ACTIVATION OF ALPHA(2)-ADRENERGIC RECEPTORS DECREASES NERVE TRAUMA-INDUCED AFFERENT BARRAGE BUT NOT AUTOTOMY

Effects of the selective alpha(2)-adrenoceptor agonist, medetomidine, on a compound volley of a tibial nerve stimulation-evoked spinal refle x, pain-induced phrenic motor responses and on postoperative neuropath ic pain behavior were studied in rats. Medetomidine (0.3 mg/kg) decrea sed the amplitude of the compound volley recorded from peroneal nerve in response to tibial stimulation in pentobarbital (40 mg/kg) anesthet ized rats. Atipamezole, an alpha(2)-adrenoceptor antagonist (1.5 mg/kg ) fully restored the response when given 60 min after the medetomidine administration. Pain-evoked phrenic motor responses were completely i nhibited upon combination anesthesia by pentobarbital (40 mg/kg) and m edetomidine (0.3 mg/kg) (PB+M) but not upon plain pentobarbital anesth esia (50 or 60 mg/kg) (PB50,PB60). To study the effect of medetomidine on postoperative neuropathic pain behavior (autotomy), transection of sciatic nerve was done under PB+M, PB50 or PB60 anesthesia. No differ ences between the groups were found in the postoperative pain behavior during eight-week follow up. The results show that activation of alph a(2)-adrenergic receptors by medetomidine under pentobarbital anesthes ia mitigates trauma-induced afferent barrage, whereas it does not redu ce the subsequent autotomy.