Effects of capsaicin on corneal wound healing

This study examined whether the depletion of neuropeptides from sensory nerve terminals induced by capsaicin modifies the healing rate of experimental corneal wounds in adult rabbits. Capsaicin (33 or 3.3 mM solutions) was administered topically and/or by a single retrobulbar injection to one eye while the fellow eye, treated with the vehicle, served as a control. After 1-3 weeks of treatment, an epithelial wound was made in the center of the cornea of both eyes with n-heptanol. Migration rates of epithelial cells surrounding the wound and estimated wound closure times were calculated by measuring the reduction in wound size. Combined treatment with 33 mM retrobulbar and 3.3 mM topical capsaicin for 3 weeks induced a significant delay in epithelial migration rates and in wound closure times (P less than 0.05). Topical or retrobulbar capsaicin alone for 3 weeks and combined treatment lasting only 1 week were not sufficient to modify wound healing times. The substance P antagonist, spantide (3 mM), applied topically for 1-3 weeks before or immediately after corneal wounding was also ineffective in changing wound closure rates. These findings suggest that the delayed wound healing observed after prolonged treatment with capsaicin could be due to a sustained depletion of neuropeptides from corneal sensory endings, supporting the hypothesis that trophic effects of sensory nerves on corneal epithelium are, at least in part, mediated by neuropeptides contained in peripheral nerve terminals.