Sexual receptivity in hamsters: Brain nuclear estrogen and cytosolic progestin receptors after single and multiple steroid treatments and during the estrous cycle

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This series of experiments investigated the relationship between various treatments consisting of estradiol benzoate (EB) and progesterone (P) on sexual receptivity and on concentrations of nuclear estrogen receptors (NER) and cytosolic progestin receptors (CPR) in the hypothalamus-preoptic area in female hamsters. The injection of 1 μg EB at 0 and 24 hr resulted in higher levels of receptivity (after 0.25 or 0.5 mg P), NER and CPR compared to those obtained after a single injection of 2 μg EB. Animals treated with 5 μg EB at 0 and 24 hr displayed greater levels of receptivity (after 0.5 mg P) and had higher NER concentrations than animals given a single injection of 10 μg EB. Groups treated with either 1 μg EB at 0 hr or 0.5 μg EB at 0 and 24 hr did not differ and showed low levels of receptivity, NER and CPR. NER and CPR were also measured on each day of the estrous cycle. NER levels rose between Days 1 and 2, again between Days 2 and 3, and remained elevated on Day 4. CPR levels increased between Days 2 and 3, and there was no difference between Days 3 and 4. Taken together, these data suggest that receptivity in hamsters after estrogen exposure is correlated with the accumulation and maintenance of relatively high NER levels and on the induction of CPR. This can be accomplished by a single large injection of EB or by smaller split doses.