Distinct Muscarinic Receptor Subtypes Differentially Modulate Acetylcholine Release from Corticocerebral Synaptosomes

The effect of McN‐A‐343 and oxotremorine on acetylcholine (ACh) release and choline (Ch) transport was studied in corticocerebral synaptosomes of the guinea pig. The synaptosomes were preloaded with [³H]Ch after treatment with the irreversible cholinesterase inhibitor, diisopropyl fluorophosphate, and then tested for their ability to release isotope‐labeled ACh and Ch in the presence and absence of these agents. The kinetics of release were determined at the resting state (basal release) and in the presence of 50 mM K⁺. Under either condition, McN‐A‐343 enhanced the release of isotope‐labeled ACh, whereas oxotremorine inhibited the K⁺‐evoked release but had no effect on the basal release. The enhancing effect of McN‐A‐343 on basal ACh release was fully blocked by the selective M1 muscarinic antagonist, pirenzepine (100 nM). In contrast to its enhancing effect on ACh release, McN‐A‐343 potently inhibited Ch efflux as well as Ch influx. These effects were not blocked by atropine, a nonselective muscarinic antagonist. Oxotremorine had no effect on Ch transport. Binding studies showed that McN‐A‐343 was 3.6‐fold more potent in displacing radiolabeled quinuclidinyl benzilate from cerebral cortex muscarinic receptors (mostly M1 subtype) than from cerebellar receptors (mostly M2 subtype), whereas oxotremorine was 2.6‐fold more potent in the cerebellum. The displacements of radio‐labeled pirenzepine and m‐dioxolane confirmed the M1 subtype preference of McN‐A‐343 and the M2 subtype preference of oxotremorine. These observations suggest that (a) activation of one subpopulation of muscarinic receptors by McN‐A‐343 enhances ACh release, whereas activation of another subpopulation by oxotremorine decreases this process (these subpopulations may correspond to the M1 and M2 subtypes, respectively), (b) the effect of the stimulatory receptors is pronounced at the resting state and that of the inhibitory receptors during depolarization, and (c) activation of the stimulatory receptors may be inversely related to the activity of the Ch transporter.