Effect of morphine sulphate eye drops on hyperalgesia in the rat cornea

In addition to their traditional role in centrally mediated analgesia, opiate compounds produce significant effects when administered peripherally. Using a recently characterized model of acute chemical injury to the rat cornea (Pain (in press)), we assessed the effects of morphine sulphate eye drops on corneal inflammation and hyperalgesia. Topical application of a 5 muM morphine sulphate eye drop preparation attenuated capsaicin-induced blinking in a concentration-dependent manner. However, morphine had no effect on capsaicin-induced blinking when applied to healthy, non-inflamed rat cornea. In addition, 5 muM morphine given every 2 h following cauterization retarded the development of both stromal edema and the infiltration of immune cells. Both the analgesic and anti-inflammatory effects of morphine were prevented by the opioid receptor antagonists naloxone, CTAP, and naltrindole. We conclude that morphine acts on R and 6 opioid receptors located in the rat cornea to attenuate inflammation and hyperalgesia. (C) 2003 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.