Inhibition by local anesthetics, phentolamine and propranolol of [3H]quinuclydinyl benzylate binding to central muscarinic receptors.

[3H]Quinuclydinyl benzylate ([3H]QNB) binding was carried out on crude synaptosomal membranes isolated from cat cerebral cortex. The specific binding showed a single type of site with KD 0.25 nM, Hill number 0.89 and Bmax 114 pmol/g protein. The local anesthetics procaine, tetracaine and dibucaine, and the adrenergic antagonists phentolamine and propranolol, in concentrations between 1 nM and 500 microM, inhibited [3H]QNB binding with Ki varying between 9 microM for procaine and 80 microM for propranolol. The Hill coefficients obtained from logit/log plots suggested that there was no cooperativity between the binding sites for local anesthetics. At various concentrations the inhibition by procaine and propranolol may appear as competitive or non-competitive. The Hill numbers obtained from the saturation curves suggest that there was no cooperativity between anesthetics and [3H]QNB binding sites. Neither 1 mM Ca2+ nor Mg2+ affected [3H]QNB binding or the action of the drugs. The effect of local anesthetics and adrenergic antagonists was reversible and these drugs did not protect the muscarinic receptor from the deleterious effect of Triton X-100 as was the case with muscarinic agents. Our findings suggest that local anesthetics inhibit [3H]QNB binding to the muscarinic receptor by acting at some accessory site but not on the true receptor site. The possible mechanism of action and local anesthetics on synaptic transmission is discussed.