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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Developmental changing of rat taste responses from chorda tympani nerve

In order to clarify developmental changing of gustatory system, histological and electrophysiological experiments were performed in the rat. Histological examination on the anterior tongue innervated by chorda tympani nerve showed that the ratio of matured taste buds which possess a taste pore were only 9% of all taste buds observed at 1 week of postnatal age, and 81.3% at 3 weeks of age. Recording integrated responses from the chorda tympani nerve reveals that taste buds with a pore at 1 week of age responded to NaCl, HCl, and quinine-HCl as well as in adult rats, which suggests that these relatively young taste buds are matured functionally for these three stimuli. However, the response magnitudes for various sugars at 1 week of age were smaller compared to those in the adult rat, reached to the maximum at 3 weeks of age, then decreased gradually with age. Also, results from the experiment of cross-adaptation among different sugars, effects of pronase-E treatment of the tongue, analysis of correlation between on- and off-responses to sugars, showed that qualitative changes for sugar responses continues after 3 weeks of age. These results suggest that functional changes occur in the gustatory processing of sugars during postnatal development in the rat chorda tympani nerve.     

Involvement of NADPH-dependent and cAMP-PKA sensitive H+ channels in the chorda tympani nerve responses to strong acids

To investigate if chorda tympani (CT) taste nerve responses to strong (HCl) and weak (CO(2) and acetic acid) acidic stimuli are dependent upon NADPH oxidase-linked and cAMP-sensitive proton conductances in taste cell membranes, CT responses were monitored in rats, wild-type (WT) mice, and gp91(phox) knockout (KO) mice in the absence and presence of blockers (Zn(2+) and diethyl pyrocarbonate [DEPC]) or activators (8-(4-chlorophenylthio)-cAMP; 8-CPT-cAMP) of proton channels and activators of the NADPH oxidase enzyme (phorbol 12-myristate 13-acetate [PMA], H(2)O(2), and nitrazepam). Zn(2+) and DEPC inhibited and 8-CPT-cAMP, PMA, H(2)O(2), and nitrazepam enhanced the tonic CT responses to HCl without altering responses to CO(2) and acetic acid. In KO mice, the tonic HCl CT response was reduced by 64% relative to WT mice. The residual CT response was insensitive to H(2)O(2) but was blocked by Zn(2+). Its magnitude was further enhanced by 8-CPT-cAMP treatment, and the enhancement was blocked by 8-CPT-adenosine-3'-5'-cyclic monophospho-rothioate, a protein kinase A (PKA) inhibitor. Under voltage-clamp conditions, before cAMP treatment, rat tonic HCl CT responses demonstrated voltage-dependence only at ±90 mV, suggesting the presence of H(+) channels with voltage-dependent conductances. After cAMP treatment, the tonic HCl CT response had a quasi-linear dependence on voltage, suggesting that the cAMP-dependent part of the HCl CT response has a quasi-linear voltage dependence between +60 and -60 mV, only becoming sigmoidal when approaching +90 and -90 mV. The results suggest that CT responses to HCl involve 2 proton entry pathways, an NADPH oxidase-dependent proton channel, and a cAMP-PKA sensitive proton channel.     

Synergistic responses of the chorda tympani to mixtures of umami and sweet substances in rats

It has been known that umami substances such as monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) elicit a unique taste called 'umami' in humans. One of the characteristics of the umami taste is synergism: the synergistic enhancement of the magnitude of response produced by the addition of 5'-ribonucleotides to MSG. In addition to this well-documented synergism, we report here for the first time on another type of synergism between a glutamate receptor agonist, L-AP4, and sweet substances, by analyzing the chorda tympani responses in rats. The results are as follows: (i) when L-AP4 was mixed with one of the sweet substances, such as sucrose, glucose, fructose and maltose, large synergistic responses were observed. (ii) These synergistic responses, except to L-AP4 + sucrose, were not suppressed by sweet taste suppressants, gurmarin and pronase E. (iii) These synergistic responses were not suppressed by either metabotropic or ionotropic glutamate receptor antagonists. (iv) Fibers that responded well to the binary mixtures of L-AP4 and sweet substances also responded well to NaCl and HCl, but very weakly to sucrose. These findings are different from the characteristics of synergism between glutamate and IMP. The multiple transduction mechanisms for the umami taste in rat taste cells are discussed.     

Loss of sensitivity to low concentrations of NaCl following bilateral chorda tympani nerve sections in rats

A reliable short-term NaCl taste test was developed for ratswhich resulted in differential responding to a variety of concentrations.The rats were required to exist on a low sodium diet for 8–10days prior to the initiation of testing. The peak response inthis test was to isotonic NaCl with lesser responding to hyper-and hypotonc solutions. After stable responding was obtained,bilateral sections were made of the chorda tympani nerves. Thissurgery resulted in a loss of sensitivity to the lowest hypotonicsolutions (0.03 and 0.06 M NaCl). Little, if any, effect wasnoted in the perception of sucrose following these nerve sections.     

