Serotonin and forskolin modulation of a chloride conductance in cultured identified Aplysia neurons.

1. The effect of serotonin (5-HT) and forskolin on a hyperpolarization activated Cl- conductance (gCl-) was studied using voltage-clamp techniques in identified Aplysia neurons maintained in primary cell culture. 2. The hyperpolarization-activated conductance induced by intracellular Cl- loading was carried by Cl- as determined by the following criteria: the extrapolated reversal potential of the current closely approximated the reversal potential of a cholinergic Cl- conductance, the current was not affected by extracellular ion substitutions other than Cl-, extracellular thiocyanate ions reversibly inhibited the current and the current exhibited slow voltage-dependent exponential kinetics similar to those described for the hyperpolarization-activated Cl- current in Aplysia neurons in situ. 3. In the identified neurons B1, B2, R15, and R2, 5-HT or forskolin reversibly inhibited gCl-, suggesting that 5-HT acted via an adenosine 3',5'-cyclic monophosphate-dependent mechanism. 4. Serotonergic inhibition resulted from a change in the voltage dependence of Cl- channel gating.