Generalization of specific taste aversions to alcohol in the rat

Two experiments were conducted to examine the taste qualitiesof alcohol in the rat. In Experiment 1, rats were trained toavoid sucrose, quinine or a sucrose + quinine mixture. In Experiment2, rats were trained to avoid sucrose, hydrochloric acid ora sucrose + hydrochloric acid mixture. In both experiments ratswere then tested for generalization to 3, 6 and 9% (v/v) alcoholsolutions. Following the alcohol tests rats were tested withthe taste solutions used during training. Results showed thatrats trained to avoid sucrose (Experiments 1 and 2), quinineor the sucrose + quinine mixture generalized that aversion tothe 6% alcohol solution. No generalization was found to the3% alcohol solution. No generalization was displayed by ratstrained to avoid either the hydrochloric acid solution or thesucrose + hydrochloric acid solution. Following the alcoholtests, all trained rats exhibited strong aversions to the solutionthey were trained to avoid. In addition, rats trained to avoida single solution generalized that aversion to the mixture andrats trained to avoid a mixture generalized to the single components.     

The sweet-bitter taste of alcohol: aversion generalization to various sweet-quinine mixtures in the rat

Rats were trained to avoid a 5% alcohol solution and then testedwith either sweet + quinine hydrochloride solutions (Experiment1) or sweet + hydrochloric acid solutions (Experiment 2). Thesweet stimuli used were sucrose, glucose, fructose and saccharin.Significant aversion generalization was found only in Experiment1 where trained rats generalized to all four test stimuli, thussuggesting that alcohol has a sweet taste (in combination withbitter) not specific to one sweetener. No significant aversiongeneralization was noted in Experiment 2 when sweet + hydrochloricacid solutions were tested. In Experiment 3, rats were trainedto avoid 6% alcohol and tested with sucrose + quinine hydrochloridemixtures with varying concentrations of each component. In general,rats showed generalization of the alcohol aversions across thevarious concentrations of sucrose and quinine hydrochloridetested.     

The contribution of latent inhibition to reduced generalization after pre-exposure to the test stimulus

Two experiments assessed the contribution of latent inhibition to the generalization-reducing effects of pre-exposure to the test stimulus using a taste aversion procedure in rats. In both experiments, lithium chloride induced illness was paired with a flavor compound (AX) of either salt or sugar (A or B) and hydrochloric acid (X). Generalization of the resulting aversion to a test compound (BX), was assessed after varying pre-exposure to BX, X, and B. Experiment 1 showed that generalization to BX was less when BX itself had been exposed than equivalent pre-exposure to either B and X separately or to B and a new compound (CX). Experiment 2 showed that levels of generalization varied directly as a function of the amount of pre-exposure to BX. The findings show that latent inhibition alone cannot account for the generalization-reducing effect of pre-exposure to BX.     

Discrimination between the tastes of sucrose and monosodium glutamate in rats

Conditioned taste aversion studies have demonstrated that rats conditioned to avoid monosodium glutamate (MSG) with amiloride added to reduce the intensity of the sodium component of MSG taste, will generalize an aversion for MSG to sucrose and vice versa. This suggests that taste transduction for sodium, sucrose and MSG may intersect at some point. Generalization of conditioned taste aversion indicates that two substances share similar taste features, but it does not reveal the extent of their differences. In this study, we tested how well rats can discriminate sucrose and MSG under a variety of conditions. Water-deprived rats were trained on a combination of water reinforcement and shock avoidance to discriminate between MSG and sucrose, both with and without amiloride, and with and without equimolar NaCl in all solutions. In the absence of amiloride, rats reliably distinguished between MSG and sucrose down to 10 mM solutions. However, they could correctly identify solutions only above 50 mM in the presence of amiloride, equimolar sodium chloride, or both. These results suggest that gustatory stimulation by MSG and sucrose interact somewhere in taste transduction, perhaps within taste receptor cells or gustatory afferent pathways.     

