Effect of antipyretic drugs on circadian rhythm in body temperature of rats

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Abstract
In many species, including the laboratory rat, body temperature varies on a circadian (24 h) basis. There is considerable evidence that the circadian rise in body temperature is attributable to an elevation in thermoregulatory set point. We hypothesized that this rise in set point may be mediated by prostaglandins. If this hypothesis is correct, then it should be possible to block or reduce the nighttime rise in body temperature by the administration of prostaglandin synthesis inhibitors. Rats were injected with the prostaglandin synthesis inhibitors sodium salicylate, acetylsalicylic acid, and indomethacin at 5:00 P.M. and at 9:00 A.M. Administration of these drugs had little effect on body temperature during the day but caused a significant fall in body temperature at night when temperature is normally in the rising or plateau phase of the cycle. We conclude that prostaglandin synthesis is an important component of the circadian rise in body temperature in the rat. In addition, evidence is presented that there exists a cryogenic factor that opposes the nighttime prostaglandin-mediated rise in body temperature.