Supported by FAPESP (Process # 2007/06519-7 and 2007/58783-0).

The objectives of the present study were to evaluate the local and analgesic effects of topical 0.5% morphine sulphate solution in rabbits with experimentally induced-ulcerative keratitis.

This study was approved by the Ethics Committee on Animal Experimentation of the Faculty of Agricultural and Veterinary Sciences of Sao Paulo State University, Jaboticabal, Brazil (protocol l# 19284-07). All animals received ophthalmic examinations prior to experiment to verify the absence of ocular lesions. Twelve male rabbits underwent general anesthesia, and an ulcer was created after a corneal button of 6 mm of diameter and 200 μm of depth was removed. Two groups were formed (n = 6), the first received 100 μL of topical 0.5% morphine (MG), 4, 6, 12 and 24 hours after surgery. The second was used as a control group (CG), and received saline at the same dose regimen and time points described to MG. In addition, both groups were treated with topical 0.3% tobramycin as prophylactic antibacterial drug, in the same periods aforementioned, respecting a 10 minute interval after instillations of morphine and saline.

Conjunctival hyperemia was recorded using a scale of 0 to 3 (none = 0; mild = 1; moderate = 2; severe = 3) by means of biomicroscopy. The corneal touch threshold (CTT) was assessed with a Cochet-Bonett esthesiometer, prior to and posteriorly the surgical procedure, 20 minutes after each treatment. Results were evaluated statistically (P < 0.05). Conjunctival hyperemia scores in both groups and all time points evaluated were considered moderate (P > 0.05). Average baselines CTT values (mean±SD) for MG and CG were 1.90±0.31 and 1.10±0.12 g/mm2, respectively (P = 1.00). Average CTT values measured 4, 6, 12 and 24 hours after surgery for MG and CG were 0.40, 2.38±0.91, 3.25±0.73, 10.95±2.65, 13.57±1.43 g/mm2; 0.40, 1.20±1.41, 1.10±0.10, 0.74±0.15, and 0.75±0.30 g/mm2, respectively. In comparison with baseline and CG CTT values, MG CCT values increased significantly 12 and 24 hours after surgery (P < 0.001). The present data suggests that 0.5% morphine sulphate acts on opioids receptors attenuating hyperalgesia in inflamed corneas of rabbits, following lamellar keratectomy.