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Bitter taste is an important warning sign for animals that food is poisonous. The transduction mechanism of bitter taste is still a contradictory matter among researchers. The major current hypothesis, based mainly on biochemical data, involves metabotropic cascades, either via cytoplasmic cyclic nucleotide monophosphate (cNMP) or via IP3. The former hypothesis involves cytoplasmic cNMP. It has been shown that a taste receptor cell expresses a specific G protein (McLaughlin et al, 1992) that activates phosphodiesterase leading to the decomposition of cNMP (Price, 1973; Ruiz-Avila et al, 1995). The frog taste receptor cell has a cationic channel, which is kept closed at a high cNMP concentration. Bitter stimuli reduce the cNMP concentration and release the cationic channel from the closed state (Kolesnikov & Margolskee, 1995). The latter hypothesis involves the cytoplasmic inositol 1,4,5-trisphosphate (IP3) as a second messenger. This hypothesis proposes that a bitter substance increases the cytoplasmic IP3 concentration by activating a G protein and phospholipase C. IP3 triggers Ca2+ release from the endoplasmic reticulum, which in turn directly or indirectly induces transmitter release from the taste receptor cells (Akabas et al, 1988; Hwang et al, 1990; Spielman et al, 1994, Spielman et al, 1996). We found recently an entirely different mechanism for the bitter taste transduction. We excised a patch membrane from an isolated taste receptor cell of the fungiform papillae of the bullfrog (Tsunenari et al, 1996, 1999) and recorded in the outside-out configuration. The patch pipette was filled with a solution containing 120 mM CsCl, 2 mM Na2-EGTA and 10 mM Na-HEPES. The outside of the patch membrane was exposed to the 120 mM NaCl solution containing 120 mM NaCl and 2 mM Na-HEPES. Thus, on the cytoplasmic face of the patch membrane none of the second messenger candidates or their precursors (eg. cyclic nucleotide, IP3, Ca2+, ATP and GTP) was present.
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