SEX-DIFFERENCES IN ESTROGENIC REGULATION OF PREPROENKEPHALIN MESSENGER-RNA LEVELS IN THE MEDIAL PREOPTIC AREA OF PREPUBERTAL RATS

Opioids have been implicated in sexual differentiation of the brain an d in the regulation of reproductive behavior and endocrinology of mamm als. Previous studies have indicated that estrogen administration in a dults regulates preproenkephalin MRNA levels in several hypothalamic b rain nuclei. We have determined preproenkephalin mRNA levels in estrog en-treated juvenile male and female rats to investigate the developmen tal pattern of estrogenic regulation of enkephalinergic neurons in the medial preoptic area. Rats were treated with estradiol benzoate (20 m u g/kg/day) or oil from day 21 to 23. Sections of the medial preoptic area (mPOA) were studied by in situ hybridization histochemistry at th e single cell level and quantified with the assistance of an image ana lysis system. Our data indicate that males contain higher levels of pr eproenkephalin mRNA per neuron than females. In addition, our results indicate that estrogen causes an upward shift in the amount of mRNA ex pressed per cell, females demonstrating a greater response to estrogen than males. An increase in soma cell area following estrogen treatmen t was observed only in female mPOA enkephalinergic neurons. Taken toge ther, these results indicate a sex difference in total preproenkephali n levels and in estrogenic regulation of preproenkephalin mRNA in the POA of juvenile rats. These results are discussed in relation to the d ifferential role opioids may play in male and female reproductive phys iology. (C) 1998 Elsevier Science B.V. All rights reserved.