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Changes in the hypothalamic-hypophyseal axis of mares in relation to the winter solstice


, : Changes in the hypothalamic-hypophyseal axis of mares in relation to the winter solstice. Journal of Reproduction & Fertility 96(1): 195-202

In mares, the amount of gonadotrophin-releasing hormone (GnRH) is low in the hypothalamus during seasonal anoestrus, but by early spring, concentrations of GnRH are high. The timing of this response was characterised more precisely by determining concentrations of GnRH in hypothalamic tissue collected immediately before and at various times after the winter solstice (22 December 1986). Ovaries, pituitary gland, hypothalamus and a blood samples were collected from six groups of mares (6-12 mares per group) at death, 1 week before day of the winter solstice and 1, 2, 3 and 12 weeks afterwards. No significant changes in weight of the interior pituitary gland or concentration of luteinizing hormone (LH) and (FSH) were observed in the anterior pituitary gland (P lt 0.1). Mean diameter of the largest follicle, number of follicles gtoreq 20 mm in diameter and concentrations of LH and FSH in serum remained unchanged for weeks -1 to +3 (P gt 0.05), then increased significantly by week 12 (P lt 0.001). Content and concentration of GnRH in the median eminence was low at -1 week, increased gradually (P lt 0.05) to a maximum by +1 week, then decreased gradually (P lt 0.05) to low values at 12 weeks. Means (+- SEM) for -1, +1 and +12 weeks were 33.5 +- 5.5, 117.7 +- 18.6 and 29.8 +- 3.7 ng GnRH, respectively. Mean content of GnRH in the preoptic area of the hypothalamus showed a reciprocal pattern. Since content of GnRH in the median eminence was increasing on the day of the winter solstice, the impetus for resurgence of GnRH synthesis may not be lengthening photoperiod, but refractoriness to the previous short photoperiod. Increased gonadotrophin content observed at 12 weeks suggest that the coincidental reduction in content in GnRH may be due to an increase in secretion. One role of lenthening photoperiod may therefore by to boost production of GnRH during a period of increasing release.

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