The estrogen receptor (ER) α, but not ER β, gene is expressed in hypothalamic growth hormone-releasing hormone neurons of the adult female rat

Growth hormone (GH) synthesis and release from pituitary somatotropes is controlled by the opposing actions of the hypothalamic neuropeptides, GH-releasing hormone (GHRH) in the arcuate nucleus (ARC), and somatostatin in the periventricular nucleus (PeV) and ARC. There is a striking sex difference in the pattern of GH secretion in rats. We have previously demonstrated in male rats that 70% of GHRH neurons in the ARC contain the estrogen receptor α (ER α) gene, whereas less than 5% of somatostatin neurons in the ARC and PeV expressed the ER α or ER β gene. In addition, it has been reported that the PeV somatostatin neurons of neither sex possess ER immunoreactivity. However, there is no available data about colocalization of ERs and GHRH and/or somatostatin in the ARC of female rats. In this study, we used in situ hybridization in the adult female rat brain to determine whether GHRH neurons and/or somatostatin neurons in the ARC coexpress the ER α or ER β gene. In the ARC, ER α mRNA was seen in the ventrolateral region where GHRH mRNA signals were also observed, and in the dorsomedial region where somatostatin mRNA signals were also observed. From studies using adjacent sections through these areas, the distribution of these cells appeared to overlap in part with that of cells containing ER α mRNA. On the other hand, few positive cells for ER β mRNA were observed in the ARC. The double-label in situ hybridization studies showed that in the ARC, 73.4% of GHRH neurons contain ER α mRNA, whereas less than 5% of somatostatin neurons express the ER α gene. These results indicated that the majority of the GHRH neurons in ARC have ER α, but not ER β, and few somatostatin neurons in ARC have ER α or ER β in either adult female or male rats, suggesting that colocalization with ERs in GHRH and/or somatostatin neurons is not an important determinant of the gender specific pattern of GH secretion.