Response properties of the pharyngeal branch of the glossopharyngeal nerve for umami taste in mice and rats

Many studies have reported the mechanism underlying umami taste. However, there are no investigations of responses to umami stimuli taste originating from chemoreceptors in the pharyngeal region. The pharyngeal branch of the glossopharyngeal nerve (GPN-ph) innervating the pharynx has unique responses to taste stimulation that differs from responses of the chorda tympani nerve and lingual branch of the glossopharyngeal nerve. Water evokes robust response, but NaCl solutions at physiological concentrations do not elicit responses. The present study was designed to examine umami taste (chemosensory) responses in the GPN-ph. Response characteristics to umami taste were compared between mice and rats. In mice, stimulation with compounds eliciting umami taste (0.1 M monosodium l-glutamate (MSG), 0.01 M inosine monophosphate (IMP) and the mixture of 0.1 M MSG + 0.01 M IMP) evoked higher responses than application of distilled water (DW). However, synergistic response of a mixture of 0.1 M MSG + 0.01 M IMP was not observed. In rats, there is no significant difference between the responses to umami taste (0.1 M MSG, 0.01 M IMP and the mixture of 0.1 M MSG + 0.01 M IMP) and DW. Monopotassium glutamate (MPG) was used in rats to examine the contribution of the sodium component of MSG on the response. Stimulation with 0.1 M MPG evoked a higher response when compared with responses to DW. The present results suggest that umami taste compounds are effective stimuli of the chemoreceptors in the pharynx of both mice and rats.     

Basic properties of umami and effects on humans.

This is a review of the taste of umami substances, and some related findings. The data demonstrate that, though the taste of the common umami substances such as MSG and IMP is mainly caused by their anions, the effects of their cations, such as Na, should not be ignored. The effects of cations approach the taste thresholds of umami substances. Although the taste threshold of MSG was slightly lower than that of Na, the threshold of IMP was found to be controlled by Na. However, the degree of saltiness was less than 10% above the threshold of the equivalent weight of NaCl. It was also found that the taste of IMP was probably caused by glutamic acid in saliva, since IMP itself has no umami taste. That is, IMP enhances the umami taste of MSG. Finally, comparison of umami sensitivity of Japanese and Americans revealed no difference.     

Canine taste nerve responses to umami substances.

The taste responses to the "umami substances" such as monosodium glutamate (MSG), guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) were recorded from the canine chorda tympani nerve. A large synergism was observed between MSG and the nucleotides in most mongrel dogs (type A dog). The extent of the synergism between MSG and the nucleotides was much larger than that observed in any other animal examined except for humans. No synergism was observed between the nucleotide (GMP) and stimuli other than MSG, such as NaCl, HCl, sucrose, quinine, and glycine. It was concluded that the dog is a suitable experimental animal for studies on the responses to umami substances. In order to differentiate umami and salt components in the responses to umami substances, effects of amiloride on the responses were examined. Amiloride inhibited the response to MSG, but did not inhibit the response to GMP alone or those induced by synergism between GMP and MSG. The present results favor a conclusion that GMP acts as an agonist and MSG acts as a modulator for the umami receptor in the dog. The synergism can be explained by an allosteric model where the umami receptor is assumed to have two binding sites, one for GMP and another for MSG.     

Gustatory information of umami substances in three major taste nerves

The chorda tympani (CT), glossopharyngeal (GL), and greater superficial petrosal (GSP) nerves, the three major branches of cranial nerves innervating taste buds, respond with considerable differences to various taste stimuli. To examine which nerve is responsible for transmitting umami taste in rats, we conducted electrophysiological and behavioral experiments. In the electrophysiological study, responses to umami substances were compared among these three nerves. The CT and GSP were more responsive to mixtures of monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) than the GL. Synergistic effects by the mixture of MSG and IMP were the most prominent in the CT followed by the GSP, whereas it was negligible in the GL. In the behavioral study, rats with a combined transection of the CT and GSP could not acquire conditioned taste aversions to umami substances. These results suggest that umami taste is conveyed more dominantly via the CT and GSP than the GL in the rat.     

