Two kinds of transient outward currents, IA and IAdepol, in F76 and D1 soma membranes of the subesophageal ganglia of Helix aspersa

Transient outward currents were characterized with twin electrode voltage clamp techniques in isolated F76 and D1 neuronal membranes (soma only) of Helix aspersa subesophageal ganglia. In this study, in addition to the transient 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 inactivation curves, holding potentials and sensitivity to 4-aminopyridine (4-AP). In contrast to I-A, I-Adepol did not require hyperpolarizing conditioning pulses to remove inactivation; it was evoked from a holding potential of -40 mV, at which I-A is completely inactivated. I-Adepol shows noticeable activation at around -5 mV, whereas I-A activates at around -50 mV. The time courses 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 notably, it does not require a period of hyperpolarization to be active.