17 beta-estradiol reduces stroke injury in estrogen-deficient female animals

Background and Purpose-The importance of postmenopausal estrogen replacemen t therapy for stroke in females remains controversial. We previously showed that female rats sustain less infarction in reversible middle cerebral art ery occlusion (MCAO) than their ovariectomized counterparts and that vascul ar mechanisms are partly responsible for improved tissue outcomes. Furtherm ore, exogenous estrogen strongly protects the male brain, even when adminis tered in a single injection before MCAO injection. The present study examin ed the hypothesis that replacement of 17 beta-estradiol to physiological le vels improves stroke outcome in ovariectomized, estrogen-deficient female r ats, acting through blood flow-mediated mechanisms. Methods-Age-matched, adult female Wistar rats were ovariectomized and treat ed with 0, 25, or 100 mu g of 17 beta-estradiol administered through a subc utaneous implant or with a single Premarin (USP) injection(1 mg/kg) given i mmediately before ischemia was induced (n = 10 per group). Each animal subs equently underwent 2 hours of MCAO by the intraluminal filament technique, followed by 22 hours of reperfusion. Ipsilateral parietal cortex perfusion was monitored by laser-Doppler flowmetry throughout ischemia. Cortical and caudate-putamen infarction. volumes were determined by 2,3,5-triphenyltetra zolium chloride staining and digital image analysis. End-ischemic regional cerebral blood flow was measured in ovariectomized females with 0- or 25-mu g implants (n=4 per group) by C-14-iodoantipyrine quantitative autoradiogr aphy. Results-Plasma estradiol levels were 3.0+/-0.6, 20+/-8, and 46+/-10 pg/mL i n the 0-, 25-, and 100-mu g groups, respectively. Caudate-putamen infarctio n (% of ipsilateral caudate-putamen) was reduced by long-term, 25-mu g estr ogen treatment (13+/-4% versus 31+/-6% in the 0-mu g group, P<0.05, and 22/-3% in the 100-mu g group). Similarly, cortical infarction (% of ipsilater al cortex) was reduced only in the 25-mu g group (3+/-2% versus 12+/-3% in the 0-mu g group, P<0.05, and 6+/-3% in the 100-mu g group. End-ischemic st riatal or cortical blood flow was not altered by estrogen treatment at the neuroprotective dose. Infarction volume was unchanged by acute treatment be fore MCAO when estrogen-treated animals were compared with saline vehicle-t reated animals. Conclusions-Long-term estradiol replacement within a low physiological rang e ameliorates ischemic brain injury in previously ovariectomized female rat s. The neuroprotective mechanism is flow-independent, not through preservat ion of residual ischemic regional cerebral blood flow. Furthermore, the the rapeutic range is narrow, because the benefit of estrogen in transient vasc ular occlusion is diminished at larger doses, which yield high, but still p hysiologically relevant, plasma 17 beta-estradiol levels. Lastly, unlike in the male brain, single-injection estrogen exposure does not salvage ischem ic tissue in the female brain. Therefore, although exogenous steroid therap y protects both male and female estrogen-deficient brain, the mechanism may not be identical and depends on long-term hormone augmentation in the fema le.