Comparison of primary and secondary stimulation of male rats by estradiol in terms of prolactin synthesis and mRNA accumulation in the pituitary

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Abstract
Male rats received acute or chronic primary or acute secondary stimulation with estradiol, and the effects on pituitary prolactin synthesis and its mRNA accumulation were examined. Prolactin synthesis was determined by the in vitro incorporation of [3H]leucine into prolactin over 1 h. Prolactin mRNA was measured by cell-free translation in a nuclease-treated rabbit reticulocyte lysate and by hybridization to the complementary DNA. The latter 2 methods gave similar results under all experimental conditions. Acute primary stimulation with estradiol produced a significant increase in pituitary prolactin mRNA accumulation at 12 h, which further increased by 2- to 3-fold over the next 48 h. In contrast, no increase in prolactin synthesis was observed during the first 24 h. Chronic stimulation with estradiol induced increases of prolactin synthesis and prolactin mRNA that were quantitatively indistinguishable over 1-4 wk, reaching a plateau at 5-fold the basal values. By the 13th day after withdrawal of therapy prolactin synthesis and mRNA had returned to the prestimulation levels. When the effects of estradiol on previously unexposed and estrogen withdrawn animals were compared, secondary stimulation not only produced a more rapid accumulation of the prolactin mRNA but also abolished the lag period of prolactin synthesis observed during the primary estrogen stimulation. A lag in the endogenous translation of newly accumulated pituitary prolactin mRNA translatable in vitro after primary estrogen stimulation of male rats was demonstrated. The mechanism for the abolition of this lag during the secondary stimulation is not known.

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