REDUCTION OF CAPSAICIN-INDUCED OCULAR PAIN AND NEUROGENIC INFLAMMATION BY CALCIUM-ANTAGONISTS

Purpose. To examine whether blockade of chemosensitivity of corneal no ciceptors by Ca2+ antagonists decreases pain and irritation induced by capsaicin. Methods. In adult rabbits, the number of lid-squeezing mov ements and the degree of palpebral opening, miotic response, and conju nctival vasodilation evoked by a bilateral instillation of 30 mul of c apsaicin (33 mM) were measured at different times (up to 5 hours) afte r the drug. Irritative responses to capsaicin in eyes pretreated with diltiazem, verapamil, or nifedipine were compared with those that rece ived only the vehicle. Protein content in aqueous humor was also measu red at the end of the experiment. Results. Diltiazem at doses of 1 to 28 mM, administered 15 minutes before the application of capsaicin, si gnificantly decreased scratching movements, conjunctival hyperemia, cl osure of the eye, and elevated aqueous protein concentration induced b y capsaicin; however, it did not significantly reduce miosis. Nifedipi ne (2.8 and 10 mM) diminished the number of scratching movements but n ot other inflammatory parameters, whereas verapamil (2.8 and 10 mM) wa s totally ineffective in attenuating ocular signs of irritation produc ed by capsaicin. Conclusions. These results suggest that by lowering c apsaicin-induced neural activity in nociceptive terminals, diltiazem d ecreases pain and neurogenic inflammation and may be useful as both an analgesic and an antiinflammatory agent in the eye.