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"Effects of estrogen on thermoregulation in ovariectomized rats: core temperature and the thermoneutral zone."
Hot flushes, a disorder of hypothalamic thermoregulation, are the most common reason for postmenopausal women to seek estrogen replacement therapy. Exposure to a warm environment provokes hot flushes in symptomatic postmenopausal women, suggesting that estrogen withdrawal alters the sensitivity of thermoregulatory networks such that heat loss mechanisms are activated at lower ambient temperatures. To test this hypothesis, we measured the effects of estrogen on core and tail skin temperatures at a range of ambient temperatures in ovariectomized rats. A major goal of this study was to adapt the method of Romanovsky et al. (J Appl Physiol 92: 2667, 2002) to study the effects of estrogen on the thermoneutral zone (TNZ) in freely moving rats. This method estimates the TNZ as the range of ambient temperatures where the cutaneous blood vessels actively fluctuate between dilatation and constriction, as measured by tail skin temperature. Twenty-four rats were ovariectomized and administered subcutaneous capsules that were either empty (OVX rats) or containing estradiol dissolved in sesame oil (OVX + E rats). Core and tail skin temperatures were recorded by Subcue Datalogers and IT-18 thermocouples, respectively, in an environmental chamber for 24 ambient temperatures ranging from 13-34 degrees C. Core temperatures were not significantly different between the two groups at most ambient temperatures. However, at the ambient temperature of 32.5 degrees C and above, the OVX rats had significantly elevated core temperatures compared to the OVX + E rats. Interestingly, calculations of tail skin temperature fluctuations showed that the TNZ of OVX rats was shifted nearly 4 degrees C lower compared to that of OVX + E rats. Our results show an impairment of core temperature regulation at high ambient temperatures in OVX rats without estrogen replacement. The shift in the TNZ to lower ambient temperatures in OVX rats supports the hypothesis that estrogen withdrawal leads to an increase in the sensitivity of thermoregulatory networks to warm ambient temperatures.
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