Nitric-oxide-dependent pial arteriolar dilation in the female rat: effects of chronic estrogen depletion and repletion.

In this study, we compared endothelial nitric oxide synthase (eNOS)-mediated cerebral vasodilating responses in intact female rats, chronically ovariectomized (OVX) rats, and OVX rats treated for 2 weeks with 17beta-estradiol (E(2)). Under anesthesia, using intravital microscopy and a closed cranial window system, pial arteriolar diameter changes were monitored during sequential cortical suffusions of an eNOS-dependent dilator [acetylcholine (ACh)] and a direct NO donor [S-nitrosoacetylpenicillamine (SNAP)]. In separate rats from the same groups, we compared eNOS and caveolin-1 (CAV-1) protein abundance in pial arterioles (via immunofluorescence analyses). In untreated and low-dose E(2)-treated (1.0 microg x kg(-1) x day(-1)) OVX rats, ACh-induced vasodilations were virtually absent. High-dose E(2) treatment (100 microg x kg(-1) x day(-1)) restored ACh-induced pial arteriolar dilations to levels seen in intact females. The vasodilations elicited by SNAP and ADO were unaffected by chronic estrogen changes, indicating no direct estrogen influence on vascular smooth muscle (VSM) reactivity. Pial arteriolar eNOS protein abundance was diminished by ovariectomy and restored by high-dose E(2) treatment. Pial arteriolar CAV-1 expression was higher in OVX versus intact and E(2)-treated OVX females. These results suggest that long-term changes in estrogen directly influence brain eNOS functional activity. The estrogen-related changes in eNOS-dependent vasodilating function appear to be related, in part, to a capacity for E(2) to increase eNOS protein expression and, in part, to an E(2)-associated diminution in endothelial CAV-1 expression.

Cerebral vasodilating capacity during forebrain ischemia: effects of chronic estrogen depletion and repletion and the role of neuronal nitric oxide synthase.

The effects of chronic 17beta-estradiol (E2) depletion, via ovariectomy (OVX), and its repletion, on cortical cerebral blood flow (CBF) and EEG activities during forebrain ischemia, as well as post-ischemic recovery and neuropathology, were assessed and compared with results obtained in normal female rats. We also examined whether neuronal nitric oxide synthase (nNOS) activity is affected by OVX and E2 replacement and whether NOS-derived NO supports vasodilation during ischemia. OVX females displayed a significantly lower CBF during ischemia (10% of baseline) than did normal females (23% of baseline). In OVX rats, given chronic low-dose E2 treatment (0.1 mg kg(-1) day(-1)), intra-ischemic CBF was similar to normal females (25% of baseline). However, at supraphysiologic E2 doses (> or = 0.5 mg kg(-1) day(-1)), that benefit was diminished or lost. Intra-ischemic EEG power reductions and post-ischemic survival rates, neurological dysfunction, and histopathology displayed similar relative differences among groups as the CBF findings. Intra-ischemic CBF was reduced by nNOS inhibition, with ARL 17477, in normal and low-dose E2-treated OVX rats (4-8% baseline). The repressed intra-ischemic vasodilating function in OVX rats may be due to reductions in nNOS activity, because untreated OVX rats showed a 50% lower cortical nNOS activity than that in normal rats and in rats treated with low or high dose (5 mg kg(-1) day(-1)) E2. However, the inability to restore vasodilating function despite normalization of nNOS activity indicates that another mechanism is responsible for the repression of vasodilatory function in the high-dose group. These findings suggest that E2, at levels within the physiological range, promotes ischemic neuroprotection via improving vasodilating capacity. One possible mechanism may relate to E2 enhancing brain nNOS expression and activity.