Monosodium glutamate and sweet taste: discrimination between the tastes of sweet stimuli and glutamate in rats

Generalization of a conditioned taste aversion (CTA) is based on similarities in taste qualities shared by the aversive substance and another taste substance. CTA experiments with rats have found that an aversion to a variety of sweet stimuli will cross-generalize with monosodium glutamate (MSG) when amiloride, a sodium channel blocker, is added to all solutions to reduce the taste of sodium. These findings suggest that the glutamate anion elicits a sweet taste sensation in rats. CTA experiments, however, generally do not indicate whether two substances have different taste qualities. In this study, discrimination methods in which rats focused on perceptual differences were used to determine if they could distinguish between the tastes of MSG and four sweet substances. As expected, rats readily discriminated between two natural sugars (sucrose, glucose) and two artificial sweeteners (saccharin, SC45647). Rats also easily discriminated between MSG and glucose, saccharin and, to a lesser extent, SC45647 when the taste of the sodium ion of MSG was reduced by the addition of amiloride to all solutions, or the addition of amiloride to all solutions and NaCl to each sweet stimulus to match the concentration of Na+ in the MSG solutions. In contrast, reducing the cue function of the Na+ ion significantly decreased their ability to discriminate between sucrose and MSG. These results suggest that the sweet qualities of glutamate taste is not as dominate a component of glutamate taste as CTA experiments suggest and these qualities are most closely related to the taste qualities of sucrose. The findings of this study, in conjunction with other research, suggest that sweet and umami afferent signaling may converge through a taste receptor with a high affinity for glutamate and sucrose or a downstream transduction mechanism. These data also suggest that rats do not necessarily perceive the tastes of these sweet stimuli as similar and that these sweet stimuli are detected by multiple sweet receptors.     

Within-compound associations are not sufficient to produce taste-mediated odor potentiation

Rats were used in two flavor-aversion experiments to determine if within-compound associations could be detected with a taste+odor compound that would not support taste-mediated odor potentation. In Experiment 1, following taste+odor compound conditioning, postconditioning taste extinction significantly weakened the odor aversion. In Experiment 2, following taste+odor compound conditioning, postconditioning taste inflation significantly strengthened the odor aversion. There was no evidence that taste potentiated the odor aversion in either Experiment 1 or 2. Thus, the results demonstrate that the presence of within-compound associations is not sufficient to produce taste-mediated odor potentiation. We offer a mediated conditioning explanation to account for the results of these two experiments.     

Rapid stimulus-bound suppression of intake in response to an intraduodenal nonnutritive sweetener after training with nutritive sugars predicting malaise

In a previous report (Schier et al., Am J Physiol Regul Integr Comp Physiol 301: R1557-R1568, 2011), we demonstrated with a new behavioral procedure that rats exhibit stimulus-bound suppression of intake in response to an intraduodenal (ID) bitter tastant predicting subsequent malaise. With the use of the same modified taste aversion procedure, the present experiments evaluated whether the sweet taste properties of ID stimuli are likewise detected and encoded. Thirsty rats licked at sipper spouts for hypotonic NaCl for 30 min and received brief (first 6 min) yoked ID infusions of either the same NaCl or an isomolar lithium chloride (LiCl) solution in each session. An intestinal taste cue was mixed directly into the LiCl infusate for aversion training. Results showed that rats failed to detect intestinal sweet taste alone (20 mM Sucralose) but clearly suppressed licking in response to a nutritive sweet taste stimulus (234 mM sucrose) in the intestine that had been repeatedly paired with LiCl. Rats trained with ID sucrose in LiCl subsequently generalized responding to ID Sucralose alone at test. Replicating this, rats trained with ID Sucralose in compound with 80 mM Polycose rapidly suppressed licking to the 20 mM Sucralose alone in a later test. Furthermore, ID sweet taste signaling did not support the rapid negative feedback of sucrose or Polycose on intake when their digestion and transport were blocked. Together, these results suggest that other signaling pathways and/or transporters engaged by caloric carbohydrate stimuli potentiate detection of sweet taste signals in the intestine.     

