ABSTRACT
Ligands on specific morphine receptors in the brain have been identified as two pentapeptides: methionine-enkephalin (Met-enkephalin) and leucine-enkephalin (Leu-enkephalin) (1-3). Other larger peptides, the endorphins, also bind to morphine receptors (4-6). Subsequent studies have described the analgesic activity in various animal models by means of the intracerebral route of administration (7). However, pronounced activity by systemic administration (intraperitoneal) was discovered by Plotnikoff et al. (8), revealing that the enkephalins exhibited marked activity as tranquilizers, antidepressants, and anticonvulsants. Most important, Plotnikoff demonstrated that enkephalins were extremely active in potentiating the central effects of dopamine, in contrast to morphine, suggesting for the first time separate enkephalins receptors.
