Topical combination of NGF and DHA increases rabbit corneal nerve regeneration after photorefractive keratectomy

PURPOSE. To investigate the effect of nerve growth factor (NGF) in combination with docosahexaenoic acid (DHA) on corneal nerve regeneration in a rabbit model after PRK and correlate the findings with functional tear test results. METHODS. Unilateral PRK was performed on 21 New Zealand albino rabbits. Three groups, each consisting of six rabbits, were randomized to receive twice-weekly treatments with DHA, NGF, or NGF plus DHA delivered by collagen shield. A fourth group, the control, received treatment with albumin. Rabbits were observed for 8 weeks, and tear secretion tests were conducted every 15 days. The eyes were prepared for immunostaining. Monoclonal antibodies for class II beta-tubulin, calcitonin gene-related peptide (CGRP), substance P (SP), and Ki-67 were used. Cell nuclei were stained with 4', 6'-diamino-2- phenylindole ( DAPI). Positive staining areas in relation to total area were calculated with image-analysis software. RESULTS. There were no significant differences in the tear-secretion test results among the four groups. However, none of the eyes treated with NGF plus DHA showed rose bengal staining 30 days after PRK, compared with 50% in the control group and 33% in the DHA-treated group. A higher percentage of Ki-67-positive cells, a marker of cell proliferation, was observed in the DHA plus NGF - and NGF-treated groups compared with DHA alone or the control. Eight weeks after PRK, tubulin-positive and CGRP-positive epithelial and subbasal nerve bundle areas were significantly higher in the DHA plus NGF group than in the control and NGF or DHA alone. No differences were noted in the SP-positive nerve bundles between the different treatments and the control treatment. CONCLUSIONS. NGF plus DHA treatment after PRK in rabbits is associated with increased corneal nerve surface area, increased epithelial proliferation, and decreased rose bengal staining compared with NGF, DHA, or vehicle control alone. The combination of NGF plus DHA yields faster nerve recovery after PRK and may have therapeutic usefulness in the treatment of post-PRK dry eye and other neurotrophic keratopathies.