Asymmetrical neural cross-adaptation of hamster chorda tympani responses to sodium and chloride salts

Cross-adaptation has occurred when exposure to an adapting chemicalstimulus (A) reduces the response to a subsequent test stimulus(B). The degree of cross-adaptation between two stimuli is thoughtto reflect the overlap of their ‘neural activation processes’.We measured self- (A—A) and reciprocal crossadaptation(A—B, B—A) of the response of the hamster chordatympani nerve with lingual presentations of stimuli elicitingequal unadapted transient responses. Adapting and test stimuliwere 0.1 M NaCl, 0.1 M NaNO₃, 0.1 M NaBr, 0.4 M Na acetate (NaAc),0.09 M LiCl and 0.4 M NH₄Cl. Nearly complete and symmetricalcross-adaptation was seen for NaCl, NaNO₃ and NaBr. Those Nasalts paired with LiCl showed strong but asymmetrical cross-adaptation.Exposure to sodium completely eliminated the response to LiClbut not vice versa, suggesting that lithium and sodium are notcompletely interchangeable taste stimuli for the hamster chordatympani. Relatively little cross-adaptation between NH₄Cl andother salts suggested relatively separate neural activationprocesses. Strongly asymmetrical cross-adaptation was foundbetween NaAc and the other sodium salts. Responses to NaCl,NaNO₃ or NaBr were eliminated after adaptation to NaAc whereasthe response to NaAc during the reciprocal cross was strong.Asymmetries are discussed in reference to sensitivities of singlenerve fibers for the chorda tympani, effects of adaptation andthe concept of anion inhibition.     

The chorda tympani and glossopharyngeal nerves in the adult hamster

The numbers and diameters of axons in the intact chorda tympani(CT) and lingual branch of the glossopharyngeal nerve (GN) arequantified with the use of electron microscopic photomontages.The cross-sectional diameters of the CT and GN average 68 and86 microns, respectively. The intact CT contains {small tilde}1050 fibers, 63% are unmyelinated and 37% are myelinated. TheGN contains {small tilde} 1600 fibers, 79% are unmyelinatedand 21% are myelinated. Both nerves are made up of relativelysmall unmyelinated and myelinated fibers, although the GN showsa broader distribution of diameters for its myelinated fibersdue to the presence of general somatosensory fibers. Followingde-efferentation, there is a 48% reduction in the number ofunmyelinated fibers in the CT. Fifty-two per cent of the unmyelinatedfibers are sensory. The number of myelinated fibers is not significantlyreduced and nearly all of the myelinated fibers are sensory.Sixty-seven per cent of the fibers within the CT are sensory.The de-efferented CT contains an equal number of unmyelinatedand myelinated axons and a total of {small tilde} 700 fibers.Comparable data in the rat indicate that its intact and de-efferentedCT are organized differently in regards to the numbers of sensoryand motor, and myelinated versus unmyelinated fibers. The findingsof the present study, together with the available data fromother species, suggest that anatomical differences in the make-upof the major gustatory nerves do not contribute in any obviousway to the known differences in the response properties betweenthe rat and hamster CT, and that the number of myelinated fibersin the visceral motor component of the CT varies considerablyacross species.     

