ENDOCRINE EFFECTS OF PREGNANCY AND EXPOSURE TO A SIMULATED OPEN-FIELDIN RATS

In adult rats, exposure to a novel environment, such as a simulated op en field, elicits an increase in body core temperature. We have recent ly shown that this response is attenuated in midpregnancy and abolishe d at term of pregnancy in rats. We postulated that this gestation-depe ndent response resulted from alterations in the hypothalamic-pituitary -adrenal axis. To test this hypothesis, we measured the effects of pre gnancy on renin, corticosterone, and arginine vasopressin (AVP) respon ses to exposure to either a simulated open field (30 or 120 min) or to the home cage (30 or 120 min) in rats. Pregnancy increased renin and corticosterone levels but not plasma AVP levels. Exposure to an open f ield decreased renin and increased plasma AVP levels in nonpregnant ra ts and on days 15 and 20 of gestation in pregnant rats, compared with home cage responses. Serum corticosterone levels were elevated after e xposure to an open field in nonpregnant and pregnant rats, compared wi th home cage rats, the effect being more prolonged on day 20 of gestat ion. These observations provide new information on endocrine changes d uring pregnancy in rats and may help to explain the attenuated stress- induced hyperthermic response to exposure to a novel environment seen near term of pregnancy.