Identification of four brain areas each enriched in a unique muscarinic receptor subtype

The affinities of muscarinic agonists and antagonists were determined by autoradiography and image analysis in selected areas of the rat brain. IC50 values and Hill coefficients for the inhibition of the binding of 0.2 nM [3H]-QNB to dentate gyrus, superior colliculus, rhomboid thalamus and substantia nigra were measured in coronal sections. Pirenzepine displayed a high affinity for receptors in the dentate gyrus and AF-DX 116, the superior colliculus. Both pirenzepine and AF-DX 116 had high affinities for the substantia nigra and low affinities for the rhomboid thalamus. Gallamine displayed a 50-fold preference for superior colliculus over dentate gyrus receptors. Amitriptyline was less selective, showing a modest preference for substantia nigra receptors and 4-DAMP was essentially nonselective. Carbachol was the most selective agonist with a 4000-fold preference for superior colliculus over dentate gyrus receptors. Other agonists except RS 86 were also selective for superior colliculus receptors in the order carbachol much greater than arecoline greater than bethanechol greater than McN A343 = oxotremorine = pilocarpine.