MMP9 and injury: Ji et al. (2009) demonstrated distinct roles of MMP-9 and MMP-2 in neuropathic pain development: (i) transient MMP-9 upregulation after nerve injury is critical for the early-phase development of neuropathic pain; (ii) sustained MMP-2 upregulation maintains neuropathic pain; and (iii) MMP-9 and MMP-2 induce the cleavage of interleukin-1β for its activation in the early and late phase of nerve injury, respectively [20].