Morphological Evidence for Functional Crosstalk Between Multiple GnRH Systems in the Male Tilapia, Oreochromis niloticus.

Satoshi Ogawa,Ishwar Parhar

doi:10.3389/fendo.2020.00586

Abstract

Gonadotropin-releasing hormone (GnRH) is a reproductive neuropeptide, which controls vertebrate reproduction. In most vertebrates, there are more than two GnRH orthologs in the brain. In cichlid fish, the Nile tilapia (Oreochromis niloticus), GnRH1 is the primary hypophysiotropic hormone, while GnRH2 and GnRH3 are non-hypophysiotropic but neuromodulatory in function. Hypophysiotropic GnRH neurons are thought to inter-communicate, while it remains unknown if hypophysiotropic and non-hypophysiotropic GnRH systems communicate with each other. In the present study, we examined interrelationship between three GnRH types using specific antibodies raised against their respective GnRH associated peptide (GAP) sequence. Double-immunofluorescence labeling coupled with confocal microscopy revealed that in sexually mature males, GnRH-GAP1-immunoreactive (-ir) processes are in proximities of GnRH-GAP3-ir cell somata in the terminal nerve, while GnRH-GAP1-ir cell somata were also accompanied by GnRH-GAP3-ir processes in the preoptic area. However, such interaction was not seen in immature males. Further, there was no interaction between GnRH-GAP2 and GnRH-GAP1 or GnRH-GAP3 neurons. Single cell gene expression analysis revealed co-expression of multiple GnRH receptor genes (gnrhr1 and gnrhr2) in three GnRH-GAP cell types. In mature males, high levels of gnrhr2 mRNA were expressed in GnRH-GAP1-ir cells. In immature males, gnrhr1 and gnrhr2 mRNAs are highly expressed in GnRH-GAP3-ir cells. These results suggest heterologous interactions between the three GnRH-GAP cell types and their potential functional interaction during different reproductive stages.

Highlights

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that primarily plays an essential role in the control of reproduction in vertebrates [1,2,3,4,5]
Distinct single bands were detected in the brain and pituitary homogenate reacted with the antisera to GnRH associated peptide (GAP) types (Figure 1A)
Detected immunoreactive bands were seen in unexpected sizes ranging from 35 to 45 KDa for all antisera, which are far bigger than the expected sizes of tilapia GAP types (7, 5.7, and 6 kDa for GAP1, GAP2, and GAP3, respectively) or prepro-GnRH types (11.4, 9.6, and 10.1 KDa for tilapia prepro-GnRH1, GnRH2, and GnRH3, respectively)

Summary

Introduction

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that primarily plays an essential role in the control of reproduction in vertebrates [1,2,3,4,5]. Most vertebrate species possess more than two different GnRH types and their receptor (GnRH receptor, GnRHR) orthologs [7, 8]. GnRH2 is structurally most conserved in vertebrates and is expressed in the midbrain region. In addition to these two GnRH types, the third type of GnRH (GnRH3 = salmon GnRH) is present in the forebrain of teleosts. Most vertebrates possess more than two GnRHR types. GnRHR orthologs have been classified into three major lineages of GnRHR types that are further subdivide into five classes: non-mammalian type I (GnRHRn1 and GnRHRn1b), non-mammalian type II (GnRHRn2), and non-mammalian type III (GnRHRn3 and GnRHRn3b) [11]

Methods

Results

Conclusion