Gustatory sensitivities of the hamster's soft palate

The response properties of taste receptors distributed on thesoft palate of the hamster were studied by recording integratedresponses from the greater superficial petrosal (GSP) nerveStimuli were concentration series of sucrose, NaCl, HCl andquinine hydrochloride (QHCl), and several other 0.1 M saltsand 0.5 M sugars. For comparison, integrated responses wererecorded from the chorda tympani (CT) nerve in many of the sameanimals from which recordings were made from the GSP. Responsesin each preparation were scaled relative to the phasic responseto 0.1 M NaCl and were then expressed for each nerve as a proportionof the total response magnitude (TRM)—the sum of all theresponses to the four concentration series. In this way, therelative response of each nerve to all of the stimuli couldbe evaluated. There were significant differences between theGSP and CT nerves in the responses to NaCl, QHCl and sucrose.Both the phasic and tonic responses to sucrose were larger inthe GSP than in the CT, whereas the tonic responses to NaCland QHCl were smaller. The slopes of the concentration-responsefunctions for NaCl, HCl and sucrose were significantly differentbetween the two nerves. The responses to 0.1 M sodium and lithiumsalts were significantly greater in the CT than in the GSP;whereas the 0.5 M sugars elicited responses in the GSP thatwere 2–3 times greater than in the CT nerve. A comparisonof the relative responsiveness to 0.3M sucrose, 0 3 M NaCl,0.01 M QHCl, 0.01 M HCl and distilled water among the GSP, CT,glossopharyngeal (IXth) nerve and superior laryngeal nerve (SLN)indicated that the vast majority of information about sucroseand NaCl is transmitted to the brainstem by the VIIth nerve. ¹Present address: Department of Oral Physiology, Kagoshima UniversityDental School, Kagoshima 890, Japan     

Taste confusions following chlorhexidine treatment.

Chlorhexidine, a bitter bis-biguanide antiseptic, is the only known blocker of the human salty taste. In order to characterize the effects of chlorhexidine on stimulus identification, taste confusion matrix (TCM) performance was measured for subjects treated with 1.34 mM chlorhexidine gluconate (n = 9) and water controls (n = 9). Ten stimuli [water, 0.1 M NaCl, 0.1 M KCl, 0.1 mM quinine-HCl (QHCl), 0.1 M monosodium glutamate (MSG), 3 mM citric acid, 0.3 M sucrose and mixtures of NaCl, QHCl and citric acid with sucrose] were presented in 10 replicates for identification from a list of 10 stimulus names. T(10), a measure of performance consistency from information theory, was lower for chlorhexidine-treated subjects (2.02 +/- 0.11 bits) than controls (2.73 +/- 0.11 bits) (P < 0.0001). T(2), an indirect measure of pairwise stimulus discrimination, approached chance levels (0.40 bit) in chlorhexidine-treated subjects for all possible pairs of NaCl, KCl, QHCl and water, as well as pairs composed of sucrose and the NaCl-sucrose and quinine-sucrose mixtures. In controls T(2) values approached perfect scores (1.00 bit) for all stimulus pairs except NaCl-KCl and NaCl-MSG. The results demonstrate a decreased ability to identify taste stimuli that is consistent with alterations in the ability of stimuli to elicit salty and bitter taste perceptions. As a selective, effective, persistent and reversible blocker of taste perceptions, chlorhexidine should prove useful in defining taste mechanisms in humans.     

