Neurobiology on the development and regeneration of taste buds.

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Taste-mGluR4, cloned from taste tissues, is a truncated variant of brain-expressed mGluR4a (brain-mGluR4), and is known to be a candidate for the receptor involved in the sense of umami taste. Although the expression patterns of taste- and brain-mGluR4 mRNAs have been demonstrated, no mention has so far been made of the expression of these two mGluR4 proteins in taste tissues. In order to examine the expression of taste-mGluR4 and brain-mGlu4 proteins in rat taste tissues, we used a specific antibody for mGuR4a, which shared C-terminus of both taste- and brain-mGluR4, for immunoblot analysis and immunohistochemistry.The immunoblot analysis showed that both brain-mGluR4 and taste-mGluR4 were expressed in the taste tissues. Taste-mGluR4 was not detected in the cerebellum. The immunoreactive band for brain-mGluR4 protein was much stronger than that for taste-mGluR4 protein. In the cryosections of fungiform, foliate and circumvallate papillae, the antibody against taste-mGluR4 gave intense labelling of the taste pores and taste hairs in all the taste buds of gustatory papillae examined ; the immunoreaction to the antibody against taste-mGluR4 was more intense at the same sites of taste buds. The portions of taste bud cells below the taste pore and surrounding keratinocytes did not show any immunoreactivities.The results of the present study strongly suggest that, in addition to taste-mGluR4 brain-mGluR4 may function, and even more importantly than the former, as a receptor for glutamate, i. e., umami, taste sensation.