[Participation of glucocorticoid receptors in morphine tolerance development through the signal pathway of extracellular signal-regulated kinase].

OBJECTIVE: To investigate the mechanism of glucocorticoid receptors (GR) participating in morphine tolerance development via the extracellular signal-regulated kinase (ERK) signal pathway. METHODS: Forty healthy male SD rats were implanted with intrathecal catheters and then randomized into 4 groups: Group C received an intrathecal injection of 10 µl saline, Group M 10 µg morphine, Group MR 10 µg morphine followed by 2 µg GR antagonist RU38486 and Group MD 10 µg morphine followed by 4 µg GR agonist dexamethasone (DEX) respectively. Each intrathecal drug was administered twice daily for 7 days. Tail-flick test was employed to evaluate the thermal hyperalgesia. After tail-flick test at Day 8, the lumbar intamescentias were isolated by Western blot to examine the protein expressions of Mu opioid receptor (MOR), GR and pERK. And terminal deoxynucleotidyl transferase mediated X-dUTP nick end labeling (TUNEL) stain was performed. RESULTS: DEX and RU38486 enhanced and inhibited morphine induced hyperalgesia to heat respectively. Compared with Group M (count of spinal dorsal horn apoptotic cells 5.4 ± 1.1, protein expression: MOR 37 ± 5, GR 20 ± 6, pERK(1) 39 ± 4, pERK(2) 41 ± 5), apoptotic cells decreased in Group MR (3.2 ± 0.4) (P < 0.01) and increased in Group MD (16.0 ± 1.6) (P < 0.01); the protein expressions of MOR (28 ± 5), GR (12 ± 6), pERK(1)(33 ± 4) and pERK(2)(34 ± 5) were down-regulated in Group MR (P < 0.01) while up-regulated in Group MD (55 ± 6, 28 ± 9, 49 ± 6, 59 ± 5) (P < 0.01). CONCLUSION: The activity of glucocorticoid receptors affects the morphine-induced spinal neural apoptosis and the expression of spinal pERK in morphine tolerance.

Spinal glucocorticoid receptor­regulated chronic morphine tolerance may be through extracellular signal­regulated kinase 1/2.

Opioid use has been limited in the treatment of chronic pain due to their side effects, including analgesic tolerance. Previous studies demonstrated that glucocorticoid receptors (GRs) may be involved in the development of chronic morphine tolerance; however, the mechanism remains unknown. It was hypothesized that the expression of spinal phosphorylated mitogen­activated protein kinase [MAPK; phosphorylated extracellular signal­regulated kinase (ERK)] is regulated through the spinal GRs, following chronic treatment with morphine. In the first experiment, the experimental rats were randomly divided into four groups: Control, morphine, morphine+GR antagonist mifepristone (RU38486) and morphine+GR agonist dexamethasone (Dex). Each group was treated with continuous intrathecal (IT) injection of the drugs for 6 days. The expression of GRs and MAPK 3/1 (p­ERK 1/2) in the spinal dorsal horn was detected by western blot analysis and immunofluorescence staining. In the second experiment, the MAPK inhibitor PD98059 was added and the rats were randomly divided into four groups: Control, morphine, PD98059+morphine and PD98059+morphine+Dex. The continuous IT injection lasted for 7 days in each group. For all experiments, the tail flick test was conducted 30 min following administration every day to assess the thermal hyperalgesia of the rats. The experimental results demonstrated that there was a co­existence of GRs and p­ERK 1/2 in the spinal cord dorsal horn by double immunofluorescence staining. The GR antagonist RU38486 attenuated the morphine analgesia tolerance by inhibiting the expression of GR and increasing the expression of p­ERK. The MAPK inhibitor PD98059 increased the effect of morphine tolerance and prolonged the duration of morphine tolerance. The present results suggest that spinal GRs may serve an important role in the development of morphine tolerance through the ERK signaling pathway.