Quantitative autoradiographic localization of the M1 and M2 subtypes of muscarinic acetylcholine receptors in the monkey brain

The distribution of muscarinic acetylcholine receptors (mAChR) was investigated in the monkey brain by means of quantitative in vitro autoradiography. 3H-QNB, 3H-pirenzepine (PZ) and 3H-AF-DX 116 were used for labelling total mAChR, M1 and M2 receptors, respectively. 3H-PZ and 3H-AF-DX 116 showed specificity to each receptor subtype in the monkey brain. On sections containing the putamen and globus pallidus, the sum of Bmax values of 3H-PZ and 3H-AF-DX 116 binding sites was almost close to that of 3H-QNB binding sites. Autoradiographic distributions of muscarinic subtype receptors in the monkey brain were similar to those reported in the rat brain; that is, M1 receptors were dominant in most areas of the telencephalon, while M2 receptors were richly distributed in the brainstem and cerebellum. However, some nuclei of the brainstem such as the central gray matter, superior colliculus, substantia nigra, nucleus of the oculomotor nerve, pontine nucleus and inferior olivary nucleus, had relatively high ratios of M1 receptors in the monkey brain. In addition, the cortical lamminar distribution of M2 receptors noticed in the rat was not observed in the monkey brain. Knowledge about the localizations of M1 and M2 receptors in various brain regions in the monkey brain will increase our understanding of the functions of the brain cholinergic system in the primate.