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

Seasonal changes in the hypothalamic-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 (October, n = 11), midanestrus (December, 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 follicle-stimulating hormone (FSH) were measured in the resulting supernatant. Content of GnRH in each of the 3 hypothalamic areas varied with season (P less than 0.01) and was lowest during midanestrus (P less than 0.05). There was no effect of season (P greater than 0.01) 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 less than 0.001). Means (+/- SEM) 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 microgram LH/mg anterior pituitary, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)