Objective To investigate the role of extracellular signal-regulated kinase(ERK)signal pathway in the spinal cord in the development of chronic morphine tolerance.Methods Thirty healthy male SD rats weighing 300-350 g in which intrathecal(IT)catheters were successfully implanted without complication were randomly divided into 3 groups(n = 10 each): group Ⅰ control(group C);group Ⅱ morphine tolerance(group M)and group Ⅲ morphine tolerance + PD98059(ERK upstream kinase MEK inhibitor)(group P).Morphine tolerance was induced with IT morphine 10 μg twice a day for 7 consecutive days.In group P PD98059 10 μg was injected IT at 30 min before each morphine administration.Tail flick latency(TFL)(the time between the onset of heat stimulus and voluntary tail withdrawal)was measured once a day at 30 min after first IT morphine administration and at 1 day after last IT morphine.Maximum analgesic effect(MPE)was calculated.MPE =(TFL after IT morphine - baseline TFL)/(12 - baseline TFL)× 100%.The animals were sacrificed after last TFL measurement.The L3-5 segment of the spinal cord was isolated for determination of the expression of μ-receptor and phosphorylated ERK 1/2(p-ERK1/2)by Western blot analysis and fluoroimmunoassay.Results Morphine tolerance was induced in group M and M + P.MPE was higher in group P than in group M.The expression of μ-receptor in spinal dorsal horn was significantly lower while the p-ERK1/2 expression was higher in group M than in group C.IT PD98059 significantly up-regulated μ-receptor expression and down-regulated p-ERK expression in group P as compared with group M.Conclusion ERK signal pathway is involved in the development of chronic morphine tolerance in rats.

Objective To investigate the role of glucocorticoid receptor (GR) in the induction of neuronal apoptosis in spinal dorsal horn in chronic morphine tolerant rats. Methods Twenty healthy male SD rats weighing 300-350 g in which intrathecal (IT) catheter was successfully implanted without complication were randomized into 4 groups ( n = 5 each): group Ⅰ control ( group C);group Ⅱ morphine ( group M );group Ⅲ morphine +RU38486 (GR antagonist) (group MR) and group Ⅳ morphine + dexamethasone (GR agonist) (group MD).Normal saline 10 μl, morphine 10 μg, morphine 10 μg + RU38486 2 μg and morphine 10μg + dexamethasone 4 μg were injected IT twice a day at 8:00 and 20:00 for6 consecutive days in group C, M, MR and MD respectively. Tail flick latency (TFL) to a thermal nociceptive stimulus was measured every day at 30 min after IT administration in the morning (8:00). Hyperalgesia was considered to be a sign of morphine tolerance. The animals were killed at 7 days after IT drug administration. The L3-5 segment of the spinal cord was isolated for determination of neuronal apoptosis in spinal dorsal horn by TUNEL staining. Apoptotic index was calculated ( the number of apoptotic neurons/the total number of neurons × 100% ). Results TFL was significantly prolonged at day 1 and 3 of IT morphine 10 μg twice a day and returned to baseline at day 5 and 7 indicating morphine tolerance. RU38486 inhibited while dexamethasone enhanced morphine tolerance. IT morphine significantly increased the number of apoptotic neurons in spinal dorsal horn. Morphine-induced neuronal apoptosis was decreased by IT RU38486 and increased by IT dexamethasone. Conclusion Glucocorticoid receptors may be involved in morphine tolerance by inducing neuronal apoptosis in spinal dorsal horn.