SYSTEMIC CLONIDINE DIFFERENTIALLY MODULATES THE ABNORMAL REACTIONS TOMECHANICAL AND THERMAL STIMULI IN RATS WITH PERIPHERAL MONONEUROPATHY

The antinociceptive action of the systemically administered alpha(2)-a drenoceptor agonist clonidine was evaluated in a rat model of peripher al unilateral mononeuropathy produced by loose ligatures around the co mmon sciatic nerve, using nociceptive tests based on mechanical (vocal ization threshold to paw pressure) or thermal (struggle latency to paw immersion in a cold (10 degrees C) or hot (44 degrees C) water bath) stimuli. Experiments were performed 2-3 weeks after surgery when pain- related behavior was fully developed. We demonstrated a dissociative a ction depending on the test used: clonidine (30-100 mu g/kg i.v.) had a moderate effect on the abnormal reactions to the mechanical stimulus . By contrast it dramatically increased the struggle latency to hot or cold stimuli. These latter effects were completely prevented by prior administration of the alpha(2)-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.).