Sodium and calcium gating currents in an Aplysia neurone.

Abstract

Currents generated by depolarizing and hyperpolarizing voltage pulses were recorded at temperatures of 4-12.degree. C in the voltage-clamped soma of R15 in Aplysia abdominal ganglia exposed to solutions which suppressed ionic currents. Subtraction of linear capacitive and leakage currents from current generated by voltage pulses to levels more positive than -20 mV revealed non-linear transient outward displacement currents at the onset of the clamp step (on-current) and transient inward displacement currents after the membrane potential returned to the holding potential (off-current). Only on-currents were studied. Pulses to membrane potential of -20 to 0 mV generated a displacement current with rapid onset and exponential decay. At membrane potentials more positive than 0 mV a 2nd displacement current with a much slower onset and slower exponential decay was seen. Because the different threshold potentials for the 2 displacement currents were close to the different threshold potentials for Na and Ca ion currents, the 2 displacement currents were called Na and Ca gating currents. The amount of charge transfer during Ca gating currents increased sigmoidally with increasing depolarization, reaching a maximum at +30 to +40 mV. Half-maximum charge transfer occurred at +15 mV. Total charge movement during Ca gating currents was maximal with holding potentials of -30 to -40 mV. More positive or more negative holding potentials produced a decrease in charge movement. The time course of the gating currents, but not the total charge displaced, was very sensitive to temperature. The time constant of decay of Ca gating currents had a Q10 of about 3, whereas the total amount of charge displaced had a Q10 of 1.2. The charge transfer during both Na and Ca gating currents and the amplitude of Na and Ca (but not K) ionic currents were reduced in solutions containing 1 mM n-octanol.