Steroid-induced alterations in mRNA expression of the long form of the prolactin receptor in the medial preoptic area of female rats: Effects of exposure to a pregnancy-like regimen of progesterone and estradiol

It is firmly established that the onset of maternal behavior in the female rat is stimulated by a combination of hormones that include prolactin (PRL), estradiol (E 2), and progesterone (P 4). Specifically, nulliparous rats display short latencies to respond to foster young when primed with Silastic capsules filled with P 4 and E 2 and then administered PRL centrally to the medial preoptic area (MPOA), an area integrally involved in the expression of maternal behavior in this species. PRL or P 4 treatments alone are ineffective in stimulating the expression of maternal care. Since the actions of PRL in the MPOA appear to be mediated by PRL receptors, it was of interest to determine whether and how treatment with P 4 and E 2 together or separately might alter mRNA expression of the long form of the PRL receptor (PRL-R L) in the MPOA. Using in situ hybridization histochemistry (ISHH), mRNA expression of the PRL-R L was measured in the MPOA of ovariectomized, nulliparous rats treated with various combinations of P 4 and E 2. Treatment of animals with P 4 alone for 10 days or with P 4 followed by E 2 for 1 or 4 days resulted in reductions in PRL receptor mRNA expression in the MPOA when compared with the expression in animals treated with E 2 alone or blank capsules. The actions of P 4 on mRNA expression of the PRL-R L were more pronounced in the dorsal MPOA. Circulating PRL levels collected at the time of sacrifice were elevated in all groups treated with E 2, but no association between PRL levels and receptor mRNA expression within the MPOA was evident. These findings indicate that the dorsal MPOA may be one site of progesterone's action in facilitating prolactin-mediated maternal behavior.