Nocturnal shifts in thermal and metabolic responses of the immature rat

Immature rats were tested at 2, 7, 11, and 15 days of age to determine steady-state thermoregulatory responses during light (L) and dark (D) phases of the daily cycle. Pups were housed with dams in a vivarium illuminated from 0700 to 1900 h. During each phase tests began approximately 1 h after the change in the light conditions of the vivarium. Duration of each test was approximately 7 h. Rats were tested individually in temperature-controlled cylinders at ambient temperatures (Ta) = 25.0, 30.0, 32.5, and 35.0 degrees C. Both colonic (Tco) and tail skin temperatures of each animal were measured continuously. O2 content of effluent air from each cylinder was determined to provide an estimate of metabolic rate (M). Immature rats, at 2 to 11 days of age, exhibited significant L:D differences in M and Tco. However, no significant L:D differences in these responses were noted at 15 days of age. In every case, nocturnal increases in Tco were associated with a rise in M. L:D differences in Tco response were not attributed to a significant change in total thermal conductance. These data support the conclusion that the immature rat exhibits daily variation in metabolic rate, which is the primary contributor to L:D shifts in Tco.