Muscarinic antagonist binding site heterogeneity as evidenced by autoradiography after direct labeling with [³H]-QNB and [³H]-pirenzepine

Saturable, high affinity binding of tritiated pirenzepine ([³H]-PZ) was obtained in slide mounted tissue sections prior to performing autoradiographic localization of these binding sites. The binding in tissue sections of rostral rat forebrain gave a K_D of 18nM and a B_max of 51 fmoles/mg tissue. These binding characteristics are similar to those previously obtained in homogenate membrane preparations and indicate the binding is taking place in a similar manner. The distribution of the binding sites labeled with [³H]-PZ represented a subpopulation of those which could be labeled with tritiated quinuclidinyl benzilate ([³H]-QNB). Thus, [³H]-PZ and [³H]-QNB both label regions of the cerebral cortex, hippocampus, striatum and dorsal horn of the spinal cord, while sites in the cerebellum, nucleus tractus solitarius, facial nucleus and ventral horn of the spinal cord are labeled with [³H]-QNB and not by [³H]-PZ. These observations indicate separate regions of the brain where antagonists bind to subtypes of muscarinic receptors.