Binding sites for [3H]AF-DX 116 and effect of AF-DX 116 on endogenous acetylcholine release from rat brain slices.
The present study shows that the putative M2 ligand, [3H]AF-DX 116, binds to two classes of muscarinic sites in homogenates of rat hippocampus, striatum and cerebral cortex: one with a high affinity (Kd less than 5 nM)/low capacity (Bmax = 30-63 fmol/mg protein), and a second of lower affinity (Kd greater than 65 nM) and higher capacity (Bmax greater than 190 fmol/mg protein). In experiments which tested the effects of the muscarinic antagonists on acetylcholine (ACh) release from brain slices, the non-selective antagonist (-)-quinuclidinyl benzylate and atropine significantly enhanced the potassium (25 mM)-evoked release of ACh. This effect was mimicked by the M2 ligand AF-DX 116, but neither the M1-selective antagonist pirenzepine, nor the putative M3-muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), altered ACh release. Also, the muscarinic agonist, oxotremorine, significantly depressed evoked ACh release from brain slices, an effect that was completely antagonized by atropine or by AF-DX 116, but not by pirenzepine or 4-DAMP. Thus, it appears that presynaptic muscarinic autoreceptors in the rat hippocampus, striatum and cerebral cortex belong to the M2 subtype of muscarinic receptors.
Lapchak
P A
PA
Department of Pharmacology, McGill University, Montreal, Que., Canada.
Araujo
D M
DM
Quirion
R
R
Collier
B
B
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D010095:Oxotremorine / Q000494:pharmacology; D010277:Parasympathomimetics / Q000378:metabolism* / Q000494:pharmacology; D010890:Pirenzepine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000145:classification / Q000187:drug effects*
D010277:Parasympathomimetics; D011976:Receptors, Muscarinic; D010890:Pirenzepine; D010095:Oxotremorine; D000109:Acetylcholine; C048593:otenzepad