Effect of different ambient temperature and lipopolysaccharide administration on the circadian rhythm of rectal temperature and hypothalamic PGE₂ production in aged male rats

Old and young male rats (22 and 7 months respectively) were exposed to ambient temperatures of 4 22, 27 and 35°C. The rats' rectal temperatures (RTs) were measured periodically, after exposure to the varying temperatures at different hours during the day. The mean circadian value of RTs in the aged rats was different from that of the young rats. Whereas exposure to low temperatures caused a decrease of 2.0°C in the RTs of the old rats, exposure to heat (35°C) caused an increase of 1°C in their RTs. An injection of 200 μg (intraperitoneally) of E. coli lipopolysaccharide caused them to experience a long period of hypothermia. Elevation in the RTs after the hypothermic period ended was significantly lower in the old rats. However no significant differences in hypothalamic PGE₂ production were to be found between the old and young groups 24 h after pyrogen administration.