In previous work we have shown that in the snail Helix aspersa neuron F1 carbamylcholine (CCh) and other muscarinic agonists enhance the inward current carried through high voltage-activated Ca2+ channels by Ba2+ (HVA-ICa). It was also found that cyclic nucleotides, inositol trisphosphate or arachidonic acid are not involved in this modulation. Moreover, despite the effect of CCh being blocked by intracellular injection of EGTA, neither protein kinase C nor Ca(2+)-calmodulin-dependent protein kinase II appeared to play a role. 2. In the present paper, the intracellular mechanism of this muscarinic modulation was investigated further by studying the effects of inhibitors of Ser-Thr protein phosphatases (PP) on both the HVA-ICa of neuron F1 and its enhancement by CCh. 3. Intracellular injections in the F1 neuron of either microcystin LR or okadaic acid, both inhibitors of PP1 and PP2A, mimic the action of CCh on the HVA-ICa and occlude the effects of CCh on this current. In contrast, cyclosporin A, an inhibitor of PP2B (calcineurin), affects neither the HVA Ca2+ current itself nor its modulation by CCh. 4. The efficacy of PP inhibitors was tested in F1 neurons in which serotonin (5-HT) induces an inward current involving intracellular increases in cAMP and a protein kinase A-dependent closing of K+ channels. We found that intracellular injection of either microcystin LR or okadaic acid mimicked the 5-HT-induced inward current and occluded the effect of further application of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)
