Dynamic responses to acute heat stress between 34 degrees C and 38.5 degrees C, and characteristics of heat stress response in mice.

We focused on dynamic responses to acute heat stress between 34 degrees C and 38.5 degrees C. Physiological and neuroendocrinological changes between 34 degrees C and 38.5 degrees C were studied in mice. The influence of humid conditions, 85% relative humidity (RH), on these changes was also investigated. Rectal temperatures increased above 34 degrees C and hematocrit levels increased at 38.5 degrees C 85% RH for 60 min. Food consumption and body weight gains decreased after a daily 60 min exposure to 34, 37 and 38.5 degrees C for 2 weeks. The corticosterone and vasopressin levels in the blood, and catecholamine and serotonin metabolite levels in the hypothalamus were not changed at 34 degrees C, but increased when above 37 degrees C for 60 min. Above 37 degrees C, these physiological and neuroendocrinological changes were accelerated by humid conditions. These results indicated that food consumption and body weight gains decreased above 34 degrees C, and the neuroendocrinological changes, which were accelerated by humid conditions, were induced above 37 degrees C. In comparison with restraint and water immersion stress, heat stress at 37 degrees C 85% RH showed a slower increase in serum corticosterone levels, smaller changes in plasma dopamine and dihydroxyphenylacetic acid levels, and, after repeated exposure, larger decreases in food consumption and body weight gains. This study clarified the relationships between temperature and humidity conditions and physiological and neuroendocrinological changes, along with the characteristics of responses in acute heat stress.