Discrimination of monosodium glutamate and sodium chloride solutions by rats

Discrimination of monosodium glutamate (MSG) and sodium chloride (NaCl) solutions by rats was studied in a series of two or three bottle preference tests in free-choice situations. In two bottle preference tests, solutions containing 0.001–0.1 M NaCl or MSG were preferred to deionized water and the peak preference was 0.05 M for both NaCl and MSG-flavored solution. The transition point from preference to aversion was found to be very close to the isotonic point. Total fluid intake of both choices (MSG or NaCl-flavored water and water) was approximately 30% above the consumption of tap water controls when solutions contained 0.1 M NaCl or MSG. However, maximum intake of moles of MSG or NaCl did not exceed approximately 2 × 10^?2 mmoles per gram body weight. In a series of three choice preference tests, MSG (0.005–0.01 M) was preferred to comparable concentration of NaCl and water. Both MSG and NaCl (0.05–0.1 M) were preferred to water. When mixtures of MSG and NaCl (0.05 and 0.1 M) were compared with equimolar solutions of MSG or NaCl, the mixtures were preferred to solutions containing a single taste substance. It is concluded from these results that the rat might recognize MSG solution as a favorable substance, differently from NaCl solution.     

MSG intake and preference in mice are influenced by prior testing experience

Monosodium glutamate (MSG), the prototypical umami substance, is used as a flavor enhancer in many foods, but when presented alone is often only weakly attractive. Yet with experience mice will develop strong preferences for MSG solution over water. The present experiments explored the conditions that change indifference to preference for MSG. C57BL/6J mice were given a series of 2-day two-bottle tests with water vs. an ascending series of MSG concentrations (0.1–450 mM) to assess preference and intake. Naive mice were indifferent to all concentrations, but following forced one-bottle exposure to 300 mM MSG they preferred most concentrations and consumed more MSG. Exposure to 100 mM MSG also increased subsequent MSG preference but not intake. Experience with other nutritive solutions (8% sucrose, 8% Polycose, 8% casein hydrolysate, and isocaloric 3.5% soybean oil emulsion) also enhanced subsequent MSG preference and intake. Polycose and sucrose experience were almost as effective as MSG experience. However, not all sapid solutions were effective; 0.8% sucralose and 10 mM MSG exposure did not alter subsequent MSG preference. The generality of the preexposure effect was tested by offering an ascending series (0.1–100 mM) of inosine monophosphate (IMP), another umami substance; initial indifference was converted to preference after forced exposure to 300 mM MSG. Together these results suggest that a combination of oral and post-oral effects may be responsible for the experience effect, with MSG itself the most potent stimulus. A final experiment revealed that MSG preference in naïve mice is enhanced by presenting the MSG and water drinking spouts far apart rather than side by side. Thus the preferences for umami solutions in mice are subject to influence from prior tastant experience as well spout position, which should be taken into account when studying acceptance of taste solutions in mice.     

Evaluation of the 'liking' and 'wanting' properties of umami compound in rats

Food reward is neurologically and psychologically divided into at least two properties; ‘liking’ and ‘wanting’. Although umami taste enhances food palatability, the liking and wanting properties of umami taste, and the underlying neural mechanisms for these properties are not clear. Here, we compared sucrose (0, 10, 30, 120 and 480 mM) and monosodium l-glutamate (MSG; 0, 10, 30, 60 and 120 mM) solutions using a taste reactivity test to evaluate liking, and fixed/progressive-ratio operant licking tasks to evaluate wanting. To determine the underlying neural mechanisms, we also conducted systemic blockade of opioid receptors in both tests. In the taste reactivity test, the hedonic reactions to 30, 60 and 120 mM MSG were greater than those to water (0 mM) but lower than those to 480 mM sucrose. In the operant task, the intake, number of licks, and breakpoint to MSG reached peaks at around 60 mM but they were lower than those to 30-480 mM sucrose. The systemic naloxone treatment decreased the hedonic responses to MSG and sucrose, and reduced the incentive salience of MSG but not sucrose. These findings indicate that the hedonic response and incentive salience of MSG is lower than those of sucrose when compared at the maximum response and that the incentive salience of MSG is lower than sucrose even where the hedonic response is similar. The present study also suggest that the hedonic response and incentive salience of umami compound is modulated by brain opioid signaling.     

Parabrachial gustatory neural responses to monosodium glutamate ingested by awake rats.

