Failure of pituitary transplants to facilitate the onset of maternal behavior in ovariectomized virgin rats

Ovariectomized virgin female rats received two pituitary glands grafted under a kidney capsule in an attempt to assess the effect of chronically increased plasma levels of prolactin on the onset of maternal behavior. When exposed to fresh foster pups on 6 consecutive days, only 5 out of 12 of these females and 2 out of 12 females in an ovariectomized control group displayed maternal behavior, whereas all 4 rats in an additional control group of lactating mothers were maternal throughout these tests. Ovariectomized animals which failed to become maternal then received subcutaneous injections of either 10 μg estradiol benzoate (EB) or oil on two consecutive days, and 1 day thereafter foster pups were again presented to these females for 18 hr. Significantly more EB-treated females displayed maternal behavior, regardless of whether or not they bore pituitary grafts, suggesting that estradiol and not prolactin may be essential for the rapid onset of maternal behavior in parturient rats.