Spontaneous release of acetylcholine (ACh) from rat basal forebrain slices in the presence of cholinesterase inhibitor was directly determined using a specific radioimmunoassay for ACh. The release was calcium dependent. A consistent amount of ACh release was observed throughout the experiment. Atropine (10-8 to 10-5 M) and pirenzepine (10-7 to 10-5 M) enhanced spontaneous ACh release. These findings indicate the presence of an M1 muscarinic autoreceptor that modulates spontaneous release of ACh in the rat basal forebrain.
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