery. The data suggest that cAMP enhances the sour taste of strong acids by activating a Ca(2+)- and amiloride-insensitive H(+) conductance and inhibiting pH(i) recovery in TRCs. However, an increase in [Ca(2+)]i stimulates pH(i) recovery, which, in turn, increases sensory adaptation to acids.     

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.     

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.     

Na+-H+ exchange activity in taste receptor cells

mRNA for two Na(+)-H(+)-exchanger isoforms 1 and 3 (NHE-1 and NHE-3) was detected by RT-PCR in fungiform and circumvallate taste receptor cells (TRCs). Anti-NHE-1 antibody binding was localized to the basolateral membranes, and the anti-NHE-3 antibody was localized in the apical membranes of fungiform and circumvallate TRCs. In a subset of TRCs, NHE-3 immunoreactivity was also detected in the intracellular compartment. For functional studies, an isolated lingual epithelium containing a single fungiform papilla was mounted with apical and basolateral sides isolated and perfused with nominally CO(2)/HCO(3)(-)-free physiological media (pH 7.4). The TRCs were monitored for changes in intracellular pH (pH(i)) and Na(+) ([Na(+)](i)) using fluorescence ratio imaging. At constant external pH, 1) removal of basolateral Na(+) reversibly decreased pH(i) and [Na(+)](i); 2) HOE642, a specific blocker, and amiloride, a nonspecific blocker of basolateral NHE-1, attenuated the decrease in pH(i) and [Na(+)](i); 3) exposure of TRCs to basolateral NH(4)Cl or sodium acetate pulses induced transient decreases in pH(i) that recovered spontaneously to baseline; 4) pH(i) recovery was inhibited by basolateral amiloride, 5-(N-methyl-N-isobutyl)-amiloride (MIA), 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), HOE642, and by Na(+) removal; 5) HOE642, MIA, EIPA, and amiloride inhibited pH(i) recovery with K(i) values of 0.23, 0.46, 0.84, and 29 microM, respectively; and 6) a decrease in apical or basolateral pH acidified TRC pH(i) and inhibited spontaneous pH(i) recovery. The results indicate the presence of a functional NHE-1 in the basolateral membranes of TRCs. We hypothesize that NHE-1 is involved in sour taste transduction since its activity is modulated during acid stimulation.     

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.     

The mammalian amiloride-insensitive non-specific salt taste receptor is a vanilloid receptor-1 variant

The amiloride-insensitive salt taste receptor is the predominant transducer of salt taste in some mammalian species, including humans. The physiological, pharmacological and biochemical properties of the amiloride-insensitive salt taste receptor were investigated by RT-PCR, by the measurement of unilateral apical Na⁺ fluxes in polarized rat fungiform taste receptor cells and by chorda tympani taste nerve recordings. The chorda tympani responses to NaCl, KCl, NH₄Cl and CaCl₂ were recorded in Sprague-Dawley rats, and in wild-type and vanilloid receptor-1 (VR-1) knockout mice. The chorda tympani responses to mineral salts were monitored in the presence of vanilloids (resiniferatoxin and capsaicin), VR-1 antagonists (capsazepine and SB-366791), and at elevated temperatures. The results indicate that the amiloride-insensitive salt taste receptor is a constitutively active non-selective cation channel derived from the VR-1 gene. It accounts for all of the amiloride-insensitive chorda tympani taste nerve response to Na⁺ salts and part of the response to K⁺, NH₄⁺ and Ca²⁺ salts. It is activated by vanilloids and temperature (> 38°C), and is inhibited by VR-1 antagonists. In the presence of vanilloids, external pH and ATP lower the temperature threshold of the channel. This allows for increased salt taste sensitivity without an increase in temperature. VR-1 knockout mice demonstrate no functional amiloride-insensitive salt taste receptor and no salt taste sensitivity to vanilloids and temperature. We conclude that the mammalian non-specific salt taste receptor is a VR-1 variant.     

