Fever response in the guinea pig before and after parturition.

The febrile response to an intramuscular injection of bacterial endotoxin (E. coli 4 microgram/kg) was tested in guinea pigs at the end of pregnancy in the time period extending from 8 days before until 3 days after parturition. In comparison to non-pregnant female controls both fever height and fever index were reduced in mother guinea pigs one week before parturition. This response was gradually reduced and reached its minimum on the last day before parturition. Immediately after parturition the fever response was still suppressed in mother animals as well as in newborns. Several hours after birth the fever response increased again in both groups of animals. The onset time and duration of fever were, however, shorter than in controls. The full fever responsivity was not reached until several days postpartum. Apparently the guinea pig develops an active antipyresis during the last phase of pregnancy. This resembles the suppression of fever in ewes at term of pregnancy where endogenous arginine-vasopressin has been proposed as an antipyretic agent. The vasopressinergic neuronal systems have therefore been localized by immunohistochemical methods in the brains of the guinea pigs whose responses to bacterial endotoxin were studied. These studies, which are described in detail in a following paper, support the involvement of vasopressin in natural antipyresis in the guinea pig.