Localization of muscarinic m3 receptor protein and M3 receptor binding in rat brain

A family of receptor subtypes, defined either by molecular (m1-m5) or pharmacological (M1-M4) analysis, mediates muscarinic cholinergic neurotransmission in brain. The distribution and functions of the m3 receptor protein in brain and its relation to M3 ligand binding sites are poorly understood. To better characterize the native brain receptors, subtype-specific antibodies reactive with the putative third inner loops were used: (i) to measure the abundance of m3 protein and its regional distribution in rat brain by immunoprecipitation; (ii) to determine the cellular and subcellular distribution of m3 protein by light microscopic immunocytochemistry; and (iii) to compare the distribution of m3 immunoreactivity with the autoradiographic distribution of M3 binding sites labeled by [3H]4-diphenylacetoxy-N-methyl piperidine methioxide in the presence of antagonists selective for the other receptor binding sites. The m3 protein, measured by immunoprecipitation, accounted for 5-10% of total solubilized receptors in all brain regions studied. Immunocytochemistry also revealed a widespread distribution of m3-like immunoreactivity, and localized the subtype to discrete neuronal populations and distinct subcellular compartments. The distribution of m3 protein was consistent with the messenger RNA expression, and like M3 binding sites, the protein was enriched in limbic cortical regions, striatum, hippocampus, anterior thalamic nuclei, superior colliculus and pontine nuclei. However, m3 immunoreactivity and M3 binding were differentially localized in regions and lamina of cortex and hippocampus. The results confirm the presence of m3 protein in brain, its low abundance compared to other muscarinic receptor subtypes, and provide the first immunocytochemical map of its precise localization. The distribution of m3 suggests that it mediates a wide variety of cholinergic processes in brain, including possible roles in learning and memory, motor function and behavioral state control. However, since the distribution of the molecularly-defined receptor protein is distinct from the pharmacologically-defined M3 binding site, investigations of the functions of m3 in brain must await development of more selective ligands or use of non-pharmacological approaches.