Modulation of rat chorda tympani NaCl responses and intracellular Na+ activity in polarized taste receptor cells by pH

Mixture interactions between sour and salt taste modalities were investigated in rats by direct measurement of intracellular pH (pH(i)) and Na(+) activity ([Na(+)](i)) in polarized fungiform taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the lingual surface with NaCl solutions adjusted to pHs ranging between 2.0 and 10.3 increased the magnitude of NaCl CT responses linearly with increasing external pH (pH(o)). At pH 7.0, the epithelial sodium channel (ENaC) blocker, benzamil, decreased NaCl CT responses and inhibited further changes in CT responses induced by varying pH(o) to 2.0 or 10.3. At constant pH(o), buffering NaCl solutions with potassium acetate/acetic acid (KA/AA) or HCO(3)(-)/CO(2) inhibited NaCl CT responses relative to CT responses obtained with NaCl solutions buffered with HEPES. The carbonic anhydrase blockers, MK-507 and MK-417, attenuated the inhibition of NaCl CT responses in HCO(3)(-)/CO(2) buffer, suggesting a regulatory role for pH(i). In polarized TRCs step changes in apical pH(o) from 10.3 to 2.0 induced a linear decrease in pH(i) that remained within the physiological range (slope = 0.035; r(2) = 0.98). At constant pH(o), perfusing the apical membrane with Ringer's solutions buffered with KA/AA or HCO(3)(-)/CO(2) decreased resting TRC pH(i), and MK-507 or MK-417 attenuated the decrease in pH(i) in TRCs perfused with HCO(3)(-)/CO(2) buffer. In parallel experiments, TRC [Na(+)](i) decreased with (a) a decrease in apical pH, (b) exposing the apical membrane to amiloride or benzamil, (c) removal of apical Na(+), and (d) acid loading the cells with NH(4)Cl or sodium acetate at constant pH(o). Diethylpyrocarbonate and Zn(2+), modification reagents for histidine residues in proteins, attenuated the CO(2)-induced inhibition of NaCl CT responses and the pH(i)-induced inhibition of apical Na(+) influx in TRCs. We conclude that TRC pH(i) regulates Na(+)-influx through amiloride-sensitive apical ENaCs and hence modulates NaCl CT responses in acid/salt mixtures.     

Acid-induced responses in hamster chorda tympani and intracellular pH tracking by taste receptor cells

HCl- and NaCl-induced hamster chorda tympani nerve responseswere recorded during voltage clamp of the lingual receptive field. Voltage perturbations did not influence responses to HCl. In contrast, responses to NaCl were decreased by submucosal-positive and increased by submucosal-negative voltage clamp. Responses to HCl were insensitive to the Na⁺ channel blockers,amiloride and benzamil, and to methylisobutylamiloride (MIA), anNa⁺/H⁺exchange blocker. Responses to NaCl were unaffected by MIA but weresuppressed by benzamil. Microfluorometric and imaging techniques wereused to monitor the relationship between external pH(pH_o) and the intracellular pH(pH_i) of fungiform papilla tastereceptor cells (TRCs) following2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein loading.TRC pH_i responded rapidly andmonotonically to changes in pH_o.This response was unaffected byNa⁺ removal or the presence ofamiloride, benzamil, or MIA. The neural records and the data fromisolated TRCs suggest that the principal transduction pathway for acidtaste in hamster is similar to that in rat. This may involve themonitoring of changes in TRC pH_i mediated through amiloride-insensitiveH⁺ transport across TRC membranes.This is an example of cell monitoring of environmental pH through pHtracking, i.e., a linear change inpH_i in response to a change inpH_o, as has been proposed for carotid bodies. In taste, the H⁺transport sites may be concentrated on the basolateral membranes ofTRCs and, therefore, are responsive to an attenuatedH⁺ concentration from diffusion ofacids across the tight junctions.

