Dissociation between muscarinic receptor-mediated inhibition of adenylate cyclase and autoreceptor inhibition of [3H] acetylcholine release in rat hippocampus.

Activation of muscarinic cholinergic receptors (mAChRs) in the central nervous system reduces the catalytic activity of membrane-bound adenylate cyclase and attenuates depolarization-dependent release of acetylcholine (ACh). Inasmuch as reports have indicated that these mAChR-mediated responses exhibit pharmacological profiles similar to the M2 subclass of mAChR, the present studies were undertaken to ascertain whether attenuation of presynaptic adenylate cyclase activity [and concurrent reduction of intraneuronal cyclic AMP (cAMP) levels] underlies mAChR-mediated autoinhibition of electrically evoked ACh release. In [3H]choline-prelabeled rat hippocampal slices, the mAChR agonists oxotremorine (EC50 = 15 microM) and carbachol (EC50 = 80 microM) caused atropine-reversible inhibition of [3H]ACh release up to a maximum of 80% reduction. The rank order of potency for antagonist reversal of this inhibitory action (N-methylatropine = atropine greater than scopolamine much greater than pirenzepine) was generally consistent with an M2 mAChR-mediated response although pirenzepine was ineffective up to 1 mM. Under these assay conditions, forskolin (1-10 microM) and 8-bromo-cAMP (30-300 microM) enhanced electrically evoked [3H]ACh release maximally by 50 to 60%; however, neither agent significantly reversed mAChR agonist-induced inhibition of [3H]ACh release. Additional studies were undertaken to determine the consequences of chemically uncoupling mAChR from their G protein-adenylate cyclase effector system in this tissue. Whereas brief pretreatment with the sulfhydryl alkylating agent N-ethylmaleimide (30 microM) or pertussis toxin (1 microgram/ml) markedly attenuated carbachol inhibition of adenylate cyclase activity in hippocampal tissue, there was no concurrent reduction of carbachol-inhibited [3H] ACh release.(ABSTRACT TRUNCATED AT 250 WORDS)