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 resp onse in rats, as well as level of activity (L-a), to prolonged (63 h) hypox ia at rat thermoneutral temperature (29 degrees C). Mini Mitter transmitter s were implanted in the abdomens of 10 adult Sprague-Dawley rats to continu ously record body temperature (T-b) and L-a. After habituation for 7 days t o 29 degrees C and 12:12-h dark-light cycles, 48 h of baseline data were ac quired from six control and four experimental rats. The mean Tb for the gro up oscillated from a nocturnal peak of 38.4 +/- 0.18 degrees C (SD) to a di urnal nadir of 36.7 +/- 0.15 degrees C. Then the experimental group was swi tched to 10% O-2 in N-2. The immediate T-b response, phase I, was a disappe arance 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 perio d, phase IV, T-b increased 1.1 +/- 0.25 degrees C. Before hypoxia, L-a decr eased 70% from its nocturnal peak to its diurnal nadir and was entrained wi th T-b. With hypoxia L-a decreased in phase I to essential quiescence by ph ase II. L-a had returned, but only to a low level in phase III, and was dev oid of any circadian rhythm. L-a resumed its circadian rhythm on reoxygenat ion. 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) t he 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 thermoneutra lity has several phases and includes both hypothermic and hyperthermic comp onents.