Acetylcholine elicits metabolically mediated M2-muscarinic hyperpolarization in isolated rabbit sympathetic neurons

Topically applied acetylcholine elicited a hyperpolarizing (ACh-HP) response in about 2/3 of the principal neurons of rabbit superior cervical ganglia isolated in a culture medium for several days. Membrane changes responsible for the ACh-HP were multiple and varied with the level of membrane potential (Vm): At, or less negative than, resting Vm (about -60 mV), membrane conductance (Gm) was increased; at Vm more negative than -70 mV, Gm either increased, decreased, or remained unchanged. The effect of altering extracellular K+ and Ca2+ concentrations suggest that a Ca2(+)-dependent increase in K+ conductance contributes to ACh-HP; however, drugs reported to block such channels (D-tubocurarine at 30 microM or apamin at 200 nM) shortened duration of the ACh-HP but did not depress peak amplitude. The ACh-HP was depressed by the M2-muscarinic antagonist AF-DX 116 and blocked by an intracellular administration of guanosine-5'-o-(2-thiodiphosphate) (GDP-beta-S) or by preincubation with pertussis toxin. Intracellularly injected inositol-1,4,5-triphosphate (IP3) elicited a hyperpolarization associated with an increase in Gm at any Vms. Activators of protein kinase C applied extracellularly, 1,2-oleoylacetylglycerol (OAG) or phorbol-12,13-dibutyrate (Pb(Bu)2) elicited a hyperpolarization with a decrease in Gm at Vms more negative than -50 mV and a reversal potential close to Ec1. In the presence of Li+, ACh-HP was smaller after a repetitive stimulation with ACh. These results suggest that, in these neurons, ACh can directly activate M2-muscarinic receptors coupled to GTP-binding proteins, which leads to both an increase in Ca2(+)-sensitive K+ conductance mediated by the intracellular messenger IP3 and a decrease in a conductance, possibly Gc1, through another phosphatidylinositide pathway.