The organization of taste sensibilities in hamster chorda tympani nerve fibers

Electrophysiological measurements of nerve impulse frequencies were used to explore the organization of taste sensibilities in single fibers of the hamster chorda tympani nerve. Moderately intense taste solutions that are either very similar or easily discriminated were applied to the anterior lingual surface. 40 response profiles or 13 stimulus activation patterns were considered variables and examined with multivariate statistical techniques. Three kinds of response profiles were seen in fibers that varied in their overall sensitivity to taste solutions. One profile (S) showed selectivity for sweeteners, a second (N) showed selectivity for sodium salts, and a third (H) showed sensitivity to salts, acids, and other compounds. Hierarchical cluster analysis indicated that profiles fell into discrete classes. Responses to many pairs of effective stimuli were covariant across profiles within a class, but some acidic stimuli had more idiosyncratic effects. Factor analysis of profiles identified two common factors, accounting for 77% of the variance. A unipolar factor was identified with the N profile, and a bipolar factor was identified with the S profile and its opposite, the H profile. Three stimulus activation patterns were elicited by taste solutions that varied in intensity of effect. Hierarchical cluster analysis indicated that the patterns fell into discrete classes. Factor analysis of patterns identified three common unipolar factors accounting for 82% of the variance. Eight stimuli (MgSO4, NH4Cl, KCl, citric acid, acetic acid, urea, quinine HCl, HCl) selectively activated fibers with H profiles, three stimuli (fructose, Na saccharin, sucrose) selectively activated fibers with S profiles, and two stimuli (NaNO3, NaCl) activated fibers with N profiles more strongly than fibers with H profiles. Stimuli that evoke different patterns taste distinct to hamsters. Stimuli that evoke the same pattern taste more similar. It was concluded that the hundreds of peripheral taste neurons that innervate the anterior tongue play one of three functional roles, providing information about one of three features that are shared by different chemical solutions.     

Anion modulation of taste responses in sodium-sensitive neurons of the hamster chorda tympani nerve

Beidler's work in the 1950s showed that anions can strongly influence gustatory responses to sodium salts. We have demonstrated "anion inhibition" in the hamster by showing that the chorda tympani nerve responds more strongly to NaCl than to Na acetate over a wide range of concentrations. Iontophoretic presentation of Cl- and acetate to the anterior tongue elicited no response in the chorda tympani, suggesting that these anions are not directly stimulatory. Drugs (0.01, 1.0, and 100 microM anthracene-9-carboxylate, diphenylamine-2-carboxylate, 4- acetamido-4'-isothiocyanatostilbene-2,2'-disulfonate, and furosemide) that interfere with movements of Cl- across epithelial cells were ineffective in altering chorda tympani responses to 0.03 M of either NaCl or Na acetate. Anion inhibition related to movements of anions across epithelial membranes therefore seems unlikely. The chorda tympani contains a population of nerve fibers highly selective for Na+ (N fibers) and another population sensitive to Na+ as well as other salts and acids (H fibers). We found that N fibers respond similarly to NaCl and Na acetate, with spiking activity increasing with increasing stimulus concentration (0.01-1.0 M). H fibers, however, respond more strongly to NaCl than to Na acetate. Furthermore, H fibers increase spiking with increases in NaCl concentration, but generally decrease their responses to increasing concentrations of Na acetate. It appears that anion inhibition applies to taste cells innervated by H fibers but not by N fibers. Taste cells innervated by N fibers use an apical Na+ channel, whereas those innervated by H fibers may use a paracellularly mediated, basolateral site of excitation.     

Sensitivities of single nerve fibers in the hamster chorda tympani to mixtures of taste stimuli

Responses of three groups of neural fibers from the chorda tympani of the hamster to binary mixtures of taste stimuli applied to the tongue were analyzed. The groups displayed different sensitivities to six chemicals at concentrations that had approximately equal effects on the whole nerve. Sucrose-best fibers responded strongly only to sucrose and D-phenylalanine. NaCl-best and HCl-best fibers, responded to four electrolytes: equally to CaCl2 and nearly equally to HCl, but the former responded more to NaCl, and the latter responded more to NH4Cl. The groups of fibers dealt differently with binary mixtures. Sucrose- best fibers responded to a mixture of sucrose and D-phenylalanine as if one of the chemicals had been appropriately increased in concentration, but they responded to a mixture of either one and an electrolyte as if the concentration of sucrose or D-phenylalanine had been reduced. NaCl- best fibers responded to a mixture as if it were a "mixture" of two appropriate concentrations of one chemical, or somewhat less. But, responses of HCl-best fibers to mixtures were greater than that, approaching a sum of responses to components. These results explain effects on the whole nerve, suggest that the sensitivity of a mammalian taste receptor to one chemical can be affected by a second, which may or may not be a stimulus for that receptor, and suggest that some effects of taste mixtures in humans may be the result of peripheral processes.     

