Acetylcholine modulates two types of presynaptic potassium channels in vertebrate motor nerve terminals

Using external microelectrodes to record local circuit currents from preterminal motor nerve axons, two distinct populations of potassium (K) channels were identified in frog motor nerve terminals: delayed rectifier and calcium-activated K channels. Both are sensitive to the transmitter acetylcholine (ACh) which, when externally applied, blocks them in concentrations in the millimolar range. As this action also is not prevented by nicotinic and muscarinic antagonists, it probably is not mediated by classical cholinergic receptors. This cholinergic sensitivity of presynaptic K channels may account for the hyperexcitability of motor nerve terminals manifested when ACh accumulates in the junctional cleft.