CAN MUSCARINIC RECEPTORS BE CLASSIFIED PHARMACOLOGICALLY?
To discuss cholinergic transduction at specific muscarinic receptor subtypes and to interpret studies with animal tissues, one must first have knowledge of the pharmacological identification of these receptors. The corollary question of that posed above is: are there drugs that specifically or selectively block or stimulate muscarinic receptor subtypes? A compound that is specific for a receptor binds to only one receptor. A compound that is selective for a receptor binds to one receptor with a higher affinity than that for its binding to another receptor. It is generally considered that a compound shows selectivity for a receptor when its affinity for one receptor is at least 10-fold greater than that for another receptor. The definitive answer to the question posed above has come from studies with different transfected cell lines, each expressing a different subtype (m1m5) of the muscarinic receptor. Currently, there is no selective or specific antagonist or agonist of the five musca
