Changes in intracellular chloride after oxygen-glucose deprivation of the adult hippocampal slice: Effect of diazepam

Ischemic injury to the CNS results in loss of ionic homeostasis and the development of neuronal death. An increase in intracellular Ca2+ is well established, but there are few studies of changes in intracellular Cl-([Cl-](i)) after ischemia. We used an in vitro model of cerebral ischemia (oxygen-glucose deprivation) to examine changes in [Cl-](i) and GABA(A) receptor-mediated responses in hippocampal slices from adult rats. Changes in [Cl-](i) were measured in area CA1 pyramidal neurons using optical imaging of 6-methoxy-N-ethylquinolinium chloride, a Cl--sensitive fluorescent indicator. Oxygen-glucose deprivation induced an immediate rise in [Cl-](i), which recovered within 20 min. A second and more prolonged rise in [Cl-](i) occurred within the next hour, during which postsynaptic field potentials failed to recover. The sustained increase in [Cl-](i) was not blocked by GABA(A) receptor antagonists. However, oxygen glucose deprivation caused a progressive downregulation of the K+-Cl- cotransporter (KCC2), which may have contributed to the Cl- accumulation. The rise in [Cl-](i) was accompanied by an inability of the GABA(A) agonist muscimol to cause Cl- influx. In vivo, diazepam is neuroprotective when given early after ischemia, although the mechanism by which this occurs is not well understood. Here, we added diazepam early after oxygen-glucose deprivation and prevented the downregulation of KCC2 and the accumulation of [Cl-](i). Consequently, both GABA(A) responses and synaptic transmission within the hippocampus were restored. Thus, after oxygen-glucose deprivation, diazepam may decrease neuronal excitability, thereby reducing the energy demands of the neuron. This may prevent the activation of downstream cell death mechanisms and restore Cl- homeostasis and neuronal function.