Modulation of synaptic output by the transient outward potassium current in aplysia.

The mechanisms involved in synaptic output modulation by presynaptic membrane potential was studied in identified Aplysia synapses, where a presynaptic hyperpolarization reduces the postsynaptic potential amplitude. The experiment reported here reveals that a presynaptic conditioning hyperpolarization induces a decreased presynaptic spike amplitude and that the reduction is due to a superimposition of a transient outward potassium current on the inward current. This is demonstrated by the external application of 4-aminopyridine which suppresses the early outward current and thus suppresses the modulation both of the presynaptic spike amplitude and the synaptic output.