Changes in the hypothalamic-hypophyseal axis of mares associated with seasonal reproductive recrudescence

Four groups of mares, representing anestrus (AN; n = 8), early transition (ET; n = 7), late transition (LT; n = 8) and estrus (EST; n = 12) were used to examine changes in the hypothalamus and anterior pituitary during the period of transition from winter anestrus into the breeding season. Mares were of mixed breeding, between the ages of 3 and 20 years, and had shown normal patterns of estrous behavior and ovulation during the breeding season previous to this experiment. Hypothalamic content of gonadotropin-releasing hormone (GnRH) and anterior pituitary content of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of receptors for GnRH in anterior pituitary tissue was also determined. There was no effect of stage of transition into the breeding season on receptors for GnRH or content of FSH (p greater than 0.05). Likewise, content of GnRH in the hypothalamus did not differ between the four groups (p greater than 0.05). However, pituitary content of LH increased progressively from anestrus to the breeding season (p less than 0.05). Means for the AN, ET, LT and EST groups were 1.1 +/- 0.2, 2.2 +/- 0.3, 6.3 +/- 1.4 and 15.2 +/- 1.8 micrograms LH/mg pituitary, respectively. In addition, serum concentrations of LH associated with the first ovulation of the year for 5 of the EST mares were significantly lower (p less than 0.01) than those associated with the second ovulation of the year.(ABSTRACT TRUNCATED AT 250 WORDS)