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

When rats, acclimated to an ambient temperature (T_a) of 29°C, are exposed to 10% O₂ for 63 h, the circadian rhythms of body temperature (T_b) and level of activity (L_a) are abolished, T_b 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°C. 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°C, occurred at 25 or 21°C. The hypoxic depression of L_a was least at 21°C 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°C as well as a persisting hypothermia may protect the brain from overt pathology.