Interaction of seasonal and ovarian factors in the regulation of LH and FSH secretion in the mare.

The study presents a working hypothesis about the regulation of Luteinizing Hormone (LH) in mares. It suggests two factors working in tandem: a primary central nervous system-pituitary component for a basal LH rhythm and a secondary ovarian component, responsible for ovulatory season fluctuations. The same theory doesn’t entirely apply to the secretion of Follicle-Stimulating Hormone (FSH) due to possible influences beyond steroids.

Regulation of Luteinizing Hormone (LH)

• The hypothesis proposes two components that drive the regulation of LH in mares. The first one is a basic physiological process steered by the central nervous system (CNS) and pituitary gland. This establishes a circannual (yearly) LH rhythm.
• According to the study, this rhythm is entrained, meaning it is synchronized with an environmental cycle (known as a Zeitgeber). The particular Zeitgeber is likely the photoperiod (which is the amount of light in a 24-hour period).
• The findings suggest that this CNS-pituitary component operates independently of any ovarian influences, meaning its function is not altered by ovarian changes or activities.
• The second component is an ovarian one, specifically concerning steroids. This secondary component influences and modifies the established primary LH rhythm during the ovulation season, leading to a variation in LH levels based on the reproductive cycle of the mare.

The Follicle-Stimulating Hormone (FSH)

• The same hypothesis was attempted on the secretion of FSH but seemed not to apply in its entirety. The CNS-pituitary component is evident within the first year after an ovariectomy in mares (a surgical procedure for the removal of one or both ovaries), but then appears to disappear.
• The ovarian contribution to the regulation of FSH is suggested to be reliant on factors that extend beyond just steroids. This could indicate that other substances or mechanisms within the ovaries may have an influence on FSH secretion in mares.