A FAST ACTIVATING PRESYNAPTIC REUPTAKE CURRENT DURING SEROTONERGIC TRANSMISSION IN IDENTIFIED NEURONS OF HIRUDO

An electrogenic serotonin (5-HT) uptake process was characterized in t he serotonergic Retzius-P cell synapse of the leech, and the simultane ous activation of this presynaptic reuptake and the postsynaptic respo nse was monitored during evoked transmitter release. A presynaptic, Na +-dependent inward current upon application of 5-HT was isolated at me mbrane potentials between -80 and +60 mV. Its identification as a tran smitter uptake current was confirmed by monitoring accumulation of the autofluorescent 5-HT analog 5,7-dihydroxytryptamine during activation of this current. To study the kinetics of 5-HT reuptake in functional synapses, transmitter release was stimulated by flash photolysis of t he Ca2+-caging DM-nitrophen. The results demonstrate that re-uptake ac tivates with a minimal delay of less than a millisecond during synapti c transmission. It acts as a rapid transmitter removal system to deter mine the time course of the postsynaptic response and monitors the kin etics of transmitter clearance at the synaptic site.