A Type IIb, but Not Type IIa, GnRH Receptor Mediates GnRH-Induced Release of Growth Hormone in the Ricefield Eel.

Dong Chen,Shiying Han,Jiang Liu,Huiyi Yang,Weimin Zhang,Lihong Zhang,Xin Cen,Wei Yang

doi:10.3389/fendo.2018.00721

Abstract

Multiple gonadotropin-releasing hormone receptors (GnRHRs) are present in vertebrates, but their differential physiological relevances remain to be clarified. In the present study, we identified three GnRH ligands GnRH1 (pjGnRH), GnRH2 (cGnRH-II), and GnRH3 (sGnRH) from the brain, and two GnRH receptors GnRHR1 (GnRHR IIa) and GnRHR2 (GnRHR IIb) from the pituitary of the ricefield eel Monopterus albus. GnRH1 and GnRH3 but not GnRH2 immunoreactive neurons were detected in the pre-optic area, hypothalamus, and pituitary, suggesting that GnRH1 and GnRH3 may exert hypophysiotropic roles in ricefield eels. gnrhr1 mRNA was mainly detected in the pituitary, whereas gnrhr2 mRNA broadly in tissues of both females and males. In the pituitary, GnRHR1 and GnRHR2 immunoreactive cells were differentially distributed, with GnRHR1 immunoreactive cells mainly in peripheral areas of the adenohypophysis whereas GnRHR2 immunoreactive cells in the multicellular layers of adenohypophysis adjacent to the neurohypophysis. Dual-label fluorescent immunostaining showed that GnRHR2 but not GnRHR1 was localized to somatotropes, and all somatotropes are GnRHR2-positive cells and vice versa at all stages examined. GnRH1 and GnRH3 were shown to stimulate growth hormone (Gh) release from primary culture of pituitary cells, and to decrease Gh contents in the pituitary of ricefield eels 12 h post injection. GnRH1 and GnRH3 stimulated Gh release probably via PLC/IP3/PKC and Ca2+ pathways. These results, as a whole, suggested that GnRHs may bind to GnRHR2 but not GnRHR1 to trigger Gh release in ricefield eels, and provided novel information on differential roles of multiple GnRH receptors in vertebrates.

Highlights

Gonadotropin-releasing hormone (GnRH) is a decapeptide best known for its action in releasing gonadotropins through binding to GnRH receptors on gonadotropes of the pituitary in vertebrates
GnRH immunoreactive signals disappeared after pre-adsorption of the antiserum with an excess of the corresponding synthetic ricefield eel GnRH peptide, suggesting the specificities of the GnRH immunostaining in the brain and pituitary of ricefield eels (Figure S3)
The presence of GnRH immunoreactive fibers in the pituitary is suggestive of hypophysiotropic roles of the corresponding GnRH form

Summary

Introduction

Gonadotropin-releasing hormone (GnRH) is a decapeptide best known for its action in releasing gonadotropins through binding to GnRH receptors on gonadotropes of the pituitary in vertebrates. In addition to stimulating gonadotropes, GnRHs have been suggested to regulate other pituitary cells including somatotropes in mammals and fishes [1]. GnRH was shown to increase GH release in vitro from hemipituitaries in combination with enkephalin [3] and perifused pituitary cells [4]. GnRHs have been shown to stimulate GH release from cultured pituitary cells of goldfish [5] and tilapia [6], and from pituitary fragments of common carp [7, 8]. GnRHs have been demonstrated to upregulate gh mRNA in some teleosts including the goldfish [9], common carp [10], blue gourami fish [11], and masu salmon [12]

Methods

Results

Conclusion