>>23Free amino acids are present in many foods and are significant gustatory stimuli. In humans, amino acids elicit complex tastes, including sweetness, saltiness, sourness and bitterness (Schiffman et al., 1981). Glutamate is also an important amino acid in many foods and is normally present in the form of the monosodium salt, monosodium glutamate (MSG). MSG is believed to elicit a taste that is distinct from sweet, sour, salty or bitter and is known as umami (Kawamura and Kare, 1986), a Japanese term roughly translated as‘ good taste’. It is likely that, at the level of sensory receptor cells, signal transduction for amino acid taste responses involves membrane receptors on the apical, chemosensitive tips of taste bud cells. Researchers have searched intensely for amino acid receptors in taste buds, and the receptors for MSG taste in particular. Several approaches have been taken to investigate putative receptors for MSG in taste buds, including receptor binding assays (Torii and Cagan, 1980; Cagan, 1986), animal behavioral studies (Ninomiya and Funakoshi, 1989a; Yamamoto et al., 1991; Chaudhari et al., 1996), electrophysiological recordings (Ninomiya and Funakoshi, 1989b; Adachi and Aoyama, 1991; Brand et al., 1991; Faurion, 1991; Hellekant and Ninomiya, 1991; Kumazawaet al., 1991; Nishijo et al., 1991; Ninomiya et al., 1992; Plata-Salaman et al., 1992; Rolls et al., 1996; Bigiani et al., 1997; Hellekant et al., 1997), Ca2+-imaging (Hayashi et al., 1996) and molecular biological experiments (Chaudhari et al., 1996). One approach has been to search for ligands other than MSG that mimic taste responses to glutamate and that might, collectively, define a pharmacological profile for the putative receptor(s) responsible for this unique taste. This strategy was applied in a study where taste-evoked activity in the chorda tympani nerve in hamsters was recorded in response to stimulating the tongue with glutamate receptor agonists, including Laspartic acid (L-Asp) (Faurion, 1991). Prior to that report, it was known from human studies that L-Asp somewhat resembles the taste of MSG (Maga, 1983), and it was believed that L-Asp activated ionotropic glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype. Faurion (Faurion, 1991) concluded that MSG taste was transduced by more than one type of glutamate receptor, including NMDA-like and non-NMDA-like sites. Electrophysiological recordings from lipid bilayer membranes in which membranes from mouse lingual tissue had been incorporated also suggested that NMDA receptors were present in taste buds (Brand et al., 1991). More recently, however, studies at the molecular biological level showed that a class III metabotropic glutamate receptor, mGluR4, is expressed in rat taste buds and not in surrounding non-taste cells, and may transduce MSG taste (Chaudhari et al., 1996). Furthermore, rat behavioral experiments carried out in this same study showed that agonists for mGluR4 mimicked MSG taste, supporting the interpretation that mGluR4 is involved in taste transduction for MSG. Human psychophysical studies have also implicated a class III metabotropic glutamate receptor(s) in taste transduction for glutamate (Kurihara and Kashiwayanagi, 1998). This combination of molecular biological, animal behavioral and human psychophysical studies provides strong support for the hypothesis that a metabotropic glutamate receptor, and in particular mGluR4, functions as a taste receptor in gustatory sensory organs.