Seasonal Variation in Hypothalamic Content of Gonadotropin- Releasing Hormone (GnRH), Pituitary Receptors for GnRH, and Pituitary Content of Luteinizing Hormone and Follicle-Stimulating Hormone in the Mare 2

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Abstract
Seasonal changes in the hypothamic-hypophyseal axis were investigated using tissue from 49 light-horse mares, of mixed breeding. Hypothalamic and pituitary tissues were collected at 5 intervals throughout the years 1981 and 1982, representing midbreeding season (July, n = 10), transition out of the breeding season (Oct., n = 11), midanestrus (Dec., n = 8), transition into the breeding season (March, n = 10), and again in the following midbreeding season (July, n = 10). The hypothalamic region was dissected into preoptic area, body and median eminence. Gonadotropin-releasing hormone (GnRH) was extracted from hypothalamic samples with methanol-formic acid and quantified by radioimmunoassay. The anterior pituitary was homogenized and receptors for GnRH were quantified in a crude membrane fraction. Concentrations of luteinizing hormone (LH) and FSH were measured in the resulting supernatant. Content of GnRH in each of the 3 hypothalamic areas varied with season (P < 0.01) and was lowest during midanestrus (P < 0.05). There was no effect of season (P > 0.1) on either concentration or total number of receptors for GnRH, or concentration of FSH in the anterior pituitary. Concentrations of LH in the anterior pituitary varied with season (P < 0.001). Means (.+-. SEM [standard error of the mean]) for the 5 collection times were 15.5 .+-. 2.7, 9.7 .+-. 2.4, 2.3 .+-. 0.5, 2.7 .+-. 0.4 and 11.7 .+-. 1.5 .mu.g LH/mg anterior pituitary, respectively. Hypothalamic content of GnRH along with anterior pituitary concentrations of LH, but not the number of GnRH-receptors or concentration of FSH in the anterior pituitary, may be 1 of the hypothalamic-hypophyseal factors responsible for seasonal anestrus in the mare. The low concentration of LH in the pituitary available for release may delay ovulation during the period of transition into the breeding season.