The amino acid L-glutamate elicits a unique taste sensation in humans that has been termed "umami". An important feature of the umami sensation is that it is potentiated by the presence of 5' ribonucleotides such as 5'-GMP and 5'-IMP. The receptors involved in glutamate taste and the mechanisms involved in their modulation by nucleotides are definitively not known, although we have obtained evidence during the previous funding period that one of the receptors involved is a taste- specific form of the metabotropic glutamate receptor mGluR4. During the next funding period we will investigate the role of mGluR4 and other glutamate receptors in the taste transduction of glutamate. The specific aims are as follows: (1) To determine the distribution of glutamate receptors in taste buds. It is possible that responses to glutamate involve activation of basolateral neurotransmitter receptors as well as apical taste receptors. We will utilize an approach that allow stimuli to be applied independently to apical and basolateral membranes of taste buds during whole cell recording to determine the cellular distribution of glutamate receptors. (2) To determine the physiological mechanisms involved in the transduction of glutamate as a taste stimulus and its modulation by 5'- ribonucleotides. We will utilize whole-cell and single-channel patch- clamp records and Ca2+ imaging of single taste cells to determine the second messengers and ion channels involved in glutamate taste transduction. (3) To determine is gustducin is involved in the transduction of glutamate. Gustducin is a taste-specific G protein that is expressed in subset of taste cells. We will use transgenic mice in which the gustducin-expressing taste cells also express green fluorescent protein, so that they can be identified for record. We also have gustducin-null mice, in which the gustducin gene has been deleted. We will compare responses to glutamate agonists in gustducin-expressing and gustducin-null mice to determine if gustducin plays a role in glutamate transduction.