'Thermal taste' predicts higher responsiveness to chemical taste and flavor

Individual differences in taste perception have been explained in part by variations in peripheral innervation associated with the genetic ability to taste the bitter substances PTC and PROP. In the present study we report evidence of another source of individual differences that is independent of taste stimulus, taste quality, or gustatory nerve. Individuals who perceived taste from thermal stimulation alone (thermal taste) gave significantly higher taste ratings to chemical stimuli--often by a factor of >2:1--than did individuals who perceived no taste from thermal stimulation. This was true for all taste stimuli tested (sucrose, saccharin, sodium chloride, citric acid, quinine sulfate, MSG and PROP), for all three gustatory areas of the mouth (anterior tongue, posterior tongue and soft palate) and for whole-mouth stimulation. Moreover, the same individuals reported stronger sensations from the olfactory stimulus vanillin, particularly when it was sensed retronasally. The generality of the thermal-taster advantage and its extension to an olfactory stimulus suggests that it arises from individual differences in CNS processes that are involved in perception of both taste and flavor.     

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.     

味觉刺激引起大鼠臂旁核神经元抑制性反应

本研究以轻度麻醉的大鼠为对象,应用细胞外微电极记录技术,观察并分析了脑桥臂旁核抑制性味觉神经元的自发活动及其对NaCl、HCl、盐酸奎宁（quinine HCl,QHCl））和蔗糖等四种基本味觉刺激的反应。共分析了18个具有自发活动的抑制性味觉神经元,自发放电频率分布在0．2～5．5Hz之间,平均放电频率（2．15±0．31）Hz。18个神经元中,1个神经元对单一味觉刺激呈抑制性反应,其余17个神经元对两种或两种以上的基本味觉刺激发生抑制性反应,且抑制具有潜伏期短、持续时间较长等特征。抑制持续时间5～80S,部分神经元表现为后抑制效应。根据神经元对四种基本味觉刺激呈抑制性反应的程度,将其分为NaCl优势神经元（n=8）,HCl优势神经元（n=3）,QHCl优势神经元n=3）和蔗糖优势神经元n=4）。其中NaCl优势神经元的反应谐宽最高（0．945）。这些神经元对欣快或厌恶刺激的区别能力较低。结果提示,在脑桥臂旁核存在对味觉刺激起抑制性反应的神经元,这些味觉神经元可能在味觉的调制及对欣快和厌恶刺激的编码中发挥重要的作用。     

A metric for the breadth of tuning of gustatory neurons

The breadth of the responsiveness of gustatory neurons to thefour basic taste stimuli (sucrose,NaCl, HCl and quinine hydrochloride)is a question of importance to current theories of gustatoryquality coding. The mathematical expression of entropy frominformation theory can provide a measure of the breadth of tuningof these neurons. The entropy measure, as applied here, variescontinuously from 0.0 for a unit that responds exclusively toone stimulus (i.e., narrowly tuned) to 1.0 for a cell that respondsequivalently to all four of the basic compounds (i.e., broadlytuned). Subtle variations in the neural response profile ofa cell, such as those produced by changes in stimulus concentration,are reflected in this measure. Thus, the use of the equationfor entropy to describe the responsiveness of gustatory neuronsprovides a quantitative measure of their breadth of tuning thatcan be meaningfully applied to the problems of gustatory qualitycoding.     

Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.     

Vasopressin increases frog gustatory neural responses elicited by NaCl and HCl.

1. The effect of arginine vasopressin (AVP) on frog gustatory responses was investigated by recording integrated responses of the whole glossopharyngeal nerve by stimulation of the tongue with tastants. 2. After AVP (100 mUnits/ml) was perfused to the basolateral side of taste cells through the lingual artery, gustatory neural responses for NaCl and hydrochloric acid (HCl) stimuli were greatly enhanced, but the responses for CaCl2, quinine hydrochloride (Q-HCl) and galactose were not affected. 3. Three hours after the onset of AVP perfusion, the responses for NaCl and HCl increased to 260% and 270% of the respective controls. 4. The NaCl response which was insensitive to amiloride during normal saline perfusion became sensitive to amiloride during AVP perfusion. 5. When membrane-permeable 8-bromo-cyclic AMP (8-Br-cAMP, 0.1 mM) was perfused to the basolateral side of taste cells, the responses for NaCl and HCl decreased to 41 and 63% of the respective controls. 6. These results suggest that AVP may regulate the gustatory responses for monovalent salts and acids by a mechanism which is not necessary to activate adenylate cyclase.     

