Induction of Corneal Lesion and Nerve Fiber Sprouting by Neonatal Capsaicin Application Depends on the Dose of the Drug and Survival Time after Treatment

Abstract

The effects of capsaicin on the rat cornea and its NsAchE (non specific acetylcholinesterase)-positive nerve fibers were investigated after long and short survival periods following subcutaneous (s.c) injection of the drug. Sixteen rats were injected once s.c with capsaicin on postnatal day 2 at a dose of 50 mg/kg. Age-matched rats were injected only vehicle, and served as control (n = 16). After 4 (n = 6), 8 (n = 6) and 12 (n = 4) mos., both sides of the corneas were examined under a binocular microscope to look for corneal abnormalities. Immediately after the enucleation, bilateral corneas were excised with a thin scleral margin and their ciliary body and iris were removed in DPBS solution. Then, they were fixed in a 4% paraformaldehyde solution containing 8% sucrose for 1 h at 4 degrees C, and processed for staining by the NsAchE method. For comparison, 15 rats were injected 3 times with capsaicin (total dose: 150 mg/kg) at a dose of 50 mg/kg on days 1, 2 and 3 after birth. Age-matched rats (n = 16) were injected vehicle as controls. Five (n = 4), 17 (n = 4), 45 (n = 3) and 75 (n = 4) days later, their corneas were similarly handled as described above. Corneal lesions and sprouting of the NsAchE-positive subepithelial nerve fibers appeared 4 mos. after the treatment with capsaicin (50 mg/kg). In particular, all the treated corneas (8/8) at 12 mos. showed corneal abnormalities. Contrary to the results from the single injection of capsaicin, corneal changes had already appeared on day 17 after treatment with capsaicin at a high dose (150 mg/kg). These data reinforce the suggestion that yielding of corneal lesions is closely associated with the longer survival time at lower doses and with the short survival time at a high dose of the drug. Further, the relationship between sprouting of corneal nerve fibers and corneal wounds was discussed on the basis of the previous reports.