Role of NKCC1 and KCC2 during hypoxia-induced neuronal swelling in the neonatal neocortex

Neonatal hypoxia causes cytotoxic neuronal swelling by the entry of ions and water. Multiple water pathways have been implicated in neurons because these cells lack water channels, and their membrane has a low water permeability. NKCC1 and KCC2 are cation-chloride cotransporters (CCCs) involved in water movement in various cell types. However, the role of CCCs in water movement in neonatal neurons during hypoxia is unknown. We studied the effects of modulating CCCs pharmacologically on neuronal swelling in the neocortex (layer IV/V) of neonatal mice (post-natal day 8-13) during prolonged and brief hypoxia. We used acute brain slices from Clomeleon mice which express a ratiometric fluorophore sensitive to Cl^- and exposed them to oxygen-glucose deprivation (OGD) while imaging neuronal size and [Cl^-]_i by multiphoton microscopy. Neurons were identified using a convolutional neural network algorithm, and changes in the somatic area and [Cl^-]_i were evaluated using a linear mixed model for repeated measures. We found that (1) neuronal swelling and Cl^- accumulation began after OGD, worsened during 20 min of OGD, or returned to baseline during reoxygenation if the exposure to OGD was brief (10 min). (2) Neuronal swelling did not occur when the extracellular Cl^- concentration was low. (3) Enhancing KCC2 activity did not alter OGD-induced neuronal swelling but prevented Cl^- accumulation; (4) blocking KCC2 led to an increase in Cl^- accumulation during prolonged OGD and aggravated neuronal swelling during reoxygenation; (5) blocking NKCC1 reduced neuronal swelling during early but not prolonged OGD and aggravated Cl^- accumulation during prolonged OGD; and (6) treatment with the "broad" CCC blocker furosemide reduced both swelling and Cl^- accumulation during prolonged and brief OGD, whereas simultaneous NKCC1 and KCC2 inhibition using specific pharmacological blockers aggravated neuronal swelling during prolonged OGD. We conclude that CCCs, and other non-CCCs, contribute to water movement in neocortical neurons during OGD in the neonatal period.