Effects of yohimbine on naloxone-induced antinociception in a rat model of inflammatory hyperalgesia

Abstract Effects of the α 2 -adrenoceptor antagonist yohimbine on the antinociception produced by a low dose of naloxone were examined in a rat model of carrageenan-induced inflammation. In rats receiving saline prior to naloxone injection, the low dose of naloxone (5 μ g/kg, i.p.) significantly prolonged paw withdrawal latency in response to noxious thermal stimuli for both the inflamed and the non-inflamed paws 4 h after carrageenan injection (6.0 mg in 0.15 ml saline). In rats receiving yohimbine, the low dose of naloxone failed to produce prolongation of paw withdrawal latencies 4 h after carrageenan, whereas naloxone produced antinociception 7 days after carrageenan. The results suggest that noradrenergic mechanisms are involved in naloxone-induced antinociception only in the early phase of carrageenan-induced inflammation.