MMP-9 deficiency does not alter opioid-induced hyperalgesia after subcutaneous morphine. (A, B) Western blot gel (A) and band density (B) showing MMP-9 expression in lumbar DRGs 24 h after morphine (MORPH) injection. *P > 0.05, compared to naive control (CTRL), Student's t-test, n = 4 mice. (C, D) Subcutaneous morphine (5 mg/kg) induces comparable heat hyperalgesia in both wild-type and Mmp9-KO mice (C) and also in wild-type mice pretreated with intrathecal MMP-9 inhibitor-I (0.2 μg) or vehicle (10% DMSO). *P < 0.05, compared to corresponding baseline (BL), Student's t-test, n.s., no significance. n = 5 mice. Morphine-induced heat hyperalgesia was tested by tail flick latency in 48°C hot water.