Metabotropic glutamate receptors are involved in the detection of IMP and L-amino acids by mouse taste sensory cells.

G-protein-coupled receptors are thought to be involved in the detection of umami and L-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1)+taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1+T1r3 is a broadly tuned lLamino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in L-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various L-amino acids and inosine 5' monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and L-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three L-amino acids (monopotassium L-glutamate, L-serine and L-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-L-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these L-amino acids and IMP. We found that L-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the L-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and L-amino acid taste.

Cyclophosphamide-induced disruption of umami taste functions and taste epithelium.

Clinical studies have reported taste dysfunctions developing in patients undergoing chemotherapy. This adverse side effect is a major concern for the doctors and patients because disrupted taste can reduce appetite, cause malnutrition, delay recovery, and affect quality of life. Cyclophosphamide (CYP) is a common atenoplastic drug used during chemotherapy and is thought to affect taste through learned tasted aversions. This study asked whether CYP also alters umami taste sensory functions and disrupts taste epithelium of mice. Behavioral tests focused on taste acuity, assessed by the ability of mice to discriminate between the taste qualities of two umami substances, monosodium glutamate (MSG) and inosine 5'-monophosphate (IMP), and taste sensitivity, assessed by detection thresholds of MSG and IMP, after an IP injection (75 mg/kg) of CYP. The behavioral results revealed a two-phase disturbance in taste acuity and loss of sensitivity, the first phase occurring within 2-4 days after injection and the second occurring 9-12 days after injection. The number of fungiform papillae (with and without pores) decreased immediately after injection and did not begin to recover until 12 days after injection. Circumvallate taste buds began to show disturbances by 8 days after injection and evidence of recovery beginning 12 days after injection. Von Ebner glands were smaller and secreted less saliva 4 days postinjection but not later. These findings suggest the initial behavioral deficits may be because of cytotoxic effects of the drug on taste sensory tissues, whereas the second phase may be because of a disturbance of the taste cell replacement cycle.

Antagonism of metabotropic glutamate receptor 4 receptors by (RS)-alpha-cyclopropyl-4-phosphonophenylglycine alters the taste of amino acids in rats.

T1R1/T1R3, taste-metabotropic glutamate receptor (mGluR) 4 and other taste receptors have been implicated in umami taste perceptionT1R1/T1R3 has also been identified as an L-amino acid receptor. We investigated the possibility that taste-mGluR4 receptors may also play a role in the taste of amino acids in Sprague-Dawley rats using conditioned taste aversion methods. Specifically, we examined whether a taste aversion generalized between L-monosodium glutamate (MSG) and one of three amino acids (glycine, L-serine, and L-arginine), and whether (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), a group III mGluR selective antagonist with a strong binding affinity for mGluR4 receptors, can impact stimulus generalization. Rats showed cross-generalization between MSG and all three amino acids (all mixed with amiloride to block the taste of sodium), although less so for L-arginine than the other two amino acids, suggesting that all of the amino acids shared at least some taste qualities with MSG. However, when 1 mM CPPG was mixed with these amino acids, the strength of the learned taste aversions and cross-generalization for all but glycine were either decreased or increased. The increase in generalization induced by CPPG indicated that the antagonist did not simply reduce the intensity of the stimulus experience but also changed the qualities of the sensory experience. These findings suggest that multiple receptors are involved in amino acid taste and that taste-mGluR4 receptors contribute to the taste of MSG and at least some l-amino acids.

Behavioral comparison of sucrose and l-2-amino-4-phosphonobutyrate (L-AP4) tastes in rats: does L-AP4 have a sweet taste?

Even though it is generally thought that umami stimuli such as monosodium glutamate (MSG) and sweet stimuli such as sucrose are detected by different taste receptors, these stimuli appear to share taste qualities when amiloride (a sodium channel blocker) is present to reduce the sodium taste. Single fiber recording studies of the facial and glossopharyngeal nerves have shown that encoding of L-2-amino-4-phosphonobutyrate (L-AP4), a potent mGluR4 agonist that elicits a taste quite similar to MSG, may occur in the same fibers that also encode sweet stimuli. This suggests that L-AP4 and sweet substances may activate common receptors or afferent signaling mechanisms. We report results of behavioral experiments that test this hypothesis. In the first study, rats conditioned to avoid sucrose or L-AP4 generalized the aversion to the opposite substance, indicating that both substances elicited similar tastes. However, two taste discrimination experiments showed that rats easily discriminated between sucrose and L-AP4 over a wide range of concentrations, even when the cue function of sodium associated with L-AP4 was reduced by amiloride and neutralized by adding equimolar concentrations of NaCl to sucrose. These data suggest that even though L-AP4 and sucrose elicit similar taste qualities, one or both substances also elicit other taste qualities not shared by the opposite substance. They also suggest that the taste-mGluR4 receptor and the signal pathway activated by L-AP4 are not the same as those activated by sucrose. These data, when combined with fiber recording data, suggest that there is convergence of L-AP4 and sucrose signals at some point early in the gustatory pathway.

