Binding and functional selectivity of himbacine for cloned and neuronal muscarinic receptors

Abstract

The binding potencies for the putative M2-selective antagonist himbacine were determined in radioligand binding and in functional response assays in neuronal tissue and Chinese hamster ovary cells containing transfected muscarinic receptors. Himbacine was shown to bind to all five cloned muscarinic receptor subtypes in the order of potencies: hM2 = hM4 > hM3 > hM1 > hM5 (Kd values were 4, 7, 59, 83 and 296 nM, respectively). Himbacine was shown to bind to M2 receptors in rat heart and brain stem with Kd values of 6.9 and 4.6 nM, respectively. In rat brain tissues with complex mixtures of muscarinic receptors, and using the radioligand [3H] +/- -5,11-dihydro-11-([(2-(2-[(dipropylamino)methyl]-1- peperidinyl)ethyl)amino]carbonyl)-6H-pyrido(2,3-b)(1,4)benzodiazep ine-6-one to demarcate M2 and M4 receptors, himbacine was shown to bind to 80% of cortical or striatal receptors with Kd values of 4.5 and 3.8 nM, respectively, consistent with the involvement of M2 and/or M4 receptors in both these brain regions. Himbacine was a potent blocker of oxotremorine-M-mediated cyclic AMP inhibition in rat striatum (4.4 nM) and in N1E-115 neuroblastoma cells (10.6 nM), responses mediated by M4 receptors. Himbacine also reversed oxotremorine-M-mediated inhibition of evoked acetylcholine release from hippocampal tissue with a Kd value of 8.6 nM, a value consistent with the involvement of M2 or M4 receptors. At the cortical postsynaptic muscarinic receptors involved with phosphoinositide turnover (putative M1 and M3 receptors), himbacine was 21-fold less potent. Himbacine appears to be a potent muscarinic antagonist that displays selectivity for M2 or M4 receptors, as compared to M1 or M3 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)