Activation of a muscarinic receptor selectively inhibits a rapidly inactivated Ca2+ current in rat sympathetic neurons.

Sympathetic neurons dissociated from the superior cervical ganglion of 2-day-old rats were studied by whole-cell patch clamp and by fura-2 measurements of the cytosolic free Ca2+ concentration, Ca2+i. Step depolarizations in the presence of tetrodotoxin and hexamethonium triggered two Ca2+ currents that differed in the voltage dependence of activation and kinetics of inactivation. These currents resemble the L and N currents previously described in chicken sensory neurons Nowycky, M. C., Fox, A. P. & Tsien, R. W. (1985) Nature (London) 316, 440-442. Treatment with acetylcholine resulted in the rapid (within seconds), selective, and reversible inhibition of the rapidly inactivated, N-type current, whereas the long-lasting L-type current remained unaffected. The high sensitivity to blocker drugs (atropine, pirenzepine) indicated that this effect of acetylcholine was due to a muscarinic M1 receptor. Intracellular perfusion with nonhydrolyzable guanine nucleotide analogs or pretreatment of the neurons with pertussis toxin had profound effects on the Ca2+ current modulation. Guanosine 5'-gamma-thiotriphosphate caused the disappearance of the N-type current (an effect akin to that of acetylcholine, but irreversible), whereas guanosine 5'-beta-thiodiphosphate and pertussis toxin pretreatment prevented the acetylcholine-induced inhibition. In contrast, cAMP, applied intracellularly together with 3-isobutyl-1-methylxanthine, as well as activators and inhibitors of protein kinase C, were without effect. Acetylcholine caused shortening of action potentials in neurons treated with tetraethylammonium to partially block K+ channels. Moreover, when applied to neurons loaded with the fluorescent indicator fura-2, acetylcholine failed to appreciably modify Ca2+i at rest but caused a partial blunting of the initial Ca2+i peak induced by depolarization with high K+. This effect was blocked by muscarinic antagonists and pertussis toxin and was unaffected by protein kinase activators. Thus, muscarinic modulation of the N-type Ca2+ channels appears to be mediated by a pertussis toxin-sensitive guanine nucleotide-binding protein and independent of both cAMP-dependent protein kinase and protein kinase C.