Is glycine "sweet" to mice? Mouse strain differences in perception of glycine taste

Glycine is an amino acid tasting sweet to humans. In 2-bottle tests, C57BL/6ByJ (B6) mice strongly prefer glycine solutions, whereas 129P3/J (129) mice do not, suggesting that they differ in perception of glycine taste. We examined this question using the conditioned taste aversion (CTA) generalization technique. CTA was achieved by injecting LiCl after drinking glycine, and next its generalization to 10 taste solutions (glycine, sucrose, saccharin, D-tryptophan, L-tryptophan, L-alanine, L-proline, L-glutamine, NaCl, and HCl) was examined by video recording licking behavior. Both B6 and 129 mice generalized the aversion to sucrose, saccharin, L-alanine, and L-proline and did not generalize it to NaCl, HCl, and L-tryptophan. This indicates that both B6 and 129 mice perceive the sweetness (i.e., a sucrose-like taste) of glycine. Thus, the lack of a glycine preference by 129 mice cannot be explained by their inability to perceive its sweetness. Strain differences were observed for CTA generalization to 2 amino acids: 129 mice generalized aversion to L-glutamine but not D-tryptophan, whereas B6 mice generalized it to D-tryptophan but not L-glutamine. 129.B6-Tas1r3 congenic mice with 2 genotypes of the Tas1r3 locus (B6/129 heterozygotes and 129/129 homozygotes) did not differ in aversion generalization, suggesting that the differences between 129 and B6 strains are not attributed to the Tas1r3 allelic variants and that other, yet unknown, genes are involved in taste perception of amino acids.     

Differences in the taste quality of maltose and sucrose in rats: issues involving the generalization of conditioned taste aversions

The present study employed a conditioned taste aversion generalizationparadigm to test the hypothesis that maltose produces tastesensations in the rat which are qualitatively distinguishablefrom sucrose. Since stimulus generalization can occur in boththe quality and intensity domains, an intrachemical (acrossconcentration) generalization gradient was established to aidin the interpretation of the interchemical (across molecules)generalization gradient. Moreover, since the commonly used intaketest is vulnerable to nontaste post-ingestional influences,the present study measured immediate responses to 100 µlstimulus samples, thus increasing our confidence that the behaviorwas under orosensory control. In Experiment 1, naive water deprivedrats were trained in a specially designed gustometer to maintaindrinking-spout contact for intermittent water reinforcement.Following this, rats in the experimental group were given threeexposures to 0.1 M sucrose on separate days, with the firsttwo exposures immediately preceding an injection of LiCl. Acontrol group was treated identically but received distilledwater instead of sucrose. Rats were then tested in the gustometerfor their avoidance of three equimolar concentrations of sucroseand maltose. Rats received ten trials of each stimulus quasi-randomlypresented in two sessions. Results indicated that all sucroseconcentrations were avoided (in experimental group only), butonly the 0.3 M concentration of maltose was avoided. The lowestsucrose concentration was significantly less avoided than thehigher concentrations. Intensity generalization gradients aresuch that intensities weaker than the conditioned stimulus (CS)produce just as much or less of a conditioned response (CR)and intensities stronger than the CS produce just as much ora greater CR than that elicited by the CS itself. Therefore,based on the results of Experimental, it was predicted thatif 0.1 M maltose served as the CS, the order of avoidance shouldbe: 0.3 M sucrose 0.1 M sucrose 0.03 M sucrose 0.3 M maltose 0.1 M maltose 0.03 M maltose, if it were true that maltoseand sucrose produce identical sensations that differ only inintensity. Experiment 2 explicitly tested this prediction usingthe same procedure as Experiment 1 except that 0.1 M maltoseserved as the CS. The observed order of avoidance was 0.3 Mmaltose > 0.1 M maltose > 0.03 M maltose = 0.3 M sucrose= 0.1 M sucrose = 0.03 M sucrose. In both experiments the intrachemicalgeneralization gradient broadened and the interchemical generalizationgradient steepened upon retesting. In conclusion, qualitativedifferences between maltose and sucrose explain the outcomesof these experiments better than differences in the relativeintensity of these sugars at isomolar concentrations.     

Blocking of acquisition of a taste aversion by a context experienced prior to the taste

This experiment tested the proposal that events taking place before a rat has access to a taste can proactively interfere with acquisition of an aversion to the taste when this has been followed by lithium chloride injection. Rats were initially given context discrimination whereby placement in one distinctive context (target) was followed by lithium injection, while placement in a second context (safe) was followed by saline injection. In the subsequent 1-trial taste conditioning session, rats were first placed in either their target context (Blocking group), their safe context (Control-Safe group) or a neutral context (Control-Neutral group), then given access to sucrose and 30 min later were injected with lithium. Subsequent tests of sucrose intakes revealed a blocking effect. These results indicate that proactive interference with taste aversion learning by a context can occur that is unlikely to be based on generalization decrement.     

