PRESYNAPTIC MUSCARINIC RECEPTORS AND THE RELEASE OF ACETYLCHOLINE FROM CEREBROCORTICAL PRISMS - ROLES OF CA2+ AND K+ CONCENTRATIONS

The mechanism by which presynaptic muscarinic autoreceptors inhibit th e release of acetylcholine (ACh) from cerebrocortical cholinergic fibr es has not been clarified. To test the view that muscarinic autorecept ors act by decreasing Ca2+ influx, we performed experiments in which m t cerebrocortical prisms were preloaded with (C-14)choline, washed, de polarized with 14-65 mM K+ in the absence of Ca2+ and then exposed (st ill under depolarization) to various concentrations of Ca2+ to evoke t he release of (C-14)ACh. The muscarinic agonist, oxotremorine, used at a 100 muM concentration, inhibited the release of (C-14)ACh by 59-86% in experiments with 14 and 26.5 mM K+ but had no significant effect a t 65.5 mM K+. No systematic changes in the inhibitory effects of oxotr emorine could be found at any of the K+ concentrations used when the c oncentration of Ca2+ was varied in the range of 0.25-4.0 mM. At 2 mM C a2+ and K+ concentrations above 14 mM, the inhibitory effect of oxotre morine was inversely related to the concentration of K+. The inhibitor y effect of oxotremorine on (C-14)ACh release was not blocked by 100 m uM 4-aminopyridine. The fact that the inhibitory effect of oxotremorin e could not be overcome by an increase in the concentration of Ca2+ su ggests that, under the conditions used, a restriction of the influx of Ca2+ did not play a major role in the muscarinic inhibition of ACh re lease; rather, oxotremorine appeared to act by decreasing membrane dep olarization. Our observations are best explained in terms of the Ca2+- voltage hypothesis of neurotransmitter release, supposing that a volta ge-activated molecule is involved in the control of the release proces s at a post-Ca2+-entry stage.