Melatonin treatment counteracts the hyperthermic effect of lipopolysaccharide injection in the Syrian hamster

The present study examined the acute response in body temperature to lipopolysaccharide (LPS) injection to Syrian hamsters at two time intervals during the light-dark cycle. Its modification by melatonin (MT) administration in the drinking water was also assessed. Hamsters were intraperitoneally (i.p.) implanted with a transmitter to measure core body temperature. MT was administered from day 8 post-surgery until the end of experiment. On day 16 after surgery, LPS or saline was injected i.p. at the beginning of the light phase (ZT 0) or of the scotophase (ZT 14). At ZT 0, LPS increased core body temperature, an effect that persisted for at least 5 h and that was blunted by MT administration. At ZT 14, the hyperthermic effect of LPS was absent. Rather, at ZT 14 the animals showed increases in core body temperature following saline or LPS during the first 2 h after injection only, which were significantly less intense in LPS-treated animals. MT administration blunted this difference. Five days after injection, hamsters that had received LPS at ZT 0 showed an increase in the mesor of core body temperature rhythm as compared to saline. This effect was suppressed by MT administration. The results demonstrate that MT prevents body temperature increase after LPS at ZT 0.