Quantitative autoradiographic distribution of [3H]AF-DX 116 muscarinic-M2 receptor binding sites in rat brain

Abstract

The distribution of muscarinic-M₂ receptors in rat brain was investigated by in vitro autoradiography using [³H]AF-DX 116, a putative probe for the muscarinic-M₂ receptor subtype. Incubation of rat brain coronal sections with 10 nM [³H]AF-DX 116 showed highest binding site densities in discrete areas such as the superior colliculus and certain thalamic and brainstem nuclei, similar to the distribution reported for [²H]acetylcholine/M₂ sites. [³H]AF-DX 116 site densities were markedly lower in forebrain areas such as cortex, striatum, and hippocampus, in contrast to the distribution seen for [³H]pirenzepine-M₁ binding sites, which were concentrated in these forebrain areas; however, differential patterns of labeling were observed for the two muscarinic-M₂ probes, [³H]AF-DX 116 and [³H]acetylcholine, in the hippocampal formation. Although [³H]AF-DX 116 binding was broadly distributed in multiple subfields of the hippocampus, [³H]acetylcholine binding was discretely distributed in a manner resembling that of acetylcholinesterase staining. This suggests the existence of muscarinic-M₂ subtypes in the CNS, especially in the hippocampal formation.