Fast and slow depolarizing responses of guinea-pig coeliac ganglion cells to 5-hydroxytryptamine.

Responses to 5-hydroxytryptamine (5-HT) applied by pressure ejection to guinea-pig coeliac ganglion cells were recorded intracellularly in vitro. Three types of responses were observed: a fast depolarization (43% of cells), a slow depolarization (25%) and a biphasic response comprising an initial fast followed by a slow depolarization (30%). Fast depolarizations had a mean duration of 6.4 s and were associated with a decrease in membrane input resistance. On hyperpolarizing the membrane, responses increased in amplitude; the mean reversal potential was -23 mV. Fast responses were nearly eliminated in a Na-free medium but were unaffected by low Ca/high Mg solution or by low Cl medium. Fast responses were depressed by MDL 72222, quipazine, metoclopramide, and by relatively high concentrations of D-tubocurarine, but unaffected by methysergide. Slow responses had a mean duration of 62 s and were associated either with no change or a slight fall in membrane resistance. hyperpolarizing the membrane, slow responses were increased in most but not all cells tested. Slow depolarizations were depressed by methysergide but unaffected by agents that suppressed the fast responses. The biphasic response displayed electrophysiological and pharmacological characteristics of both the fast and slow depolarizations. It is concluded that guinea-pig coeliac ganglion cells display either a rapid depolarization, a slow depolarization or a biphasic response to 5-HT, that the fast depolarization appears to be mediated by a conductance increase to Na and K, while a different mechanism appears to underlie the slow depolarization, and that the two responses are mediated by different kinds of 5-HT receptor.