Influence of pregnancy on the febrile response to intracerebroventricular administration of PGE1 in rats.

Rats have an attenuated or absent febrile response to exogenous pyrogen (e.g., bacterial endotoxin) and endogenous pyrogen (e.g., interleukin-1 beta) near term of pregnancy. The present experiments have been carried out on 19 nonpregnant and 18 time-bred pregnant Long-Evans rats to investigate the febrile response to intracerebroventricular (ICV) administration of prostaglandin E1 (PGE1). Each rat was anesthetized, a biotelemetry device was placed in the peritoneal cavity for measurement of body core temperature (Tbc), and guide cannulas were placed above the lateral cerebral ventricles for ICV injection of PGE1. At least 6 days were allowed to lapse between surgery and the experiments. ICV injection of 0.2 micrograms PGE1 produced significant increases in Tbc in both nonpregnant and pregnant animals (day 19 of gestation). The increase in Tbc as well as the fever index, however, were significantly attenuated in the pregnant compared with the nonpregnant rats. Vehicle had no effect on Tbc or fever index in either group of animals. The attenuated febrile response to PGE1 in the pregnant rats may have resulted from a pregnancy-related activation of endogenous antipyretics and/or impaired thermoregulatory effector mechanisms.