The gonadotropin-inhibitory hormone (Lpxrfa) system's regulation of reproduction in the brain–pituitary axis of the zebrafish (Danio rerio)†

Abstract

Gonadotropin-inhibitory hormone (GNIH) was discovered in quail with the ability to reduce gonadotropin expression/secretion in the pituitary. There have been few studies on GNIH orthologs in teleosts (LPXRFamide (Lpxrfa) peptides), which have provided inconsistent results. Therefore, the goal of this study was to determine the roles and modes of action by which Lpxrfa exerts its functions in the brain-pituitary axis of zebrafish (Danio rerio). We localized Lpxrfa soma to the ventral hypothalamus, with fibers extending throughout the brain and to the pituitary. In the preoptic area, Lpxrfa fibers interact with gonadotropin-releasing hormone 3 (Gnrh3) soma. In pituitary explants, zebrafish peptide Lpxrfa-3 downregulated luteinizing hormone beta subunit and common alpha subunit expression. In addition, Lpxrfa-3 reduced gnrh3 expression in brain slices, offering another pathway for Lpxrfa to exert its effects on reproduction. Receptor activation studies, in a heterologous cell-based system, revealed that all three zebrafish Lpxrfa peptides activate Lpxrf-R2 and Lpxrf-R3 via the PKA/cAMP pathway. Receptor activation studies demonstrated that, in addition to activating Lpxrf receptors, zebrafish Lpxrfa-2 and Lpxrfa-3 antagonize Kisspeptin-2 (Kiss2) activation of Kisspeptin receptor-1a (Kiss1ra). The fact that kiss1ra-expressing neurons in the preoptic area are innervated by Lpxrfa-ir fibers suggests an additional pathway for Lpxrfa action. Therefore, our results suggest that Lpxrfa may act as a reproductive inhibitory neuropeptide in the zebrafish that interacts with Gnrh3 neurons in the brain and with gonadotropes in the pituitary, while also potentially utilizing the Kiss2/Kiss1ra pathway.

Ontogeny of gonadotrophin‐inhibitory hormone in the cichlid fish Cichlasoma dimerus

“…These results are supported by previous studies showing the presence of RFamide peptides in the NOR or nervus terminalis, where they co‐localise with GnRH, although not completely in some species . Recently, Spicer et al . demonstrated the lack of co‐localisation of GnIH and GnRH3, and the absence of a GnIH‐IR cell cluster in the nervus teminalis of Danio rerio , after using a specific antiserum raised against zebrafish‐LPXRFa .…”

“…Although GnIH was first characterised in this group in 2002, knowledge of its role increased only recently. Most studies focused on the reproductive function of GnIH and reported both stimulatory and inhibitory effects of the hormone . This dual effect has been proposed to depend on the action of sex‐steroids and neuro‐oestrogens and on the abundance of GnIH, GnRH and oestrogen receptors in the plasmatic membrane …”

“…Most studies focused on the reproductive function of GnIH and reported both stimulatory and inhibitory effects of the hormone. [6][7][8][9][10][11][12] This dual effect has been proposed to depend on the action of sex-steroids and neuro-oestrogens and on the abundance of GnIH, GnRH and oestrogen receptors in the plasmatic membrane. 13 Little is known about spatial-temporal expression patterns and roles of GnIH during development 1,14 because most published work focused on GnIH anatomical distribution and function in adult vertebrates.…”

Gonadotrophin‐inhibitory hormone and its mammalian orthologue RFamide‐related peptide‐3: Discovery and functional implications for reproduction and stress

“…Examination of GnIH:GnRH actions in the fish brain have been particularly informative. In zebrafish ( Danio rerio ), an elegant study on the GnIH orthologue (LPXRFamide, or LPXRFa) demonstrated that, in the preoptic area, LPXRFa fibres interact with gonadotrophin‐releasing hormone 3 (GnRH3) cell bodies and LPXRFa‐3 reduced GnRH3 expression in brain slices . Furthermore, zebrafish LPXRFa‐2 and LPXRFa‐3 antagonises kisspeptin‐2 (Kiss2) activation of kisspeptin receptor‐1a (Kiss1ra) and kiss1ra‐expressing neurones in the preoptic area are innervated by LPXRFa‐immunoreactive (‐ir) fibres; an overview of teleost kisspeptin is provided elsewhere .…”

“…The GnIH homologue in teleost fish, LPXRF, acts on the anterior pituitary gland across species. In pituitary explants of zebrafish, LPXRFa‐3 down‐regulates LHβ subunit and gonadotrophin common α subunit expression . In tilapia, the actions of GnIH on the anterior pituitary are likely more widespread, with LPXRFa‐receptor expressed in LH, adrenocorticotrophin‐releasing hormone and α‐melanocyte‐stimulating hormone cells, and LPXRFa in this species appears to be a positive regulator of reproduction …”