Stroke is a leading cause of permanent disability and death. It is well accepted that the principal mammalian estrogen (E2), 17-b estradiol, provides robust neuroprotection in a variety of brain injury models in animals of both sexes. E2 enhances neurogenesis after stroke in the subventricular zone; however, it is unknown if these cells survive long-term or enhance functional recovery. In this study, we examined stroke-induced neurogenesis in male, gonadally intact female, and ovariectomized female mice 2 and 6 weeks after stroke. Treatment with 17-b estradiol increased 5-bromo-2 0 -deoxyuridine-labeled cells at both time points in both the dentate gyrus and subventricular zone; the majority were colabeled with doublecortin at 2 weeks and with NeuN at 6 weeks. Stroke-induced neurogenesis was reduced in estrogen receptor knockout mice, as well as in mice lacking the gene for aromatase, which converts testosterone into E2. Improved behavioral deficits were seen in E2-treated mice, suggesting that E2-induced increases in poststroke neurogenesis contribute to poststroke recovery.

The effect of oestrogen replacement therapy (ERT) on stroke incidence and severity has been extensively debated. Clinical trials of ERT have demonstrated an increased risk of stroke in treated women, although the study participants were well past menopause when therapy was initiated. It has been suggested that detrimental effects of ERT may be unmasked after prolonged periods of hypoestrogenicity. To date, very few studies have examined the effect of ERT in aged animals, although the timing of replacement may be critical to the neuroprotective effects of ERT. We hypothesised that chronic ERT initiated in late middle age would decrease infarct size in the brain after an induced stroke, whereas acute ERT would have no beneficial effects in aged females. To test this hypothesis, two paradigms of ERT were administered to aged mice of both sexes aiming to determine the effects on stroke outcome and to explore the possible mechanisms by which ERT interacts with age. Female mice that received chronic ERT from 17–20 months of age showed improved stroke outcomes after experimental stroke, whereas females that had acute ERT initiated at 20 months of age did not. Chronic ERT females exhibited diminished levels of nuclear factor kappa B (NF-κB) translocation compared to acute ERT females after stroke. Acute ERT females demonstrated both an increase in nuclear NF-κB and enhanced expression of pro-inflammatory cytokines. In addition, a sexual dimorphic effect of ERT was seen because males benefited from ERT, regardless of the timing of initiation. Aged males had significantly reduced expression of pro-inflammatory markers after stroke compared to age-matched females, suggesting a pro-inflammatory milieu emerges with age in females. These results are consistent with the emerging clinical literature suggesting that ERT should be initiated at the time of menopause to achieve beneficial effects. The present study demonstrates the importance of using appropriate animal models in preclinical studies.