Ambient temperature modulates hypoxic-induced changes in rat body temperature and activity differentially

When rats, acclimated to an ambient temperature (T-a) of 29 degreesC, are exposed to 10% O-2 for 63 h, the circadian rhythms of body temperature (T-b) and level of activity (L-a) are abolished, Tb falls to a hypothermic nadir followed by a climb to a hyperthermic peak, L-a remains depressed (Bishop B, Silva G, Krasney J, Salloum A, Roberts A, Nakano H, Shucard D, Rifkin D, and Farkas G. Am J Physiol Regulatory Integrative Comp Physiol 279: R1378-R1389, 2000), and overt brain pathology is detected (Krasney JA, Farkas G, Shucard DW, Salloum AC, Silva G, Roberts A, Rifkin D, Bishop B, and Rubio A. Soc Neurosci Abstr 25: 581, 1999). To determine the role of T-a in these hypoxic-induced responses, T-b and L-a data were detected by telemetry every 15 min for 48 h on air, followed by 63 h on 10% O2 from rats acclimated to 25 or 21 degreesC. Magnitudes and rates of decline in T-b after onset of hypoxia were inversely proportional to T-a, whereas magnitudes and rates of T-b climb after the hypothermic nadir were directly proportional to T-a. No hyperthermia, so prominent at 29 degreesC, occurred at 25 or 21 degreesC. The hypoxic depression of L-a was least at 21 degreesC and persisted throughout the hypoxia. In contrast, T-a was a strong determinant of the magnitudes and time courses of the initial fall and subsequent rise in T-b. We propose that the absence of hyperthermia at 21 and 25 degreesC as well as a persisting hypothermia may protect the brain from overt pathology.