Prolactin regulation of the HPA axis is not mediated by a direct action upon CRH neurons: evidence from the rat and mouse

Centrally acting prolactin has been shown to have anti-stress effects by modulating the activity of the hypothalamic–pituitary–adrenal axis. We tested the hypothesis that prolactin directly targets hypothalamic corticotropin-releasing hormone (CRH) neurons. In situ hybridisation confirmed expression of mRNA encoding the long, but not the short, isoform of the prolactin receptor (PRLR) within the paraventricular nucleus (PVN) of the virgin rat; however, only 6% of CRH neurons expressed long-form Prlr mRNA. Examination of the functional response of CRH neurons to intracerebroventricular prolactin (500 ng) showed that these neurons did not respond with activation of phosphorylated signal transducer and activator of transcription 5 (pSTAT5), a marker of long-form PRLR activation. However, as only a subset of neurons expressing Crh mRNA could be detected using immunohistochemistry, we utilised a transgenic mouse model to label CRH neurons with a fluorescent reporter (CRH-Cre-tdTomato). In lactating animals, chronically elevated prolactin levels resulted in significantly increased pSTAT5 expression in the PVN. Overall, few tdTomato-labelled CRH neurons were double-labelled, although a small subset of CRH neurons in the caudal PVN were pSTAT5 positive (approximately 10% of tdTomato neurons at this level, compared to 1% in the rostral PVN). These data suggest that most CRH neurons do not respond directly to prolactin. To confirm that prolactin was not activating another signalling pathway, we used a transgenic mouse line to label PRLR-expressing neurons with Cre-dependent green fluorescent protein (GFP) expression (CRH-Cre-Prlr ^lox/lox ). No GFP-expressing cells were evident in the PVN, indicating that in the mouse, as in the rat, the CRH neurons do not express either PRLR isoform. Together these data showed that the anti-stress effects of prolactin are not the result of prolactin directly regulating CRH neurons.