Competitive and allosteric interactions of 6-chloro-5,10-dihydro-5-[(1-methyl-4-piperidinyl)acetyl]-11H-dibenzo[b,e][1,4]diazepine-11-one hydrochloride (UH-AH 37) at muscarinic receptors, via distinct epitopes

6-Chloro-5,10-dihydro-5-[(1-methyl-4-piperidinyl)acetyl]-11H-dibenzo[b,e][1 ,4]diazepine-11-one hydrochloride (UH-AH 37) is an analog of pirenzepine th at has previously been reported to interact with classical muscarinic antag onists in a competitive manner, yet its binding has also been found to be s ensitive to the same epitope as is that of the allosteric ligand gallamine. The present study was carried out with wild-type and chimeric muscarinic r eceptors to determine whether UH-AH 37 might also have an allosteric mode o f action. In assays that detect only allosteric interactions, UH-AH 37 slow ed the rate of dissociation of [H-3]N-methylscopolamine (NMS) from all five muscarinic receptor subtypes, with the highest apparent affinity at m2. By contrast, studies carried out under equilibrium conditions have found UH-A H 37 to have the lowest affinity for the m2 subtype. Studies with m2/m5 chi meric receptors found the allosteric potency of UH-AH 37 to be sensitive to an epitope in the seventh transmembrane domain (TM). Again, this contrasts with equilibrium studies, wherein an epitope in the sixth TM has been impl icated. Simultaneous analysis of the interactions between UH-AH 37 and [3H] NMS at the m2 receptor under equilibrium and non-equilibrium conditions fou nd that a simple allosteric model could not accommodate both sets of data. On the other hand, the model did accommodate such data for gallamine; galla mine also displays concordance in order-of potency and epitope sensitivity between equilibrium and non equilibrium assays. Based on these results, we conclude that UH-AH 37 interacts at the classical muscarinic binding site w ith high affinity and at a second (allosteric) sire with lower affinity. BI OCHEM PHARMACOL 57;2:181-186, 1999. (C) 1998 Elsevier Science Inc.