Slow cholinergic and peptidergic transmission in sympathetic ganglia.

Experiments of voltage-clamped bullfrog sympathetic neurons suggest that the "slow depolarization" produced by orthodromic stimulation, by muscarinic agonists, or by the peptide luteinizing hormone-releasing hormone (LHRH), results from the suppression of a time- and voltage-dependent outward K+ current, the "M current" (IM). This current is activated between -60 and -10mV, with a half-maximal activation voltage of -35 mV, a minimum time constant (TM) of 150 ms at -35 mV, and a voltage sensitivity corresponding to a single gating particle with a minimum valency of 4.IM does not show time-dependent inactivation within its activation range and provides the sole potential-sensitive component of the steady outward membrane conductances between -60 and -25 mV. Muscarinic agonists and LHRH selectively depress IM via different receptors, without altering their voltage sensitivity. Although not dependent on external Ca2+ ion, IM is also selectively depressed by Ba2+ ions, so accounting for the cholinomimetic action of Ba2+. It is suggested that IM acts as a braking control on spike discharges and that removal of this control during slow cholinergic and peptidergic transmission provides a unique synaptic tuning mechanism.