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

Abstract

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.ARTICLE HISTORY

Faecal glucocorticoid metabolites and body temperature in Australian merino ewes (Ovis aries) during summer artificial insemination (AI) program

“…Activation of the HPA-axis system via the physiological stress response, plays a crucial role in the regulation of reproduction in ewes [ 30 ]. Stress negatively affects reproductive physiology via the modulation of reproductive steroids GnRH, LH, progesterone and oestrogen levels within the hypothalamic-pituitary-gonadal axis (HPG-axis) system [ 31 ].…”

Small Ruminants: Farmers’ Hope in a World Threatened by Water Scarcity

“…This became very noticeable in the water restricted groups of Malpura ewes [64] and undernourished ewes [97] having decreased plasma estradiol levels. Generally and during stress, productive functions such as growth and reproduction are suppressed by endocrine hormones in favour of survival and maintenance [98]. During periods of water scarcity or deprivation, the hormones are mobilized, giving their critical roles, to ensure that the energy needs are satisfied and water losses minimized.…”

Hypogonadism in Exercising Males: Dysfunction or Adaptive-Regulatory Adjustment?

“…Furthermore, evidence exists for circulating testosterone and cortisol to be negatively associated with athletes even in the resting, basal state (82). In these scenarios the inhibitory effect of cortisol appears twofold; i.e., to impact LH and FSH via GnRH suppression as well as a compromise of Leydig cell function via direct steroidogenesis inhibition (79,83). Prolactin is another hormone that can induce reductions in testosterone levels, and this hormone's release is also stimulated by exercise (see review article- (84)).…”