Irreversible and quaternary muscarinic antagonists discriminate multiple muscarinic receptor binding sites in rat brain.

The maximal number of binding sites (Bmax) of [3H]quinuclidinyl benzilate (QNB) binding was greater than the Bmax of N-[3H]methylscopolamine (NMS) binding to homogenates of rat brain. The competition of NMS for [3H]QNB demonstrated that NMS discriminates multiple muscarinic binding sites. Similarly, pirenzepine competition of [3H]QNB binding also revealed multiple muscarinic binding sites. Pirenzepine competition for [3H]NMS also was shallow and demonstrated the presence of binding sites with similar affinities to those labeled by [3H]QNB. These data were consistent with the presence of at least three populations of muscarinic binding sites with similar affinities for [3H]QNB: the M1 and M2 binding sites having high and low affinity for pirenzepine, respectively, but which cannot be discriminated by [3H]NMS, and a third site with high affinity for [3H]QNB which has low affinity for NMS. The classical muscarinic antagonists, atropine and scopolamine, also appear to have slightly different affinities for the putative M1 and M2 binding sites. The use of the irreversible antagonists, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) and propylbenzilylcholine mustard (PBCM), were used to elucidate the distinct properties of these multiple muscarinic binding sites. Both PBCM and EEDQ irreversibly decreased the Bmax of [3H]QNB and [3H]NMS binding in cortex. PBCM did not appear to discriminate putative M1 and M2 binding sites but selectively alkylated the high affinity NMS and QNB binding sites. In contrast, EEDQ modified the low affinity NMS binding sites such that they still bound [3H]QNB but their affinity for other muscarinic antagonists was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)