Endocrine response to acute cold exposure by lactating and non-lactating Norway rats.

Plasma levels of corticosterone, prolactin and thyroxine (T4) were measured in lactating and non-lactating Norway rats at 22 degrees C and 4 degrees C. Acute cold exposure increased corticosterone secretion in all groups, although non-lactating female levels rose higher than those of mother rats. While prolactin levels are unaffected by acute cold exposure in non-lactating females, mothers with their litters had lower prolactin levels in the cold. T4 levels increased during cold exposure in lactating females, suggesting that the low T4 levels observed during lactation may not be due to lactational competition for available iodine.

Thermal control of mother-young contact in Norway rats: factors mediating the chronic elevation of maternal temperature.

The chronically elevated heat production of lactating Norway rats makes them vulnerable to acute hyperthermia during pup contact and thereby limits the duration of such interactions. High lactational levels of progesterone and corticosterone may act in concert to increase maternal heat load. Specifically, progesterone appears to increase maternal thermal set point and corticosterone is necessary for the increase in maternal heat production. Thyroid hormones and brown adipose tissue do not seem to contribute to the chronic increase in maternal heat production. While mammary tissue does contribute to maternal heat load, it is no more hypermetabolic than other maternal tissues.