Influences of the stress endocrine system on the reproductive endocrine axis in sheep (Ovis aries)

The hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-pituitary-gonadal (HPG) axis systems are inversely related in humans and animals. Although livestock animals, such as sheep (Ovis aries), tend to be well adapted to their environment, it is known that the livestock production processes subject animals to a multitude of physical and psychological stressful stimuli that have the potential to elevate the HPA axis activity. Chronic stress is one of the major challenges in sheep production, as it is difficult to detect and can result in prolonged dysfunction of the HPA axis, causing downstream negative physiological effects such as immunosuppression, increased susceptibility to disease and reproductive dysfunction. The elevation of HPA axis activity during chronic stress has been suggested as the primary neuroendocrine mechanism underlying the aetiology of reproductive dysfunction in sheep. Research in sheep has demonstrated that glucocorticoids act on the HPG axis at the level of the hypothalamus and hypophyseal portal system to decrease gonadotrophin secretion and at the level of the pituitary gland to reduce responsiveness and sensitivity of gonadotroph cells and their receptors to GnRH. Sheep farming enterprises rely on the breeding efficacy of ewes to optimise lambing percentage and reproductive success in order to ensure a profitable business. This review discusses the influences of the HPA axis on the HPG axis and defines any significant reproductive function consequences caused by stress in ewes and places them into perspective for sheep management and productivity.