EFFECTS OF CYCLIC-GMP ON THE SUGAR TASTE RECEPTOR CELL OF THE FLY PHORMIA-REGINA

In some sensory receptors of vertebrates, cyclic GMP (cGMP) and/or cyclic AMP (cAMP) is demonstrated to mediate information from the receptor molecule to the ion channel. The labellar chemosensillum of the fly, Phormia regina, is a hair-like organ which contains four taste cells, i.e. the sugar, the salt, the water receptor cells and the fifth cell. They generate spike potentials of different sizes which are easily discriminated from each other. When a membrane-permeable cGMP analogue, dibutyryl cyclic GMP, was applied to the tip of sensillum, large spikes were evoked. Membrane-impermeable nucleotides also evoked the same spikes but their stimulating effect was less than that of dibutyryl cyclic GMP. Judging from the size of the spikes and feeding behaviour of flies to the nucleotides, the spikes were identified as those from the sugar receptor cells. When the mixture of dibutyryl cyclic GMP and phosphodiesterase inhibitor, both of which are membrane-permeable, was repeatedly applied to the sensillum tip, "remaining spikes" were observed after the removal of the mixture. Furthermore, application of dibutyryl cyclic GMP to the sensillum induced extremely slow adaptation of the sugar receptor cell while adaptation of the cell by sugar stimulation is usually rapid. Based upon these results, we discuss the possibility that cGMP works as a second messenger for the sugar receptor excitation of the fly. © 1990.