The role of μ-opioid receptors in inflammatory hyperalgesia and α2-adrenoceptor-mediated antihyperalgesia

The purpose of the present study was to investigate the role of μ-opioid receptor in inflammatory hyperalgesia in intact and in spinalized animals and the interaction between μ-opioid and α2-adrenergic receptor in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and heat stimuli were studied in μ-opioid receptor knockout mice and wildtype control mice. Thermal nociception was evaluated by measuring paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured by von Frey monofilament applications to the hindpaws. Intraplantar carrageenan-induced (1 mg/40 μl) mechanical and heat hyperalgesia were compared in μ-opioid knockout and wildtype mice. The effect of systemically administered α2-adrenergic receptor agonist dexmedetomidine (1–10 μg/kg) was evaluated on mechanical and thermal withdrawal responses under normal and inflammatory state in knockout and wildtype mice. The role of μ-opioid receptor in descending modulation of nociception was studied by assessing mechanical and heat withdrawal responses before and after mid-thoracic spinalization.