Reepithelialization of the human cornea is regulated by endogenous opioids

To determine the influence of endogenous opioid modulation on reepithelialization of the human cornea.

Eight-millimeter-diameter epithelial defects were created with a trephine and mechanical scraping in the center of human corneas. Resurfacing was studied in organ culture. The size of the defect, the number of specimens with complete reepithelialization, and rate of closure were evaluated using topical fluorescein and morphometric analysis. The influence of opioid receptor blockade was studied using the potent and long-acting opioid antagonist, naltrexone (NTX; 10(-6) M), and the effects of excess (10(-6) M) opioid growth factor (OGF), [Met5]enkephalin, also were determined. The modulatory activity of NTX and OGF on DNA synthesis was evaluated by monitoring the labeling index (LI) using radioactive thymidine. The presence and location of OGF and its receptor (OGFr) were ascertained by immunocytochemistry 1 hour and 24 hours after abrasion.

NTX accelerated the wound-healing process, with 21% to 89% less defect than controls observed from 24 to 96 hours. At 72 hours, 62% of the subjects in the NTX group had complete closure of the corneal defects, in contrast to only 19% of the control specimens. All epithelial abrasions were resurfaced in the NTX group between 96 and 120 hours, whereas all controls were not closed until 168 hours. The rate of healing in the NTX group was 1.06 mm2/h compared to a rate of 0.68 mm2/h in the control group. OGF delayed corneal wound healing, with 24% to 260% more defect recorded than in control specimens at day 7. The healing rate of the OGF group was 0.42 mm2/h compared to 0.82 mm2/h for control subjects. The corneal epithelium adjacent to the wound had an LI that was 152% greater than control specimens, whereas OGF decreased the LI of this region by 75%. OGF and OGFr were detected in the epithelium bordering the damaged region at 1 hour, and both peptide and receptor were noted in the regenerating epithelium at 24 hours.

These results indicate that an endogenous opioid is present and functions as a tonically active, receptor-mediated, negative growth factor during reepithelialization of the abraded human cornea.