Comparison of L-monosodium glutamate and L-amino acid taste in rats.

T1R2/T1R3 heterodimers are selectively responsive to sweet substances whereas T1R1/T1R3 receptors are selective for umami substances, represented by monosodium glutamate (MSG), and for L-amino acids. If a single receptor is responsible for detection of umami and L-amino acids, then it would be predicted that MSG and L-amino acids elicit similar tastes in rats. The present study compared the taste profile of MSG with four amino acids (glycine, L-proline, L-serine and L-arginine) using conditioned taste aversion, detection threshold, and taste discrimination methods. These experiments were designed to either reduce or neutralize the taste of sodium associated with MSG and the other amino acids. Detection threshold studies showed that rats were most sensitive to L-arginine and least sensitive to L-proline. Glycine and L-serine thresholds were similar to those previously reported for MSG. Like MSG, a conditioned taste aversion to each of the four amino acids generalized to sucrose in the presence of amiloride, a sodium channel blocker. Rats showed moderate generalization of aversion between MSG and L-arginine, suggesting that these two amino acids taste only moderately alike. However, the taste aversion experiments indicated that glycine, L-serine, and L-proline elicit taste sensations similar to MSG when amiloride is present. Discrimination experiments further compared the tastes of these three amino acids with MSG. When the sodium taste associated with MSG was reduced or neutralized, glycine and L-proline elicited tastes very similar but not identical to the taste of MSG. Low (but not higher) concentrations of L-serine were also difficult for rats to discriminate from MSG. While there are taste qualities common to all of these amino acids, the perceptual differences found in this study, combined with previous reports, suggest either multiple taste receptors and/or multiple signaling pathways may be involved in umami and amino acid taste perception in rats.

An analysis of 5'-inosine and 5'-guanosine monophosphate taste in rats.

Inosine monophosphate (IMP) and guanosine monophosphate (GMP) elicit an umami taste in humans and synergistically increase the intensity of the umami taste of monosodium glutamate (MSG). Conditioned taste aversion (CTA) studies in rodents indicate that these nucleotides and MSG elicit quite similar tastes, but recent physiological evidence suggests that these nucleotides and MSG may not activate the same population of taste receptors and therefore may not elicit identical taste qualities. This study reports the findings of several behavioral experiments with rats that compared the taste properties of IMP and GMP with each other and with those of MSG. Well-trained rats were able to detect both nucleotides at nanomolar concentrations, but they did not respond to either nucleotide in two-bottle preference tests or brief-access CTA tests at concentrations less than 0.5 mM. Discrimination experiments found that the tastes of these nucleotides could not be discriminated from each other, but both could be discriminated from MSG, even when the taste of Na(+) was controlled. Overall, these experiments indicate the taste properties of the two 5'-ribonucleotides are quite similar to each other, and even though they may elicit an umami sensation, these sensations are not identical to the taste of MSG.

Sucrose and monosodium glutamate taste thresholds and discrimination ability of T1R3 knockout mice.

Molecular and behavioral studies have identified heterodimers of the T1R family as receptors for detecting the tastes of sweet (T1R2 + T1R3) and umami (T1R1 + T1R3). However, behavioral studies have reported conflicting findings with T1R3 knockout (KO) mice. One study showed a complete or nearly complete loss of preference for sweet and umami substances by KO mice, whereas KO mice in another study showed only a partial reduction in preferences for sucrose and monosodium glutamate (MSG), the prototypical umami substance. The present experiments used psychophysical methods to assess how sensitive T1R1-KO mice are to sucrose and MSG and discrimination methods to determine if these mice could distinguish between the tastes of sucrose and MSG. Detection thresholds of T1R3-KO mice and wild-type (WT) C57Bl mice were nearly identical for sucrose and MSG. Mice of both genotypes were easily able to discriminate between the tastes of sucrose and MSG. When amiloride (a sodium channel blocker) was added to all solutions to reduce the taste of Na+, discrimination accuracy of both genotypes of mice decreased but more so for the T1R3-KO mice than the WT mice. However, even when the sodium taste of MSG was neutralized, both genotypes could still discriminate between the two substances well above chance performance. These results suggest that sucrose and MSG can be detected by taste receptors other than T1R2 + T1R3 and T1R1 + T1R3 and that the conflicts between the previous studies may have been due to the methodological limitations.

