Fever and behavioral thermoregulation in young and old rats

At standard laboratory ambient temperatures (T-a) of 20-24 degreesC, periph eral injections of lipopolysaccharide (LPS) reliably produce fever in young rats. In contrast, old rats may show a blunted fever, no fever, or even hy pothermia after LPS. In the present study we hypothesized that old rats mig ht use behavioral thermoregulation to help them develop a fever. Young and old rats were implanted with temperature transmitters. At least 1 wk postop eratively they were placed in a thermally graded alleyway (T-a 10-40 degree sC). On the third and sixth day they were taken out of the gradient, placed at an T-a of 23 degreesC, injected intraperitoneally with LPS or saline, a nd left at 23 degreesC 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 tha t was similar to fever in young rats left at 23 degreesC. The old animals c hose significantly warmer positions in the thermal gradient than did the yo ung 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 dif ferences were not significant. However, the differences in the old rats wer e significant. These results suggest that the LPS had increased the thermal set point in the old rats, but they could develop febrile responses only a t the warm T-a they selected.