Endocrine Regulation of Estrus Cycles
As day length increases in the spring, the mare responds with decreasing melatonin from the pineal gland. The changes in melatonin result in increased GnRH release from the hypothalamus. The GnRH stimulates LH and FSH release from the anterior pituitary. The FSH stimulates follicular growth that is completed by LH. The follicle produces estrogen which induces estrus behavior. The increasing estrogen stimulates further LH release that triggers ovulation of the follicle and formation of the corpus luteum (CL). The CL produces progesterone which inhibits estrus behavior and promotes diestrus behavior. The progesterone also inhibits GnRH production and the decreased in GnRH leads to reduced LH and FSH levels. Low levels of LH and FSH result in no follicular growth. After 14 days, the CL regresses, progesterone drops precipitously, GnRH increases and the entire cycle is repeated.
Anestrus occurs when day length deceases in the fall and winter. Melatonin is high and GnRH release is inhibited. As a mare moves into the months of February - March, melatonin begins to decrease and GnRH will increase. However levels of GnRH will at first not be sufficient to support normal follicular growth and ovulation. Hence we have the transitional type cycle occurring. Once melatonin is low enough and GnRH is high enough, the mare will have repeated estrus cycles during the breeding season.
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