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.

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.

Cross-modal transfer effects on visual discrimination depends on lesion location in the rat visual system.

The effects of postoperative visual and auditory training on a brightness discrimination task were examined after lesions of various structures in the visual system. In Experiment 1, rats were trained to avoid shock with visual intensity cues. Twenty-four hours later, each rat received bilateral lesions in one of the following areas of the visual system: (1) sham, (2) visual cortex (VC), (3) pretectal (PT) area, (4) combined PT/VC, (5) superior colliculus (SC), or (6) combined SC/VC. Six days later, each rat received either training with visual or auditory intensity cues, or no training. The next day all rats were retrained on the preoperative visual avoidance task. All lesions except those in the SC condition produced relearning deficits. Auditory training reduced these deficits significantly more than visual training, except in rats with combined SC/VC lesions. In Experiment 2, sham and combined PT/VC lesion rats were given either direct or reversal intensity training using visual or auditory cues before relearning the visual discrimination. Rats given auditory direct training relearned the task faster than rats given reversal training or visual direct training. Postinjury training with an intact sensory system can enhance functional recovery more effectively than training with the damaged system. The differential effects of direct and reversal training suggest that cross-modal training involves both specific and nonspecific transfer that may be mediated through the VC or the SC.

Taste preference synergy between glutamate receptor agonists and inosine monophosphate in rats.

Monosodium glutamate (MSG) elicits a taste called umami and interacts synergistically with nucleotide monophosphates such as 5'-inosine monophosphate (IMP) to potentiate this taste intensity. Indeed, the synergistic interaction of nucleotide monophosphates and MSG is a hallmark of umami. We examined interactions between MSG and other taste stimuli, including IMP, by measuring the lick rates of non-deprived rats during 30 s trials. To control for non-linear psychophysical functions, the concentration of one taste stimulus in a binary mixture was systematically increased while the concentration of the second taste stimulus was decreased (stimulus substitution method). Synergy between two stimuli was detected if the lick rate for a binary mixture exceeded that expected from the sum of the lick rates for each stimulus alone. In initial experiments, taste synergy was observed when rats were presented with mixtures of MSG and IMP but not with mixtures of MSG and sucrose. In subsequent experiments, glutamate receptor agonists other than MSG were presented with IMP to test for taste synergy. No evidence of synergy was seen when rats were presented with mixtures of IMP and kainic acid or IMP and N:-methyl-D-aspartate. However, taste synergy between IMP and L-AP4, a potent agonist at mGluR4 receptors, was observed. These results suggest that a metabotropic glutamate receptor similar to mGluR4 may be involved in the taste synergy that characterizes umami.

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.

Endogenous cannabinoids as an aversive or counter-rewarding system in the rat.

Human use of marijuana (Cannabis sativa) is widely assumed to have rewarding properties, a notion supported by its widespread recreational use. However, no study has clearly demonstrated such effects in animal models. The purpose of this study was to test for the presumed rewarding effect of cannabinoids using a conditioned place preference paradigm. The results showed that animals failed to develop place conditioning at a low dose (1.5 mg/kg) and developed a place aversion at a high dose (15 mg/kg) of the active principle in marijuana, delta 9-tetrahydrocannabinol (delta 9-THC), a finding consistent with most previous studies. Moreover, the administration of the cannabinoid antagonist SR141716A induced a conditioned place preference at both a low (0.5 mg/kg) and a high (5 mg/kg) dose. In summary, cannabinoid antagonism produced place preference while cannabinoid agonism induced place aversion. These results suggest that endogenous cannabinoids serve normally to suppress reward or to induce aversion.

Comparison of compound and cross-modal training on postoperative visual relearning of visual decorticate rats.

