Two kinds of transient outward currents, I-A and I-Adepol, in F76 and D1 soma membranes of the subesophageal ganglia of Helix aspersa

Transient outward currents were characterized with twin electrode voltage c lamp techniques in isolated F76 and D1 neuronal membranes (soma only) of He lix aspersa subesophageal ganglia. In this study, in addition to the transi ent outward current (A-current, I-A) described by Connor and Stevens (1971b ), another fast outward current, referred to as I-Adepol here, is described for the first time. This is similar to the current component characterized in Aplysin (Furukawa, Kandel & Pfaffinger, 1992). The separation of these two current components was based on activation and steady-state inactivatio n curves, holding potentials and sensitivity to 4-aminopyridine (4-AP). In contrast to I-A, I-Adepol did not require hyperpolarizing conditioning puls es to remove inactivation; it was evoked from a holding potential of -40 mV , at which I-A is completely inactivated. I-Adepol shows noticeable activat ion at around -5 mV, whereas I-A activates at around -50 mV. The time cours es of I-Adepol activation and inactivation were similar but slower than I-A . It was found that I-Adepol was more sensitive than I-A to 4-AP. 4-AP at a concentration of 1 mM blocked I-Adepol completely, whereas 5-6 mM 4-AP was needed to block I-A completely. This current is potentially very important because it may, like other A currents, regulate firing frequency but notab ly, it does not require a period of hyperpolarization to be active.