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

Abstract

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 (E₂), and progesterone (P₄). Specifically, nulliparous rats display short latencies to respond to foster young when primed with Silastic capsules filled with P₄ and E₂ 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 P₄ 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 P₄ and E₂ 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 P₄ and E₂. Treatment of animals with P₄ alone for 10 days or with P₄ followed by E₂ 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 E₂ alone or blank capsules. The actions of P₄ 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 E₂, 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.