2 KINETICALLY DISTINCT 5-HYDROXYTRYPTAMINE-ACTIVATED CL- CONDUCTANCESAT RETZIUS P-CELL SYNAPSES OF THE MEDICINAL LEECH

The properties of serotonin (5-HT)-activated Cl- receptor/ion channel complexes in neurons of the CNS of the medicinal leech were analyzed. These channels mediate the postsynaptic response at the serotoninergic Retzius P-cell as well as Retzius-Retzius cell synapses. 5-HT-induced Cl- currents were activated by fast superfusion of transmitter on cel ls dissociated from embryonic leeches including histochemically identi fied Retzius cells. Whole-cell currents elicited by partial superfusio n of the membrane and summated single-channel currents of outside-out patches showed times to peak of 18 +/- 4 msec and 10 +/- 5 msec, respe ctively, and desensitized with time constants of 28 +/- 3 msec and 20 +/- 11 msec. Persistence of single-channel openings in the outside-out configuration as well as lack of effect of dialysis of the whole cell with AMP-PNP or GDP-beta-S indicated that 5-HT directly gates the Cl- channels without involving second messenger cascades. In outside-out patches, two single-channel conductances of 13 pS and 32 pS were ident ified. While the 13 pS conductance desensitized, the 32 pS conductance activated within several tens of msec and showed no desensitization, We postulate that two subtypes of channels are coactivated by 5-HT and that the activation of the fast desensitizing channel could be respon sible for the fast decaying component of the postsynaptic response. Th e slow conductance explains the second slower decay time constant of t he postsynaptic response and could account for the tonic component som etimes observed at Retzius P-cell synapses.