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.
Gerschenfeld
H M
HM
Laboratoire de Neurobiologie (URA 295 CNRS), Ecole Normale Supérieure, Paris, France.
Paupardin-Tritsch
D
D
Yakel
J L
JL
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002217:Carbachol / Q000494:pharmacology; D003600:Cytosol / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006372:Helix, Snails / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D011976:Receptors, Muscarinic / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects
D015220:Calcium Channels; D011976:Receptors, Muscarinic; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D002217:Carbachol