ABSTRACT
Neurotransmitter, hormone, and drug receptors are critical sites for the transmission of chemical signals that regulate the normal functioning of the body. A binding site may be identified and considered a receptor for a drug or neurotransmitter or hormone only after a careful comparison of the properties of the binding site with the properties expected of the related receptor. The selection of the radiolabeled ligand for receptor binding is always a very important consideration, and normally one selects the most potent drug studied in the phase one experiments. Muscarinic cholinergic receptor binding has been identified biochemically in vitro by the binding of cholinergic agonists and antagonists to membrane fragments of brain tissue. Labeling drug, hormone, and neurotransmitter receptors in vivo is a desirable goal with many potential applications. Many experiments of various types have indicated the presence of cholinergic muscarinic drugs in brain.
