Abstract
Estrogen receptors (ERs) ERalpha and ERbeta are important for sexual differentiation of the brain. Previous studies in rats reported that the locus coeruleus (LC), a catecholaminergic nucleus in the brain stem, is sexually dimorphic such that females have more neurons than males. We hypothesized that ERs may be important for sexual differentiation of this nucleus in mice. Since previous studies reported conflicting results regarding ER protein expression in the mouse LC, we evaluated ERalpha and ERbeta gene expression by in situ hybridization and real-time RT-PCR. We demonstrated that both ERalpha and ERbeta mRNAs are present in tyrosine hydroxylase (TH)-positive cells in the male LC. In the female LC, ERalpha is present at levels similar to males while ERbeta expression is significantly lower than in males. Like rats, male mice have fewer TH-positive cells in the LC than females at 60 days after birth, but the difference is absent at 120 days after birth when females exhibit a similar reduction in TH-positive cells. This transient sex difference is abolished in ERbeta knockout mice (betaERKOs). In addition, the ERbeta-dependent sex difference in wild type (WT) mice results from regulation of TH expression and not from death of TH-positive cells. Testicular hormones produced at adolescence are necessary for the regulation of TH expression in the male LC since orchidectomy of pre-pubertal males prevented the decrease in TH-positive cells, while treatment of gonadectomized males with testosterone restored the intact male phenotype. In contrast, treatment of ovariectomized females with testosterone or its metabolites, dihydrotestosterone (DHT) and 5alpha-androstane-3beta, 17beta-diol (3beta-diol), did not decrease the number of TH-positive cells in the LC. Testosterone could be converted to DHT or 3beta-diol in the adolescent LC since the enzymes necessary for this reaction, 5alpha-reductase and 3alpha-hydroxysteroid dehydrogenase, are locally expressed. Overall, these studies indicate that ERbeta is differentially expressed in males and females and is important in directing morphological changes in the mouse LC.