Gnrh1-induced responses are indirect in female medaka Fsh cells
Reproductive function in vertebrates is stimulated by gonadotropin-releasing hormone (GnRH) that controls the synthesis and release of the two pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH, which regulates different stages of gonadal development, are produced by two different cell types in the fish pituitary, in contrast to mammals and birds, thus allowing the investigation of their differential regulation. In the present work, we show by fluorescent in situ hybridization that Lh cells in adult female medaka express Gnrh receptors, whereas Fsh cells do not. This is confirmed by patch clamp recordings and cytosolic Ca2+ measurements on dispersed pituitary cells, where Lh cells, but not Fsh cells, respond to Gnrh1 by increased action potential frequencies and cytosolic Ca2+ levels. In contrast, both Fsh and Lh cells are able to respond electrically and by elevating the cytosolic Ca2+ levels to Gnrh1 in brain-pituitary tissue slices. Using Ca2+ uncaging in combination with patch clamp recordings and cytosolic Ca2+ measurements, we show that Fsh and Lh cells form homo- and heterotypic networks in the pituitary. Taken together, these results show that the effects of Gnrh1 on Fsh release in adult female medaka is indirect, likely mediated via Lh cells.