Expression of metabotropic glutamate receptor group I in rat gustatory papillae

Abstract

Taste-metabotropic glutamate receptor 4 (taste-mGluR4) and the heteromers of T1R1 and T1R3 are candidate receptors involved in the sense of umami (monosodium glutamate) taste. Although the expression of group III mGluRs (taste-mGluR4) has been demonstrated in taste tissues, no mention has been made of the expression of group I mGluRs (mGluR1 and mGluR5) in taste tissues. We examined the expression of mGluR1 and mGluR5 in rat gustatory tissues by using reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, immunohistochemistry and immunoelectron microscopy. RT-PCR assay showed that mGluR1 alpha and mGluR1 beta mRNAs were expressed in circumvallate papillae, but mGluR5 mRNA was not expressed. The positive signals of mGluR1 mRNA were detected only in circumvallate taste buds by in situ hybridization analysis. In cryosections of fungiform, foliate and circumvallate papillae, the antibody against mGluR1 alpha gave intense labeling on the taste hairs in all taste pores examined. In the developing taste buds, the positive signals of mGluR1alpha in taste hairs gradually increased with the increase in number of taste bud cells. These results show that, in addition to taste-mGluR4 and the heteromer of T1R1 and T1R3, mGluR1 alpha may function as a receptor for glutamate (umami) taste sensation.

Multiple Receptor Systems for Glutamate Detection in the Taste Organ

“…Brain-expressed mGluR1 and mGluR4 are shown to be expressed in rat taste cells in the posterior tongue. 36,37) Taken together, all these results indicate involvement of mGluR1 and mGluR4 in addition to T1R1/T1R3 in umami detection.…”

Brain Activation by Umami Substances via Gustatory and Visceral Signaling Pathways, and Physiological Significance

“…9) Now, several types of GLU receptors are found to exist at the apical membrane of the taste cells (type II taste cells) including the T1R1/T1R3, metabotropic GLU receptor type 1 and 4 (mGluR1 and mGluR4), taste mGluR1, and taste mGluR4. [10][11][12][13][14] Activation of receptors produces changes in intracellular second messenger levels and open/close state of membrane channels to cause excitation of taste cells. The activation of taste cells is transmitted to the taste nerves through the release of a transmitter ATP.…”

Nerve and Behavioral Responses of Mice to Various Umami Substances

“…We investigated changes in palatability, taste quality, and taste intensity of umami substances using a 48-h 2-bottle preference test, a CTA test, and gustatory nerve Previous studies have identified three putative G protein-coupled receptors for umami substances: tastemGluR1, 14) taste-mGluR4, 15,16) and the amino-acid receptor T1R1 þ T1R3 heterodimer. 17,18) Synergy, a hallmark of umami taste, was observed not only at the individual level but also at the cellular level.…”