LPS fever in old rats depends on the ambient temperature

In earlier work, we found that following lipopolysaccharide (LPS) injection at an ambient temperature (Ta) of 23 degrees C, old rats developed blunted fevers compared with those of young rats. However, the old rats did become febrile if placed in a thermally graded alleyway: they spent more time in the warm end of the gradient and developed a significantly higher body temperature (Tb) than they did following saline injections. In the present experiments, we maintained old and young rats for 3 days at 20 or 31 degrees C (the Ta preferred by the old rats given LPS). After LPS (50 microg/kg ip), the young rats developed equivalent fevers at both Ta's. The old rats developed fevers that were equivalent to those of the young rats at 31 degrees C. At 20 degrees C, their fever was significantly lower. These results suggest that Ta plays a decisive role in the ability of old rats to mount febrile responses.