The effects of postoperative bimodal compound conditioning and cross-modal transfer of learning on behavior were compared by training rats prior to visual decortication to avoid shock with visual intensity cues. On Postop Day 6, rats were given avoidance training in one of three cue conditions: auditory intensity cues (cross-modal), paired auditory and visual cues (compound conditioning), or no cues (no-training control). On Postop Day 7 rats in the no-training control and the cross-modal transfer conditions were retrained with the visual discrimination while rats in the compound conditioning group were either retrained with the visual intensity cue or trained with the auditory intensity cue. Postoperative cross-modal transfer training enhanced visual relearning whereas bimodal compound conditioning interfered with relearning. However, compound conditioning facilitated subsequent auditory discrimination learning. These results support the notion of an injury-induced neurological bias that is increased after bimodal compound conditioning and reduced after cross-modal training. Potential implications for neurological rehabilitation are also discussed.

Sucrose threshold variation during the menstrual cycle.

Sucrose thresholds were measured at three points during the menstrual cycle for 14 women who were not taking oral contraceptives. Thirteen men also were tested at similar intervals. Sucrose thresholds of the men remained constant throughout the experiment. During menstruation and postovulation, the thresholds of the women were similar to the thresholds of the men, whereas during preovulation the thresholds of the women are significantly lower. The stable sucrose thresholds of the men suggest that ovarian hormones may be involved in the variation in sucrose sucrose thresholds of the women. The increase in sensitivity of the women during preovulation may have been related to the high level of estrogen, whereas the decrease in sensitivity during postovulation may be due to some type of interaction between estrogen and progesterone.

Crossmodal training reduces behavioral deficits in rats after either auditory or visual cortex lesions.

Rats were trained with either visual or auditory intensity cues until they emitted nine avoidance responses in 10 trials (9/10) prior to bilateral ablation of the corresponding sensory neocortex. Six days after surgery, rats were trained to 5/10 criterion in one of the following conditions: within-modality direct, within-modality reversal, crossmodality direct, crossmodality reversal, or no training control. The next day all rats were retrained to 9/10 on their preoperative tasks. For visual decorticate rats, the no training and the visual within-modality direct groups relearned the discrimination at the same rate as preoperative learning. Auditory crossmodal direct training enhanced relearning more than other forms of training and visual within-modality reversal training hindered retraining. For auditory decorticate rats, similar postoperative auditory within-modality and visual crossmodality training effects were seen during retraining of the auditory discrimination. These findings suggest crossmodality training facilitates functional recovery through relational information and learning sets transferred from experimental training to the relearning task.

Seashore Rhythm test: comparison of Signal Detection Theory and standard scoring procedures.

The effects of age and instructional set on Seashore Rhythm test performance were examined with standard scoring and Signal Detection Theory (SDT) procedures. Neurologically intact young (17-28 years) and old (65-92 years) subjects were given standard test instructions with sentences added which stated that specific proportions of the test stimuli would be identical or different. Age differences in d' scores were highly correlated with standard Rhythm scores. While standard scores were unaffected by instructional set, all three SDT measures of response criteria detected effects of instructional set, or an age by instructional set interaction. SDT scoring of the Rhythm test yields a measure of auditory sensitivity comparable to standard methods and detects response bias on the lest that standard procedures cannot.

Recovery of a temporally based visual discrimination after visual cortex lesion in the rat.

In Expt. 1, rats were conditioned to emit a shock avoidance response when the pulse rate of a light was increased. Then, after bilateral visual cortex lesions, the rats were given 10, 20, or 40 days recovery before relearning the discrimination. While all rats were able to relearn the discrimination response, lesion rats had a performance deficit after all recovery periods. Expt. 2 compared the effects of postoperative visual pulse rate training to those of auditory pulse rate training on relearning of the photic pulse-rate discrimination 10 days after visual decortication. Recovery of discrimination responding was better after auditory pulse rate training than after visual training. These data suggest that visual cortex lesions in the rat disrupt perceptual or associational functions involving the temporal features of a visual stimulus. In addition, generalization of relational properties during cross-modal training through multimodal CNS structures appears to enhance recovery of behavior after brain insult.

Response confidence and the Ponzo illusion.

The number of longer line judgments and the ratings of confidence in these judgements were significantly higher when the comparison line was located near the apex of the Ponzo wedge but only confidence ratings were affected by the orientation of the wedge (ns = 24 men, 24 women). Differences between the two measures suggest that perceptual and nonperceptual processes may be involved in the illusion.