Amiloride-sensitive and amiloride-insensitive responses to NaCl + acid mixtures in hamster chorda tympani nerve

Component signaling in taste mixtures containing both beneficial and dangerous chemicals depends on peripheral processing. Unidirectional mixture suppression of chorda tympani (CT) nerve responses to sucrose by quinine and acid is documented for golden hamsters (Mesocricetus auratus). To investigate mixtures of NaCl and acids, we recorded multifiber responses to 50 mM NaCl, 1 and 3 mM citric acid and acetic acid, 250 μM citric acid, 20 mM acetic acid, and all binary combinations of each acid with NaCl (with and without 30 μM amiloride added). By blocking epithelial Na(+) channels, amiloride treatment separated amiloride-sensitive NaCl-specific responses from amiloride-insensitive electrolyte-generalist responses, which encompass all of the CT response to the acids as well as responses to NaCl. Like CT sucrose responses, the amiloride-sensitive NaCl responses were suppressed by as much as 50% by citric acid (P = 0.001). The amiloride-insensitive electrolyte-generalist responses to NaCl + acid mixtures approximated the sum of NaCl and acid component responses. Thus, although NaCl-specific responses to NaCl were weakened in NaCl-acid mixtures, electrolyte-generalist responses to acid and NaCl, which tastes KCl-like, were transmitted undiminished in intensity to the central nervous system. The 2 distinct CT pathways are consistent with known rodent behavioral discriminations.     

Taste responses of human chorda tympani nerve

Records from humans of summated action potential dischargesof the chorda tympani nerve were examined. The magnitudes ofneural and psychophysical responses were well related only whenthe comparison was made within a given taste quality. The responseto a mixture of 0 02 M citric acid and 0.5 M sucrose was lessthan the sum of the separate responses to the mixture components.Citric acid failed to cross-adapt the response to sucrose, implyingthe receptor sites for sucrose are independent of citric acid.The human chorda tympani nerve shows vigorous responses to mechanicalstimulation and cooling of the tongue that are maintained aftertreatment of the tongue with a water extract of the herb Gymnemasylvestre. Gymnema extract selectively suppressed the responseto all sweeteners tested (sucrose, fructose, saccharin and cyclamate)and also suppressed by – 50% the water-after-citric-acidresponse which has a predominantly sweet taste. Gymnema suppressedby 0 – 10% the water-rinse response following NaCl. fructoseand sucrose that have a predominantly bitter-sour taste. Water-rinseresponses were present even when mechanical and thermal stimulationof the tongue were minimized. The human chorda tympani nerveappears to have positive water-rinse taste responses. Theseare solute-specific off-responses that are probably mediatedby receptor sites independent of those responsible for the on-responseto the given solute.     

Sugar best single chorda tympani nerve fiber responses to various sugar stimuli in rat and hamster

1. Sugar best single chorda tympani nerve fiber of rat and hamster were tested with six sugars. 2. Fibers were selected for this experiment, only if they responded to 1.0 M sucrose or 1.0 M maltose and they responded poorly to 0.1 M NaCl. 3. In rat, some single fibers gave larger responses to maltose than to sucrose, while in hamster nearly all nerve fibers responded best to sucrose. 4. The order of effectiveness of sugars was maltose greater than fructose greater than or equal to lactose greater than sucrose greater than glucose greater than galactose in rat and sucrose greater than fructose greater than or equal to glucose greater than or equal to galactose greater than maltose greater than lactose in hamster.     

Direct measurement of translingual epithelial NaCl and KCl currents during the chorda tympani taste response.

