Autoradiographic mapping of M3 muscarinic receptors in the rat brain

The regional distribution of putative M3 receptors was studied by quantitative autoradiography of rat brain sections labeled with 4-diphenylacetoxy-N-methyl-piperidine methioxide. The radioligand appears to label multiple muscarinic receptor subtypes in direct binding assays, as evidenced by competition experiments with unlabeled pirenzepine and 11-([2-[(2-(diethylamino)methyl]-1- piperidinyl]acetyl-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodi azepine-6-on. Conditions favoring the selective labeling of M3 receptors were determined on the basis of the competition studies and were subsequently applied to regional mapping of M3 receptor binding. Under the conditions used, autoradiographic images may be obtained with both 9-fold M3-to-M1 selectivity and 12-fold M3-to-M2 selectivity. Although distributed widely at relatively low prevalence throughout the neuraxis, M3 receptors are apparently enriched relative to M1 and M2 receptors in several diencephalic and brain stem regions including thalamic and hypothalamic nuclei, the substantia nigra, superior colliculus, periaqueductal gray and the pons. The present results suggest that the binding of 4-diphenylacetoxy-N-methyl-piperidine methioxide, under appropriate conditions, may permit direct estimation of M3 receptors in mammalian brain.