Mechanisms of bitter taste reception

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Mechanisms of taste transduction in gustatory receptor cells are not well described. One reason is that most of the past studies have been carried out on isolated taste receptor cells, in which identification of cell types and the cell location are unidentifiable. In the present study we developed a technique to making a slice preparation of the bullfrog fungi form papilla. Voltage-gated ionic current and the response to quinine were studied on the four types of morphologically identified taste cells by whole-cell patch clamp recording with Lucifer yellow-filled pipette. Dye-coupled type la cells (mucous cells) did not show voltage-activated currents. Type Ib cells (wing cells), type II cells (rod cells) and type III cells had voltage-gated sodium (I_<Na>) and potassium currents (I_K) and generated action potentials. The amplitude of Nawas significantly larger in type Ib and II cells than in type III cells. Type Ib and II cells responded to quinine but Type III cells did not.We also studied the cation current from the frog taste receptor cell activated by quinine. From the variance/mean ratio of the quinine-activated current, the single-channel conductance was estimated to be 12 pS in zero extracellular Ca^<2+>. In the presence of 1.8 mM Ca^<2+>, this conductance decreased to 5 pS. The dependence of the current on quinine concentration had a K_<1/2> of 0.48 mM in the absence of extracellular Ca^<2+>, which increased to 2.8 mM in 1.8 mM external Ca^<2+>. The spectral power density distribution of the quinine-activated current could be described by the sum of two Lorentzian functions, with corner frequencies not substantially different in the absence and presence of 1.8 mM external Ca^<2+>. The above results support the notion that the major component of the response of frog taste receptor cells to quinine comes from an ion channel directly activated by quinine.