Modulation of potassium current kinetics in bag cell neurons of Aplysia by an activator of adenylate cyclase.

The bag cell neurons of Aplysia are neurosecretory cells which control egg-laying behavior. In their resting state, the cells have a high resting potential and show no spontaneous activity. In response to brief stimulation of a neural input, the cells depolarize and fire repetitively for up to 60 min. This afterdischarge is thought to be controlled by elevations of intracellular adenosine 3':5'-monophosphate (cAMP). A voltage clamp study of bag cells in primary culture was undertaken in order to characterize the effects of cAMP on the cells' electrical properties. The transient outward potassium current (A-current) was studied before and after the application of forskolin (an activator of adenylate cyclase) and RO20-1724 (a phosphodiesterase inhibitor). These drugs reduced the amplitude of the A-current, primarily by speeding the inactivation process. The time constants for inactivation were speeded at all potentials, but the largest effects were seen at the more positive potentials (-40 to -15 mV), where the time constants were reduced 5-fold. Neither the activation process nor the steady-state parameters of inactivation were altered by the drugs. It is suggested that these changes in the A-current could explain the ability of the bag cells to fire repetitively during the afterdischarge.