Behavioural and electrophysiological experiments on the sense of taste in Saguinus midas tamarin (Callitrichidae) (author's transl)]

The activity in the gustatory nerve from the anterior part of the tongue, the chorda tympani proper nerve, has been recorded during stimulation of the tongue of a New World monkey, Saguinus midas tamarin. A series of 0.3 M sugars and 0.07 M NaCl, 0.07 M sucrose, 0.004 M acetic acid and 0.00005 M quinine hydrochloride were used as taste stimuli. The concentrations of the last four stimuli were the same as those earlier found to be the lowest concentrations at which this monkey in behavioural experiments discriminates between them and water. The records showed that these threshold concentrations all elicited a neural response. Further, the neural activities during stimulation with the series of 0.3 M sugars were recorded. Using the amplitudes of the responses as a measure, the order between them was found to be fructose greater than glucose greater than lactose greater than arabinose greater than sucrose = galactose greater than raffinose. This order was discussed and related with the order found in behavioural experiments. With both methods, fructose seemed to be the strongest stimulus, but then the order among the sugars differed.     

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.     

Parabrachial unit activities after the acquisition of conditioned taste aversion to a non-preferred HCl solution in rats

Abstract In a behavioral experiment, rats reliably acquired a taste aversion to non-preferred 0.01 M HCl that had been previously paired with intraperitoneal injection of 0.15 M LiCl. These rats showed aversions to other acidic solutions such as malic acid and tartaric acid. In a neurophysiological experiment, the neuronal activities of the parabrachial nucleus (PBN) were recorded after the acquisition of conditioned taste aversion (CTA) to 0.01 M HCl in urethane-anesthetized rats. Neuronal responses to the conditioned stimulus (CS) did not change on the whole but decreased in the dorsal region to the brachium conjunctivum. The proportion of HCl-best to NaCl-best units was lower in the CTA group than in controls. The spontaneous firing rate was lower in the CTA group than in controls. Correlation coefficients between the HCl CS and normally preferred tastes (sucrose and NaCl) were more negative and those between HCl and quinine were more positive in the CTA group than in the controls. These results may be explained by the notion that gustatory responses of PBN neurons are concerned with alterations in taste hedonics after the acquisition of conditioned taste aversions.     

Tonic GABAergic inhibition of taste-responsive neurons in the nucleus of the solitary tract

The effects of gamma-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline methiodide (BICM) on the activity of taste- responsive neurons in the nucleus of the solitary tract (NST) were examined electrophysiologically in urethane-anesthetized hamsters. Single neurons in the NST were recorded extracellularly and drugs (21 nl) were microinjected into the vicinity of the cell via a multibarrel pipette. The response of each cell was recorded to lingual stimulation with 0.032 M NaCl, 0.032 M sucrose, 0.0032 M citric acid and 0.032 M quinine hydrochloride (QHCl). Forty-six neurons were tested for the effects of GABA; the activity of 29 cells (63%) was inhibited by 5 mM GABA. Whether activity was elicited in these cells by repetitive anodal current stimulation (25 microA, 0.5 s, 0.1 Hz) of the tongue (n = 13 cells) or the cells were spontaneously active (n = 13 cells), GABA produced a dose-dependent (1, 2 and 5 mM) decrement in activity. Forty- seven NST neurons were tested for the effects of BICM on their responses to chemical stimulation of the tongue; the responses of 28 cells (60%) were enhanced by 10 mM BICM. The gustatory responses of 26 of these cells were tested with three concentrations (0.2, 2 and 10 mM) of BICM, which produced a dose-dependent increase in both spontaneous activity and taste-evoked responses. Nine of these neurons were sucrose- best, seven were NaCl-best, eight were acid-best and two responded best to QHCl. The responses to all four tastants were enhanced, with no difference among neuron types. For 18 cells that were tested with two or more gustatory stimuli, BICM increased their breadth of responsiveness to their two most effective stimuli. These data show that approximately 60% of the taste-responsive neurons in the rostral NST are inhibited by GABA and/or subject to a tonic inhibitory influence, which is mediated by GABAA receptors. The modulation of these cells by GABA provides a mechanism by which the breadth of tuning of the cell can be sharpened. Modulation of gustatory activity following a number of physiological changes could be mediated by such a GABAergic circuit.      

