Presynaptic modulation of transmitter release by the early outward potassium current in Aplysia.

The mechanism involved in presynaptic modulation of transmitter release was studied in an identified synapse of Aplysia californica. Presynaptic hyperpolarization induces a decrease in he evoked postsynaptic potential amplitude. This is shown to be due to a reduction in the presynaptic spike amplitude during the hyperpolarization. The decreased presynaptic spike amplitude with hyperpolarization is explained s resulting from the superimposition of an early outward potassium current on the transient inward current. It is suggested that the presynaptic hyperpolarizing conditioning pulse decreases inactivation of the early outward current, which shunts the transient inward current. The superimposition of these two currents (transient inward current and the early outward current) induces a decrease in presynaptic spike amplitude, which in turn reduces the synaptic output from the terminal.