The effects of estrogen on the synthesis of pituitary PRL mRNA and PRL and the release of PRL into plasma were investigated in male and female normal and hypogonadal (hpg) mice. Because the hpg mouse is totally deficient in hypothalamic GnRH, it provides an excellent model for investigating the effects of estrogen and GnRH on PRL synthesis and release in the presence of a totally inactive gonadotropingonadal system. Estrogen stimulated the synthesis and release of pituitary PRL in both sexes of normal and hpg mice, and this marked increase in PRL synthesis correlated with an equally dramatic increase in pituitary PRL mRNA levels. Administration of GnRH alone produced a slight but significant increase in pituitary PRL content, which is consistent with an action of GnRH on prolactotropes, either directly or by way of a paracrine action involving gonadotropes.
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