Effects of acute estradiol on 5-hydroxytryptamine and dopamine receptor subtype mRNA expression in female rat brain

The aim of the present study was to determine the effect of estradiol-17beta, in its positive feedback mode for the release of prolactin and luteinizing hormone (LH), on gene expression of 5-hydroxytryptamine (5-HT) and dopamine receptors. Gene expression was determined by measurement of the levels of receptor mRNA by in situ hybridization in brain sections from adult female rats. The animals were ovariectomized under halothane anesthesia on the morning of diestrus, given a sc injection of either estradiol benzoate (EB) or oil (vehicle), and killed between 1600-1700 h of the next day, presumptive proestrus. The injection of EB in this preparation is known to stimulate a massive surge of LH and prolactin in the late afternoon of presumptive proestrus. The injection of EB produced a significant increase in silver grain densities representing 5-HT(2) receptor mRNA in the dorsal raphe nucleus (DRN) and a significant reduction in these grain densities in the medial septum and diagonal band of Broca. The number of cells expressing 5-HT(2) receptor mRNA was increased by 290% in the DRN and decreased by 25% in the medial preoptic area. Estradiol had no effect on silver grain densities, or numbers or percentages of cells expressing 5-HT(2) receptor mRNA in any of the other nine brain regions or choroid plexus studied, and had no effect on the levels of 5-HT(1A), 5-HT(1C), D(1), or D(2) receptor mRNA in any region studied. These data suggest that the 5-HT(2) receptor is a key receptor involved in mediating the positive feedback stimulation by estradiol-17beta of LH and prolactin release. Since estradiol increases 5-HT levels in the DRN, the data raise the intriguing possibility that estrogen positive feedback on LH and prolactin release is mediated by a feed-forward 5-HT drive which results in increased 5-HT release at DRN-derived nerve terminals on hypothalamic neurons which control LH and prolactin release.