The regional distributions of muscarinic receptor subtypes (M1 and M2) in the macaque brain were investigated by in vitro receptor autoradiography. Putative muscarinic receptor subtypes were distinguished by their differential affinities for pirenzepine and carbachol in competition with [3H]-quinuclidinyl benzilate. Autoradiographic visualization of muscarinic receptor subtype demonstrated marked regional and laminar variations that respected architectonic boundaries. The M1 receptor subtype was widely distributed throughout most cortical areas and was most intense over the superficial layers. Almost all limbic and paralimbic regions including the amygdala, hippocampus, orbitofrontal, temporopolar, parahippocampal, cingulate, and parolfactory areas displayed peak densities of the M1 receptor subtype. The M2 receptor subtype was selectively elevated in the primary sensory areas of all five sensory modalities, including the visual (area 17, V1), auditory (A1), and somatosensory (3b,S1) koniocortices, the anterior olfactory nucleus, and the gustatory area. The primary motor area also displayed a relative peak of M2 receptor subtype labeling. In the hippocampal formation, M1, M2, and nicotine receptors were distributed differentially, with each subdivision having a specific complement of cholinergic receptor subtype. The M1 receptor subtype was prevalent in the dentate gyrus, the CA4-CA3 region, and the CA1 ammonic sector. The M2 receptor subtype was concentrated in the CA2 sector, the subiculum, the rhinal cortices, and the parasubiculum. Putative neural nicotinic receptors, tagged with L-[3H]-nicotine, were most concentrated within the presubiculum.
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