Abstract
The hypothalamic-pituitary-gonadal (HPG) axis is primarily regulated by kisspeptin neurons. In addition, activin and inhibin within the central nervous system might contribute to the regulation of the HPG axis because they are expressed near kisspeptin and gonadotropin-releasing hormone (GnRH) neurons. We investigated the effects of inhibin and activin within the hypothalamus in the estradiol (E2)-induced negative feedback mechanism. Inhibin α subunit gene within the posterior hypothalamus in female rats increased after ovariectomy, and this increase was completely suppressed by E2 supplementation. In contrast, inhibin βA subunit decreased after ovariectomy and this reduction was recovered by E2. In ovary-intact rats, E2 reduced inhibin α subunit and increased inhibin βA expression within the hypothalamus. In the rHypoE8 and GT1-7 hypothalamic cell models, E2 stimulation increased inhibin α subunit gene expression. Activin and inhibin A increased Kiss1 gene expression in GT1-7 cells, while inhibin B reduced it. Kisspeptin increased inhibin α subunit expression in rHypoE8 cells, GT1-7 cells, and the mHypoA55 hypothalamic KNDy neuron cell model. Our findings suggest that the expression of inhibin subunits, especially inhibin α, could be increased by E2 in hypothalamic cells and that kisspeptin, inhibin, and activin mutually influence each other under the actions of E2, but their regulation might be controlled mainly by kisspeptin neurons in vivo. Although the effects of activin and inhibin on Kiss1 gene expression varied depending on the hypothalamic cell model examined, intracerebral inhibin and activin might have potential roles in the E2-induced negative feedback mechanism under the influence of kisspeptin neurons.