Individual differences in response to LPS and psychological stress in aged rats.

Old rats may show blunted fever or hypothermia after injection of lipopolysaccharide (LPS), a fever-producing agent, and have a reduced body temperature (Tb) rise in response to psychological stress. These results may partly be a consequence of aging per se, partly a sex difference, and partly an effect of differences in types and doses of pyrogen. Here we tested age and gender differences in Tb responses to 30-min exposure to a novel environment and to injection of several doses of LPS. There were age-related reductions in novelty-induced hyperthermia, and some old rats even became hypothermic. Sensitivity to the pyrogenic activity of LPS and to the toxic effects of endotoxin (manifested by hypothermia) both increased in aged female rats. A major finding was that there were no correlations between age-related changes in Tb in response to novelty and to LPS injection. Tb responses in aged rats were variable; in each situation, there were old rats whose Tb rose as high as did younger ones. We did not observe significant gender differences in response either to novelty or to LPS in young or old rats.