CONSERVED EXPRESSION OF THE OPIOID GROWTH-FACTOR, [MET(5)]ENKEPHALIN,AND THE ZETA (ZETA)-OPIOID RECEPTOR IN VERTEBRATE CORNEA

In addition to neuromodulation, endogenous opioids serve as growth fac tors. The naturally occurring opioid peptide, [Met(5)]enkephalin, term ed opioid growth factor (OGF), has been found to be a potent and tonic inhibitor of processes related to growth and renewal, particularly ce ll proliferation. OGF mediates its actions through the zeta (zeta) opi oid receptor. In order to determine if OGF and/or the zeta receptor ar e present in human corneal epithelium, immunocytochemistry was utilize d. Immunoreactivity with regard to OGF and to the zeta receptor could be detected in the cortical cytoplasm of both basal and suprabasal epi thelial cells, but was not associated with the cell nucleus. Investiga tion of the ubiquity of OGF and zeta receptor in the vertebrate cornea showed that both elements are present in a wide variety of classes of the phylum Chordata, including mammalia, aves, reptilia, amphibia, an d osteichthyes. These results suggest that an endogenous opioid system related to growth may have originated as early as 300 million years a go, and that the function of this system in cellular renewal and homeo stasis is a requirement of the vertebrate corneal epithelium.