Multiple components of delayed potassium current in peptidergic neurons of Aplysia: modulation by an activator of adenylate cyclase.

The neurosecretory bag cell neurons of the mollusk, Aplysia, control egg-laying behavior in the animal. In these cells, elevation of cAMP greatly enhances the height and width of action potentials. A similar enhancement of action potentials is seen during the bag cell afterdischarge, a 30 min period of repetitive activity that may be triggered by peptides from the reproductive tract or by brief extracellular stimulation. The enhancement of action potentials during an afterdischarge is well correlated with the observed elevations of cAMP. In the present study, we have examined the effects of forskolin (an activator of adenylate cyclase) and theophylline (a phosphodiesterase inhibitor) on the delayed outward currents that are likely to control repolarization of the action potential. Isolated bag cell neurons, maintained in primary culture, were studied with a whole-cell patch clamp technique. High intracellular concentrations of EGTA were used to block potassium current activated by calcium entry. Analysis of the remaining voltage-dependent delayed outward current revealed two major components, which could be separated on the basis of their different kinetic properties. Both currents (IK1 and IK2) were carried by potassium. IK1, which did not inactivate during 100 msec depolarizations, was reduced in amplitude by application of forskolin and theophylline. Ik2, a current defined by its faster kinetic properties, partially inactivated during 100 msec depolarizations. This inactivation was markedly speeded by application of forskolin and theophylline. It is suggested that such changes in outward current in response to cAMP could explain the enhancement of spike width seen during an afterdischarge in vivo.