EFFECTS OF OESTRADIOL-17β, OESTRADIOL BENZOATE AND THE SYNTHETIC OESTROGEN RU 2858 ON SEXUAL DIFFERENTIATION IN THE NEONATAL FEMALE RAT

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Abstract
SUMMARY: Groups of neonatal female rats were treated for the first 5 days of life with oestradiol-17β, oestradiol benzoate or a synthetic oestrogen, 11β-methoxy-17-ethynyl-1,3,5(10)-oestratriene-3,17β-diol (RU 2858), in daily doses ranging from 0·5 to 1000 ng. Oestradiol-17β had no effect on adult ovarian cyclicity or sexual receptivity after ovariectomy and oestrogen + progesterone treatment. Ovarian cyclicity was prevented by 100 ng or more oestradiol benzoate/day, and by all doses of RU 2858. Only rats receiving 50 ng oestradiol benzoate/ day or 0·5 ng RU 2858/day showed normal receptivity. The defeminizing action of RU 2858 was at least 100 times greater than that of oestradiol benzoate; it is suggested that this greater potency is due to the low affinity of RU 2858 for the oestradiol-binding protein in the plasma of neonatal rats. These results indicate that defeminization of the neonatal rat brain can be induced by physiological amounts of oestrogen, and are discussed with reference to the action of testosterone.