Intrathecal administration of MMP-9 siRNA reduces MMP-9 expression in DRGs and enhances morphine analgesia. (A-D) Western blotting showing MMP-9 expression in DRGs (A, B) and spinal cord dorsal horns (C, D) after intrathecal injections of MMP-9 siRNA or control siRNA (3 μg, 24 and 2 h before the s.c. morphine injection. B and D are quantification of the MMP-9 bands in DRGs (B) and spinal cords (D). *P < 0.05, compared to control (CTRL); Mann-Whitney test, n = 4 mice. (E) Enhanced morphine analgesia after intrathecal treatment of MMP-9 siRNA. *P < 0.05, compared to control siRNA, Student's t-test, n = 5 mice. BL, baseline.