Heat adaptation patterns in infant rats during the first postnatal period.

In 129 infant rats divided into two groups housed to a constant ambient temperature of 21 degrees C, dynamics of body temperature and hypothermic reaction were studied. Beginning with day 1, the experimental group (66 animals), was exposed 15 min. daily for four weeks to an ambient temperature of +40 degrees C. Rectal temperature, temperature of hair-covered skin areas and tail temperature were monitored in either group. A statistically significant difference, which increased with age, between rectal and skin temperature, was found in experimental 3-week-old rats, while in control animals only in 45-day-old ones. Moreover, rectal and skin temperatures in the experimental group were significantly higher than in control group. The body-temperature difference between the two groups disappeared 17 days after cessation of exposure to heat. The hypothermic reaction induced by an weekly 2-min exposure to an ambient temperature of -1 degrees C, was significantly more important and extensive, in the experimental group than in the control one, during the first week of life only. The changes observed suggested a retarded development of the pituitary-corticoadrenal axis.