Potency ratio for the inhibition of 3H-QNB and 3H-cis-methyldioxolane binding predicts agonist or antagonist activity on muscarinic receptors.

3H-Quinuclidinyl benzylate (3H-QNB) and 3H-cis-methyldioxolane (3H-CD) binding to cholinergic muscarinic receptors have been carried out on the same membrane preparation from the rat brain cortex. Inhibition of 3H-QNB and 3H-CD binding by cholinergic muscarinic agonists and antagonists has been studied with determination of the IC50 values for each ligand. Antagonists were as potent at inhibiting 3H-QNB binding as 3H-CD binding. Muscarinic agonists were much less potent at inhibiting 3H-QNB binding than 3H-CD binding. The ratio of the IC50 values for the inhibition of the binding of 3H-QNB and 3H-CD was between 1 and 10 for antagonists and between 500 and 5000 for agonists. It is suggested that the determination of this ratio predicts agonist or antagonist activity of unknown compounds and represents a useful tool for their screening.