Regional binding of 4-diphenylacetoxy- N-methylpiperidine methobromide (4-DAMP) to muscarinic receptors in rat brain and comparative analysis of minimum energy conformations

The binding of the muscarinic antagonist 4-diphenylacetoxy- N-methylpiperidine methobromide (4-DAMP), which has been suggested as an M 3-selective antagonist in peripheral tissues, was examined through quantitative autoradiographic techniques in brain. The ability of 4-DAMP to displace [ 3H](R)-quinuclidinyl benzilate (QNB) binding to rat brain sections was compared with the known distribution of M 1 and M 2 muscarinic receptor subtypes as measured previously with pirenzepine and AF-DX 116 (Messer et al., 1989a). 4-DAMP displayed a high affinity for [ 3H](R)-QNB binding sites in rat brain sections. Analysis of 4-DAMP binding to various brain regions revealed heterogeneous binding profiles, suggesting an interaction with multiple receptor sites. Quantification of the autoradiograms indicated that 4-DAMP bound with the highest affinity to muscarinic receptors in the midline thalamus (IC 50 values < 30 nM), and had a slightly lower affinity for hippocampal receptors (IC 50 values between 30 and 46 nM). 4-DAMP also displayed a lower affinity for cortical receptors with IC 50 values between 30 and 50 nM. The binding profile of the putative M 3 muscarinic antagonist did not exhibit a marked selectivity for any single region of brain. The data suggest that whereas 4-DAMP may be selective for M 3 receptors in peripheral tissues, it has limited selectivity in the CNS. Minimum energy conformations for 4-DAMP were calculated using molecular mechanics calculations. 4-DAMP displayed two global minimum energy conformations, differing in the relative position of the piperidine ring with respect to the aromatic rings. The minimum energy conformations of 4-DAMP were compared with conformations generated for pirenzepine (Messer et al., 1989a). The lowest energy conformation of 4-DAMP was superimposable on the lowest energy conformation of pirenzepine ( RMS = 0.297 A ̊ ). It is suggested that the conformations available to 4-DAMP permit binding to several muscarinic receptors in the CNS.