Deleterious effect of β-estradiol in a rat model of transient forebrain ischemia

Estrogen has demonstrated great potential as a therapeutic agent in focal ischemic brain injury, as exogenous β-estradiol has proven beneficial in a variety of focal stroke models. In contrast, the relatively few studies of estrogen's efficacy in transient forebrain ischemia have produced inconsistent results. The present study was therefore designed to clarify estrogen's neuroprotective potential in selective hippocampal neuronal injury resulting from four-vessel occlusion in the rat. Female Wistar rats (normal, ovariectomized, or ovariectomized and estradiol-treated) received 5 or 10 min of ischemia. No differences in hippocampal cell loss were found amongst the groups with 10 min of ischemia. Amongst the groups with 5 min of ischemia, the mildest injury was found in the ovariectomized animals, which lost only 32% of their CA1 pyramidal cells. In comparison, mean cell losses were 54% and 49%, respectively, in intact females and in ovariectomized animals with estradiol replacement. Linear regression analysis demonstrated a highly significant relationship between cell loss and plasma estradiol levels. The mechanism by which exogenous and endogenous estrogen exacerbated the injury is unclear, as estrogen has many neuroprotective effects. On the other hand, many other reported effects of estrogen in hippocampal area CA1 might confer increased sensitivity to ischemia, either by modulating the excitatory effects of glutamate or by modifying the inhibitory effects of GABA. Determining how to modulate the various competing effects of estrogen is of both theoretical and practical importance, as it is now clear that one cannot assume that estrogen administration will always improve outcome in cerebral ischemia.