title: Dissociation between muscarinic receptor-mediated inhibition of adenylate cyclase and autoreceptor inhibition of [3H] acetylcholine release in rat hippocampus.
abstract: 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)
authors.affiliation: University of Florida, Department of Physiological Sciences, J. Hillis Miller Health Center, Gainesville.
authors.forename: T W
authors.initials: TW
authors.lastname: Vickroy
authors.forename: E D
authors.initials: ED
authors.lastname: Cadman
mesh_terms: D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000109:Acetylcholine / Q000378:metabolism*; D037361:Adenylate Cyclase Toxin; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D005033:Ethylmaleimide / Q000494:pharmacology; D006624:Hippocampus / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D037342:Pertussis Toxin; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000502:physiology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D037361:Adenylate Cyclase Toxin; D000067956:Adenylyl Cyclase Inhibitors; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D037342:Pertussis Toxin; D000109:Acetylcholine; D005033:Ethylmaleimide