Abstract
Muscarinic receptors in the rat cerebral cortex, reacted with p-chloromercuribenzoate (PCMB) under different conditions (Phase I and II), have modified binding sites. These exhibit remarkable changes in the structural dependence of the binding of drugs. In Phase I, the structure-binding profile of agonists for both the high and low affinity agonist sites are altered. In Phase II, the structure-binding profile of antagonists is also observed. In Phase II, the ability of potent agonists to discriminate between sub-classes of agonist binding sites is eliminated. There is also a loss of heterogeneity in the binding of the selective antagonist pirenzepine. Of the 16 agonists examined, only pilocarpine has a heterogeneous binding profile in Phase II, the dispersity of binding being increased. The changes in binding properties of the receptors are discussed in terms of general theories of drug-receptor interactions.