ESTROGEN AND CEREBRAL VASODILATION

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The mechanisms involved in estrogen-related brain protection are unclear, but based on findings in the periphery, improved vasodilating capacity through upregulation of a constitutive nitric oxide synthase (cNOS) is an attractive possibility. The proposed studies are designed to examine the effects of estrogens on cerebral vasodilating function and the role of NO in this regard. Results will be compared in 17 beta- estradiol (E2)-treated and untreated ovariectomized (OVX) rats and normal females. There are 3 principal aims. For aim 1, the studies will evaluate: a) No-dependent cerebral vasodilatory responses, in the 3 groups, via monitoring changes in pial arteriolar diameters during suffusions of a NO donor (SNAP), a NO-independent vasodilator (adenosine), and endothelial cNOS (eNOS) and neuronal cNOS (nNOS)- dependent vasodilators (acetylcholine and NMDA, respectively); b) NOS activity and eNOS/nNOS mRNA and protein expression in the 3 groups; c) dose (0.01 to 5 mg kg -1 day) and time (1-14 days) -related changes in eNOS/nNOS activity and expression and vasodilating function associated with E2 treatment and withdrawal in OVX rats; d) variations in cNOS isoform expression and NO-dependent vasodilation that may occur during the estrous cycle in normal females; and e) using estrogen receptor (ER) antagonists (1C1 182780, tamoxifen--co-administered with E2), whether ERs are involved in E2-related changes in cNOS expression/function. In addressing aim 2, experiments will examine acute dose-dependent effects of E2 suffusions on pial arteriolar relaxation in the presence and absence of NOS inhibition and in the presence and absence of ER antagonists. NOS isoform specificity will be tested using the nNOS inhibitor, ARL 17477AR and the non-specific inhibitor, nitro-L-arginine (L-NA) eNOS contributions are estimated by difference. For aim 3, a transient forebrain ischemia (TFI) model will be used (carotid occlusion + hypotension). Using laser-Doppler flowmetry (LDF), intra-ischemic cerebral cortical blood flow (CCBF) changes will be monitored and compared in the 3 groups, followed by postischemic assessments of neuropathology. As for aim 1 studies, the data will be evaluated with respect to estrous cycle stage (normals), time and dose-related effects of E2 treatment and withdrawal (OVX), and ER antagonist effects (chronic co-administration with E2). The relative roles of nNOS vs eNOS in contributing to intra-ischemic vasodilation (using iv ARL 17477AR and L- NA) will also be examined. In several groups (i.e., those showing the greatest CCBF fall during ischemia [e.g., untreated OVX]), the intra- ischemic CCBF reductions (instead of arterial pressure) will be matched to that observed in the OVX group showing the greatest E2 treatment benefit. These results should provide important information about the cerebrovascular influences of estrogen.