Muscarinic suppression of a novel voltage-sensitive K+ current in a vertebrate neurone

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Abstract
Cholinergic excitation of vertebrate neurones is frequently mediated through the action of acetylcholine on muscarinic (atropine-sensitive) receptors. This type of excitation differs substantially from the better known nicotinic excitation. One difference is that, instead of an increased membrane conductance, a decreased conductance (to K+ ions) frequently accompanies muscarinic depolarisation. This has been detected in sympathetic1–4, cortical5 and hippocampal6 neurones. Using voltage-clamped frog sympathetic neurones we have now identified a distinctive voltage-sensitive K+ -current, separate from the delayed rectifier current, as the prime target for muscarinic agonists. We have termed this current the M-current, IM.