We have measured the NaCl or KCl currents under voltage clamp across the dorsal lingual epithelium of the rat and simultaneously the response of the taste nerves. Under short-circuit conditions a NaCl stimulus evoked an inward current (first current) that coincided with excitation of the chorda tympani. This was followed by a slower inward current (second current) that matched the kinetics of taste nerve adaptation. The peak first current and the coincident neural response satisfied the same saturating NaCl concentration dependence. Both first and second currents were partially blocked by amiloride as were the phasic and tonic components of the neural response. The NaCl-evoked second current was completely blocked by ouabain. Investigation of the NaCl-evoked current and the neural response over a range of clamped voltages showed that inward negative potentials enhanced the inward current and the neural response to 0.3 M NaCl. Sufficiently high inward positive potentials reversed the current, and made the neural response independent of further changes in voltage. Therefore, one of the NaCl taste transduction mechanisms is voltage dependent while the other is voltage independent. A KCl stimulus also evoked an inward short-circuit current, but this and the neural response were not amiloride-sensitive. The data indicate that neural adaptation to a NaCl stimulus, but not a KCl stimulus, is mediated by cell Na/K pumps. A model is proposed in which the connection between the NaCl-evoked second current and cell repolarization is demonstrated.     

Responses of the rat chorda tympani nerve to glutamate-sucrose mixtures

Monosodium glutamate (MSG) has a multifaceted, unusual taste to humans. Rats and other rodents also detect a complex taste to MSG. Responses of the chorda tympani nerve (CT) to glutamate applied to the front of the tongue were recorded in 13 anesthetized rats. Whole-nerve responses to 30 mM, 100 mM and 300 mM MSG mixed with 300 mM sucrose were recorded before and after adding 30 micro M amiloride to the rinse and stimulus solutions. Responses of CT single fibers were also recorded. Predictions from models of whole-nerve responses to binary mixtures were compared to the observed data. Results indicated that MSG-elicited CT responses have multiple sources, even in an amiloride-inhibited environment in rats. Those sources include responses of sucrose-sensitive CT neural units, which may provide the substrate for a sucrose-glutamate perceptual similarity, and responses of sucrose-insensitive CT neural units, which may respond synergistically to MSG-sucrose mixtures.     

Intracellular pH modulates taste receptor cell volume and the phasic part of the chorda tympani response to acids

The relationship between cell volume and the neural response to acidic stimuli was investigated by simultaneous measurements of intracellular pH (pHi) and cell volume in polarized fungiform taste receptor cells (TRCs) using 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) in vitro and by rat chorda tympani (CT) nerve recordings in vivo. CT responses to HCl and CO2 were recorded in the presence of 1 M mannitol and specific probes for filamentous (F) actin (phalloidin) and monomeric (G) actin (cytochalasin B) under lingual voltage clamp. Acidic stimuli reversibly decrease TRC pHi and cell volume. In isolated TRCs F-actin and G-actin were labeled with rhodamine phalloidin and bovine pancreatic deoxyribonuclease-1 conjugated with Alexa Fluor 488, respectively. A decrease in pHi shifted the equilibrium from F-actin to G-actin. Treatment with phalloidin or cytochalasin B attenuated the magnitude of the pHi-induced decrease in TRC volume. The phasic part of the CT response to HCl or CO2 was significantly decreased by preshrinking TRCs with hypertonic mannitol and lingual application of 1.2 mM phalloidin or 20 microM cytochalasin B with no effect on the tonic part of the CT response. In TRCs first treated with cytochalasin B, the decrease in the magnitude of the phasic response to acidic stimuli was reversed by phalloidin treatment. The pHi-induced decrease in TRC volume induced a flufenamic acid-sensitive nonselective basolateral cation conductance. Channel activity was enhanced at positive lingual clamp voltages. Lingual application of flufenamic acid decreased the magnitude of the phasic part of the CT response to HCl and CO2. Flufenamic acid and hypertonic mannitol were additive in inhibiting the phasic response. We conclude that a decrease in pHi induces TRC shrinkage through its effect on the actin cytoskeleton and activates a flufenamic acid-sensitive basolateral cation conductance that is involved in eliciting the phasic part of the CT response to acidic stimuli.     

Inhibitory effect of the extract from Zizyphus jujuba leaves on sweet taste responses of the chorda tympani in the rat and hamster

1. One of the fractions obtained from the extract of Zizyphus jujuba leaves suppressed the response of the chorda tympani to sucrose, both in the rat and hamster. 2. In the rat and man, suppressive effect was found to be significant in responses to various sugars and artificial sweeteners but not in some sweet amino acids.     

