Two experiments were conducted: 1) to assess the ovulation-blocking ability and steroidogenesis-inhibiting activity of aminoglutethimide in the laying hen; and 2) to determine whether LHRH or progesterone (P4) administration can overcome the ovulation-blocking effect of aminoglutethimide. Aminoglutethimide inhibited ovulation and suppressed the secretion of P4 and testosterone (T) in a dose-related fashion. In the absence of any increase in plasma P4 and T, there was no preovulatory increase in plasma LH. These results indicate that the preovulatory surge of LH is initiated by an increase in steroid. The effectiveness of P4 and LHRH to stimulate LH release and overcome the ovulation-blocking effect of aminoglutethimide was tested in the second experiment. Administration of 500 µg P4 (im) to aminoglutethimide-treated hens resulted in a significant and sustained release of LH [peak, 3.08 ± 0.62 (±SEM) ng/ml; 120 min after injection] and induced ovulation in the absence of any increase in plasma T or estrogen. In contrast, injection of 20 µg LHRH (iv) failed to overcome the blocking effect of aminoglutethimide and caused an attenuated (peak, 2.17 ± 0.37 ng/ml; 60 min after injection) and short-lived increase in plasma LH. These results are consistent with the model for a true positive feedback mechanism in which P4 initiates and sustains the preovulatory LH surge of the hen.
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