title: Endogenous opioids regulate cell proliferation in the retina of developing rat.
abstract: The role of endogenous opioids and opioid receptors (endogenous opioid systems) in modulating cell proliferation in the developing mammalian retina was examined in 1-day-old rats. In contrast to a labeling index (LI) of 35.8% in control animals, administration of the opioid peptide [Met5]-enkephalin (100 micrograms/kg) significantly reduced (10.6%) the proportion of cells incorporating [3H]thymidine; concomitant injection of 1 mg/kg naloxone blocked the inhibitory effects of [Met5]-enkephalin on cell division. Naloxone (1 mg/kg) alone did not alter the LI. The interruption of endogenous opioid-opioid receptor interaction by naltrexone (50 mg/kg), a potent opioid antagonist, was accompanied by a significant increase (6.4%) in the LI relative to control levels. Immunocytochemical experiments revealed the presence of enkephalin-like immunoreactivity, with staining of the cortical cytoplasm of proliferating and differentiating retinal cells recorded; no immunoreactivity was noted in the adult retina. In vitro autoradiography using 125I-[Met5]-enkephalin indicated that [Met5]-enkephalin binding sites were localized to the developing retina; no binding of the radiolabeled ligand was recorded in the adult retina. These results demonstrate the presence of growth-related endogenous opioids and opioid receptors in the developing mammalian retina, but not in adult retina, and suggest that endogenous opioids serve as natural inhibitory trophic factors that tonically regulate cell proliferation.
authors.affiliation: Department of Anatomy, Pennsylvania State University, M.S. Hershey Medical Center, Hershey 17033.
authors.forename: T
authors.initials: T
authors.lastname: Isayama
authors.forename: P J
authors.initials: PJ
authors.lastname: McLaughlin
authors.forename: I S
authors.initials: IS
authors.lastname: Zagon
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001345:Autoradiography; D002455:Cell Division / Q000187:drug effects; D004261:DNA Replication / Q000187:drug effects*; D004723:Endorphins / Q000502:physiology*; D004744:Enkephalin, Methionine / Q000378:metabolism / Q000494:pharmacology*; D005260:Female; D007457:Iodine Radioisotopes; D008297:Male; D008940:Mitotic Index / Q000187:drug effects; D009270:Naloxone / Q000494:pharmacology*; D009271:Naltrexone / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000166:cytology* / Q000187:drug effects / Q000502:physiology; D012165:Retinal Ganglion Cells / Q000166:cytology* / Q000187:drug effects; D013936:Thymidine / Q000378:metabolism
chemical_list: D004723:Endorphins; D007457:Iodine Radioisotopes; D011957:Receptors, Opioid; D009270:Naloxone; D004744:Enkephalin, Methionine; D009271:Naltrexone; D013936:Thymidine