Activin A regulation of gonadotropin-releasing hormone synthesis and release in vitro

Activin is essential for the regulation of normal mammalian reproductive function at both the pituitary and gonadal levels. However, its central actions in the control of the hypothalamic-pituitary-gonadal axis remain largely unexplored. The present study aims to determine whether activin could regulate the reproductive axis at the level of the hypothalamus, through control of the GnRH neuroendocrine system. Using the GnRH-secreting GT1-7 neuronal cell line as a model system, we demonstrate expression of mRNAs encoding activin receptor types I, IB, and II. We examined the effects of activin A on GnRH protein secretion and mRNA levels in GT1-7 cells. Treatment with rh-activin A regulated both GnRH protein secretion and GnRH mRNA expression in the GT1-7 cells in a time-dependent fashion. Using transient transfection assays, we explored a potential transcriptional basis for these changes. Activin A increased reporter gene activity driven by minimal GnRH enhancer and promoter elements, suggesting that activin may regulate GnRH gene expression at the level of transcription. Lastly, activin A treatment of male rat hypothalami, in vitro, increased GnRH protein secretion. Collectively, molecular and physiological evidence support the presence of an activin system which might act at a hypothalamic site to regulate mammalian reproduction via activation of GnRH synthesis and release.