Abstract
Prolactin is a peptide hormone released from the anterior pituitary that belongs to the somatotropin family of hormones. It mediates its actions via prolactin receptors which are expressed on a diverse range of cell types. While prolactin is classically known for its role in promoting lactation, it is now recognised to have many additional actions in both male and females. One of these roles may be associated with the stress response since many forms of stress have been shown to increase prolactin secretion from the pituitary. The functional significance of this stress-induced prolactin elevation are still unclear. Central to a stress response are the adrenal glands which are responsible for releasing both glucocorticoids and catecholamines. This study has examined the distribution of prolactin receptor expression in the mouse adrenals using both a prolactin receptor reporter mouse and RNAscope to measure prolactin receptor mRNA expression Building on previous work conducted in our laboratory, comparisons were made between diestrus mice and their lactating counterparts. Results obtained from the reporter mouse showed that prolactin receptor expression was located within the adrenal cortex while much lower levels were detected within the medulla. RNAscope showed that prolactin receptor mRNA was predominantly located within the zona fasciculata of the adrenal cortex. Expression levels within this region were compared between diestrus and lactating mice and found to be significantly downregulated during lactation. No similar downregulation was observed in the zona glomerulosa. Interestingly, RNAscope illustrated the presence of prolactin receptor mRNA within the X-zone region of the adrenal, a finding that was absent in the reporter mouse. Given that the prolactin receptor was predominantly located within the adrenal cortex, the study was extended to determine the expression of key steroidogenic enzymes using immunohistochemistry. These experiments showed the presence of CYP11A1 and CYP11B1 and the transport protein StAR predominantly within the zona fasciculata and a population of cells at the corticomedullary border. It was observed that during lactation there was an overall increase in the expression levels for all three while the number of stained cells within the corticomedullary boundary decreased. We speculate that the partial downregulation in prolactin receptor mRNA expression is a homeostatic mechanism to prevent over stimulation of the adrenal glands and thus rise in circulating glucocorticoids during periods of chronically high prolactin levels such as lactation. This may be physiologically significant as excessive glucocorticoid levels can be detrimental to health of the mother and offspring.