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

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