Extensive Lesions in Rat Insular Cortex Significantly Disrupt Taste Sensitivity to NaCl and KCl and Slow Salt Discrimination Learning

While studies of the gustatory cortex (GC) mostly focus on its role in taste aversion learning and memory, the necessity of GC for other fundamental taste-guided behaviors remains largely untested. Here, rats with either excitotoxic lesions targeting GC (n = 26) or sham lesions (n = 14) were assessed for postsurgical retention of a presurgically LiCl-induced conditioned taste aversion (CTA) to 0.1M sucrose using a brief-access taste generalization test in a gustometer. The same animals were then trained in a two-response operant taste detection task and psychophysically tested for their salt (NaCl or KCl) sensitivity. Next, the rats were trained and tested in a NaCl vs. KCl taste discrimination task with concentrations varied. Rats meeting our histological inclusion criterion had large lesions (resulting in a group averaging 80% damage to GC and involving surrounding regions) and showed impaired postsurgical expression of the presurgical CTA (LiCl-injected, n = 9), demonstrated rightward shifts in the NaCl (0.54 log₁₀ shift) and KCl (0.35 log₁₀ shift) psychometric functions, and displayed retarded salt discrimination acquisition (n = 18), but eventually learned and performed the discrimination comparable to sham-operated animals. Interestingly, the degree of deficit between tasks correlated only modestly, if at all, suggesting that idiosyncratic differences in insular cortex lesion topography were the root of the individual differences in the behavioral effects demonstrated here. This latter finding hints at some degree of interanimal variation in the functional topography of insular cortex. Overall, GC appears to be necessary to maintain normal taste sensitivity to NaCl and KCl and for salt discrimination learning. However, higher salt concentrations can be detected and discriminated by rats with extensive damage to GC suggesting that the other resources of the gustatory system are sufficient to maintain partial competence in these tasks, supporting the view that such basic sensory-discriminative taste functions involve distributed processes among central gustatory structures.     

Chemosensory similarities among oils: does viscosity play a role?

It is widely thought that viscosity plays an important rolein the perception of fats. Rats were conditioned to avoid oilsuspensions by treating them with lithium chloride after theyhad ingested the oil suspension. Control rats received the samelithium chloride injection after they drank vehicle. Three,chemically different, oils were examined, triglyceride oil,silicone oil and mineral oil. Rats trained to avoid a 4% aqueoussuspension of one of these oils, reliably avoided suspensionscontaining any of the other oils, although the rats could discriminatebetween the oils. The viscosity of the oil suspension was slightlygreater than the viscosity of the vehicle alone. However, ratstrained to avoid an oil suspension, did not avoid a fluid havinga viscosity similar to that of the oil suspension. In orderto assess the possibility that rats could sense the viscosityof the oil separately from that of the vehicle, rats were testedfor their aversion to oils having viscosities much higher andmuch lower than that of the oil they were trained to avoid.Rats trained to avoid triolein having a viscosity of 67 cp,reliably avoided silicone oils having viscosities of 5 and 203cp. However, rats trained to avoid oil did not avoid an oil-freefluid having a viscosity of 22–29 cp. A final experimentexamined whether the use of viscous or non-viscous vehiclesinfluenced the conditioned aversion. No significant effect ofvehicle viscosity appeared. Thus, chemically-diverse oils areperceived, by rats, to have some perceptible attribute in common.It is proposed that this common physical attribute is boundarylubrication rather than viscosity.     

The taste of monosodium glutamate (MSG), L-aspartic acid, and N-methyl-D-aspartate (NMDA) in rats: are NMDA receptors involved in MSG taste?

Monosodium glutamate (MSG) is believed to elicit a unique taste perception known as umami. We have used conditioned taste aversion assays in rats to compare taste responses elicited by the glutamate receptor agonists MSG, L-aspartic acid (L-Asp), and N-methyl-D-aspartate (NMDA), and to determine if these compounds share a common taste quality. This information could shed new light upon the receptor mechanisms of glutamate taste transduction. Taste aversions to either MSG, L-Asp or NMDA were produced by injecting rats with LiCl after they had ingested one of these stimuli. Subsequently, rats were tested to determine whether they would ingest any of the above compounds. The results clearly show that a conditioned aversion to MSG generalized to L-Asp in a dose-dependent manner. Conversely, rats conditioned to avoid L-Asp also avoided MSG. Conditioned aversions to MSG or L-Asp generalized to sucrose when amiloride was included in all solutions. Importantly, aversions to MSG or L-Asp did not generalize to NMDA, NaCl or KCl, and aversions to NMDA did not generalize to MSG, L-Asp, sucrose or KCl. These data indicate that rats perceive MSG and L-Asp as similar tastes, whereas NMDA, NaCl and KCl elicit other tastes. The results do not support a dominant role for the NMDA subtype of glutamate receptors in taste transduction for MSG (i.e. umami) in rats.     

