Prolactin and prolactin receptor expression in the HPG axis and crop during parental care in both sexes of a biparental bird (Columba livia)

AbstractDuring breeding, multiple circulating hormones, including prolactin, facilitate reproductive transitions in species that exhibit parental care. Prolactin underlies parental behaviors and related physiological changes across many vertebrates, including birds and mammals. While circulating prolactin levels often fluctuate across breeding, less is known about how relevant target tissues vary in their prolactin responsiveness via prolactin receptor (PRLR) expression. Recent studies have also investigated prolactin (PRL) gene expression outside of the pituitary (i.e., extra-pituitaryPRL), but howPRLgene expression varies during parental care in non-pituitary tissue (e.g., hypothalamus, gonads) remains largely unknown. Further, it is unclear if and how tissue-specificPRLandPRLRvary between the sexes during biparental care. To address this, we measuredPRLandPRLRgene expression in tissues relevant to parental care, the endocrine reproductive hypothalamic-pituitary-gonadal (HPG) axis and the crop (a tissue with a similar function as the mammalian mammary gland), across various reproductive stages in both sexes of a biparental bird, the rock dove (Columba livia). We also assessed how these genes responded to changes in offspring presence by adding chicks mid-incubation, simulating an early hatch when prolactin levels were still moderately low. We found that pituitaryPRLexpression showed similar increases as plasma prolactin levels, and detected extra-pituitaryPRLin the hypothalamus, gonads and crop. Hypothalamic and gonadalPRLRexpression also changed as birds began incubation. CropPRLRexpression correlated with plasma prolactin, peaking when chicks hatched. In response to replacing eggs with a novel chick mid-incubation, hypothalamic and gonadalPRLandPRLRgene expression differed significantly compared to mid-incubation controls, even when plasma prolactin levels did not differ. We also found sex differences inPRLandPRLRthat suggest gene expression may allow males to compensate for lower levels in prolactin by upregulatingPRLRin all tissues. Overall, this study advances our understanding of how tissue-specific changes in responsiveness to parental hormones may differ across key reproductive transitions, in response to offspring cues, and between the sexes.