Electrophysiological characterization of 5-HT receptors on rat petrosal neurons in dissociated cell culture

The petrosal ganglion supplies chemoafferent pathways via the glossopharyngeal (IXth) nerve to peripheral targets which release various neurotransmitters including serotonin (5-HT). Here, we combined rapid 5-HT application with patch clamp, whole-cell recording to investigate whether 5-HT receptors are expressed on isolated petrosal neurons (PN), cultured from 7-12 day-old rat pups, In responsive cells, the dominant effect of 5-HT was a rapid depolarization associated with a conductance increase in similar to 43% of the neurons (53/123); however, in a minority population (similar to 6% 8/123), 5-HT Caused membrane depolarization associated with a conductance decrease. In the former group, 5-HT produced a transient inward current (I5-HT) in neurons voltage-clamped near the resting potential (similar to -60 mV); the effect was mimicked by the 5-HT3 receptor-specific agonist, 2-methyl-5-HT, suggesting it: was mediated by 5-HT3 receptors. Further, I5-HT was selectively inhibited by the 5-HT3 receptor-specific antagonist MDL72222(1-10 mu M), but was unaffected by either 5-HT1/5-HT2 receptor antagonist, spiperone, or by 5-HT2 receptor-specific antagonist, ketanserin (50-100 mu M). I5-HT displayed moderate inward rectification and had a mean reversal potential (+/- S.E.M.) of -4.3 +/- 6.6 mV (n = 6). Application of 5-HT (dose range: 0.1-100 mu M) produced a dose-response curve that was fitted by the Hill equation with EC50 = similar to 3.4 mu M and Hill coefficient = similar to 1.6 (n = 8). The activation phase of I5-HT (10 mu M 5-HT at -60 mV) was well fitted by a single exponential with mean (+/-S.E.M,) time constant of 45 +/- 30 ms (n = 6). The desensitization phase of I5-HT was best fitted by a single exponential with mean (+/- S.E.M.) time constant of 660 +/- 167 ms (n = 6). Fluctuation analysis yielded an apparent mean single-channel conductance (+/- S.E.M) of 2.7 +/- 1.5 pS (n = 4) at -60 mV. In the minority (- 6%) population of neurons which responded to 5-HT with a conductance decrease, the depolarization was blocked by the 5-HT2 receptor antagonist, ketanserin (50 mu M). Taken together, these results suggest that 5-HT3 receptors are the major subtype expressed by rat petrosal neurons, and therefore are candidates for facilitating chemoafferent excitation in response to 5-HT released from peripheral targets. (C) 1999 Published by Elsevier Science B.V. All rights reserved.