Up-regulation of MMP-9 expression in DRGs but not spinal cords after intrathecal morphine or remifentanil and potentiation of intrathecal morphine analgesia in Mmp9 deficient mice. (A-D) Western blotting showing MMP-9 expression in DRGs (A, B) and spinal cord dorsal horns (C, D) 2 h after intrathecal injection of morphine (10 nmol) or remifentanil (1 nmol). B and D are quantification of the MMP-9 bands in DRGs (B) and spinal cords (D). *P < 0.05, compared to control, ANOVA followed by Bonferroni post hoc test, n = 4 mice. CTRL, control; MORPH, morphine; REMIF, remifentanil. (E) Mmp9-KO mice exhibit enhanced and prolonged analgesia after intrathecal morphine (10 nmol). *P < 0.05, compared to Wild-type control, Student's t-test, n = 7 mice. BL, baseline.