Factors Influencing the Positive Feedback Action of Estrogen upon the Luteinizing Hormone Surge in the Ovariectomized Guinea Pig*

Abstract

Factors influencing the positive feedback action of estrogen, especially those related to dose and the time of day of injection, were investigated in the ovariectomized female guinea pig Adult guinea pigs were housed in a light-controlled room (12 h light, 0600–1800 h) and ovariectomized 2 weeks before estradiol benzoate (EB) injections. EB doses of 1.5, 10, or 50 μg were administered sc at 1000 or 2200 h. Blood samples were obtained through a chronic indwelling venous catheter. Plasma LH was estimated by RIA. The mean latency from the time of EB injection to the occurrence of the LH surge was not influenced by the time of day of injection but was clearly related to the dose of estrogen administered. The mean latency to the LH surge from the time of injection decreased with increasing doses of estrogen but the mean dose-dependent latency was comparable in the same dose groups regardless of the time of day of injection. However, at each dose of estrogen, the temporal distributions of the individual LH surges were clearly different between morning and evening groups. Overall, a greater percentage of animals showed the LH surge during the dark phase than light phase. At all doses tested, 100% of the responders showed LH surges in the dark phase when the estrogen injection was timed so that the dosetypical latency was coincident with the midpoint of the dark phase of the cycle. When the dose-typical latency was coincident with the midpoint of the light phase, episodes of the LH surge were widely distributed and occurred in the dark as well as in the light phases of the cycle. This tendency for episodes of the LH surge to be shifted out of the light phase into the preceding and subsequent dark phases was inversely related to the dose of estrogen administered. Thus, the effect was most clearly observed in animals given 1.5 μg EB. These data indicate that the timing of the estrogen-induced LH surge in the ovariectomized guinea pig is regulated by two major factors: 1) the dose of estrogen given and 2) the lighting conditions (environmental light-dark cycle).