Ionic mechanism of inhibition of long duration in Aplysia synapse

In the abdominal ganglion of Aplysia kurodai, three neurons were identified that responded with inhibition of long duration (ILD) to a single stimulation of the siphon nerve and to bath and iontophoretic application of dopamine. The ILD is a long-lasting (15-60 sec) hyperpolarizing potential, typically associated with a conductance increase but with a reversal potential, typically negative than the potassium equilibrium potential. Exchange of half the external chloride with acetate induced no change in the ILD. The ILD was reduced in amplitude in the presence of 0.1 normal K+ in the external medium and enhanced in two times normal K+ solution. The ILD amplitude, but not conductance, was progressively impaired by cooling and by the metabolic inhibitors ouabain and strophanthidin. These results suggest that the ILD is composed of an increase in K++ conductance and activity of the electrogenic Na-pump. theophylline potentiated the ILD, especially its duration. Furthermore, bath application of dibutyryl cAMP augmented the amplitude of the synaptically elicited ILD and the dopamine responses by 30% over controls without any effect on the change in membrane conductance. Intracellularly injected cAMP also increased the ILD, but 5'-AMP showed no such effect. These data suggest that dopamine acts on the ILD neurons by increasing K+ conductance and the activity of the electrogenic Na-pump and that cAMP might selectively stimulate the pump component of the ILD.