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.