ABSTRACT
Objective To evaluate the role of extracellular signal-regulated protein kinase 5 (ERK5) in the spinal cord in withdrawal responses in morphine-dependent rats.Methods Ninety-six adult male SpragueDawley rats in which intrathecal catheters were successfully placed,weighing 200-250 g,were randomly divided into 4 groups (n =24 each) using a random number table:normal saline group (group A),withdrawal group (group B),dimethyl sulfoxide (DMSO) group (group C) and ERK5 inhibitor BIX02188 group (group D).Morphine dependence (MD) was induced by increasing doses of subcutaneous morphine for 6 days.The initial dose of morphine was 10 mg/kg once a day and was increased by 10 mg/kg once a day from the 2nd to 5th days until 50 mg/kg on the 6th day in B,C and D groups.Morphine withdrawal response (MW) was induced by intraperitoneal naloxone 4 mg/kg at 4 h after last morphine administration in B,C and D groups.In addition,BIX02188 and 1％ DMSO 10 μl were injected intrathecally at 1 h before naloxone injection in D and C groups,respectively.MW and morphine withdrawal-induced hypemlgesia were scored.The rats were then sacrificed after hyperalgesia was scored and the spinal cord was removed for determination of ERK5 and phosphorylated ERK5 (p-ERK5) expression.Results Compared with group A,MW and hyperalgesia scores were significantly increased and the expression of pERK5 was up-regulated in B,C and D groups (P ＜ 0.05).Compared with group B,MW and hyperalgesia scores were significantly decreased and the expression of p-ERK5 was down-regulated in D group (P ＜ 0.05),and no significant change was found in group C (P ＞ 0.05).Conclusion ERK5 in the spinal cord is involved in withdrawal responses in morphine-dependent rats.