Glutamate taste: Discrimination between the tastes of glutamate agonists and monosodium glutamate in rats

Taste aversion studies have demonstrated that rats conditioned to avoid monosodium glutamate (MSG) with amiloride added to reduce the intensity of the sodium component of MSG taste, generalize this aversion to aspartic acid and to L-AP4, but not to ionotropic glutamate receptor agonists. That is, MSG, L-AP4 and aspartate have similar tastes to rats. However, conditioned taste aversion methods are unable to show to what extent the tastes of two substances are different. If two substances activate the same afferent processes (e.g. taste receptors), they are likely to produce the same tastes, but if they activate different afferent processes, the subject may detect differences between the tastes of the substances. In this study, rats were tested to determine if they could discriminate between the tastes of these agonists and MSG. We also established the detection thresholds for NMDA, aspartic acid and L-AP4, with and without amiloride (a sodium channel antagonist). Taste threshold values were 1-4 mM for NMDA and aspartic acid and 0.5-2.5 microM for L-AP4. None were affected by 30 micro M amiloride. Rats could readily distinguish between the tastes of MSG and NMDA but they had difficulty discriminating between the tastes of aspartic acid and MSG. Rats could also easily distinguish between 10-100 mM MSG and 0.01-5 mM L-AP4. However, in two separate experiments error rates increased significantly when L-AP4 concentrations were between 10-100 mM, indicating that the tastes of L-AP4 and MSG were similar at these concentrations.     

Monosodium glutamate and sweet taste: generalization of conditioned taste aversion between glutamate and sweet stimuli in rats

Even though monosodium glutamate (MSG) is a prototypical umami substance, previous studies reported that a conditioned taste aversion (CTA) to MSG, mixed with amiloride to block the taste of sodium, generalizes to sucrose. These findings suggest that the taste of glutamate mimics the taste of sucrose and raise the question of whether glutamate has a broadly tuned sweet taste component. To test this hypothesis, CTA experiments were conducted to test for generalization between MSG and several sweet stimuli: sucrose, glucose, maltose, saccharin and SC-45647. Strong bidirectional generalization was seen between MSG mixed with amiloride and sucrose, glucose, saccharin and SC-45647. Weak generalization was seen between MSG and maltose, and sucrose and maltose. None of the CTAs generalized to NMDA. These findings support the hypothesis that the taste of MSG has broadly tuned, sweet-like characteristics, possibly due to the convergence of afferent signals for MSG, natural sugars and artificial sweeteners.     

Learned aversions and taste qualities in hamsters

Interralations among taste perceptions in gloden hamsters (Mesocricetusauratus) were examined using generalizaions of learned tasteaversions. If stimulus A is avoided given a taste aversion hasbeen established to stimulus B, and vice versa, A and B ‘cross-generalize’.Stimuli within five groups cross-generalized. The groups ofcompounds were (i) sweeteners (fructose, saccharin, sucrose);(ii) sodium salts (NaCl, NaNO₃, Na₂SO₄): (iii) non-sodium salts(KCl, MgSO₄ NH₄Cl) plus quinine HCl; (iv) acids (acetic, hydrochloric,citric); and (v) urea. Only two pairs of stimuli from differentgroups cross-generalized (HCl—NH₄Cl. quinine HCl—urea).Neural patterns of response recorded form chorda tympam nervefibers in hamsters suggest that taste receptors on the anteriortongue distinguish among three groups of taste stimuli: sweeteners,sodium salts, and a group including non-sodium salts, acids,quinine HCl and urea. Neurons innervating other taste fieldsare likely to provide the information that hamsters use to discriminateamong the tastes of non-sodium-salt and non-sweetener stimuli.     

