Na+-K+-2Cl− Cotransporter in Immature Cortical Neurons: A Role in Intracellular Cl−Regulation

Abstract

Na+-K+-2Cl− cotransporter in immature cortical neurons: a role in intracellular Cl−regulation. Na+-K+-2Cl−cotransporter has been suggested to contribute to active intracellular Cl− accumulation in neurons at both early developmental and adult stages. In this report, we extensively characterized the Na+-K+-2Cl− cotransporter in primary culture of cortical neurons that were dissected from cerebral cortex of rat fetus at embryonic day 17. The Na+-K+-2Cl− cotransporter was expressed abundantly in soma and dendritic processes of cortical neurons evaluated by immunocytochemical staining. Western blot analysis revealed that an ∼145-kDa cotransporter protein was present in cerebral cortex at the early postnatal (P0–P9) and adult stages. There was a time-dependent upregulation of the cotransporter activity in cortical neurons during the early postnatal development. A substantial level of bumetanide-sensitive K+ influx was detected in neurons cultured for 4–8 days in vitro (DIV 4–8). The cotransporter activity was increased significantly at DIV 12 and maintained at a steady level throughout DIV 12–14. Bumetanide-sensitive K+influx was abolished completely in the absence of either extracellular Na+ or Cl−. Opening of γ-aminobutyric acid (GABA)-activated Cl− channel or depletion of intracellular Cl− significantly stimulated the cotransporter activity. Moreover, the cotransporter activity was elevated significantly by activation of N-methyl-d-aspartate ionotropic glutamate receptor via a Ca2+-dependent mechanism. These results imply that the inwardly directed Na+-K+-2Cl− cotransporter is important in active accumulation of intracellular Cl− and may be responsible for GABA-mediated excitatory effect in immature cortical neurons.