Expression of ATP-gated P2X3 receptors in rat gustatory papillae and taste buds

It has recently become evident that ATP and other extracellular nucleotides could play an important role in signal transductions. ATP mediates excitatory signaling by means of P2X receptors. P2X(3), one of its subtypes, a membrane ligand-gated ion channel, is strongly expressed in peripheral sensory neurons. The aim of the present study was to examine the distribution of nerve fibers expressing P2X(3) receptors in taste buds in the gustatory papillae and soft palate of rats by immunohistochemistry. We found that the fluorescence ATP marker quinacrine stained subsets of taste bud cells. Numerous nerve fibers innervating taste buds were intensely immunostained with the P2X(3) receptor antibody. These nerve fibers ascended among intragemmal cells and terminated just below the taste pores. In order to examine whether P2X(3) receptors are involved in signal modulation within taste buds, we used fluorescent double stainings to analyze the distribution of P2X(3) receptors and their relationship to alpha-gustducin immunopositive taste receptor cells. Many varicose nerve fibers expressing P2X(3) receptor-immunoreactivities were entangled with alpha-gustducin immunopositive taste receptor cells and ended closely below the taste pores. In fungiform papillae, nerve fibers expressing both P2X(3) receptors and PGP 9.5 were observed. In contrast, only PGP 9.5 immunoreactive nerve fibers were recognized in filiform papillae. These results suggest that P2X(3) receptors might be involved in taste transmission pathways within taste buds. ATP may act as a neurotransmitter, co-transmitter, or neuromodulator at P2X(3) receptors to generate activating gustatory nerve fibers.