The taste characteristics of sodium-saccharin in the rat: a re-examination of the dual taste hypothesis

A conditioned flavor aversion (CFA) was used to measure thetaste characteristics of 0.005 and 0.025 M Na-saccharin in therat. At neither concentration did the rats show generalizationto NaCl, quinine HCl or SO₄, sucrose octa-acetate or HCl, whereasgeneralization to sucrose was found at both concentrations.When the CFA was formed to 0.0001 M quinine HCl, no direct evidencewas found for generalization of the aversion to either saccharinconcentration. Thus at the two concentrations used Na-saccharinwould appear to be primarily sucrose-like in the rat taste systemwhen behavioral criteria are used.     

Prenatal Alcohol Exposure Increases Postnatal Acceptability of Nicotine Odor and Taste in Adolescent Rats

Human studies indicate that alcohol exposure during gestation not only increases the chance for later alcohol abuse, but also nicotine dependence. The flavor attributes of both alcohol and nicotine can be important determinants of their initial acceptance and they both share the component chemosensory qualities of an aversive odor, bitter taste and oral irritation. There is a growing body of evidence demonstrating epigenetic chemosensory mechanisms through which fetal alcohol exposure increases adolescent alcohol acceptance, in part, by decreasing the aversion to alcohol''s bitter and oral irritation qualities, as well as its odor. Given that alcohol and nicotine have noteworthy chemosensory qualities in common, we investigated whether fetal exposure to alcohol increased the acceptability of nicotine''s odor and taste in adolescent rats. Study rats were alcohol-exposed during fetal development via the dams'' liquid diet. Control animals received ad lib access to an iso-caloric, iso-nutritive diet throughout gestation. Odorant-induced innate behavioral responses to nicotine odor (Experiment 1) or orosensory-mediated responses to nicotine solutions (Experiment 2) were obtained, using whole-body plethysmography and brief access lick tests, respectively. Compared to controls, rats exposed to fetal alcohol showed an enhanced nicotine odor response that was paralleled by increased oral acceptability of nicotine. Given the common aversive component qualities imbued in the flavor profiles of both drugs, our findings demonstrate that like postnatal alcohol avidity, fetal alcohol exposure also influences nicotine acceptance, at a minimum, by decreasing the aversion of both its smell and taste. Moreover, they highlight potential chemosensory-based mechanism(s) by which fetal alcohol exposure increases the later initial risk for nicotine use, thereby contributing to the co-morbid expression with enhanced alcohol avidity. Where common chemosensory mechanisms are at play, our results suggest broader implications related to the consequence of fetal exposure with one substance of abuse and initial acceptability of others.     

Salt Appetite Is Reduced by a Single Experience of Drinking Hypertonic Saline in the Adult Rat

Salt appetite, the primordial instinct to favorably ingest salty substances, represents a vital evolutionary important drive to successfully maintain body fluid and electrolyte homeostasis. This innate instinct was shown here in Sprague-Dawley rats by increased ingestion of isotonic saline (IS) over water in fluid intake tests. However, this appetitive stimulus was fundamentally transformed into a powerfully aversive one by increasing the salt content of drinking fluid from IS to hypertonic saline (2% w/v NaCl, HS) in intake tests. Rats ingested HS similar to IS when given no choice in one-bottle tests and previous studies have indicated that this may modify salt appetite. We thus investigated if a single 24 h experience of ingesting IS or HS, dehydration (DH) or 4% high salt food (HSD) altered salt preference. Here we show that 24 h of ingesting IS and HS solutions, but not DH or HSD, robustly transformed salt appetite in rats when tested 7 days and 35 days later. Using two-bottle tests rats previously exposed to IS preferred neither IS or water, whereas rats exposed to HS showed aversion to IS. Responses to sweet solutions (1% sucrose) were not different in two-bottle tests with water, suggesting that salt was the primary aversive taste pathway recruited in this model. Inducing thirst by subcutaneous administration of angiotensin II did not overcome this salt aversion. We hypothesised that this behavior results from altered gene expression in brain structures important in thirst and salt appetite. Thus we also report here lasting changes in mRNAs for markers of neuronal activity, peptide hormones and neuronal plasticity in supraoptic and paraventricular nuclei of the hypothalamus following rehydration after both DH and HS. These results indicate that a single experience of drinking HS is a memorable one, with long-term changes in gene expression accompanying this aversion to salty solutions.     

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.