The results show that Eb was effective in eliciting an afternoon surge of LH 1 and 2 days after treatment while morning levels remained low, and that the LH response was clearly reduced by the second day after Eb treatment. Although estrogen caused a minimal increase in pituitary responsiveness to LRH, the responsiveness did not prove significantly different when tested in pentobarbital blocked animals either a) in the morning (1000) and afternoon (1700) of the same day after Eb treatment, or b) in the afternoon of Days 1 and 2 after Eb injections. This stability in pituitary responsiveness to LRH was maintained in spite of dramatic differences in LH levels at these times. Studies on the priming effects of exogenous LRH showed that the Eb-induced LH surge could be significantly potentiated by giving a morning injection of 100 ng LRH; curiously, rats receiving 200 ng LRH showed an LH response similar to that of the saline control animals.

These results suggest that after estrogen has sensitized the pituitary to LRH, the responsiveness remains stable for at least 2 days, but that an appropriate dose of LRH can further sensitize the pituitary to endogenous LRH. These findings support the hypothesis that the daily LH surges in Ovx rats are controlled primarily by neural mechanisms that require estrogen for their expression.