RESUMO
The role of midbrain reticular formation, which includes the nucleus cuneiformis [NCF], as a crucial antinociceptive region in descending pain modulation has long been investigated. In this study, we tried to highlight the role of NCF in morphine-induced antinociception in formalin-induced pain model in rats. A total of 201 male Wistar rats weighing 260-310 g were used in this study. The effective dose of morphine in systemic administration [intraperitoneal; i.p.] was determined after a dose- and time-response protocol. In consequent groups, bilateral electrolytic lesion [500 microA, 30 sec] or reversible inactivation [lidocaine 2%] were used in the NCF before systemic administration of morphine, and then, the nociceptive test was immediately carried out. The results showed that administration of 6 mg/kg morphine, 30 min before the formalin test, is the best dose- and time-response set in these experiments. The obtained data also indicated that bilateral electrical destruction or reversible inactivation of the NCF significantly decreased antinociceptive responses of systemic morphine [6 mg/kg; i.p.] during the second phase of formalin test [P<0.05]. Therefore, it seems that opioid receptors located in the NCF may be involved in modulation of central sensitization which occurred in inflammatory pain in rats