Sex hormones underlying 17a-Estradiol effects on neuroinflammation

17--estradiol (17aE2) treatment extends lifespan in male mice and can reduce neuroinflammatory responses in the hypothalamus of 12-month-old males. Although 17aE2 improves longevity in males, female mice are unaffected, suggesting a sexually dimorphic pattern of lifespan regulation. We tested whether the sex-specific effects of 17aE2 on neuroinflammatory responses are mediated by sex hormones and whether hypothalamic changes extend to other brain regions in old age. Manipulating sex hormone levels through gonadectomy, we show that sex-specific effects of 17aE2 on age-associated gliosis are brain region-specific and are partially dependent on gonadal hormone production. 17aE2 treatment started at 4 months of age protected 25-month-old males from hypothalamic inflammation. Castration prior to 17aE2 exposure reduced the effect of 17aE2 on hypothalamic astrogliosis. By contrast, sex-specific changes in microgliosis with 17aE2 were not significantly affected by castration in males. While 17aE2 treatment had no effect of hypothalamic astrocytes or microglia in intact females, ovariectomy significantly increased the occurrence of hypothalamic gliosis evaluated in 25-month-old females, which was partially reduced by 17aE2. In the hippocampus, both male and female gonadally-derived hormones influenced the severity of gliosis and the responsiveness to 17aE2 in a regiondependent manner. The male-specific effects of 17aE2 correlate with changes in hypothalamic ER expression, highlighting a receptor through which 17aE2 could act. The results of this study demonstrate that neuroinflammatory responses to 17aE2 are partially controlled by the presence of sex-specific gonads. Interactions between sex-steroids and neuroinflammation could, therefore, influence late-life health and disease onset, leading to sexual dimorphism in aging.