Effect of chorda tympani nerve transection on salt taste perception in mice

Effects of gustatory nerve transection on salt taste have been studied extensively in rats and hamsters but have not been well explored in the mouse. We examined the effects of chorda tympani (CT) nerve transection on NaCl taste preferences and thresholds in outbred CD-1 mice using a high-throughput phenotyping method developed in our laboratory. To measure taste thresholds, mice were conditioned by oral self-administration of LiCl or NaCl and then presented with NaCl concentration series in 2-bottle preference tests. LiCl-conditioned and control NaCl-exposed mice were given bilateral transections of the CT nerve (LiCl-CTX, NaCl-CTX) or were left intact as controls (LiCl-CNT, NaCl-CNT). After recovery from surgery, mice received a concentration series of NaCl (0-300 mM) in 48-h 2-bottle tests. CT transection increased NaCl taste thresholds in LiCl-conditioned mice and eliminated avoidance of concentrated NaCl in control NaCl-exposed mice. This demonstrates that in mice, the CT nerve is important for detection and recognition of NaCl taste and is necessary for the normal avoidance of high concentrations of NaCl. The results of this experiment also show that the method of high-throughput phenotyping of salt taste thresholds is suitable for detecting changes in the taste periphery in mouse genetic studies.     

Selective procaine inhibition of rat chorda tympani responses to electric taste stimulation

1. The lingual treatment of 1% procaine for 10 min selectively suppressed responses of the rat chorda tympani nerve to anodal current applied to the tongue with NaCl in the bathing medium to about 50% of control but the drug produced no significant suppression in responses to chemical taste stimuli. 2. The magnitude of suppression of response to anodal current varied with concentration of procaine and kind of bathing medium for the current stimulation (larger in the order of NaCl greater than KCl greater than CaCl2 greater than HCl). 3. Such ion specificity in procaine suppression suggests that responses of the chorda tympani nerve to anodal current are provoked through the taste cell (not direct action on the taste nerve), and that the receptor mechanisms for anodal current are at least partly different from that for chemical taste stimuli.     

Developmental changes in sugar responses of the chorda tympani nerve in preweanling rats

To clarify developmental changes in the gustatory system of the rat, integrated taste responses from the chorda tympani (CT) nerve were recorded and analyzed at different postnatal ages. The response magnitude was calculated relative to the response to the standard, 0.1 M NH4Cl. Even at 1 week of age, the CT responded well to all tested 0.1 M chloride salts (NH4Cl, NaCl, LiCl, KCl, RbCl and CsCl). The responses to 0.1 M NaCl and LiCl increased with increasing age of the rat while response magnitudes to KCl, RbCl and CsCl did not change up to 8 weeks. At 1 week, the integrated response pattern was quite similar to that in adult rats for NaCl, HCl and quinine hydrochloride (QHCl). The concentration-response functions for NaCl, HCl, QHCl and sucrose at 2 weeks were essentially the same as those at 8 weeks. These results suggest that taste buds in the 2-week-old rat are functionally mature for the detection of the four basic taste stimuli. The relative magnitude of the responses to the various sugars was smaller at 1 week compared to the adult rat and reached a maximum at weeks 3-4, then decreased gradually with age. Among the six sugars, sucrose was the most effective followed by lactose. From weeks 1-4, the magnitude of the integrated taste response to fructose was smaller than that to lactose except at 3 weeks of age. Maltose, galactose and glucose were less potent stimuli than the other sugars tested. The response magnitude to lactose at 4 weeks had decreased compared to that for the other sugars. Taste responses to the sugars in preweanling and adult rats were not cross-adapted by the individual sugars. These results suggest that after 1 week of age during postnatal development in the rat, taste information from the CT rapidly increases in its importance for feeding behavior.     

