nists). All are now known to be GPCRs.
The discovery of receptors that respond to chemicals found in pop-
pies and produced by the pharmaceutical industry prompted a search
for whatever molecules might be the endogenous (made within
the body) neurotransmitter ligands for the opioid receptors. By the
mid-1970s, molecules began to be discovered that were found in the
brain and functioned as agonists at opioid receptors. About a dozen
such molecules have now been identified from animals, collectively
called endorphins, a word derived from “endogenous” and “morphine.”
These endogenous opioid neurotransmitters were the first of anew
class of neurotransmitters—the neuropeptides. The endorphins are
chains of amino acids (polypeptides), the shortest of which are five
amino acids in length (leu-enkephalin and met-enkephalin), and the
longest of which is thirty-one amino acids long (beta-endorphin).
Figure 9.3. Coca plant, Erythroxylum coca.