TLR4 and injury: Toll-like receptor 4 (TLR4) is located primarily on microglia and has been implicated in pain pathology, with evidence that systemic and intrathecal administration of lipopolysaccharide (LPS) results in TRL4 activation leading to pain hypersensitivity in rats [62,63], and that TLR4 knockout mice exhibit decreased pain hypersensitivity after nerve injury.