Rats have an attenuated febrile response to intravenous endogenous pyrogen [e.g., interleukin-1 (IL-1)] near the term of pregnancy. The present experiments were carried out on 25 nonpregnant and 32 pregnant rats to test the hypothesis that arginine vasopressin functioning as an endogenous antipyretic substance in the central nervous system mediates this attenuated febrile response. An intravenous injection of recombinant rat IL-1 (rrIL-1) after intracerebroventricular vehicle produced a significant increase in core temperature in both nonpregnant and pregnant animals, the magnitude and duration of which was greater in the nonpregnant rats. In nonpregnant rats, intravenous rrIL-1 after intracerebroventricular vasopressin V₁-receptor antagonist accentuated the core temperature response compared with that observed with intravenous rrIL-1 after intracerebroventricular vehicle. In pregnant animals, however, intravenous rrIL-1 after intracerebroventricular vasopressin V₁-receptor antagonist produced a decrease in core temperature rather than an increase in core temperature, which was observed with intravenous rrIL-1 after intracerebroventricular vehicle. Thus our data do not support the hypothesis that a pregnancy-related activation of arginine vasopressin as an endogenous antipyretic substance in the central nervous system attenuates the febrile response to intravenous rrIL-1 near the term of pregnancy in rats.

endogenous antipyretic; endogenous pyrogen; interleukin

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