Picric acid-evoked release of [14C]acetylcholine from the isolated synaptosome of rat cerebral cortex.

Picric acid stimulated, in a dose-dependent manner, the release of [14C]acetylcholine (ACh) from isolated synaptosomes of rat cerebral cortex pre-loaded with labelled choline. Radioactive ACh was separated for counting from choline in the synaptosomal supernatants by a liquid cation-exchange method. Neither the nicotinic antagonist (hexamethonium) nor the muscarinic antagonists (atropine and scopolamine) affected the effectiveness of picric acid, suggesting that the action of picric acid does not occur through a cholinoceptor-mediated mechanism. Moreover, oxotremorine, but not pilocarpine, inhibited ACh release in a concentration-dependent manner in either basal- or picric acid-evoked conditions, indicating the presence of muscarinic M2-receptors for auto-regulation of ACh release. The effect of picric acid was compared with high-K+ depolarization which also initiated a non-receptor-mediated release of ACh. Deletion of calcium ion from the medium negated the effects of both drugs. The ACh-releasing effect of picric acid was totally abolished, whereas high-K+ depolarization was reduced to some extent, when tetrodotoxin was added to the medium. These results indicate that picric acid acts as a releaser of ACh in the cerebrocortex of rat.