Expression of relaxin-3 and its receptors in the hypothalamic-pituitary-ovary axis in layers and broiler breeders

Layers can regulate their feed intake and maintain well-organized ovaries. Conversely, broiler breeders are prone to overeating, leading to disorganized ovaries. Relaxin-3 (RLN3) is a neuropeptide hormone involved in metabolism, energy balance, and reproduction in mammals. In chickens, RLN3 was identified in the ovary, pituitary gland, and hypothalamus. This study aimed to explore the potential role of RLN3 in feeding behavior and reproduction. Expression of RLN3 and relaxin family peptide receptors (RXFP1 and 3) were examined in ovarian follicles, the pituitary gland, and the hypothalamus between layers (L) and broiler breeders (BB). Gene expression analysis revealed RLN3 was highest in the granulosa cells (GC) of 9-12 mm follicles compared with other follicles, with BB had the highest overall expression. In the pituitary, RLN3 was higher in L compared with BB and minimally expressed in follicle theca tissue (TH), cortex, and hypothalamus. Expression of RXFP1 was highest in TH of all follicles without a significant difference between L and BB. The expression of RXFP3 was highest in the cortex without a significant difference between L and BB. The hypothalamus had the highest expression of RXFP3 across all tissues in both L and BB. Immunofluorescence staining for RLN3, RXFP1, and RXFP3 revealed their presence in the GC, TH interna and externa of 3-12 mm follicles in both L and BB. Interestingly, RLN3 was localized in small vesicles in the ooplasm of 3-12 mm follicles. Within the cortex, RXFP1 was localized in the GC and TH of cortical follicles while RXFP3 was exclusively localized in the stromal and muscle cells. Surprisingly, RXFP3 was also localized in the nucleus. Overall, RLN3 and its receptors were differentially expressed across the hypothalamic-pituitary-ovarian axis and between layers and broiler breeders. This suggests potential role in nutrition and ovarian dysregulation. Further, RLN3 action is likely mediated through autocrine and paracrine effects. Modulating RLN3 could lead to novel strategies of regulating feed intake in broiler breeders.