Endothelin-A receptors and NO mediate decrease in arterial pressure during recovery from restraint

Emotional Stress and Cardiovascular Complications in Animal Models: A Review of the Influence of Stress Type

Emotional stress has been recognized as a modifiable risk factor for cardiovascular diseases. The impact of stress on physiological and psychological processes is determined by characteristics of the stress stimulus. For example, distinct responses are induced by acute vs. chronic aversive stimuli. Additionally, the magnitude of stress responses has been reported to be inversely related to the degree of predictability of the aversive stimulus. Therefore, the purpose of the present review was to discuss experimental research in animal models describing the influence of stressor stimulus characteristics, such as chronicity and predictability, in cardiovascular dysfunctions induced by emotional stress. Regarding chronicity, the importance of cardiovascular and autonomic adjustments during acute stress sessions and cardiovascular consequences of frequent stress response activation during repeated exposure to aversive threats (i.e., chronic stress) is discussed. Evidence of the cardiovascular and autonomic changes induced by chronic stressors involving daily exposure to the same stressor (predictable) vs. different stressors (unpredictable) is reviewed and discussed in terms of the impact of predictability in cardiovascular dysfunctions induced by stress.

Does nitric oxide act in the ventromedial preoptic area to mediate oestrogen negative feedback in the seasonally anoestrous ewe?

Seasonal anoestrus in the ewe results from enhanced oestrogen negative feedback. Recent data have implicated the ventromedial preoptic area (vmPOA) as an important site of oestrogen action. This study addressed whether NO acts within the vmPOA to inhibit LH during seasonal anoestrus. In Experiment 1, microimplants containingNω-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor),S-methyl thiocitrulline (SMTC, neural NOS (nNOS) inhibitor) or empty implants (control) were administered during mid-anoestrus to the vmPOA.l-NAME, but not SMTC, significantly increased LH pulse frequency. For Experiment 2, ewes in late anoestrus were administered 7-nitroindazole (7NI; nNOS inhibitor),l-NAME, SMTC, or empty implants. 7NI, but notl-NAME or SMTC, increased LH pulse frequency. In Experiment 3, the effects of microimplants and microinjections ofl-NAME were compared in mid-anoestrus. Microinjections ofl-NAME (300 nl at 10 μg/μl) increased LH pulse frequency, but microimplants did not. In late anoestrus, similar microinjections were ineffective. Taken together, the results of Experiments 1–3 suggested that NO inhibition may be stronger during the middle than at the end of seasonal anoestrus. To test this hypothesis, ewes in Experiment 4 received microinjection ofl-NAME or vehicle thrice during the non-breeding season; none of the treatments increased LH pulse frequency. These results indicate that NO plays a role in the vmPOA in suppressing LH secretion during seasonal anoestrus because NOS inhibitors were consistently stimulatory when LH pulse frequency was low. However, the inconsistent and modest effects of these inhibitors suggest that NO actions in this area cannot completely account for the effects of inhibitory photoperiod.

Closed-loop analysis of cardiovascular variability in rats under restraint stress

Causal transfer function analysis was applied to the heart rate variability and blood pressure variability in normotensive male Sprague-Dawley rats those were measured before, during, and after acute restraint stress. The causal transfer gain (CTG) from systolic blood pressure (SBP) to RR interval (RRI) and CTG from RRI to SBP were estimated. The mean value of the CTG from SBP to RRI in the low-frequency (LF) band (0.27-0.74 Hz) was significantly lower during the restraint period than during the baseline period and remained significantly lower during the recovery period. The mean value of the CTG from RRI to SBP in the LF band, in contrast, was significantly higher during the restraint period than during the baseline period, and during the recovery period it tended to return relatively rapidly to the baseline level. Arterial pressure tended to increase only at the onset of the restraint stress and then to decline not only during the restraint period but also during the recovery period, finally leveling off significantly below the baseline level. These results suggest that restraint stress suppresses the baroreflex control gain not only during the restraint period but also during the recovery period and that this results in the arterial hypotension during the recovery period.

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 NG-nitro-l-arginine (88 microg/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.