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

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

Development of rat chorda tympani sodium responses: evidence for age-dependent changes in global amiloride-sensitive Na(+) channel kinetics

In rat, chorda tympani nerve taste responses to Na(+) salts increase between roughly 10 and 45 days of age to reach stable, mature magnitudes. Previous evidence from in vitro preparations and from taste nerve responses using Na(+) channel blockers suggests that the physiological basis for this developmental increase in gustatory Na(+) sensitivity is the progressive addition of functional, Na(+) transduction elements (i.e., amiloride-sensitive Na(+) channels) to the apical membranes of fungiform papilla taste receptor cells. To avoid potential confounding effects of pharmacological interventions and to permit quantification of aggregate Na(+) channel behavior using a kinetic model, we obtained chorda tympani nerve responses to NaCl and sodium gluconate (NaGlu) during receptive field voltage clamp in rats aged from 12-14 to 60 days and older (60+ days). Significant, age-dependent increases in chorda tympani responses to these stimuli occurred as expected. Importantly, apical Na(+) channel density, estimated from an apical Na(+) channel kinetic model, increased monotonically with age. The maximum rate of Na(+) response increase occurred between postnatal days 12-14 and 29-31. In addition, estimated Na(+) channel affinity increased between 12-14 and 19-23 days of age, i.e., on a time course distinct from that of the maximum rate of Na(+) response increase. Finally, estimates of the fraction of clamp voltage dropped across taste receptor apical membranes decreased between 19-23 and 29-31 days of age for NaCl but remained stable for NaGlu. The stimulus dependence of this change is consistent with a developmental increase in taste bud tight junctional Cl(-) ion permeability that lags behind the developmental increase in apical Na(+) channel density. A significant, indirect anion influence on apical Na(+) channel properties was present at all ages tested. This influence was evident in the higher apparent apical Na(+) channel affinities obtained for NaCl relative to NaGlu. This stimulus-dependent modulation of apical Na(+) channel apparent affinity relies on differences in the transepithelial potentials between NaCl and NaGlu. These originate from differences in paracellular anion permeability but act also on the driving force for Na(+) through apical Na(+) channels. Detection of such an influence on taste depends fundamentally on the preservation of taste bud polarity and on a direct measure of sensory function, such as the response of primary afferents.     

Effect of nicotine on chorda tympani responses to salty and sour stimuli

The effect of nicotine on the benzamil (Bz)-insensitive (transient receptor potential vanilloid-1 variant cation channel, TRPV1t) and the Bz-sensitive (epithelial Na(+) channel, ENaC) salt taste receptors and sour taste was investigated by monitoring intracellular Na(+) and H(+) activity (pH(i)) in polarized fungiform taste receptor cells (TRCs) and the chorda tympani (CT) nerve responses to NaCl, KCl, and HCl. CT responses in Sprague-Dawley rats and both wildtype and TRPV1 knockout (KO) mice were recorded in the presence and absence of agonists [resiniferatoxin (RTX) and elevated temperature] and an antagonist (SB-366791) of TRPV1t, the ENaC blocker (Bz), and varying apical pH (pH(o)). At concentrations <0.015 M, nicotine enhanced and at >0.015 M, it inhibited CT responses to KCl and NaCl. Nicotine produced maximum enhancement in the Bz-insensitive NaCl CT response at pH(o) between 6 and 7. RTX and elevated temperature increased the sensitivity of the CT response to nicotine in salt-containing media, and SB-366791 inhibited these effects. TRPV1 KO mice demonstrated no Bz-insensitive CT response to NaCl and no sensitivity to nicotine, RTX, and elevated temperature. We conclude that nicotine modulates salt responses by direct interaction with TRPV1t. At pH(o) >8, the apical membrane permeability of nicotine was increased significantly, resulting in increase in TRC pH(i) and volume, activation of ENaC, and enhancement of the Bz-sensitive NaCl CT response. At pH(o) >8, nicotine also inhibited the phasic component of the HCl CT response. We conclude that the effects of nicotine on ENaC and the phasic HCl CT response arise from increases in TRC pH(i) and volume.     

Two different receptor sites for Ca2+ and Na+ in frog taste responses

Distilled water, 1 mM CaCl2 and 500 mM NaCl (pH 4.5) are effective stimuli which excite chemoreceptors of the frog tongue. To learn whether or not these taste stimuli react with different taste receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. Responses were recorded from the frog glossopharyngeal nerve during taste stimulation. After application of 0.1% pronase E to the dorsal tongue surface, the magnitude of the NaCl response remained unchanged, but the magnitude of the water and CaCl2 responses was markedly decreased. The selective suppression by the pronase E treatment indicates that there are two different receptor sites for Ca2+ and Na+ in the frog taste receptor cell and the receptor sites responsible for the generation of the water and the Ca2+ response may be composed of a protein.     