     

Effects of chorda tympani nerve anesthesia on taste responses in the NST

Human clinical and psychophysical observations suggest that the taste system is able to compensate for losses in peripheral nerve input, since patients do not commonly report decrements in whole mouth taste following chorda tympani nerve damage or anesthesia. Indeed, neurophysiological data from the rat nucleus of the solitary tract (NST) suggests that a release of inhibition (disinhibition) may occur centrally following chorda tympani nerve anesthesia. Our purpose was to study this possibility further. We recorded from 59 multi- and single- unit taste-responsive sites in the rat NST before, during and after recovery from chorda tympani nerve anesthesia. During anesthesia, average anterior tongue responses were eliminated but no compensatory increases in palatal or posterior tongue responses were observed. However, six individual sites displayed increased taste responsiveness during anesthesia. The average increase was 32.9%. Therefore, disinhibition of taste responses was observed, but infrequently and to a small degree in the NST At a subset of sites, chorda tympani-mediated responses decreased while greater superficial petrosal-mediated responses remained the same during anesthesia. Since this effect was accompanied by a decrease in spontaneous activity, we propose that taste compensation may result in part by a change in signal-to-noise ratio at a subset of sites.      

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.     

Linoleic acid increases chorda tympani nerve responses to and behavioral preferences for monosodium glutamate by male and female rats

Previous studies suggest that the chorda tympani nerve (CT) is important in transmitting fat taste information to the central nervous system. However, the contribution of the CT in this process may depend upon the presence of other taste stimuli and/or differ in males and females. Accordingly, the present study investigated the role of the CT in free fatty acid taste processing by examining electrophysiological activity of the CT in response to the free fatty acid linoleic acid (LA), as well as by measuring behavioral responses to LA-taste mixtures. We recorded whole nerve responses from the CT in response to lingual application of LA with or without monosodium glutamate (MSG) in anesthetized male and female rats. In addition, we examined preferences for MSG + LA taste mixtures in behavioral tests. Although lingual application of LA alone did not produce CT whole nerve responses, coapplication of LA and MSG elicited greater CT responses than did MSG alone. These findings were paralleled by greater preferences for MSG + LA taste mixtures than for MSG alone. In both cases, the effect was particularly pronounced in male rats. Thus LA enhances CT activity and behavioral responses to LA + MSG taste mixtures, although there are sex differences in the effects. These results suggest that CT input is important in mediating behavioral responses to fat taste, but the effects depend upon other taste stimuli and differ in males and females.     

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.     

Gustatory responses to amino acids in the chorda tympani nerve of C3H mice

Integrated neural responses to various amino acids were recorded from the chorda tympani (facial) nerve in C3H mice. The basic amino acids hydrochlorides L-Arg-HCl and L-Lys-HCl evoked large magnitude integrated taste responses, similar to that for NaCl, and had estimated electrophysiological thresholds of 0.0001 M. No significant difference was indicated between the response magnitudes for the L- and D-forms of the basic amino acid hydrochlorides; however, responses to the basic amino acid hydrochlorides cross-adapted with NaCl. Responses to neutral L-amino acids (Ser, Ala, Gly), which taste sweet to humans, showed higher thresholds (>0.0003 M), similar to that for sucrose, and did not cross-adapt with basic amino acid hydrochlorides or with NaCl. Responses to the neutral amino acids L-Ser and L-Ala were larger than those to their D-amino acid enantiomers. The acidic amino acids L-Asp and L-Glu showed concentration-response functions different from that for HCl. Both acidic amino acids were more stimulatory than HCl at the same pH, although the responses to them were cross-adapted by HCl, indicating a pH effect. A comparison of the stimulatory effectiveness among amino acid derivatives and analogues suggested that the alpha- amino group is essential for the stimulatory effectiveness of neutral amino acids.      

The K+-H+ exchanger, nigericin, modulates taste cell pH and chorda tympani taste nerve responses to acidic stimuli

The relationship between acidic pH, taste cell pH(i), and chorda tympani (CT) nerve responses was investigated before and after incorporating the K(+)-H(+) exchanger, nigericin, in the apical membrane of taste cells. CT responses were recorded in anesthetized rats in vivo, and changes in pH(i) were monitored in polarized fungiform taste cells in vitro. Under control conditions, stimulating the tongue with 0.15 M potassium phosphate (KP) or 0.15 M sodium phosphate (NaP) buffers of pHs between 8.0 and 4.6, KP or NaP buffers did not elicit a CT response. Post-nigericin (500 × 10(-6) M), KP buffers, but not NaP buffers, induced CT responses at pHs ≤ 6.6. The effect of nigericin was reversed by the topical lingual application of carbonyl cyanide 3-chloro-phenylhydrazone, a protonophore. Post-nigericin (150 × 10(-6) M), KP buffers induced a greater decrease in taste cell pH(i) relative to NaP buffers and to NaP and KP buffers under control conditions. A decrease in pH(i) to about 6.9 induced by KP buffers was sufficient to elicit a CT response. The results suggest that facilitating apical H(+) entry via nigericin decreases taste cell pH(i) and demonstrates directly a strong correlation between pH(i) and the magnitude of the CT response.