Failure of estradiol-17 beta implants located in the medial preoptic area to induce ovulation in cyclic rats

Female rats exhibiting regular 4-day vaginal cycles were injected with progesterone on day 1 (estrus = day 0) and implanted bilaterally with estradiol-17 beta (E2) in the medial preoptic area (MPOA) or, for control, in the medial amygdaloid nucleus on day 2. The implants which contained different quantities of E2 were prepared by different methods and were either expelled immediately after introduction of the capillary tubes into the brain, or the tubes were left in place till autopsy. Evaluation of the ovulatory response at autopsy on day 4 revealed that E2 placed in the MPOA had not abolished the delay of ovulation produced by progesterone, whereas implants located in the medial amygdala were effective in this respect. The findings confirm former results obtained with estradiol benzoate and suggest that the MPOA may not be the primary site of action of estrogen in its ovulation-inducing effect in rats.