Different responsiveness of the chorda tympani and glossopharyngeal nerves to L-lysine in mice

Differential taste responsiveness and functional role of thetwo taste nerves, the chorda tympani (CT) and die glossopharyngeal(GL), were studied in mice by examining neural and behavioralresponses to an essential amino acid, L-lysine (Lys). Relativeresponses to Lys were larger in the GL than in the CT nerve.The neural threshold for the Lys response was about 2.5 logunits lower in the GL (about 1.0 µM) than in the CT nerve(about 300 µM). An analysis of concentration-responserelationships suggests a possibility that there are two differentreceptors (high and low affinity types) for Lys showing differentdissociation constants. The posterior tongue region possessesboth types, while the anterior region possesses only the lowaffinity type. Behavioral aversion threshold for Lys in intact mice, measuredby use of a single bottle test, was about 1.0 µM. Thisthreshold was the same as its neural threshold in the GL nerve.Animals whose bilateral GL nerves were sectioned showed a higheraversion threshold (about 300 µM) which was the same asthe neural threshold in the CT nerve. An aversion conditionedto Lys significantly generalized to L-arginine in the intactand CT-denervated mice, and L-arginine and L-histidine in theGL-denervated mice, but the generalization pattern across varioustaste stimuli including the four basic taste stimuli (NaCl,HCl, quinine HCl and sucrose) did not prominently differ amongthe intact, the GL-denervated and CT-denervated mice. These results suggest that taste sensitivity to Lys is higherin the GL than in the CT nerve, but taste quality informationfor Lys conveyed by two taste nerves is not largely different.     

Inhibition of hamster fertilization by phytoagglutinins

Unfertilized eggs of the golden hamster were treated with phytoagglutinins and inseminated in vitro with capacitated spermatozoa. Ricinus communis agglutinin was most effective in preventing fertilization, followed by wheat germ agglutinin, Dolichos biflorus agglutinin and finally concanavalin A. The agglutinin-mediated block to fertilization is related to the saccharide-binding activities of agglutinins, because inclusion of the appropriate saccharide inhibitor counteracted the actions of the agglutinins. It is proposed that the agglutinins bind to specific oligosaccharides of the zona pellucida and induce extensive cross-linking of adjacent saccharide chains in such a way that sperm-born zona lysins can no longer depolymerize the zona material. Spermatozoa failed to bind to zona surfaces following treatment of eggs with high concentrations of wheat germ agglutinin, but not with the other agglutinins, suggesting that N-acetyl- -glucosamine-like or N-acetylneuraminic acid-like residues may be essential or sterically close to sperm attachment sites on the zona pellucida of the hamster egg.     

Taste discrimination between NaCl and KCl is disrupted by amiloride in inbred mice with amiloride-insensitive chorda tympani nerves

The amiloride-sensitive salt transduction pathway is thought to be critical for the discrimination between sodium and nonsodium salts in rodents. In rats, lingual application of amiloride appears to render NaCl qualitatively indistinguishable from KCl. In this study, we tested four strains of mice for salt discriminability. In one strain (C57BL/6J), chorda tympani nerve (CT) responses to NaCl are attenuated by amiloride, and in the other three strains (BALB/cByJ, 129P3/J, DBA/2J) they are not. Under water-restriction conditions, these mice (7 mice/strain) were trained in a gustometer to lick for water from one reinforcement spout in response to a five-lick presentation of NaCl and to lick from another in response to KCl [salt concentration was varied (0.1-1 M) to render intensity irrelevant]. Mice were then tested with the stimuli dissolved in amiloride hydrochloride, and the latter was used as the reinforcer as well. Each concentration of amiloride (0.1-100 microM) was used on 2 separate days with control sessions interposed. Mice from all four strains were able to discriminate NaCl from KCl reliably. Amiloride impaired this discrimination in a dose-dependent fashion. Moreover, performance on NaCl trials appeared to be more affected by amiloride than that on KCl trials in all four strains. Thus, in contrast to the predictions based on CT recordings, discrimination in all four strains appeared to depend on the amiloride-sensitive transduction pathway, which, in the case of BALB/cByJ, 129P3/J, and DBA/2J (and perhaps C57BL/6 as well), may exist in taste buds innervated by nerves other than the CT.