Regulation of prolactin receptor gene expression in the rat choroid plexus via transcriptional activation of multiple first exons during postnatal development and lactation.

Prolactin (PRL) has numerous physiological functions that are mediated by its receptors in target cells. Expression of the rat PRL receptor (PRLR) gene is regulated in a tissue-specific manner via the transcriptional activation of five distinct first exons, i.e., E1(1), E1(2), E1(3), E1(4), and E1(5). In the present study, we investigated the expression profiles of these first exon variants of PRLR mRNA in the rat choroid plexus, which is considered to be a site of receptor-mediated PRL transport from the blood to cerebrospinal fluid. Real-time PCR analysis revealed that E1(3)-, E1(4)-, and E1(5)-PRLR mRNA expression levels increased in the choroid plexus in male and female rats during postnatal development, with markedly higher level of E1(4)-PRLR mRNA. In female rats, the E1(4)-PRLR mRNA expression levels increased markedly during lactation compared with the diestrus state, whereas there was no increase in the E1(3)- and E1(5)-PRLR mRNA levels. The E1(4)-PRLR mRNA expression pattern was similar to that of the total PRLR mRNA. The PRL plasma concentration generally correlated with the E1(4)-PRLR mRNA expression levels in both sexes. These findings suggest that PRLR gene expression in the choroid plexus is upregulated mainly via the transcriptional activation of the E1(4)-first exon.