Abstract

The properties of the specific binding of the muscarinic receptor ligands [3H]quinuclidinyl benzilate and N-[3H]methylscopolamine in rat brain were compared. The specific binding of both ligands was affected equally by heat, phospholipase A2 and trypsin. N-[3H]methylscopolamine labeled only a fraction of the total muscarinic receptors recognized by [3H]quinuclidinyl benzilate in different brain areas and in the heart. Evidence is presented that N-[3H]methylscopolamine, in fact, binds to a subpopulation of [3H]quinuclidinyl benzilate binding sites. The distribution of the high-affinity binding sites of N-[3H]methylscopolamine did not show a different tissue dependence as compared to the total receptor population, and did not parallel the distribution of the pirenzepine-sensitive M1 receptor subtype. Similarly, the affinity of both [3H]quinuclidinyl benzilate and N-[3H]methylscopolamine varied from one tissue to another by a maximum of 2-fold. Although (-)-quinuclidinyl benzilate competed for the specific binding of [3H]quinuclidinyl benzilate in different tissues according to the law of mass-action, N-methylscopolamine showed an anomalous interaction with two binding sites. The low-affinity binding sites of N-methylscopolamine showed saturability of [3H]quinuclidinyl benzilate binding and stereoselectivity. When the binding characteristics of these N-methylscopolamine-inaccessible binding sites of [3H]quinuclidinyl benzilate in the brain were investigated further, it was found that N-methylscopolamine bound exclusively with a single low affinity, whereas pirenzepine still interacted with two receptor populations incorporated in these sites. It is concluded from several lines of evidence that the heterogeneity of binding of N-methylscopolamine to muscarinic receptors does not represent an interaction with the muscarinic M1 and M2 receptor subtypes defined by pirenzepine. Thus, the unique binding profile of pirenzepine to muscarinic receptors cannot be explained merely on the basis of its hydrophilic nature.