NO mediates effects of estrogen on central regulation of blood pressure in restrained, ovariectomized rats

We tested the hypotheses that estrogen replacement in ovariectomized (OVX) rats attenuates cardiovascular responses to psychological stress and that nitric oxide (NO) in the brain mediates these effects. Female rats were OVX; one group received 17beta-estradiol (OVX-E) for 11-12 days and the other received vehicle (OVX-V). Seven days after OVX, OVX-E and OVX-V rats were chronically instrumented for arterial pressure measurements and intracerebroventricular injections. Later (4-5 days), OVX-E and OVX-V rats received intracerebroventricular injections of N-G-nitro-L-arginine (88 mug/kg), an inhibitor of constitutive NO production, or vehicle. Mean arterial pressure (MAP) and heart rate responses were then measured in conscious rats exposed to two cycles of 1-h restraint/1-h rest. We show that MAP responses in restrained OVX-E rats were attenuated both during restraint and during rest. Although inhibition of NO production in the brain had no effect on MAP responses to restraint in OVX-V rats, it augmented responses in restrained OVX-E rats, especially during periods of rest, so that MAPs in restrained OVX-E and OVX-V rats were indistinguishable. Finally, NO levels in hypothalami and brain stems were elevated in restrained OVX-E, but not OVX-V, rats compared with their respective unrestrained controls. These results show that estrogen replacement in OVX rats reduces arterial pressure responses to psychological stress and that these effects are mediated, at least in part, by NO.