Muscarinic M2 receptors in rat brain labeled with [3H] AF-DX 384.

The muscarinic antagonist AF-DX 384 (5,11-dihydro-11-(((2-(2-((dipropylamino)methyl)-1- piperidinyl)ethyl)amino)carbonyl)-6H-pyrido(2,3b)(1,4)-benzodiazepin+ ++-6-one methansulfonate) was used to label cholinergic muscarinic receptors of the M2 subtype in rat brain. In the brainstem [3H] AF-DX 384 labeled a single population of binding sites with Kd = 3-4 nM and Bmax = 430-610 fmol/mg of protein. The pharmacological profile of these sites was similar to that observed with the muscarinic M2 agonist [3H] AF-DX 116. Muscarinic M2 receptors were unequally distributed in rat brain regions: in brainstem, about 80% of total muscarinic receptors (measured with [3H] quinuclidinyl benzilate) were labeled by [3H] AF-DX 384, while in other brain areas they represented only a fraction of total binding. Because of its high specific binding, high affinity and specificity, [3H] AF-DX 384 represents a useful novel ligand to study M2 receptors in brain tissue.