A sample of 41 gustatory neurons isolated in the parabrachial nuclei of awake, behaving rats was tested with sapid solutions of 0.1 M monosodium glutamate (MSG), 0.5 mM of guanosine 5'-monophosphate (GMP), and a mixture of MSG and GMP as well as with 0.3 M sucrose, 0.1 M NaCl, 0.01 M citric acid, and 0.0001 M QHCl. Interneuronal correlation coefficients and factor analysis indicated that both the sodium cation and glutamic anion contributed to the activity elicited by MSG. Guanosine potentiated the responses to MSG, but only in neurons that also responded to sucrose. These results suggest that the gustatory contribution to the flavor denoted by the Japanese word "umami" may be mediated, in part, by neurons that also respond to chemical described by humans as sweet.     

Gustatory neural responses in preweanling mice.

Taste sensitivity of preweanling mice was studied by examining responses of the chorda tympani (CT) and glossopharyngeal (GL) nerves to various taste stimuli, and was compared to that of adult mice. In mice of 7-10 days of age, comparing to that of the CT nerve, threshold of the GL nerve for monosodium l-glutamate (MSG) was low, but those for sucrose and NaCl were high. Sensitivities to HCl and quinine-HCl were similar between the CT and GL nerves, although that to quinine-HCl was larger in the GL nerve than in the CT nerve in adult mice. Enhancement of MSG responses by addition of GMP was observed in the CT nerve but not in the GL nerve in this age group. In mice of 8-16 weeks of age, threshold of the GL nerve for MSG became higher but that for NaCl became lower. Enhancement of MSG responses by addition of GMP appeared also in the GL nerve. Inhibition of NaCl responses by amiloride was observed in the CT nerve. These results suggest that, in mice, the GL nerve is important taste input for umami substances especially during the preweanling period, whereas the CT nerve is for sweet and salty substances. Properties of umami and salt receptor systems change during the postweanling period.     

Taste perception of monosodium glutamate and inosine monophosphate by129P3/J and C57BL/6ByJ mice

Our previous studies have shown that in long-term two-bottle preference tests, mice from the C57BL/6ByJ (B6) inbred strain drink more monosodium glutamate (MSG) and inosine monophosphate (IMP) than mice from the 129P3/J (129) inbred strain. The goal of this study was to examine whether this variation in consumption could be attributed to strain differences in perception of the taste quality of MSG and IMP. We developed a conditioned taste aversion (CTA) in B6 and 129 mice to 100 mM MSG or 10 mM IMP and used a brief-access taste assay to examine CTA generalization. B6 and 129 mice did not differ in the generalization patterns following CTA to MSG: mice from both strains generalized CTA from MSG to NaCl. In contrast, strain differences in the generalization patterns were evident following the CTA to IMP: while mice from both strains generalized CTA from IMP to MSG, 129 mice tended to have stronger CTA generalization to saccharin and d-tryptophan, both of which are perceived as sweet by humans. These data suggest that the strain differences in MSG consumption are not due to variation in perception of the taste quality of MSG. Instead, the differential intake of IMP likely reflects strain differences in the way the taste quality of IMP is perceived. Our data suggest that mice perceive MSG and IMP as complex taste stimuli: some taste components are shared between these two substances, but their relative intensity seems to be different for MSG and IMP. The amiloride-sensitive salt taste component is more prevalent in MSG than in IMP taste, and in B6 compared with 129 mice.     

Neuronal responses of the nucleus tractus solitarius to oral stimulation with umami substances

In order to investigate coding mechanisms of special taste modality (umami), responses of neurons within the nucleus tractus solitarius (NTS) to oral stimulation with monosodium glutamate, disodium 5'-inosinate (IMP) or their mixture were recorded in the conventional electrophysiological method. Results obtained were as follows: Neither MSG-best nor IMP-best neuron was recorded within the NTS as in the primary taste afferents. Some of the sucrose-best neurons, NaCl-best neurons and HCl-best neurons responded to oral stimulation with MSG or IMP. A remarkable synergistic effect was observed in all of the sucrose-best neurons and in some of the NaCl-best neurons but not in all of the HCl-best neurons, when the mixed solution of MSG and IMP was applied into the oral cavity. As to the sucrose-best neurons, potency of the synergism was positively correlated with the responsiveness to sucrose. No correlation was recognized between them in the case of NaCl-best neurons. These results suggest a view that the sucrose-best neurons and the NaCl-best neurons which show the synergism may participate in coding umami taste.     

