Suppression of lordosis in guinea pigs by ethamoxy-triphetol (MER-25) given at long intervals (34-46 hr) after estradiol benzoate treatment

Ovariectomized guinea pigs were given estradiol benzoate (EB) followed 40 hr later by progesterone (P). Behavioral testing commenced 1 hr after P injection and continued at hourly intervals for 8 hr. This treatment activated lordosis in almost 100% of animals. Administration of the antiestrogen MER-25 (75 mg/kg body wt per injection) between 2 hr before and 6 hr after EB treatment did not cause a significant decline in proportion of animals displaying lordosis, but did cause a decrease in length of time the lordosis position was held (maximum lordosis, sec). In contrast, $\text{13}\text{14}$ animals given MER-25 at 2 hr before and 2 hr after P and $\text{8}\text{10}$ animals given MER-25 simultaneously with and 2 hr after P, failed to show lordosis. Administration of supplementary EB at around the time of P injection, partially alleviated these behavior-blocking effects of MER-25. When MER-25 was given 2–6 hr after administration of P there was a significant decrease in duration of heat (hr). These results suggest that in addition to its early “triggering” effects, estrogen has important “maintenance” effects which determine the character of heat in guinea pigs. Continued presence of estrogen in the nervous system may be a requirement for the facilitatory actions of P on sexual behavior in guinea pigs, but such a requirement may not exist in other rodents such as rats.