Pituitary Responsiveness of Mares Challenged with GnRH at Various Stages of the Transition into the Breeding Season

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Abstract
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 release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) after a bolus injection of gonadotropin releasing hormone (GnRH) during the transition from anestrus into the breeding season. Estrous mares received GnRH on d 2 or 3 of estrus in the cycle immediately preceding slaughter. Anestrous, ET and LT mares received GnRH exactly 1 wk prior to slaughter. A single injection of GnRH (Sigma LHRH, L-0507, 2.0 µg/kg body weight in .9% saline, iv) was given to each mare. Blood samples were collected at −2, h, −1 h, directly prior to GnRH, then 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, 300, 360,420 and 480 min post-injection. Maximum release of LH and FSH was observed within 30 min after injection of GnRH. Except for the LH response in EST mares, concentrations of both hormones had returned to pre-injection baseline levels within 8 h. Group means for area under the curve (AUC) of concentrations of LH in serum, and the maximum amount (MAX) of LH quantified in serum, post-GnRH, increased (P<.05) progressively from AN to the breeding season. The AUC and MAX responses for FSH showed a reverse pattern, decreasing (P<.05) from AN to the breeding season. Coefficients of correlation for the linear regression of AUC with gonadotropin concentrations in the anterior pituitary were high for LH (r = .72) and low for FSH (r = .22). From these results it appears that, in the mare, release of LH in response to an injection of GnRH during the transition into the breeding season is a direct function of the concentration of LH present in the anterior pituitary gland. In contrast, concentrations of FSH in the anterior pituitary do not change from AN to the breeding season; however, GnRH-induced release of FSH decreases as the breeding season approaches, suggesting that regulation of FSH secretion during this period is a more complex process which may involve an inhibitory substance. Copyright © 1987. American Society of Animal Science . Copyright 1987 by American Society of Animal Science.