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.     

Gustatory projections from the nucleus of the solitary tract to the parabrachial nuclei in the hamster.

Taste-responsive cells in the nucleus of the solitary tract (NST) either project to the parabrachial nuclei (PbN) of the pons, through which taste information is transmitted to forebrain gustatory nuclei, or give rise to axons terminating locally within the medulla. Numerous anatomical studies clearly demonstrate a substantial projection from the rostral NST, where most taste-responsive cells are found, to the PbN. In contrast, previous electrophysiological studies in the rat have shown that only a small proportion (21-45%) of taste-responsive NST cells are antidromically activated from the PbN, suggesting that less than half the cells recorded from the NST are actually involved in forebrain processing of gustatory information. In the present experiment we investigated the projections from the NST to the PbN electrophysiologically in urethane anesthetized hamsters. Responses of 101 single neurons in the rostral NST were recorded extracellularly following lingual stimulation with 32 mM NaCl, sucrose and quinine hydrochloride (QHCl) and 3.2 mM citric acid. The taste-responsive region of the PbN was identified electrophysiologically and stimulated with a concentric bipolar electrode to antidromically activate each NST cell. Of the 101 taste-responsive NST cells, 81 (80.2%) were antidromically activated from the ipsilateral PbN. The mean firing rates to taste stimulation and the spontaneous activity of these projection neurons were significantly greater than those of non-projecting cells. Every sucrose-best neuron in the sample projected to the PbN. The mean conduction velocity of the 23 QHCl-best neurons was significantly lower than that of the other 58 PbN projection neurons, suggesting that the most QHCl-responsive cells are a subset of smaller neurons. These data show that a large majority of NST cells responsive to taste stimulation of the anterior tongue project to the gustatory subdivisions of the PbN and that these cells have the most robust responses to gustatory stimulation.     

Gurmarin suppression of licking responses to sweetener-quinine mixtures in C57BL mice

Gurmarin (Gur) is a peptide that selectively suppresses responses of the chorda tympani nerve to sweet substances in rats and mice. In the present study, we examined the effect of Gur on behavioral responses to sweet substances in C57BL mice. To accomplish this, we developed a new short-term lick test and measured numbers of licks for 10 s for sweet substances mixed with quinine hydrochloride (QHCl) in water-deprived mice. Numbers of licks for sucrose mixed with 1 or 3 mM QHCl increased with increasing concentration of sucrose from 0.01 to 1.0 M. Oral infusion with 30 micro g/ml Gur produced significant decreases in responses to concentration series for sucrose mixed with 3 mM QHCl, whereas no such effect by Gur was observed in responses to QHCl alone or QHCl-mixed HCl, NaCl or monosodium glutamate. The Gur suppression of QHCl-mixed sucrose responses, which otherwise lasted for 2-3 h, rapidly returned to approximately 80% of control levels after oral infusion with beta-cyclodextrin. These results are comparable to neural data previously found in chorda tympani responses, and thereby provide further evidence for Gur as a sweet response inhibitor in C57BL mice. In the other aspect, our newly developed short-term test can also provide a tool for measurements of taste-guided behavioral responses to sweeteners.     

