Cell swelling exacerbates hypoxic neuronal damage in rat hippocampal slices

Abstract

In the present study we investigated the effect of acute cell swelling on the sensitivity of rat hippocampal slices to hypoxia. Hippocampal slices were exposed to different degrees of hypo- or hyperosmolality 15 min prior to and during a 15-min hypoxia followed by reoxygenation under isosmotic (293 mosm) conditions. Recovery of neuronal function (an electrically evoked population spike) after hypoxia was significantly diminished in slices exposed to hyposmotic conditions as 57% of control (isosmotic) slices showed recovery compared with 51%, 35%, and 13% recovery rate in slices made hyposmotic (273, 253, and 233 mOsm, respectively). Of slices exposed to a medium made hyperosmotic by the addition of 20, 40, 60, and 80 mM mannitol, only those exposed to the most hyperosmotic treatment (373 mOsm) exhibited a recovery rate significantly greater than control (70% vs. 57%). The competitive NMDA antagonist CGS-19755 (50 μM) completely protected both isosmotic and hyposmotic (233 mOsm) slices against hypoxic damage. However, a threshold dose (15 μM) of the antagonist provided no protection to isosmotic slices (51% vs. 57% recovery rate) while affording substantial protection to hyposmotic slices (233 mOsm), as 54% of the treated slices recovered their neuronal function after hypoxia compared to 13% recovery rate of the untreated slices. These results suggest an increase in activation of the NMDA receptor under hyposmotic conditions. We conclude that acute osmotic swelling of neuronal tissue predisposes it to hypoxic damage, possibly by activation of NMDA receptors that are not usually activated by hypoxia alone.