Blockade of the Pre-ovulatory LH Surge in Hamsters by an Inhibitory Analog of LH-RH 1

Summary

Cycling hamsters were injected sc with 750 μg of d-Phe²-d-Leu⁶-LH-RH, an inhibitory analog of LH-RH at 3, 4, 5, and 6 pm on the day of proestrus. Serum LH levels were determined by radioimmunoassay in samples collected throughout the afternoon of proestrus. Treatment with d-Phe²-d-Leu⁶-LH-RH led to 82.8% suppression of the physiological surge of LH as compared with the integrated LH levels of diluent-treated animals. When ova were counted, treatment with d-Phe²-d-Leu⁶-LH-RH was shown to produce 29.6% (P < 0.05) inhibition of ovulation as compared with diluent-treated animals. No rebound phenomenon in serum LH was observed in the evening of proestrus in animals treated with the analog. Incomplete inhibition of ovulation may be due to the possibility that the small LH surge was well in excess of that needed to produce ovulation or could be synergized with an incomplete FSH surge which was not measured in this study. The small peaks of LH still present in the analog-treated animals may be explained by the intrinsic gonadotropin-releasing activity of the analog and/or the schedule of treatment used.