Augmenting effect of serotonin on the voltage-dependent Ca²⁺ current and subsequently activated K⁺ current in Aplysia neurons

Receptor-induced activation of protein kinase C (PKC) plays an important role in modulation of various types of ionic channels in neurons. For example, PKC causes facilitation or long-lasting activation of certain ionic channels involved in spike firing after the receptor stimulation. We investigated the effect of serotonin (5-HT) on the voltage-dependent Ca²⁺ channels in RB and RC neurons of Aplysia ganglia under voltage clamp. An outward current response was induced by voltage change of the cell membrane from -60 mV to +10 mV. Application of 5-HT significantly augmented the outward current response to the voltage change. Both the outward current and the augmenting effect of 5-HT markedly decreased when examined in either Ca²⁺-free, 10 mM tetraethylammonium, or 0.3 mM Cd²⁺-solution, indicating the current to be Ca²⁺-activated K⁺ current produced by Ca²⁺ entry. Intracellular application of either guanosine 5′-O-(2-thiodiphosphate) or cholera toxin (CTX), reagents for G-proteins, irreversibly blocked the augmenting effect of 5-HT. Application of phorbol dibutylate (PDBu), an activator of PKC, augmented the outward current and the effect of 5-HT was occluded after PDBu application. Staurosporine, a specific inhibitor of PKC, markedly suppressed the augmenting effects of both 5-HT and PDBu on the outward current. However, either 5-HT or PDBu did not augment the Ca²⁺-activated K⁺ current induced by intracellular injection of Ca²⁺ but rather depressed it. These results suggest that stimulation of 5-HT receptor may activate a novel type of CTX-sensitive G-protein and subsequent PKC, and that phosphorylation of voltage-dependent Ca²⁺ channels may result in the increase in Ca²⁺ entry and subsequent C²⁺-activated K⁺ current. The mechanism may contribute to retain the long-lasting activation without broadening of the spike width during the excitatory response to 5-HT in these neurons.