Higher vertebrates are believed to possess at least five basic tastes: Sweet, bitter, sour, salty, and unami (the taste of monosodium glutamate). Taste receptor cells that may selectively reside in various parts of the tongue and respond to different tastants and perceive these taste modalities. Taste receptor cells are organized into taste buds that extend into different papillae in the tongue epithelium. Circumvallate papillae are found at the very back of the tongue, contain hundreds to thousands of taste buds, and are particularly sensitive to biter substances. Foliate papillae are found at the posterior lateral edge of the tongue, contain dozens to hundreds of taste buds, and are sensitive to sour and bitter. Fungiform papillae are found at the front of the tongue, contain a few taste buds, and specialize in sweet taste.

The taste buds may contain 50-150 cells, including precursor cells, support cells, and taste receptor cells. Receptors cells are innervated at their base by afferent nerve endings that transmit information to the taste centers of the cortex through synapses in the in the brain stem and thalamus. At the molecular level, sour and salty tastants modulate taste function by direct entry of H+ and Na+ ions through specialized membrane ion channels on the apical surface of the cells. In the case of sour compounds, taste cell depolarization may be due to the result of H+ blockage of K+ channels. Salty taste seems to be mediated by the entry of Na+ via amiloride-sensitive Na+ channel. Sweet, bitter and unami transduction is believed to mediated by G protein coupled receptors.

G proteins are heterotrimeric GTP binding proteins that couple receptors to effector in many diverse signal transduction processes. The alpha-subunit of G proteins confers most of the specialty of interaction between receptors and the effector. Recently, a novel G protein a-subunit, termed alpha-Gustducin, has been cloned from taste tissue. It is expressed in taste buds of all taste papillae (circumvallate, foliate, and fungiform). It is not expressed in non-sensory proteins of the tongue or other tissues. Gustducin is involved in bitter and sweet transduciton, since gustducin knockout mice show decreased sensitivity to sweet and bitter tastants. Gustducin encodes a 354 aa protein with close resemblance to transducins (rod and cone photoreceptor G proteins). Rat gustducin has ~80% homology with bovine a-transducin.

Recently, two novel taste receptors, TR1 and TR2, have been cloned with distinct topographical distribution in taste receptor cells and taste buds. TRs are members of a new group of 7 TM domain containing GPCR distantly related to the Ca+-sensing receptor (CaSR), a family of putative hormone receptor (V2R), and metabotropic glutamate receptors. All of these proteins share low but distinct sequence homology and distinguished form the other GPCRs by the presence of very long N-terminal extracellular domain - a putative ligand-binding site.

TR1 (rat 840 aa) and TR2 (rat 843 aa) share ~40% homology with each other, and ~30% with CaSR, and 22-30% with V2R pheromone receptors and mGLURs. TR1 and TR2 show a differential expression taste cells. TR1 is rare in taste buds of circumvallate papillae but is expressed in all fungiform taste buds. In contrast, TR2 is almost undetectable in fungiform papillae but is expressed in all circumvallate taste buds. This differential expression is also found in the palate. TR1 is expressed in all gesenchmackstreifen taste buds, while TR2 is rare in these cells. No correlation can be found with respect to the expression of Gustducin- TR1 and TR2 are expressed both Gustducin positive and negative cells.

ADI has produced highly specific rabbit antibodies to rat TR1, TR2-7, alpha-Gustducin using peptide sequences specific to each protein. These antibodies should be useful in studying taste transduciton.