Orexin receptor expression in the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes of free-living European beavers (Castor fiber L.) in different periods of the reproductive cycle

Orexins are hypothalamic neuropeptides acting via two G protein-coupled receptors in mammals: orexin receptor 1 (OX1R) and orexin receptor 2 (OX2R). In European beavers, which are seasonally breeding animals, the presence and functions of orexins and their receptors remain unknown. Our study aimed to determine the expression of OXR mRNAs and the localization of OXR proteins in hypothalamic-pituitary-adrenal/gonadal (HPA/HPG) axes in free-living beavers. The expression of OXR genes (OX1R, OX2R) and proteins was found in all analysed tissues during three periods of beavers' reproductive cycle (April, July, November). The expression of OXR mRNAs in the beaver HPA axis varied seasonally (P < 0.05). The levels of OX1R mRNA also differed between the sexes (P < 0.05). In the mediobasal hypothalamus, OX1R transcript content increased in pregnant females in April (P < 0.05) and OX2R expression increased in males in July (P < 0.05). In the pituitary and adrenals, OX1R mRNA levels were relatively constant in females and peaked in July in males (P < 0.05), whereas the OX2R was most highly expressed in males in November and in females in April (P < 0.05). In gonads, OX1R expression did not fluctuate between seasons or sexes, but transcript levels were elevated in the testes in November and in the ovaries in July (P < 0.05). In turn, OX2R mRNA levels varied between the sexes (P < 0.05) and were higher in females (July and November) than in males (P < 0.05). The circannual variations in OXR mRNA levels in HPA and HPG axes suggest that the expression of these receptors is associated with sex-specific changes in beavers' reproductive activity and their environmental adaptations.