A fundamental concept of pharmacology is that to initiate 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 combination of the drug and the receptor results in a molecular change in the receptor, such as an altered configuration 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 naturally 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, hormones, or other physiological substances. Thus, the discovery 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 endogenous peptides with morphinelike activity. A series of these peptides have since been identified and are collectively termed endorphins and enkephalins (see Chapter 26). It is now clear that drugs such as morphine merely mimic endorphins or enkephalins by combining with the same receptors.

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