Efficacy of nalmefene antagonizing postoperative respiratory depression induced by opioids:a multicenter,randomized,double-blind,positive drug-controlled study

Objective To investigate the role of NO and extracellular signal-regulated kinase (ERK) signaling pathways in the spinal cord in naloxone-induced withdrawal response in morphine-dependent rats.Methods Ninety male adult SD rats weighing 200-250 g in which IT catheters were successfully implanted without complication were randomly divided into 9 groups (n ＝ 10 each):group control (group C); group morphine dependence (group MD); group morphine withdrawal (group MW); group N(G)-nitro-L-arginine methyl ester (eNOS inhibitor) (L-NAME group) ; group 7-nitroindazole (nNOS inhibitor) (group 7-Ni) ; group aminoguanidine(iNOS inhibitor) (group AG); group U0126 (ERK signaling pathway blocker); group cremophor (solvent for 7-Ni) and group DMSO (solvent for U0126).Morphine dependence was induced by increasing doses of subcutaneous morphine for 6 days.The initial dose of morphine was 10 mg/kg twice a day and was increased by 10 mg/kg twice every other day until 50 mg/kg on the 6th day in groups MD,MW,L-NAME,7-Ni,AG,U0126,cremophor and DMSO.Morphine withdrawal response was induced by intraperitoneal (IP) naloxone 4 mg/kg at 4 h after last morphine administration in groups MW,L-NAME,7-Ni,AG,U0126,cremophor and DMSO.L-NAME 400 μg,7-Ni 400 μ g,AG 400 μg,U0126 150 μg,cremopher 10 μl and DMSO 10 μl were administered IT at 30 min before naloxone administration in groups L-NAME,7-Ni,AG,U0126,cremophor and DMSO respectively.Morphine withdrawal response (0 ＝ no withdrawal response,3 ＝ severe response) and touch evoked agitation (0 ＝ no agitation,2 ＝ severe agitation) were observed and scored during 1 h after naloxone administration.The animals were then sacrificed and the spinal cord was removed for determination of the expression of iNOS,nNOS and phosphor-ERK (p-ERK) by immunohisto-chemistry and Western blot.Results Morphine withdrawal significantly increased withdrawal response score and touch evoked agitation score in group MW as comparedwith group MD.L-NAME,7-Ni,AG and U0126 pretreatment significantly attenuated naloxone-induced increase in withdrawal response score and touch evoked agitation score in groups L-NAME,7-Ni,AG and U0126 as compared with group MW.Morphine withdrawal significantly up-regulated the nNOS and iNOS expression in group MW compared with groups C and MD.L-NAME,7-Ni and AG pretreatment significantly down-regulated p-ERK expression in groups L-NAME,7-Ni and AG as compared with group MW.Conclusion The interaction between NO and ERK signaling pathways may be involved in morphine withdrawal response in morphine-dependent rats. Key words: Nitric oxide; Extracellular signal-regulated MAP kinases; Spinal cord; Morphine dependence; Substance withdrawal syndrome