VOLTAGE-ACTIVATED POTASSIUM CURRENTS IN ISOLATED MOTOR-NEURONS FROM THE JELLYFISH POLYORCHIS-PENICILLATUS

1. We describe two voltage-activated potassium currents in the swim motor neurons (SMNs) of the hydrozoan jellyfish, Polyorchis penicillatus. Recordings from neurons isolated in primary cultures were made using the tight-seal, whole-cell technique. 2. One current, I-K-fast, turned on rapidly (time to peak = 6-15 ms), was half-activated at -10 to 0 mV, decayed with two exponential phases (tau were similar to 70 ms and similar to 1 s), and was half-inactivated by prepulses around -53 mV. It likely plays an important role in regulating the duration of SMN action potentials. I-K-fast has features shared by delayed rectifiers and A-like currents in other invertebrates and vertebrates. 3. Another current, I-K-slow, elicited from a holding potential of -30 mV, exhibited a slow onset (tau = 65-250 ms), was half-activated similar to+24 mV, exhibited a shallower voltage dependence than I-K-fast, and did not inactivate. It was slower than most known delayed rectifiers.