Heat Stress during Summer Attenuates Expression of the Hypothalamic Kisspeptin, an Upstream Regulator of the Hypothalamic–Pituitary–Gonadal Axis, in Domestic Sows

SIMPLE SUMMARY: The present study compared the expression of hypothalamic kisspeptin, an upstream regulator of the hypothalamic–pituitary–gonadal axis, in different seasons (summer and spring) to assess the effect of heat stress on reproductive performance in domestic sows. Sows in summer showed a decreased pregnancy rate and litter size and increased secretion of stress-related hormones than those in spring. Heat stress in summer lowered hypothalamic kisspeptin expression and reduced the activity of kisspeptin neurons. Due to the lowered expression of hypothalamic kisspeptin, the downstream reproductive hormones and the number of large ovarian follicles were reduced in summer. These findings may have significant implications in the fields of reproductive biology and the livestock industry for heat stress control. ABSTRACT: The release of reproductive hormones in the hypothalamic–pituitary–gonadal (HPG) axis is regulated by its upstream regulator, kisspeptin, and influenced by external stresses, including heat stress. Since the effect of heat stress (summer infertility) on hypothalamic kisspeptin expression in domestic sows is not yet understood, the present study attempted to identify changes in kisspeptin expression in different seasons (summer and spring). The high atmospheric temperature in summer decreased the pregnancy rate and litter size and increased stress-related hormones as a chronic stressor to domestic sows. The hypothalamic kisspeptin expression in summer was decreased regardless of the estrus phase and negatively correlated with atmospheric temperature, indicating that high temperature decreased kisspeptin. When the activity of hypothalamic kisspeptin neurons in the follicular phase was assessed using c-Fos staining, a decreased number of kisspeptin neurons coexpressing c-Fos was observed in domestic sows in summer. Accordingly, lower expression of kisspeptin induced decreased levels of HPG axis-related reproductive hormones, such as gonadotropins and estrogen, and fewer large ovarian follicles. In conclusion, the present study demonstrated that reduced kisspeptin expression and its neuronal activity in the hypothalamus under heat stress in summer induced downregulation of the HPG axis and caused summer infertility in domestic sows.