Procaine effects on sodium and chloride transport in frog skin

Procaine, a tertiary amine, has previously been shown to stimulate reversibly transepithelial Na transport across frog skin after application from the epithelial side. In the present study with intracellular recording from principal, i.e. amiloride-sensitive cells, we demonstrate that the stimulation results from increase in apical membrane Na permeability. A second effect of procaine (10–25 mmol/l) in the outside perfusion solution is a reversible increase of transepithelial conductance which drastically exceeds the predicted response of the transcellular Na pathway. It requires presence of chloride on the epithelial side and depends on the non-ionized molecule of procaine. Abolition of apical membrane Na uptake by amiloride or Na-free mucosal inbubation decreases the magnitude but does not prevent the stimulatory effect of procaine. The origin of this gain in conductance from stimulation of a Cl-specific pathway is demonstrated by a highly significant correlation between the increases in electrically determined tissue conductance and partial Cl conductance, obtained from measurements of influx and efflux of Cl-36. Measurements with microelectrodes indicate that the stimulated Cl-specific pathway is distinct from the principal cells. Since procaine activates a conductive pathway with similar response pattern as spontaneously existing Cl conductance, it might be a valuable tool for investigating mode and way of Cl movement across epithelial tissues.     

Amiloride sensitivity of the chorda tympani response to sodium chloride in sodium-depleted Wistar rats.

Peripheral gustatory mechanisms that may contribute to the expression of sodium (Na) appetite have been a focus of interest for many years. Because amiloride-sensitive Na transport is involved in the generation of neural signals in response to NaCl stimulation, the present study assessed whether changes in amiloride sensitivity of the neural response to NaCl accompany the induction of a Na appetite in the rat. Na deprivation was achieved by acute depletion with the diuretic furosemide. The magnitude of the whole-nerve chorda tympani response to 0.5 M NaCl was reduced in Na-depleted, compared with Na-replete, rats, which provides qualified support for previous reports that the induction of a Na appetite is associated with reduced neural responses to NaCl. However, changes in sensitivity to the specific Na channel blocker amiloride hydrochloride as a result of Na depletion were not evident. These findings suggest that the behavioral and neural changes that occur after Na depletion are not based on changes in amiloride sensitivity in the taste bud.     

Taste quality and intensity: lessons from the Morrison technique

The nontoxic and nonshock Morrison operant technique was used to evaluate taste quality in rat and marmoset: response to a tastant test solution in pursuit of a pellet reward was dependent on making a choice between two bars that had been linked in discrimination training to qualitatively different stimulus pairs (NaCl versus either HCl, QHCl, or NH(4)Cl). The percentage distribution of bar-press responses to test stimuli showed: (1) stability of quality across 0.069-0.3 M NaCl, 0.003-0.1 M HCl, and 0.0001-0.003 M QHCl; (2) for LiCl, a quality change consistent with human reports of a "sour" to "salty" shift; (3) a suggestion that the "salty-like" quality of NH(4)Cl and NaCl are not perceptually equivalent; (4) NaNO(3) shares NaCl-like, QHCl-like, and NH(4)Cl-like components; (5) CaCl(2), KCl, and MgCl(2) share QHCl-like and NH(4)Cl-like components; and (6) responses to HCl and QHCl were not hedonically driven in the rat. Comparison of rank order correlations of single-unit firing rates to the distribution of bar-press responses for the same test stimulus concentration revealed that (7) no single level of the gustatory pathway exclusively accounts for the operant response distribution pattern to either simple or complex tastants, and (8) discriminations between tastants, one of which may be qualitatively complex, are not necessarily mediated only at levels proximal to the solitary nucleus. Thus, the Morrison discrimination technique effectively yields statements about gustatory quality without use of negative reinforcers and largely uninfluenced by tastant hedonics.     

Abnormal taste preference for saccharin in hypothyroid rats.

Taste preferences for saccharin in concentrations ranging from 0.16 mM to 50 mM were determined in rats made hypothyroid with radioactive iodine and in their littermate controls. Hypothyroid rats demonstrated taste preferences for saccharin which were similar to those of controls only at very low (0.016 mM) or very high (49.0 mM) saccharin concentrations. At these concentrations of tastant, the preferences for tastant and water were similar to one another. At a concentration of 5.1 mM, preferences were also very similar in both groups but were very high. At intermediate saccharin concentrations of 1.1 and 3.0 mM, hypothyroid animals showed significantly lower percent preferences for the sweet tastant than did controls, mean +/- SEM (62.48 +/- 5.97 vs. 82.92 +/- 4.60, p = 0.0002) for the 1.1 mM concentration and (74.98 +/- 5.12 vs. 89.40 +/- 2.54, p = 0.0029) for the 3.0 mM concentration. These changes in taste preference for saccharin in hypothyroid rats were similar in direction and magnitude to those previously published by this laboratory using sucrose as the tastant. Thus, hypothyroid rats demonstrate abnormalities in taste preference for both the nonnutritive sweetener, sodium saccharin, as well as for the nutritive sweetener, sucrose.     

