A new probe for heterogeneity in muscarinic receptors: 2-methyl-spiro-(1,3-dioxolane-4,3′)-quinuclidine

The title compound (MSDQ) is a new muscarinic agonist related to 3-acetoxyquinuclidine (3-AcQ) but also of a highly rigid structure. In view of this, it may constitute a probe for the detection of heterogeneity among muscarinic receptors. Indeed, equipotent molar ratios (EPMR) for ACh, 3-AcQ and MSDQ were as follows: guinea pig ileum, 1 : 14 : 240; vasodepressor effect in the cat, 1: 6 : 188. But EPMR for 3-AcQ and MSDQ as stimulants of the superior cervical ganglion in the cat were 1 : 1 and for the induction of tremors in mice 9 : 5. No such subtle differences in receptor specificity were detected when the probe used was the 2,2-diphenyl analogue (DiPSDQ) of MSDQ which was a poerful competitive antagonist in all systems, more potent than atropine, but with a CNS/PNS activity of 1.1 compared to 26 for atropine. In view of this, the use of potent antagonists as probes for muscarinic receptor heterogeneity is questionable.