Prolactin (PRL) regulation of maternal behavior in rats: bromocriptine treatment delays and PRL promotes the rapid onset of behavior.

Recent findings indicate that PRL helps stimulate the onset of maternal behavior in inexperienced hypophysectomized steroid-treated female rats. In a series of five experiments we have further examined the involvement of PRL in maternal behavior using nonhypophysectomized ovariectomized rats treated concurrently (type I) or sequentially (type II) with progesterone (P) and estradiol (E2) and administered either bromocriptine (to suppress endogenous PRL secretion) or bromocriptine plus ovine PRL. In Exp 1 plasma PRL concentrations were measured in ovariectomized rats treated for 2 weeks with a combination of E2 and P Silastic implants. Type I steroid-treated (2mm E2, days 1-24; three 30 mm P, days 3-13) rats exhibited elevated plasma PRL levels throughout the sampling period compared with nonsteroid-treated controls. In contrast, PRL concentrations in type II steroid-treated (P, days 3-13; E2, days 13-24) females were low (similar to controls) from days 3-13 when the type II steroid-treated females were exposed to P only. Like type I treated rats, PRL levels in type II steroid-treated rats were elevated from day 13 onward after E2 capsule insertion. In Exp 2, treatment of both type I and type II steroid-treated rats with bromocriptine (2 mg/kg, sc) twice daily beginning on treatment day 13 suppressed basal PRL concentrations and prevented the estrogen-induced diurnal PRL surge. Whereas PRL was effectively suppressed by bromocriptine in both steroid-treated groups, the absolute levels of PRL were lower in rats treated with the type II steroid regimen. Behavioral analyses in Exp 3, 4, and 5 revealed that bromocriptine administration, while failing to interfere with the onset of maternal behavior in rats treated with the type I concurrent steroid regimen, disrupted the onset of maternal care in rats treated with the type II sequential steroid regimen. When a separate set of type II steroid-treated rats was given both bromocriptine (2 mg/kg) plus ovine PRL (0.5 mg, sc) twice daily, maternal behavior rapidly appeared. Thus, suppression of endogenous PRL secretion delays the onset of maternal behavior in nonhypophysectomized steroid-primed rats, an effect prevented by concurrent administration of ovine PRL. In addition to providing further experimental support for PRL's role in maternal behavior, the development of this endocrine regimen provides researchers with a potentially fruitful model to examine neural sites and mechanisms of PRL regulation of maternal behavior in mammals.