Calcium-dependent inactivation of a potassium current in the Aplysia neuron R15

Abstract

The endogenously bursting pacemaker neuron R15 of Aplysia exhibits an inwardly rectifying K+ current (I(R)) that was shown previously to be enhanced by various neurotransmitters via the intracellular second messenger, cyclic AMP (Drummond et al., 1980; Benson and Levitan, 1983; Levitan et al., 1987). Here we present evidence that Ca2+ influx, either caused by spontaneous bursting activity or elicited by depolarizing voltage-clamp pulses, causes a large, long-lasting inactivation of I(R). The ionic current inactivated by bursts is identified as I(R) by several criteria: it activates steeply at membrane potentials more negative than the K+ equilibrium potential, has very fast kinetics, is reduced by lowering external K+ from 10 to 2 mM, and is blocked by adding 1 mM Ba2+, 10 mM Cs+, or 5 mM Rb+ to the bathing medium. The peak inactivation of I(R) is delayed following a single burst of spikes in R15, such that I(R) decreases maximally by about 20% after 60-90 sec, and then recovers gradually over more than 10 min. The inactivation caused by many bursts of spikes can reduce I(R) to less than 50% of its initial amplitude. The delay in onset and slow time course of recovery from inactivation of I(R) suggest that a complex biochemical mechanism underlies the effect of Ca2+ on I(R). The effect of depolarization on I(R) is due specifically to the influx and intracellular accumulation of Ca2+. Depolarizing voltage-clamp pulses are maximally effective at reducing I(R) when they elicit a large influx of Ca2+, while pulses approaching the Ca2+ equilibrium potential have little effect. The effect of depolarizing pulses on I(R) is blocked by removing Ca2+ from the bathing medium and adding 3 mM Mn2+ to block Ca2+ channels, or by intracellular injection of the Ca2+ chelator EGTA. The results indicate that changes in intracellular Ca2+ occurring during normal bursting activity in R15 cause a profound inactivation of I(R). Hence, I(R) is modulated in opposite directions by 2 intracellular messengers in the same neuron; it is enhanced by intracellular cAMP and it is inactivated by intracellular Ca2+.