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.