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

Summary. 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 characterized 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 sample 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 anterior pituitary gland or concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were observed in the anterior pituitary gland (P > 0·1). Mean diameter of the largest follicle, number of follicles ≥ 20 mm in diameter and concentrations of LH and FSH in serum remained unchanged for weeks −1 to +3 (P > 0·05), then increased significantly by week 12 (P < 0·001). Content and concentration of GnRH in the median eminence was low at −1 week, increased gradually (P < 0·05) to a maximum by + 1 week, then decreased gradually (P < 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 suggests that the coincidental reduction in content of GnRH may be due to an increase in secretion. One role of lengthening photoperiod may therefore be to boost production of GnRH during a period of increasing release. Keywords: mare; LH; GnRH; hypothalamus; pituitary; photoperiod