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.     

Effects of binary taste stimuli on the neural activity of the hamster chorda tympani

Binary mixtures of taste stimuli were applied to the tongue of the hamster and the reaction of the whole corda tympani was recorded. Some of the chemicals that were paired in mixtures (HCl, NH4Cl, NaCl, CaCl2, sucrose, and D-phenylalanine) have similar tastes to human and/or hamster, and/or common stimulatory effects on individual fibers of the hamster chorda tympani; other pairs of these chemicals have dissimilar tastes and/or distinct neural stimulatory effects. The molarity of each chemical with approximately the same effect on the activity of the nerve as 0.01 M NaCl was selected, and an established relation between stimulus concentration and response allowed estimation of the effect of a "mixture" of two concentrations of one chemical. Each mixture elicited a response that was smaller than the sum of the responses to its components. However, responses to some mixtures approached this sum, and responses to other mixtures closely approached the response to a "mixture" of two concentrations of one chemical. Responses of the former variety were generated by mixtures of an electrolyte and a nonelectrolyte and the latter by mixtures of two electrolytes or two nonelectrolytes. But, beyond the distinction between electrolytes and nonelectrolytes, the whole-nerve response to a mixture could not be predicted from the known neural or psychophysical effects of its components.     

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.     

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.

     

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.     

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.     

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.     

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.     

Response properties of macaque monkey chorda tympani fibers

Many of the chorda tympani fibers of crab-eating monkeys respond to more than one of the four basic stimuli (NaCl, sucrose, HCl, and quinine hydrochloride) as well as cooling or warming of the tongue. Fibers could be classified into four categories depending on their best sensitivity to any one of the four basic stimuli. Sucrose-best and quinine-best fibers are rather specifically sensitive to sucrose and quinine, respectively, while salt-best and acid-best fibers respond relatively well to HCl and NaCl, respectively. Saccharin, dulcin, and Pb acetate produce a good response in sucrose-best fibers, but quinine- best and salt-best fibers also respond to saccharin. Highly significant positive correlations exist between amounts of responses to sucrose and those to saccharin, dulcin, and Pb acetate, indicating that these substances produce in the monkey a taste quality similar to that produced by sucrose. Compared with chroda tympani fibers of rats, hamsters, and squirrel monkeys, macaque monkey taste fibers are more narrowly tuned to one of the four basic taste stimuli and more highly developed in sensitivity to various sweet-tasting substances. Also LiCl and NaCl are more effective stimuli for gustatory receptors in macaque monkeys than NH4Cl and KCl. This contrasts with a higher sensitivity to KCl and NH4Cl than to NaCl in chorda tympani fibers of squirrel monkeys.     

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.     

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.     

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.     

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.     

咸味觉厌恶模型大鼠鼓索神经对味觉刺激的电生理反应特性

目的:探索大鼠咸味觉厌恶建立后外周鼓索神经(CT)对咸味觉及其他味觉刺激的电生理反应特性的改变。方法:将14只SD成年雄性大鼠分为咸味觉厌恶模型组(CTA)和对照组(n=7/group)。实验第1日给予大鼠30min的0.1mol/LNaCl饮食,随后CTA组和对照组大鼠分别腹腔注射2ml0.15mol/LLiCl和同等量生理盐水。在第2、3和4日,测量两组大鼠每天30min内对NaCl和蒸馏水饮用量。于第4日行为学测试后,分别记录CTA组大鼠和对照组大鼠CT对口内给予系列浓度NaCl溶液、0.3mol/LNaCl与0.1mmol/L阿米洛利(一种舌上皮钠通道阻断剂)混合液和其他四种基本味觉刺激溶液的电生理反应。结果:与对照组相比,CTA组大鼠CT对系列浓度NaCl和其他4种基本味觉刺激的电生理反应特性没有发生明显变化(P>0.05);舌上皮钠通道阻断剂阿米洛利强烈抑制CTA大鼠对NaCl的反应(P<0.01)。结论:条件性咸味觉厌恶模型大鼠CT对各种味觉刺激的电生理反应特性没有发生明显改变。     

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.