A fundamental concept of pharmacology is that to ini-tiate an effect in a cell, most drugs combine with some molecular structure on the surface of or within the cell. This molecular structure is called a receptor. The combi-nation of the drug and the receptor results in a molecu-lar change in the receptor, such as an altered configura-tion or charge distribution, and thereby triggers a chain of events leading to a response. This concept applies not only to the action of drugs but also to the action of nat-urally occurring substances, such as hormones and neu-rotransmitters. Indeed, many drugs mimic the effects of hormones or transmitters because they combine with the same receptors as do these endogenous substances.
It is generally assumed that all receptors with which drugs combine are receptors for neurotransmitters, hor-mones, or other physiological substances. Thus, the dis-covery of a specific receptor for a group of drugs can lead to a search for previously unknown endogenous substances that combine with those same receptors. For example, evidence was found for the existence of en-dogenous peptides with morphinelike activity. A series of these peptides have since been identified and are col-lectively termed endorphins and enkephalins . It is now clear that drugs such as morphine merely mimic endorphins or enkephalins by combining with the same receptors.