Antisense oligodeoxynucleotides against muscarinic m2 and m4 receptors were used to investigate the role of these receptor subtypes as negative autoreceptors in the regulation of acetylcholine (ACh) release in the rat hippocampus. Following the continuous infusion of antisenses into the third ventricle (1 microgram microliter-1 h-1, 3 days), 3H-AF-DX 384/muscarinic M2-like binding was significantly decreased in the medial septum by the antisense against the m2 receptor whereas M2-like binding in the dorsal striatum was decreased by the antisense against the m4 receptor. In contrast, 3H-pirenzepine/muscarinic M1-like binding was unaffected by either antisense treatment in any of the brain areas investigated. When perfused into the hippocampus via a dialysis probe, the purported muscarinic M2 receptor antagonist AF-DX 384 (100 nM) increased hippocampal ACh release in freely moving rats. This effect of AF-DX 384 was significantly attenuated by the m2, but not the m4, receptor antisense treatment. Hippocampal choline acetyltransferase activity was not affected by either antisense treatments. Taken together, these results suggest that the molecularly defined muscarinic m2 receptor regulates hippocampal ACh release by acting as a negative autoreceptor. In contrast, the molecularly defined m4 receptor is unlikely to be directly involved in the negative regulation of ACh release in the rat hippocampus. Therefore, inhibiting muscarinic m2 receptor function may be an alternative approach to regulate the release of ACh in neurodegenerative diseases associated with impaired cholinergic functions.
Copyright 1999 Elsevier Science B.V.