Fever and behavioral thermoregulation in young and old rats.

At standard laboratory ambient temperatures (T(a)) of 20-24 degrees C, peripheral injections of lipopolysaccharide (LPS) reliably produce fever in young rats. In contrast, old rats may show a blunted fever, no fever, or even hypothermia after LPS. In the present study we hypothesized that old rats might use behavioral thermoregulation to help them develop a fever. Young and old rats were implanted with temperature transmitters. At least 1 wk postoperatively they were placed in a thermally graded alleyway (T(a) 10-40 degrees C). On the third and sixth day they were taken out of the gradient, placed at an T(a) of 23 degrees C, injected intraperitoneally with LPS or saline, and left at 23 degrees C for 3 h. At the end of that time, all young rats had become febrile, whereas the old rats had not. When the rats were replaced in the thermal gradient, the young animals continued to develop a fever that was similar to fever in young rats left at 23 degrees C. The old animals chose significantly warmer positions in the thermal gradient than did the young animals and only then became febrile. Although there was a tendency for the young rats to prefer higher T(a) after LPS than after saline, these differences were not significant. However, the differences in the old rats were significant. These results suggest that the LPS had increased the thermal set point in the old rats, but they could develop febrile responses only at the warm T(a) they selected.