Muscarinic enhancement of the voltage‐dependent calcium current in an identified snail neuron.

1. In the F1 neuron of the snail Helix aspersa bathed in a Ba2+ and 4‐aminopyridine‐containing saline, carbamylcholine (CCh) enhanced the inward current carried by Ba2+ through the voltage‐dependent Ca2+ channels. 2. This effect of CCh on the F1 neuron was not affected by the nicotinic antagonists (+)‐tubocurarine and hexamethonium, but it was mimicked by oxotremorine and blocked by both atropine and pirenzepine. 3. The intracellular injection of GTP gamma S (guanosine 5'‐O‐(3‐ thiotriphosphate] into the F1 neuron caused both a decrease in Ca2+ current and a blockade of the CCh‐induced enhancement of the Ca2+ current. 4. Neither cyclic AMP, cyclic GMP nor arachidonic acid mimicked the effect of CCh on the Ca2+ current in the F1 neuron. In contrast, the intracellular injection of EGTA blocked the CCh‐induced enhancement of the Ca2+ current thus suggesting that cytosolic Ca2+ is involved in the CCh‐induced response. 5. We then investigated the possible role of inositol 1,4,5‐trisphosphate (InsP3) and Ca(2+)‐dependent protein kinases in the CCh‐induced enhancement of the Ca2+ current. The intracellular injection of InsP3 in the F1 neuron elicited no consistent change in the Ca2+ current. Diacylglycerol analogues (OAG and DOG) decreased the Ca2+ current amplitude, i.e. an effect opposite to that produced by CCh. This effect of the diacylglycerol analogues resulted from the activation of protein kinase C (PKC) since it was blocked by staurosporine. In addition, staurosporine did not affect the CCh‐induced increase in Ca2+ current. 6. The intracellular injection of either Ca(2+)‐calmodulin‐dependent protein kinase II (Ca(2+)‐CaM‐PK) or a peptide inhibitor of this enzyme into the F1 neuron affected neither the Ca2+ current nor its enhancement by CCh. 7. We conclude that the CCh‐induced enhancement of the Ca2+ current in the snail F1 neuron involves the activation via muscarinic receptors of an intracellular transduction mechanism in which cytosolic Ca2+ plays a key role. However, InsP3, protein kinase C and Ca(2+)‐CaM‐PK do not appear to be directly involved in this CCh‐induced response.