Stimulating Action of Atropine on the Release of Acetylcholine by Rat Cerebral Cortex in Vitro

1. In cortical slices from rat brain incubated in a medium containing the irreversible cholinesterase inhibitor, soman (0.005 mM) and a high concentration of KCl (25 mM), atropine exerts a stimulating action on the release of acetylcholine (ACh).2. Two possible explanations for this action were examined. Atropine might expel ACh from the nerve endings by occupying its storage sites or it might prevent an inhibitory action of the released ACh on its further release by occupying muscarinic receptors at the presynaptic endings; its action would then be a kind of ;disinhibition.'3. The stimulating action of atropine on ACh release persisted during prolonged incubation (up to 3.5 h) provided choline was added to the medium. This finding would be difficult to explain if atropine acted by expelling ACh from its storage sites.4. The stimulating action of atropine on ACh release was inhibited by oxotremorine and by methacholine, added to the medium in high concentrations. This finding is readily explained if atropine acts by ;disinhibition.'