Decreased Release of Gonadotropin-Releasing Hormone During the Preovulatory Midcycle Luteinizing Hormone Surge in Normal Women

To investigate the contribution of hypothalamic gonadotropin-releasing hormone (GnRH) secretion to the midcycle gonadotropin surge in the human, the response of luteinizing hormone (LH) to competitive GnRH receptor blockade achieved by administration of a range of doses of a pure GnRH antagonist was used to provide a semiquantitative estimate of endogenous GnRH secretion. The LH response to 5, 15, 50, and 150 μ g/kg s.c. of the NAL-GLU GnRH antagonist ([Ac-D-2Nal1,D-4ClPhe2,D-Pal3,Arg5, D-4-p-methoxybenzoyl-2-aminobutyric acid6,D-Ala10]GnRH, where 2Nal is 2-naphthylalanine, 4ClPhe is 4-chlorophenylalanine, and 3Pal is 3-pyridylalanine) was measured in normal women in the early and late follicular phases of the menstrual cycle, at the time of the midcycle LH surge and in the early luteal phase. LH decreased in a dose-response fashion after administration of the GnRH antagonist in all cycle phases (P < 0.0001). When this suppression was expressed as maximum percent inhibition, there was no difference in response during the early and late follicular and early luteal phases. However, at the midcycle surge, there was a leftward shift of the dose-response curve with significantly greater suppression of LH at the lower antagonist doses in comparison to the other cycle phases (P < 0.005), but no difference at the highest dose. Thus, we draw the following conclusions. (i) There is a consistently greater degree of LH inhibition by GnRH antagonism at the midcycle surge at submaximal degrees of GnRH receptor blockade than at other phases of the menstrual cycle in normal women. (ii) This leftward shift of the dose-response relationship to GnRH receptor blockade suggests that the overall amount of GnRH secreted at the midcycle surge is less than at other cycle stages. (iii) These data confirm the importance of pituitary augmentation of the GnRH signal at the time of the midcycle gonadotropin surge in the human.