When rats, acclimated to an ambient temperature (Ta) of 29°C, are exposed to 10% O2 for 63 h, the circadian rhythms of body temperature (Tb) and level of activity (La) are abolished, Tb falls to a hypothermic nadir followed by a climb to a hyperthermic peak, Laremains 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 Ta in these hypoxic-induced responses, Tb and La 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 Tb after onset of hypoxia were inversely proportional to Ta, whereas magnitudes and rates of Tb climb after the hypothermic nadir were directly proportional to Ta. No hyperthermia, so prominent at 29°C, occurred at 25 or 21°C. The hypoxic depression of La was least at 21°C and persisted throughout the hypoxia. In contrast, Ta was a strong determinant of the magnitudes and time courses of the initial fall and subsequent rise in Tb. 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.
Temperature-Induced Changes in Reperfused Stroke: Inflammatory and Thrombolytic Biomarkers