ABSTRACT
Objective To evaluate the role of sonic hedgehog (SHH) signaling pathway in spinal neurons in morphine tolerance (MT) in mice.Methods Pathogen-free healthy female Kunming mice,weighing 20-25 g,aged 8-10 weeks,were used in the study.MT was induced with morphine 10 mg/kg injected subcutaneously twice a day for 7 consecutive days.The experiment was performed in two parts.Experiment Ⅰ Forty-eight mice were randomly assigned into 2 groups:control group (group C,n =8) and MT group (group M,n=40).The thermal pain threshold (TPT) was measured at 1 day before morphine injection and 1,3,5,7 and 14 days after the end of injection.Eight mice in each group were sacrificed at 2 h after measurement of TPT at each time point after the end of injection in group M or at 2 h after the last measurement of TPT in group C,and the lumbar segment (L4-6) of the spinal cord was removed.Experiment Ⅱ Forty-eight mice were randomly assigned into 6 groups (n=8 each):SHH inhibitor cyclopamine plus MT group (group CP+M),cyclopamine solvent plus MT group (group D1 +M),SHH agonist SAG plus MT group (group SAG+M),SAG solvent plus MT group (group D2+M),MT plus cyclopamine group (group M+CP) and morphine plus cyelopamine solvent group (group M+D1).At 15 min before morphine injection,cyclopamine 10 mg/kg was injected subcutaneously in group CP+M,and SAG 5 mg/kg was injected subcutaneously in group SAG+M.Cyclopamine 10 mg/kg was injected subcutaneously once a day during the 1-3 days after the end of morphine injection in group M+CP.The TPT was measured before injection of morphine,at 30 min after the first injection of morphine every day and at 1-3 days after the end of morphine injection.The animals were sacrificed at 2 h after the last measurement of TPT,and the lumbar segment (L4-6) of the spinal cord was removed for determination of the expression of SHH signaling pathway-related proteins SHH,ptch1,smo,gli1 and gli3 using Western blot.Results Experiment Ⅰ Compared with group C,the TPT was significantly decreased at 1 and 3 days after the end of morphine injection (P＜0.05),no significant change was found in TPT at 5-14 days after the end of morphine injection (P＞0.05),and the expression of SHH,smo and glil at 1-5 days after the end of morphine injection,of ptchl at 1 and 3 days after the end of morphine injection and of gli3 at 7 days after the end of morphine injection was up-regulated in group M (P＜0.05).Experiment Ⅱ Compared with group D1+M,the TPT was significantly increased,the expression of SHH,ptchl,smo and glil was down-regulated,and gli3 expression was up-regulated in group C P+M (P＜0.05).Compared with group D2+M,the TPT was significantly decreased,the expression of SHH,ptch1,smo and glil was up-regulated,and gli3 expression was down-regulated in group SAG+M (P＜0.05).There was no significant difference in the parameters mentioned above between group M+CP and group M+D1 (P＞0.05).The TPT was significantly lower on 3rd-7th days after beginning of morphine injection and 1-3 days after the end of morphine injection than at 30 min after the first injection of morphine in group CP+M (P＜0.05).Conclusion The mechanism underlying the development of MT is partially related to activation of SHH signaling pathway in spinal neurons of mice,however,the maintenance mechanism has no marked relationship with it.