Circadian rhythms of body temperature and activity levels during 63 h of hypoxia in the rat.

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Abstract
The hypothermic response of rats to only brief (∼2 h) hypoxia has been described previously. The present study analyzes the hypothermic response in rats, as well as level of activity (La), to prolonged (63 h) hypoxia at rat thermoneutral temperature (29°C). Mini Mitter transmitters were implanted in the abdomens of 10 adult Sprague-Dawley rats to continuously record body temperature (Tb) and La. After habituation for 7 days to 29°C and 12:12-h dark-light cycles, 48 h of baseline data were acquired from six control and four experimental rats. The mean Tb for the group oscillated from a nocturnal peak of 38.4 ± 0.18°C (SD) to a diurnal nadir of 36.7 ± 0.15°C. Then the experimental group was switched to 10% O2 in N2. The immediate Tb response, phase I, was a disappearance of circadian rhythm and a fall in Tb to 36.3 ± 0.52°C. In phase II, Tb increased to a peak of 38.7 ± 0.64°C. In phase III, Tb gradually decreased. At reoxygenation at the end of the hypoxic period, phase IV, Tb increased 1.1 ± 0.25°C. Before hypoxia, La decreased 70% from its nocturnal peak to its diurnal nadir and was entrained with Tb. With hypoxia La decreased in phase I to essential quiescence by phase II. La had returned, but only to a low level in phase III, and was devoid of any circadian rhythm. La resumed its circadian rhythm on reoxygenation. We conclude that 63 h of sustained hypoxia 1) completely disrupts the circadian rhythms of both Tb and La throughout the hypoxic exposure, 2) the hypoxia-induced changes in Tb and La are independent of each other and of the circadian clock, and 3) the Tb response to hypoxia at thermoneutrality has several phases and includes both hypothermic and hyperthermic components.