Serotonin differentially modulates the electrical properties of different subsets of taste receptor cells in bullfrog

Abstract

Serotonin (5-hydroxytryptamin, 5-HT) is localized in taste bud cells of vertebrates. Effects of the external application of 5-HT on the membrane currents of frog taste receptor cells (TRCs) were investigated using patch-clamp technique in whole-cell con®guration. The 5-HT (0.1±1 mM) and 5-HT1A receptor agonist (+/±)-8-OH-2-(D1-n-propyl-amino)tetralin (8-OH-DPAT) (1± 20 mM) inhibited both voltage-gated sodium current (INa) and voltage-gated potassium current (IK) in 50% of TRCs, but potentiated IK without any signi®cant effect on INa in another subset of 18% of TRCs. Voltage-gated currents in the residual TRCs were not affected by 5-HT or 8-OH-DPAT. External application of 10 mM forskolin and 300 mM 8-cpt cAMP [8-(4- chlorophenylthio)adenosine 3¢:5¢-cyclic monophosphate] mimicked the inhibitory effect of 5-HT and 8-OH-DPAT on IK and INa while internal dialysis with 50 mM protein kinase A inhibitor prevented the 5-HT-mediated inhibitory effects on IK and INa in TRCs. Internal dialysis of TRCs with high Ca2+-pipette solution (1 mM) increased the IK in 58% of TRCs. The 5-HT reversibly increased the [Ca2+]i in 17% of TRCs when measured by Ca2+-imaging using a Ca2+-sensitive dye (fura-2 AM). These results suggest that 5-HT differentially modulates the voltage-gated membrane currents in different subsets of TRCs.