Receptive fields and gustatory responsiveness of frog glossopharyngeal nerve. A single fiber analysis

Receptive fields and responsiveness of single fibers of the glossopharyngeal (IXth) nerve were investigated using electrical, gustatory (NaCl, quinine HCl, acetic acid, water, sucrose, and CaCl2), thermal, and mechanical stimulation of the single fungiform papillae distributed on the dorsal tongue surface in frogs. 172 single fibers were isolated. 58% of these fibers (99/172) were responsive to at least one of the gustatory stimuli (taste fibers), and the remaining 42% (73/172) were responsive only to touch (touch fibers). The number of papillae innervated by a single fiber (receptive field) was between 1 and 17 for taste fibers and between 1 and 10 for touch fibers. The mean receptive field of taste fibers (X = 6.6, n = 99) was significantly larger than that of touch fibers (X = 3.6, n = 73) (two-tailed t test, P less than 0.001). In experiments with natural stimulation of single fungiform papillae, it was found that every branch of a single fiber has a similar responsiveness. Taste fibers were classified into 14 types (Type N, Q, A, NA, NCa, NCaA, NCaW, NCaAW, NCaWS, NQ, NQA, NQAS, NQWarm, Multiple) on the basis of their responses to gustatory and thermal stimuli. The time course of the response in taste fibers was found to be characteristic of their types. For example, the fibers belonging to Type NQA showed phasic responses, those in Type NCa showed tonic responses, etc. These results indicate that there are several groups of fibers in the frog IXth nerve and that every branch of an individual fiber has a similar responsiveness to the parent fiber.     

Taste function in patients with oral burning

Burning mouth syndrome (BMS) is an oral pain disorder occurring primarily in post-menopausal women and is frequently accompanied by taste complaints. This association of symptoms suggests an interaction between the mechanisms of nociception and gustation, two senses with strong hedonic components. Seventy-three patients of the Taste and Smell Clinic at the University of Connecticut Health Center who reported experiencing 'unexplained oral burning' were evaluated for taste function. Both intensity ratings and quality identifications were measured for a concentration series of sucrose ('sweet'), NaCl ('salty'), citric acid ('sour') and quinine-HCl ('bitter'). The 57 women with BMS gave lower intensity ratings to NaCl and sucrose than comparably aged, same sex controls. Concentrations of NaCl and sucrose >0.10 M were most affected; concentrations of sucrose and NaCl <0.10 M were rated similarly by BMS and control women. No intensity differences were found for citric acid or quinine-HCl at any concentration and no differences were evident between the 16 BMS men and the 14 control men for any stimulus. The BMS women also misidentified the quality of 19% of the stimuli that were detected whereas control women misidentified 8%. Both groups detected a similar proportion of stimuli and found lower stimulus concentrations more difficult to identify than higher concentrations. Identification of NaCl as 'salty' and citric acid as 'sour' was particularly difficult for BMS women. The present findings are consistent with the hypothesis that pain pathway activation may affect neural and behavioral taste function.     

Regional specificity of chlorhexidine effects on taste perception

Chlorhexidine (CHX) gluconate, a bitter bis-biguanide antiseptic, reduces the intensity of the salty taste of NaCl and bitter taste of quinine in humans. This study addresses regional specificity of CHX's effects on taste. Perceptual intensity and quality were measured for separate taste bud containing oral loci innervated either by afferent fibers of cranial nerve (CN) VII or CN IX. Measurements were obtained following three 1-min oral rinses with either 1.34 mM CHX or water, the control rinse. CHX rinse reduced the intensity of NaCl more at the tongue tip and palate than at posterior oral sites. Thus, fungiform and palatal salt-taste receptors may differ from salt-taste receptors of the foliate and circumvallate taste papillae. The intensity of quinine.HCl was reduced equally by CHX at all sites tested but was frequently tasteless on the less sensitive anterior sites, suggesting quinine receptor diversity. In rodents, a portion of NaCl-taste receptors in the receptive field of CN VII is sensitive to the epithelial Na+ channel blocker amiloride and a portion is amiloride insensitive; all CN IX receptors are amiloride insensitive. The current results are the first to suggest that there may also be distinct, regionally specific populations of NaCl-taste receptors in humans.     

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.