The goal of this application is to determine the mechanisms by which reproductive aging and estrogen replacement alter the inflammatory response and consequently the neuronal environment. In a series of studies, we have established that estrogen replacement to young adult animals increases trophic support in the forebrain and attenuates inflammation following neural injury. However estrogen replacement at reproductive senescence, which is physiologically akin to menopause, fails to increase trophic factors and paradoxically, increases inflammatory mediators following neural injury. Collectively these data suggest that the timing of estrogen replacement in relation to reproductive aging may critically determine whether estrogen has a benign or deleterious outcome. Our central hypothesis is that the age-related decline in endogenous hormones triggers compensatory changes in estrogen receptor systems in specific immune cells, thus increasing the central and peripheral inflammatory response. This hypothesis will be tested in three Specific Aims, using animal and human tissue models that span the reproductive spectrum, namely, normally cycling (pre-menopause). irregularly cycling (perimenopause) and reproductive senescent (postmenopause). In Specific Aim 1. we will test the hypothesis that permissive changes in the blood brain barrier will cause a more rapid and robust neural inflammation in reproductive senescent animals as compared to normally cycling or irregularly cycling animals. Animals will be injected systemically with the bacterial pathogen lipopolysaccharide (LPS) and inflammatory mediators will be measured in peripheral organs and the brain. Additionally, we will examine endothelial cells of the blood-brain barrier for reproductive age-related changes in this barrier. In Specific Aim 2. we will determine if the inflammatory response of peripheral blood mononuclear cells (PBMC) is affected by clinically-relevant variables namely, the route of hormone administration (oral versus transdermal) and diet (regular versus high cholesterol). The Response Quotient, derived from an ex vivo LPS challenge assay, will be measured in rat and human blood samples to determine if salient lifestyle variables increase the risks associated with reproductive aging. Finally, in Specific Aim 3 we will test the hypothesis that compensatory alterations of the estrogen receptor system, resulting from ovarian decline, is a principal mechanism underlying estrogen's deleterious effects in reproductive senescence. Changes in the pattern and levels of estrogen receptor (ER)-alpha will be evaluated by immunohistochemistry and Western blots, while functional changes will be evaluated using signaling arrays. Human and rodent PBMC's and rodent cerebral endothelial cells from each reproductive stage will be studied. Collectively, these studies will test the hypothesis that in order for estrogen replacement to be beneficial, therapy must be initiated before compensatory responses to ovarian decline.