RESUMEN
Post operative pain and its control remains one of the most important issues in the field of surgery and health care system. Formalin test has been used as a method for assessing pain and analgesia in rats. Systemic and spinally administered clonidine, an a2-adrenergic agonist, is proved to enhance postoperative analgesia. It has also been shown that morphine-dependent rats experience more chronic pain than naive rats. In this study we have explored the effect of certain doses of i.t. and i.p. clonidine on postoperative pain in morphine-dependent and naive rats. Rats were addicted by oral morphine. For one group, an intrathecal catheter was inserted 48 hours before surgery. A 1-cm longitudinal incision was made through skin, fascia and muscle of the plantar aspect of the right hind paw. Both morphine-dependent and naive rats received i.t. [17microg.kg[1] or i.p. [0.7 mg.kg[1] clonidine immediately and 15 minutes befor formalin test, respectively. Then, formalin-induced behaviors were recorded. In both phases of formalin test in morphine -depondent rats, i.p clonidine produced more analgesic effect, compared to i.t. injection [P<0.01]. On the other hand i.p. clonidine caused more analgesic effect in phase II of formalin test in morphine-dependent than naive rats [P<0.05]. Clonidine may potentiate the analgesic effect of chronically administered morphine. Also it seems that clonidine acts better in supraspinal level, compared to the spinal level