Dexamethasone Aggravates Ischemia-Induced Neuronal Damage by Facilitating the Onset of Anoxic Depolarization and the Increase in the Intracellular Ca2+ Concentration in Gerbil Hippocampus

Abstract

The Ca²⁺ mobilization across the neuronal membrane is regarded as a crucial factor in the development of neuronal damage in ischemia. Because glucocorticoids have been reported to aggravate ischemic neuronal injury, the effects of dexamethasone on ischemia-induced membrane depolarization, histologic outcome, and changes in the intracellular Ca²⁺ concentration in the gerbil hippocampus were examined in vivo and in vitro. The effects of metyrapone, an inhibitor of glucocorticoid synthesis, were also evaluated. Changes in the direct-current potential shift in the hippocampal CA1 area produced by transient forebrain ischemia for 2.5 minutes were compared among animals pretreated with dexamethasone (3 μg, intracerebroventricularly), metyrapone (100 mg/kg, intraperitoneally), and saline. The histologic outcome was evaluated 7 days after ischemia by assessing the delayed neuronal death in the hippocampal CA1 pyramidal cells of these animals. A hypoxia-induced intracellular Ca²⁺ increase was evaluated by in vitro microfluorometry in gerbil hippocampal slices, and the effect of dexamethasone (120 μg/L in the medium) on the cytosolic Ca²⁺ accumulation was examined. The effect in a Ca²⁺-free ischemialike condition was also investigated. Preischemic administration of dexamethasone reduced the onset latency of ischemia-induced membrane depolarization by 22%, and aggravated neuronal damage in vivo. In contrast, pretreatment with metyrapone improved the histologic outcome. The onset time of the increase in the intracellular concentration of Ca²⁺ provoked by in vitro hypoxia was advanced in dexamethasone-treated slices. The Ca²⁺-free in vitro hypoxia reduced the elevation compared with that in the Ca²⁺-containing condition. Treatment with dexamethasone facilitated the increase on both the initiation and the extent in the Ca²⁺-free condition. Aggravation of ischemic neuronal injury by endogenous or exogenous glucocorticoids is thus thought to be caused by the advanced onset times of both the ischemia-induced direct-current potential shift and the increase in the intracellular Ca²⁺ concentration.