Recently, mu-, delta- and
kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major
opioid receptor types, more extensive studies have suggested the possible existence of other
opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous
opioid peptide,
beta-endorphin, has been postulated to account for the unique characteristics of the
opioid receptor defined as a putative
epsilon-opioid receptor. Many epsilon-
opioid receptors are functionally coupled to
G-proteins. The functional epsilon-
opioid receptors in the brain are stimulated by
bremazocine and
etorphine as well as
beta-endorphin, but not by selective mu-, delta- or
kappa-opioid receptor agonists.
Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-
opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous
opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-
opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the
epsilon-opioid receptor-mediated
pain control system is different from that of other
opioid systems. Although there appears to be no epsilon-selective
ligand currently available, these findings provide strong evidence for the existence of the putative
epsilon-opioid receptor and its unique function in the brain.