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There is sexual dimorphism of specific species of mRNA in the neonatal rat brain and this sexual dimorphism may be imprinted by steroids of testicular origin during the perinatal period. According to current theories, only aromatizable androgens may cause sexual differentiation of sexual behaviour and function in the adult. The effects of oestradiol benzoate on mRNA synthesis in the neonatal female limbic system were therefore studied. In addition, cytosolic and nuclear oestrogen receptors were measured after administration of testosterone propionate, oestradiol benzoate or dihydrotestosterone (DHT). An attempt was made to distinguish between the brain oestrogen receptor and the plasma oestrogen-binding protein, alphafoetoprotein (AFP) by isoelectric focussing. After injection of 50 µg oestradiol benzoate s.c. to neonatal female rats, the expression of mRNA coding for sexually dimorphic proteins appeared to be changed to a male-type pattern. The overall density of labelling was noticeably greater and specific changes in labelled proteins were observed. These effects were observed within 3 h of injection. Both testosterone and oestradiol caused a marked depletion of cytosolic oestrogen receptors in the limbic system whereas DHT was ineffective in this respect. Nuclear receptors were present in equal abundance in male- and female-derived nuclei and only oestradiol was able to cause a significant (P < 0·025) increase in nuclear oestrogen receptors. The receptor and AFP could be distinguished by isoelectric focussing, since the pI of the receptor was 7·05, while that of AFP was 4·5. These results are consistent with the possibility that oestradiol alters transcription in the neonatal rat brain and may do this through the oestrogen receptor. Nevertheless, it is also possible that oestradiol could alter post-transcriptional events such as the stability of mRNA or the binding of tRNA to the polysomal complex.

Journal of Endocrinology (1989) 120, 83–88