Final answer:
The regulation of Luteinizing Hormone (LH) in horses during spring involves increased daylight reducing melatonin production, which promotes GnRH and subsequently LH secretion from the pituitary gland, leading to the initiation of the estrous cycle.
Step-by-step explanation:
In horses, the regulation of Luteinizing Hormone (LH) secretion during the longer days of spring is closely linked to photoperiodicity (day length). The process begins with light detection in the retina, which sends signals to the pineal gland through the circadian rhythm pathway.
This influences the secretion of melatonin, which is inversely proportional to day length—meaning it decreases as days get longer.
During spring, the extended daylight leads to a drop in melatonin production, which in turn affects the hypothalamus. The hypothalamus begins to increase the release of Gonadotropin-Releasing Hormone (GnRH). The GnRH stimulates the anterior pituitary gland to produce and release LH. In mares (female horses), LH is crucial for the initiation of the estrous cycle leading to ovulation.
LH regulation is also a subject of negative and positive feedback loops involving estrogen and other hormones. However, for modeling purposes in reproductive physiology, estrogen is usually not considered a good candidate due to its complex role in both positive and negative feedback mechanisms.