The activation of spinal glia (microglia and astrocytes) and thermal hyperalgesia by unilateral intra-articular injection of complete Freund's adjuvant (CFA)-induced monoarthritis (MA). A, Immunohistochemistry reveals an extensive increase in Iba-1- (a microglial marker) immunoreactivity (Iba-1-IR) in the ipsilateral lumbar spinal dorsal horn at 4 hrs and in both sides by days 1 and 4 after MA. B, Immunohistochemistry reveals a marked increase in GFAP- (a microglial marker) immunoreactivity (GFAP-IR) in both sides of the lumbar spinal dorsal horn on day 4 after MA. C and D, Quantification of Iba-1-IR (C) and GFAP-IR (D) expression on both sides of the spinal dorsal horn ** p < 0.01 vs. naïve control; + p < 0.05, ++ p < 0.01 vs. contralateral. E, MA induced significant thermal hyperalgesia in the ipsilateral hindpaw ** p < 0.01 vs. sham MA.