Brain-derived neurotrophic factor (BDNF) is released from activated microglia during neuropathic pain and is hypothesized to downregulate the expression of the potassium chloride cotransporter 2 (KCC2) via the TrkB receptor. Previous studies reported that KCC2 is downregulated 5 min after the plantar injection of formalin in rats; however, the mechanism behind this decrease in KCC2 expression during acute inflammatory pain remains unknown. In this study, we determined whether the TrkB receptor contributes to the expression of KCC2 during the acute pain. Five minutes after the plantar injection of formalin in rats, the ratio of KCC2-immunoreactive area in layer II of the spinal cord significantly decreased on the stimulated side compared to the unaffected side. On the other hand, this response was inhibited by the injection of a kinase inhibitor, K252a, in the subarachnoid space 15 min before the formalin injection. These findings suggest that in acute pain, the TrkB receptor may contribute to the decrease in the expression of KCC2.Cation chloride cotransporters, such as the sodiumpotassium chloride cotransporter 1 (NKCC1) and potassium-chloride cotransporter 2 (KCC2), play an important role in the movement of chlorine ions in neurons (8,14,17). NKCC1 is mainly expressed in the central nervous system, and KCC2 is mainly expressed in the superficial dorsal horn (12). NKCC1 increases intracellular sodium ions (Na + ), potassium ions (K + ) and chlorine ions (Cl − ), whereas KCC2 decreases intracellular K + and Cl − ions. Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that is present in the dorsal horn of the spinal cord, but the expression level of these cotransporters sometimes reverses the effect of GABA to have an excitatory effect. Compared with adult neural cells, immature neural cells express more NKCC1 and less KCC2; therefore, the intracellular concentration of Cl − is maintained at a high level, and the changes in the direction and amount of Cl − inflow when the GABA-A receptor opens its ion channel reverses the inhibitory effect of GABA to an excitatory effect (14,17). In addition, the excitatory effect of GABA has been confirmed in several experimental pain conditions, such as neuropathic pain (3). Previous reports have shown that in neuropathic pain, brain-derived neurotrophic factor (BDNF), which is released from activated microglia, switches the effect of GABA on spinal cord layer II neurons to the excitatory mode (2) and that via the TrkB receptor, BDNF reduces KCC2 expression in the dorsal horn of the spinal cord (11). These data suggest that in neuropathic pain, the reduced expression of KCC2 may switch the effect of GABA from the inhibitory mode to the excitatory mode and cause the aggravation of pain and that microglia are a major regulator of the expression levels of KCC2. According to recent studies, chloride cation co-
