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.

Genetically-increased taste cell population with G(alpha)-gustducin-coupled sweet receptors is associated with increase of gurmarin-sensitive taste nerve fibers in mice.

BACKGROUND: The peptide gurmarin is a selective sweet response inhibitor for rodents. In mice, gurmarin sensitivity differs among strains with gurmarin-sensitive C57BL and gurmarin-poorly-sensitive BALB strains. In C57BL mice, sweet-responsive fibers of the chorda tympani (CT) nerve can be divided into two distinct populations, gurmarin-sensitive (GS) and gurmarin-insensitive (GI) types, suggesting the existence of two distinct reception pathways for sweet taste responses. By using the dpa congenic strain (dpa CG) whose genetic background is identical to BALB except that the gene(s) controlling gurmarin sensitivity are derived from C57BL, we previously found that genetically-elevated gurmarin sensitivity in dpa CG mice, confirmed by using behavioral response and whole CT nerve response analyses, was linked to a greater taste cell population co-expressing sweet taste receptors and a G(alpha)- protein, G(alpha)--gustducin. However, the formation of neural pathways from the increased taste cell population to nerve fibers has not yet been examined. RESULTS: Here, we investigated whether the increased taste cell population with G(alpha)--gustducin-coupled sweet receptors would be associated with selective increment of GS fiber population or nonselective shift of gurmarin sensitivities of overall sweet-responsive fibers by examining the classification of GS and GI fiber types in dpa CG and BALB mice. The results indicated that dpa CG, like C57BL, possess two distinct populations of GS and GI types of sweet-responsive fibers with almost identical sizes (dpa CG: 13 GS and 16 GI fibers; C57BL: 16 GS and 14 GI fibers). In contrast, BALB has only 3 GS fibers but 18 GI fibers. These data indicate a marked increase of the GS population in dpa CG. CONCLUSION: These results suggest that the increased cell population expressing T1r2/T1r3/G(alpha)--gustducin in dpa CG mice may be associated with an increase of their matched GS type fibers, and may form the distinct GS sweet reception pathway in mice. G(alpha)--gustducin may be involved in the GS sweet reception pathway and may be a key molecule for links between sweet taste receptors and cell type-specific-innervation by their matched fiber class.

Gurmarin sensitivity of sweet taste responses is associated with co-expression patterns of T1r2, T1r3, and gustducin.

Gurmarin (Gur) is a peptide that selectively suppresses sweet taste responses in rodents. The inhibitory effect of Gur differs among tongue regions and mouse strains. Recent studies demonstrated that co-expression levels of genes controlling sweet receptors (T1r2/T1r3 heterodimer) versus Galpha-protein, gustducin, are much lower in Gur-insensitive posterior circumvallate papillae than in Gur-sensitive anterior fungiform papillae. Here, we investigated the potential link of Gur-sensitivity with the co-expression for T1r2/T1r3 receptors and gustducin by comparing those of taste tissues of Gur-sensitive (B6, dpa congenic strains) and Gur-weakly-sensitive (BALB) strains. The results indicated that co-expression ratios among T1r2, T1r3, and gustducin in the fungiform papillae were significantly lower in Gur-weakly-sensitive BALB mice than in Gur-sensitive B6 and dpa congenic mice. This linkage between Gur-sensitivity and co-expression for T1r2/T1r3 receptors versus gustducin suggests that gustducin may be a key molecule involved in the pathway for Gur-sensitive sweet responses.

Induction of salivary kallikreins by the diet containing a sweet-suppressive peptide, gurmarin, in the rat.

Gymnema sylvestre (gymnema) contains gurmarin that selectively inhibits responses to sweet substances in rodents. The present study investigated possible interaction between gurmarin and the submandibular saliva in rats fed diet containing gymnema. Electrophoretic analyses demonstrated that relative amounts of two proteins in the saliva clearly increased in rats fed the gymnema diet. However, rats previously given section of the bilateral glossopharyngeal nerve showed no such salivary protein induction. Analyses of amino acid sequence indicate that two proteins are rat kallikrein 2 (rK2) and rat kallikrein 9 (rK9). rK2 and rK9, a family of serine proteases, have a striking resemblance of cleavage site in the protein substrates. Interestingly, gurmarin possesses comparable residues with those rK2 and rK9 prefer. The kallikreins significantly inhibited immunoreaction between gurmarin and antigurmarin antiserum. These results suggest that rK2 and rK9 increased by chemosensory information for the gymnema diet via the glossopharyngeal nerve might cleave gurmarin or at least cause specific binding with it.

Conditioned food aversion elicited by the temperature of drinking water as a conditioned stimulus in rats.

It is known that taste can act as a conditioned stimulus (CS) for conditioned food aversion. In the present study, in order to examine whether or not the temperature of drinking water can be a CS, we conducted behavioral experiments in Wistar rats. The following results were obtained: (1) The rats subjected to aversive conditioning to 5 or 40 degrees C distilled water could learn to avoid these CSs, but they did not avoid any taste stimuli. (2) The rats subjected to aversive conditioning to 5 or 40 degrees C 0.1 M sucrose developed a generalized avoidance to sucrose at any temperature. (3) When rats familiarized to 25 degrees C 5 mM saccharin-Na (Sacc) were subjected to aversive conditioning to 5 or 40 degrees C Sacc, they avoided the respective CS, but they did not generalize it to any other stimuli even if having the same temperature as the CS. (4) The rats which had undergone transection of the taste nerves (chorda tympani and glossopharyngeal nerves) could acquire the conditioned response to the temperature of the CS. These results suggest that rats can be conditioned to temperature aversion and that the taste nerves are not needed in the formation of this conditioning.

Recovery of amiloride-sensitive neural coding during regeneration of the gustatory nerve: behavioral-neural correlation of salt taste discrimination.

The chorda tympani (CT) nerve innervating the anterior tongue contains two types of NaCl-responsive fibers: one, the N-type, receives input from receptor cells, the NaCl responses of which are strongly inhibited by amiloride, whereas the other, the E-type, receives input from cells poorly sensitive or insensitive to amiloride. To investigate the formation of this differentially responsive neural system, we crushed the mouse CT nerve and examined the subsequent recovery of NaCl responses and amiloride sensitivity of the regenerated nerve and behavioral discrimination between NaCl and KCl. At 2 weeks after the nerve crush, no significant response of the nerve to chemical stimuli was observed. At 3 weeks, responses to salts gradually reappeared. In this period, almost all single fibers responding to NaCl were insensitive to amiloride (E-type). At 4 weeks, some of the single fibers showed amiloride sensitivity (N-type) and behavioral discrimination between NaCl and KCl reappeared. After >or=5 weeks, the number of N-type fibers had reached the control level and became approximately equal to that of E-type fibers. During the course of recovery, N-type and E-type fibers were clearly distinguishable on the basis of their amiloride sensitivities, their KCl/NaCl response ratios, and their concentration-response relationships to NaCl. These results suggest that two salt-responsive systems are independently reformed after the nerve crush. The selective synapse reformation may account for recovery of behavioral discrimination between NaCl and KCl after taste nerve crush and regeneration. It may also explain stable sensory coding for taste quality during the continuous turnover of receptor cells in the healthy animal.