Pulmonary oedema fluid induces non-α-ENaC-dependent Na+ transport and fluid absorption in the distal lung

To determine if pulmonary oedema fluid (EF) alters ion and fluid transport of distal lung epithelium (DLE), EF was collected from rats in acute heart failure. EF, but not plasma, increased amiloride-insensitive short circuit current ( I _sc) and Na⁺-K⁺ ATPase protein content and pump activity of DLE grown in primary culture. Inhibitors of  Cl⁻  transport or cGMP-gated cation channels had a significant (   P < 0.05  ), but limited ability to block the increased I _sc. EF increased amiloride-insensitive, but not amiloride-sensitive, DLE apical membrane Na⁺ conductance. The level of mRNA encoding epithelial sodium channel (ENaC) subunits was unchanged (α, β), or decreased (γ,   P < 0.05  ) in EF-exposed DLE. EF also induced an amiloride-insensitive increase in the potential difference across murine tracheal cysts. Distal lung explants from late gestation wild-type and α-ENaC-deficient fetal mice, which normally expand due to liquid secretion, decreased in size due to liquid absorption when exposed to EF. Trypsin digestion or heat treatment of EF abrogated the ability of EF to increase amiloride-insensitive I _sc in DLE and liquid absorption by distal lung explants. Thus proteins or protein-dependent factors within cardiogenic EF induce an α-ENaC-independent and amiloride-insensitive apical membrane Na⁺ conductance and liquid absorption in the distal lung.     

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)     

Modification of behavioral and neural taste responses to NaCl in C57BL/6 mice: effects of NaCl exposure and DOCA treatment

To investigate the possible role of peripheral gustatory responsiveness to changes in NaCl acceptance, we studied NaCl consumption and the chorda tympani nerve responses to lingual application of NaCl in C57BL/6ByJ mice. The mice were treated with 300 mM NaCl (given to drink in 96-h two-bottle tests with water) or with injections of deoxycorticosterone acetate (DOCA; 33 mg/kg daily). Naive mice were neutral to 75 mM NaCl, but mice previously exposed to 300 mM NaCl avoided 75 mM NaCl. The NaCl-exposed (300 mM for 4 days and 75 mM for 2 days) mice had enhanced amiloride-sensitive components of the chorda tympani responses to 10-30 mM NaCl applied at room temperature (24 degrees C). DOCA injections increased acceptance of 300 mM NaCl, but did not change the chorda tympani responses to 100-1000 mM NaCl. However, the DOCA-treated mice had enhanced amiloride-sensitive components of the chorda tympani responses to cold (12 degrees C) 10-30 mM NaCl. These data suggest that peripheral gustatory responsiveness possibly contributes to the NaCl aversion induced by exposure to concentrated NaCl, but not to the DOCA-induced increase of NaCl acceptance.     

NaCl and sugar release,salivation and taste during mastication of salted chewing gum

Salt perception impacts on food acceptability and nutrition and depends upon salt release from foods that was determined in situ during mastication of chewing gum with up to 10% (1800 mmol/kg) added NaCl. The mechanical action of chewing increased salivation, which was further increased by the presence of salt, particularly above 180 mmol NaCl/kg gum or above 100 mM NaCl in saliva. The average resting salivary flow rate was 1 ml/min, increasing to 4 and 6 ml/min with gums containing low and high salt, respectively. Thus, stimulation of salivation by salt occurred at a concentration well above the taste threshold of 20 mM NaCl. NaCl concentration in nonstimulated saliva was about 10 mM and increased to 500 mM after 30 s chewing of the 10% NaCl gum and returned to near nonstimulated levels after 4 min chewing. Changes in pH of saliva were more gradual, increasing to a maximum at about 2 min and remaining elevated after 4 min. Salty taste was related to the free chloride ion concentration in saliva irrespective of the initial salt concentration in the gum with an indication of adaptation after 3 min chewing. During chewing, salty taste increased ahead of the increase in salivary conductivity and the salt concentration in the sublingual saliva varied in a cyclic fashion about every 20 s. This is consistent with a cyclic swallowing of saliva and replacement with newly secreted saliva of low salt content and mastication releasing further salt from the gum.