Representation of umami taste in the human brain

Umami taste stimuli, of which an exemplar is monosodium glutamate (MSG) and which capture what is described as the taste of protein, were shown using functional MRI (fMRI) to activate similar cortical regions of the human taste system to those activated by a prototypical taste stimulus, glucose. These taste regions included the insular/opercular cortex and the caudolateral orbitofrontal cortex. A part of the rostral anterior cingulate cortex (ACC) was also activated. When the nucleotide 0.005 M inosine 5'-monophosphate (IMP) was added to MSG (0.05 M), the blood oxygenation-level dependent (BOLD) signal in an anterior part of the orbitofrontal cortex showed supralinear additivity; this may reflect the subjective enhancement of umami taste that has been described when IMP is added to MSG. These results extend to humans previous studies in macaques showing that single neurons in these taste cortical areas can be tuned to umami stimuli.     

Effects of intragastric infusion of inosine monophosphate and <Emphasis Type="SmallCaps">l</Emphasis>-glutamate on vagal gastric afferent activity and subsequent autonomic reflexes

In this study we investigated the effects of intragastric infusion of palatable basic taste substances (umami, sweet, and salty) on the activity of the vagal gastric afferent nerve (VGA), the vagal celiac efferent nerve (VCE), and the splanchnic adrenal efferent nerve (SAE) in anesthetized rats. To test the three selected taste groups, rats were infused with inosine monophosphate (IMP) and l-glutamate (GLU) for umami, with glucose and sucrose for sweet, and with sodium chloride (NaCl) for salty. Infusions of IMP and GLU solutions significantly increased VGA activity and induced the autonomic reflex, which activated VCE and SAE; these reflexes were abolished after sectioning of the VGA. Infusions of glucose, sucrose and NaCl solutions, conversely, had no significant effects on VGA activity. These results suggest that umami substances in the stomach send information through the VGA to the brain and play a role in the reflex regulation of visceral functions.     

Are umami taste receptor sites structurally related to glutamate CNS receptor sites?

Umami tasting substances, MSG (monosodium glutamate), HG (glutamic acid), LGDE (1-glutamic acid diethyl ester), DLHCA (dl-homocysteic acid), DLAAA (dl-aminoadipic acid) and 5'GMP, were tested on the hamster and the human. Ten mM MSG was routinely used in the hamster as it elicited strong chorda tympani responses. Similar response amplitudes were found for MSG, HG, LGDE, DLAAA 10 mM, DLHCA 8 mM and sucrose 100 mM. A 5 microM concentration of 5'GMP eventually was an efficient stimulus on a few preparations. Such a low concentration is very seldom efficient as a taste stimulus in rodents, indicating a higher specificity of receptor mechanisms than what is usually found for sweet taste, for example. The synergy between MSG and 5'GMP was found in the hamster CT only for concentrations lower than those of the literature, i.e., a mixture of 12 microM 5'GMP and 2.5 mM MSG showed a reinforcement of 50% in response amplitude equivalent to a 100% increase in concentration. We take this as an evidence of an umami component in the hamster CT response to glutamate; in accordance with literature data, we could not find reinforcement for higher concentrations which were in fact near saturation. Responses to MSG, HG, LGDE, DLAAA and DLHCA, among 38 other organic stimuli, were studied in 42 hamster chorda tympani. Responses to HG, LGDE and 5'GMP, among chemoreception of these compounds used as umami tasting stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

On the taste of umami in chimpanzee.

Whole and single fiber chorda tympani nerve recordings were obtained in 5 chimpanzees to stimulation with MSG (monosodium phosphate) and GMP (guanosine 5'-monophosphate, disodium salt) alone and in combination. The overall chorda tympani nerve activity was recorded to 5 concentrations of MSG, ranging from 1 to 100 mM with and without 0.3 mM GMP, and to 5 concentrations of GMP, ranging from 0.1 to 10 mM, with and without 30 mM MSG. A synergistic effect was recorded between MSG and GMP in 3 out of 4 animals. The effect of stimulation with MSG and GMP alone and mixed was studied in approximately 25 single fiber recordings against a background of the stimulating effects of 11 different sweeteners, 3 acids, 3 bitter compounds and 3 different salts. The fibers showed a high taste specificity and fell into groups which corroborated with the human concepts of the taste qualities. The umami compounds elicited moderate responses which were largest in the sweet fibers. In the 6 sweet fibers that responded to the umami compounds. 0.3 mM GMP was a more effective stimulus than 10 mM MSG. In 3 of these fibers a synergistic effect was recorded to the mixture of GMP and MSG. It is interesting that the response to GMP and MSG was unaffected by gymnemic acid, although it blocked the response to the sweet compounds. Three out of 10 salt fibers responded to MSG and GMP but no synergistic effect was recorded. No specific umami fibers were recorded. However, more data must be collected before the final conclusion on the presence or absence of specific umami fibers can be drawn.     

