Reduced potassium-chloride co-transporter expression in spinal cord dorsal horn neurons contributes to inflammatory pain hypersensitivity in rats.

Cation chloride co-transporters are important determinants for the efficacy of inhibitory neurotransmission in the spinal cord and alterations in their expression levels contribute to allodynia and hyperalgesia associated with neuropathy. However, it remains unknown whether these co-transporters contribute to chronic inflammatory pain. We investigated the expression of potassium-chloride co-transporter 2 (KCC2) and sodium-potassium-chloride co-transporter 1 (NKCC1) in the rat spinal cord after peripheral inflammation induced by complete Freund's adjuvant (CFA) injection. Our results suggest that the expression of KCC2, but not that of NKCC1, was significantly reduced in CFA-injected rats. We also found that blockade of endogenous brain-derived neurotrophic factor-tyrosine receptor kinase B pathway inhibited the inflammation-induced KCC2 downregulation. Moreover, intrathecal injection of KCC2 antisense oligodeoxynucleotides into naïve rats reduced KCC2 expression in the spinal cord, leading to behavioral hypersensitivity similar to the hyperalgesia induced by peripheral inflammation. Taken together, these results indicate that peripheral inflammation induces downregulation of KCC2 in the dorsal horn of the spinal cord, which may in turn facilitate the development and/or maintenance of chronic inflammatory pain. The data also support the notion that disinhibition in the spinal cord is a general feature of inflammatory and neuropathic pain conditions, and suggest new therapeutic intervention.