Abstract
Gonadotropin-releasing hormone (Gnrh) plays a major role in the regulation of physiological and behavioural processes related to reproduction. In the pituitary, it stimulates gonadotropin synthesis and release via activation of Gnrh receptors (Gnrhr), belonging to the G protein-coupled receptor superfamily. Evidence suggests that differential regulation of the two gonadotropins (Fsh and Lh) is achieved through activation of distinct intracellular pathways and, probably, through the action of distinct receptors. However, the roles of the different Gnrhr isoforms in teleosts are still not well understood. This study investigates the gene expression of Gnrhr in the pituitary gland of precociously maturing Atlantic salmon (Salmo salar) male parr. A total of six Gnrhr paralogs were identified in the Atlantic salmon genome and named according to phylogenetic relationship; gnrhr1caα, gnrhr1caβ, gnrhr1cbα, gnrhr1cbβ, gnrhr2bbα, gnrhr2bbβ. All paralogs, except gnrhr1caα, were expressed in male parr pituitary during gonadal maturation as evidenced by qPCR analysis. Only one gene, gnrhr2bbα, was differentially expressed depending on maturational stage (yearly cycle), with high expression levels in maturing fish, increasing in parallel with gonadotropin subunit gene expression. Additionally, a correlation in daily expression levels was detected between gnrhr2bbα and lhb (daily cycle) in immature fish in mid-April. Double fluorescence in situ hybridization showed that gnrhr2bbα was expressed exclusively in lhb gonadotropes in the pituitary, with no expression detected in fshb cells. These results suggest the involvement of receptor paralog gnrhr2bbα in the regulation of lhb cells, and not fshb cells, in sexually maturing Atlantic salmon male parr.
Highlights
The complex series of morphological and physiological changes occurring during sexual maturation in vertebrates is driven through increased activity in the brain-pituitary-gonad axis
follicle-stimulating hormone (Fsh) is involved in the induction of spermatogonial proliferation and spermatogenesis (García-López et al, 2009; Loir, 1999; Mazón et al, 2014), while luteinizing hormone (Lh) is involved in the final stages of gamete maturation, regulating spermiogenesis and spermiation (Schulz et al, 2010)
The Gnrh receptors (Gnrhr) are divided into two main types, GnRHR1 and GnRHR2, each divided into three subtypes: GnRHR1a, GnRHR1b, GnRHR1c for the GnRHR type 1, and GnRHR2a, GnRHR2b and GnRHR2c for the type 2
Summary
The complex series of morphological and physiological changes occurring during sexual maturation in vertebrates is driven through increased activity in the brain-pituitary-gonad axis. Fsh is involved in the induction of spermatogonial proliferation and spermatogenesis (García-López et al, 2009; Loir, 1999; Mazón et al, 2014), while Lh is involved in the final stages of gamete maturation, regulating spermiogenesis and spermiation (Schulz et al, 2010). In male salmonids, both fshb mRNA in the pituitary (Maugars and Schmitz, 2008a; Melo et al, 2014) and Fsh plasma protein levels (Campbell et al, 2003) increase during early stages of maturation. On the basis of early presence in plasma and involvement in steroidogenesis and gametogenesis, Fsh is considered to be the main actor during early stages of maturation while Lh is mainly involved during final stages of maturation and spawning (for review see Levavi-Sivan et al, 2010; Weltzien et al, 2004)
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