Behavioral responses to glutamate receptor agonists and antagonists implicate the involvement of brain-expressed mGluR4 and mGluR1 in taste transduction for umami in mice

Recent molecular studies have identified many candidate receptors for umami, typically the taste of monosodium glutamate (MSG). The candidate receptors, including taste-mGluR4, T1R1+T1R3, and truncated mGluR1, respond to MSG in the millimolar concentration range. Expression of brain-expressed mGluR4 and mGluR1 with much higher sensitivities to glutamate has also been reported in taste papillae. To test the involvement of brain-expressed mGluRs in umami taste, we tested glutamate agonists and antagonists at concentration ranges relevant to both types of the receptors using a combination of a detection threshold and conditioned taste aversion (CTA) methods in mice. The detection threshold experiment showed that mice could detect the group III mGluR agonist L(+)-2-amino-4-phosphonobutyrate (L-AP4) taste thresholds at 0.0009-0.0019 mM. Mice conditioned using CTA methods to avoid either MSG or MPG showed aversive responses to MSG with and without amiloride or to MPG, respectively, at concentrations of 0.0001 mM and above. A CTA to L-AP4 or MSG showed comparable concentration-response ranges for L-AP4 and MSG. The Group III mGluR antagonist, (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG), and the mGluR1 antagonist, 1-aminoindan-1,5-dicarboxylic acid (AIDA), suppressed aversive responses to glutamate agonists at concentrations between 0.0001 and 100mM in the CTA experiments. Our results suggest the possibility that brain-expressed mGluR4 and mGluR1 may contribute to umami taste in mice.     

Analyses of taste nerve responses with special reference to possible receptor mechanisms of umami taste in the rat

Umami substances such as monopotassium L-glutamate (MPG) and 5'-inosine monophosphate (IMP) elicit a unique taste called 'umami' in humans. To elucidate the umami receptor mechanism in rats, we examined taste responses of the chorda tympani nerve by using three ionotropic glutamate receptor agonists, NMDA, KA and AMPA, a mGluR4 agonist, L-AP4, and a specific mGluR4 antagonist, MAP4, and an anti-sweet peptide, gurmarin. When IMP was added, synergistic responses were shown only for MPG and L-AP4, but not for NMDA, KA and AMPA. MAP4 enhanced the responses to MPG and L-AP4. Gurmarin suppressed the synergistic responses to mixtures of MPG and IMP or L-AP4 and IMP. These results suggest that glutamate and L-AP4 bind both the sweet-responsive macromolecule and mGluR4, but the synergism occurs only on the macromolecule.     

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.     

Chorda tympani responses in two inbred strains of mice with different taste preferences.

Behavioral studies suggest that there are significant differences in the taste systems of the inbred mouse (Mus musculus) strains: C57BL/6J (B6) and DBA/2J (D2). In an attempt to understand the biological basis of the behavioral differences, we recorded whole-nerve chorda tympani responses to taste solutions and compared the results to intake of similar solutions in nondeprived mice. Stimuli included a test series composed of 0.1 M sodium chloride, 0.3 M sucrose, 10 mM sodium saccharin, 3 mM hydrochloric acid, and 3 mM quinine hydrochloride, as well as concentration series for the same substances. Neural activity of the chorda tympani that was evoked by sucrose, saccharin, or NaCl was greater in B6 than D2 mice; and neural threshold for sucrose was lower in B6 mice, but neural thresholds for HCl and quinine were lower in D2 mice. B6 mice drank more sucrose and saccharin but less quinine than D2 mice; thus, sucrose and saccharin preference were positively correlated, but NaCl and quinine aversiveness were negatively correlated with the chorda tympani results. Nonetheless, genes involved in the structuring of taste receptors and/or the chordae tympani, which transduce taste stimuli having diverse perceptual qualities, differ for the two mouse strains.