Stereoselective binding and activity of oxotremorine analogs at muscarinic receptors in rat brain

Abstract

The activities of the enantiomers of BM-5 were examined to measure muscarinic cholinergic selectivity in the central nervous system. Autoradiographic studies assessed the ability of each enantiomer to inhibit the binding of [3H]-(R)-quinuclidinyl benzilate ([3H]-(R)-QNB) to muscarinic receptors in the rat brain. (+)-(R)-BM-5 inhibited [3H]-(R)-QNB binding to rat brain sections at concentrations below 1.0 microM, while 100-fold higher concentrations of (-)-(S)-BM-5 were required for comparable levels of inhibition. Analysis of the autoradiograms indicated that both stereoisomers had a similar distribution of high affinity binding sites. Each enantiomer displayed higher affinity for muscarinic receptors in the superior colliculi and lower affinity for receptors in the cerebral cortex and hippocampus. (+)-(R)-BM-5 and oxotremorine inhibited adenylyl cyclase activity in the cerebral cortex with efficacies comparable to that for acetylcholine. (+)-(R)-BM-5 was 26-fold more potent than (-)-(S)-BM-5 in inhibiting adenylyl cyclase. Oxotremorine-M and carbamylcholine stimulated phosphoinositide turnover in the cerebral cortex. Oxotremorine had lower activity and (+)-(R)-BM-5 was essentially inactive at comparable concentrations. The difference in activity of the two enantiomers indicates a remarkable stereochemical selectivity for muscarinic receptors. The stereoselectivity index is comparable for both the autoradiographic assays (48) and measures of adenylyl cyclase activity (26) in the cerebral cortex.

Pharmacological Characterisation of the Enantiomers of BM‐5, a Muscarinic Partial Agonist with Opposed Enantioselectivity between Affinity and Efficacy*

“…This has previously been reported for the ileum Ringdahl 1984;Messer et al 1992). Both enantiomers of BM-5 were partial agonists in the isolated bladder and ileum.…”

“…In the isolated bladder and ileum, (S)-BM-5 showed higher maximal agonist effect but lower potency than (R)-BM-5. This has previously been reported for the ileum Ringdahl 1984;Messer et al 1992). Both enantiomers of BM-5 were partial agonists in the isolated bladder and ileum.…”

“…The latter, but not (S)-BM-5, significantly increased the basal release of acetylcholine in rat striatum in vivo , an effect mediated by muscarinic antagonists. Both enantiomers of BM-5 acted as muscarinic agonists in rat cerebral cortex, as demonstrated by inhibition of adenylyl cyclase activity (Messer et al 1992). Thus, racemic BM-5, which displays negligible receptor selectivity, can act both as an agonist and as an antagonist, depending on which system tested.…”

Rapid Throughput Analysis Demonstrates that Chemicals with Distinct Seizurogenic Mechanisms Differentially Alter Ca2+ Dynamics in Networks Formed by Hippocampal Neurons in Culture

“…4D). The potency is similar to pilocarpine action on the inhibition of [ 3 H](R)-quinuclidinyl benzilate binding to muscarinic receptor in rat brain membranes (IC 50 5 7.6 mM) (Messer et al, 1992).…”

Injection of oxotremorine in nucleus accumbens shell reduces cocaine but not food self-administration in rats

“…OXO is a relatively non-selective muscarinic receptor agonist (Garvey et al 1992) but the methiodide derivative (which was used in this study) has been reported to stimulate phosphoinositide turnover (Messer et al 1992), an effect that is consistent with activation of M 1 -like receptors. Coupled with the ability of PIRENZ, a potent M 1 antagonist (Gil and Wolfe 1985;Noronha-Blob et al 1988) to block oxotremorine's reduction in the BP for cocaine, these findings suggest that the reduction was mediated through activation of the M 1 receptor subtype.…”