Characteristics of multiple voltage‐activated K+ currents in acutely dissociated chick ciliary ganglion neurones.

1. The properties of voltage‐activated K+ currents were examined using whole‐cell recording techniques in acutely isolated chick ciliary ganglion neurones. 2. Application of depolarizing voltage pulses from a holding potential of ‐60 mV evoked sustained outward currents that inactivated with time constants of hundreds of milliseconds (IDR). Bath application of 10 mM tetraethylammonium (TEA) caused a 70‐90% reduction of IDR. Application of depolarizing voltage steps from a holding potential of ‐120 mV revealed a second class of TEA‐resistant outward currents. These currents activated quickly but inactivated completely within tens of milliseconds (IA). IA activated at more negative command potentials than IDR. However, IDR exhibited a steeper voltage dependence of activation than IA. 3. The midpoint of the steady‐state inactivation curve of IA was between ‐95 and ‐110 mV. By contrast the midpoint of the steady‐state inactivation curve of IDR was between ‐80 and ‐90 mV. It was not possible to produce a complete inactivation of IDR using prepulses of up to 2 s duration. 4. The time course of IA inactivation could only be fitted with double‐exponential curves with time constants of 5‐18 ms and 30‐60 ms at membrane potentials positive to ‐30 mV. The inactivation of IA was slower at more positive membrane potentials because of a greater contribution of the long time constant. The individual time constants were not markedly voltage dependent. 5. Bath application of 5 mM 4‐aminopyridine (4‐AP) caused a 70‐100% block of IA whereas 1 mM 4‐AP was ineffective. Bath application of 560 nM alpha‐dendrotoxin (DTX) produced a 50‐70% reduction of IA, but application of 280 nM DTX had no effect on IA. 6. Application of 1 mM 4‐AP produced a reversible 55‐80% block of IDR measured at the end of a 500 ms depolarizing pulse. The 4‐AP‐sensitive components of IDR activated rapidly and exhibited a gradual inactivation with continued depolarization. The 4‐AP‐resistant components of IDR activated much more slowly and showed very little tendency to inactivate. Significant blockade of IDR was produced by 10 microM 4‐AP. 7. The decay of IDR tail currents could only be fitted with double exponential curves with time constants of 3‐6 and 40‐60 ms, respectively. The fast and slow components of the tail currents behaved independently with respect to the duration of the depolarizing voltage step. 8. Application of 1 mM 4‐AP eliminated the fast, but not the slow component of IDR tail currents.(ABSTRACT TRUNCATED AT 400 WORDS)