Recently, μ-, δ- and κ-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-μ, non-δ and non-κ. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, β-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative ε-opioid receptor. Many ε-opioid receptors are functionally coupled to G-proteins. The functional ε-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as β-endorphin, but not by selective μ-, δ- or κ-opioid receptor agonists. ε-Opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to ε-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 ε-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 ε-opioid receptors, stimulates spinal δ2-opioid receptors for the production of antinociception. It is noteworthy that the ε-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no ε-selective ligand currently available, these findings provide strong evidence for the existence of the putative ε-opioid receptor and its unique function in the brain.
The Japanese Pharmacological Society 1998