Effect of Spinal and Systemic Clonidine Administration on the Postoperative Analgesia in Morphine-dependent and Naïve Rats

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 ?2-adrenergic agonist, is proved to enhance postoperative analgesia. It has also been shown that morphine-dependent rats experience more chronic pain than naïve 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 naïve 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 naïve rats received i.t. (17µg.kg­¹) or i.p. (0.7 mg.kg­¹) 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 naïve 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.