5-Hydroxytryptamine mediates inhibitory postsynaptic potentials in rat dorsal raphe neurons.

In rat dorsal raphe neurons, focal electrical stimulation elicited a slow inhibitory postsynaptic potential (IPSP) associated with increased membrane conductance. On the basis of data in the literature this IPSP is presumably caused by a recurrent inhibitory circuit. 5-Hydroxytryptamine (5-HT) application caused a hyperpolarization of the cell membrane, also associated with increased membrane conductance. Both responses could be abolished by lysergic acid diethylamide or methysergide, and enhanced by imipramine. The reversal potentials for the responses were at the same level of - 95 mV which suggests an increase in membrane potassium conductance as the mechanism of both responses. These data suggest that 5-HT may be the mediator of the IPSP evoked by focal stimulation.