Abstract
The effect of short and repetitive exposure to cold (5°C, 4 hr/day for 2 weeks) from the birth up to the 14th day of newborn rats on the thermal regulation in adulthood and on the tolerance to cold was investigated. After being exposed to cold, they were transferred to a room at 25°C (N-CA). The control rats were raised at 25°C (N-WA). An acute cold exposure test was performed by placing the animals in a room at 5°C under urethane anesthesia. Electrical activity of neck muscles as an index of shivering was recorded.
The colonic temperature fell at a significantly slower rate in N-CA rats with less shivering than in N-WA ones. Nonshivering thermogenesis tested by norepinephrine was significantly greater in N-CA rats than in N-WA ones. These results suggest that N-CA rats developed improved cold tolerance accompanied by greater nonshivering thermogenesis. Such a phenomenon in N-CA lasted for 18 weeks after the termination of cold exposure. Adult rats subjected to the same scheme of cold exposure (A-CA)(5°C, 4 hr/day, 2 weeks) showed essentially the same results as seen in N-CA, but its improved cold tolerance and elevated nonshivering thermogenesis disappeared 4 weeks after the termination of cold exposure. Extirpation of interscapular brown adipose tissue immediately before the cold test did not appreciably affect the cold tolerance in N-CA and A-CA rats. The colonic temperature at the onset of shivering was significantly lower in N-CA as well as A-CA rats than in each of the corresponding control rats, indicating a shift of the shivering threshold to lower temperature values in the animals exposed intermittently to cold.
These results indicate that an infantile experience with cold results in a greater and longer sustained ability to tolerate cold in adulthood, characterized by enhanced nonshivering thermogenesis.
