Leptin plasma concentrations, leptin gene expression, and protein localization in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes of the European beaver (Castor fiber).

The European beaver (Castor fiber) is the largest seasonal free-living rodent in Eurasia. Since the physiology and endocrine system of this species remains unknown, the present study aimed to determine plasma leptin concentrations and the expression of the leptin gene and protein in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal (HPG and HPA) axes of beavers during breeding (April), postbreeding (July), and prebreeding (November) seasons. Leptin plasma concentrations did not change in females, whereas in males, leptin plasma concentrations were higher in July than those in April. The presence of leptin mRNA and protein was found in all examined tissues. In females, leptin mRNA expression in the hypothalamus, pituitary, ovaries, and myometrium was markedly higher in July than that in April. In males, leptin mRNA levels varied across the examined tissues of the HPG and HPA. Leptin synthesis increased in the hypothalamus during breeding and postbreeding seasons, but seasonal changes were not observed in the pituitary. In turn, testicular leptin levels were higher during breeding and prebreeding stages. Seasonal differences in the concentrations of leptin mRNA were also observed in the adrenal cortex. In males, leptin mRNA levels were higher in November than those in April or July. In females, leptin synthesis increased in the adrenal cortex during pregnancy relative to other seasons. This is the first ever study to demonstrate seasonal differences in leptin expression in beaver tissues, and our results could suggest that leptin is involved in the regulation of the HPG and HPA axes during various stages of the reproductive cycle in beavers.