REGULATION OF STIMULUS-DEPENDENT HIPPOCAMPAL ACETYLCHOLINE-RELEASE BYOKADAIC ACID-SENSITIVE PHOSPHOPROTEIN PHOSPHATASES

Isolated nerve endings (synaptosomes) from rat hippocampus were used t o characterize the influence by serine/threonine-specific phosphoprote in phosphatase (PP) inhibitors on acetylcholine release. Brief exposur e to low concentrations of selective PP inhibitors (okadaic acid and c alyculin A) caused a concentration-dependent attenuation of stimulus-d ependent (calcium-evoked or potassium-evoked) [H-3]acetylcholine ([H-3 ]ACh) release, while having no effect on the rate of basal transmitter efflux. In view of the observed potencies for okadaic acid and calycu lin A (pseudo-IC50 values near 3 nM), these data indicate that Type 1 (PP1) or Type 2A (PP2A) enzymes play a permissive role in exocytotic [ H-3]ACh release. In contrast, the absence of any measurable effect by sodium orthovanadate argues against a similar influence by tyrosine-sp ecific phosphoprotein phosphatases. While the neuronal substrate(s) re sponsible for PP regulation of [H-3]ACh release are unknown, the under lying mechanism clearly differs from that through which muscarinic aut oreceptors act since inhibition by okadaic acid and oxotremorine (an a utoreceptor agonist) are additive and the former is not blocked by the muscarinic receptor antagonist atropine. Based upon these results, we conclude that dephosphorylation steps catalyzed by okadaic acid-sensi tive PP represent an important regulatory mechanism for stimulus-depen dent transmitter release in septo-hippocampal cholinergic neurons.