Effects of Aminoglutethimide on Luteinizing Hormone and Steroid Secretion, and Ovulation in the Hen,Gallus Domesticus*

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Abstract
Two experiments were conducted: to assess the ovulation-blocking ability and steroidogenesis-inhibiting activity of aminoglutethimide in the laying hen; and 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 [luteinizing hormone]. The preovulatory surge of [luteinizing hormone] 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 2nd experiment. Administration of 500 .mu.g P4 i.m. to aminoglutethimide-treated hens resulted in a significant and sustained release of LH [peak, 3.08 .+-. 0.62 (.+-. standard error of the mean) 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 .mu.g LHRH i.v. 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.