Central prolactin infusions stimulate maternal behavior in steroid-treated, nulliparous female rats

A series of experiments were conducted to determine whether and under what conditions central prolactin (PRL) administration would stimulate the onset of maternal behavior in female rats and to identify possible neural sites of PRL action. In each experiment ovariectomized, nulliparous rats whose endogenous PRL levels were suppressed with bromocriptine were tested for maternal behavior toward foster young. In experiments 1, 2, and 4, females were also exposed to pregnancy-like levels of progesterone (days 1-11) followed by estradiol (days 11-17). In experiment 1 infusions (days 11-13) of four doses of ovine PRL (400 ng, 2 micrograms, 10 micrograms, or 50 micrograms, but not 80 ng) into the lateral ventricle resulted in a rapid onset of maternal behavior (behavioral testing, days 12-17). The stimulatory action of these doses of PRL appears to be central, since subcutaneous injections of 50 micrograms of ovine PRL failed to affect maternal responsiveness (experiment 2). Experiment 3 indicated that the stimulatory effect of intracerebroventricularly administered PRL is steroid dependent. Infusions of either 10 micrograms of ovine PRL or 10 micrograms of rat PRL failed to induce maternal behavior in nonsteroid-treated animals. In the final experiment (no. 4) bilateral infusions of 40 ng of ovine PRL into the medial preoptic area of steroid-treated rats resulted in a pronounced stimulation of maternal behavior. These findings demonstrate a central site of PRL action in the stimulation of maternal responsiveness and point to the medial preoptic area as a key neural site for PRL regulation of maternal behavior.