PRESYNAPTIC RECORDINGS FROM DROSOPHILA - CORRELATIN OF MACROSCOPIC AND SINGLE-CHANNEL K+ CURRENTS

We have performed direct electrophysiological recordings from Drosophi la peptidergic synaptic boutons in situ, taking advantage of a mutatio n, ecdysone, which causes an increase in size of these terminals. Usin g patch-clamp techniques, we have analyzed voltage-dependent potassium currents at the macroscopic and single-channel level, The synaptic me mbrane contained at least two distinct voltage-activated potassium cur rents with different kinetics and voltage sensitivity: an I-A-like cur rent with fast activation and inactivation kinetics and voltage-depend ent steady-state inactivation; a complex delayed current that includes a slowly inactivating component, resembling the I-K described in othe r preparations; and a noninactivating component, The I-A-like current in these peptidergic boutons is not encoded by the gene Shaker, becaus e it is not affected by null mutations at this locus. Rather, synaptic I-A has properties similar to those of the Shal-encoded I-A. Single-c hannel recordings revealed the presence in synaptic membranes of three different potassium channel types (A(2), K-D, K-L), with biophysical properties that could account for the macroscopic currents and resembl e those of the Shal, Shab, and Shaw channels described in heterologous expression systems and Drosophila neuronal somata. A(2) channels (6-9 pS) have brief open times, and like the macroscopic I-A they exhibite d voltage-dependent steady-state inactivation and a rapidly inactivati ng ensemble average current profile. K-D channels (13-16 pS) had longe r open times, activate and inactivate with much slower kinetics, and m ay account for the slowly inactivating component of the macroscopic cu rrent. K-L (44-54 pS) channels produced a noninactivating ensemble ave rage and may contribute to the delayed macroscopic current observed.