Results of previous studies provided evidence for the existence of a functional gonadotropin-inhibitory hormone (GnIH) system in the European sea bass,
Dicentrarchus labrax
, which exerted an ...inhibitory action on the brain-pituitary-gonadal axis of this species. Herein, we further elucidated the intracellular signaling pathways mediating in sea bass GnIH actions and the potential interactions with sea bass kisspeptin (Kiss) signaling. Although GnIH1 and GnIH2 had no effect on basal CRE-luc activity, they significantly decreased forskolin-elicited CRE-luc activity in COS-7 cells transfected with their cognate receptor GnIHR. Moreover, an evident increase in SRE-luc activity was noticed when COS-7 cells expressing GnIHR were challenged with both GnIH peptides, and this stimulatory action was significantly reduced by two inhibitors of the PKC pathway. Notably, GnIH2 antagonized Kiss2-evoked CRE-luc activity in COS-7 cells expressing GnIHR and Kiss2 receptor (Kiss2R). However, GnIH peptides did not alter NFAT-RE-luc activity and ERK phosphorylation levels. These data indicate that sea bass GnIHR signals can be transduced through the PKA and PKC pathways, and GnIH can interfere with kisspeptin actions by reducing its signaling. Our results provide additional evidence for the understanding of signaling pathways activated by GnIH peptides in teleosts, and represent a starting point for the study of interactions with multiple neuroendocrine factors on cell signaling.
Previous studies have revealed the stimulatory and inhibitory actions of gonadotropin-releasing hormone (GnRH) and gonadotropin-inhibitory hormone (GnIH) on the control of reproduction in European ...sea bass (Dicentrarchus labrax) and other vertebrates, respectively. However, information on the possible interactions between GnRH and GnIH on cell signaling is sparse in vertebrates. In the current study, we investigated if activation of sea bass GnIH receptor (GnIHR) can interfere with GnRH receptor II-1a (GnRHR-II-1a) involving the PKA pathway. Our results showed that GnIH and GnRH functioned via their cognate receptors, respectively. However, it appears that neither GnIH1 nor GnIH2 can block GnRH/GnRHR-II-1a-induced PKA signaling in sea bass. This is the first study to examine the potential interactions of GnIH with GnRH receptor signaling in teleosts. Further research seems necessary to shed light on unknown interactions in other signaling pathways and other GnIH/GnRH receptors involved in the physiological functions of these two relevant neuropeptides, not only in sea bass but also in other species.
Display omitted
•Potential interactions of GnIH with GnRH receptor signaling are unknown in teleosts.•GnIH and GnRH functioned via their cognate receptors in sea bass.•Neither GnIH1 nor GnIH2 can block GnRH/GnRHR-II-1a-induced PKA signaling in sea bass.•Interactions in other signaling pathways and other GnIH/GnRH receptors should be elucidated.
Gonadotropin-inhibitory hormone, GnIH, is named because of its function in birds and mammals; however, in other vertebrates this function is not yet clearly established. More than half of the ...vertebrate species are teleosts. This group is characterized by the 3R whole genome duplication, a fact that could have been responsible for the great phenotypic complexity and great variability in reproductive strategies and sexual behavior. In this context, we revise GnIH cell bodies and fibers distribution in adult brains of teleosts, discuss its relationship with GnRH variants and summarize the few reports available about the ontogeny of the GnIH system. Considering all the information presented in this review, we propose that in teleosts, GnIH could have other functions beyond reproduction or act as an integrative signal in the reproductive process. However, further studies are required in order to clarify the role of GnIH in this group including its involvement in development, a key stage that strongly impacts on adult life.
The role of rearing temperature on fish development, sex differentiation and puberty has been largely addressed, but the impact of water temperature on the ontogeny of the main neuroendocrine systems ...controlling reproduction has received little attention. Gonadotropin-inhibitory hormone (GnIH) has been shown to act on gonadotropin-releasing hormone (GnRH) neurons and on the pituitary to inhibit gonadotropin release and synthesis in vertebrates, including sea bass, Dicentrarchus labrax. In the present study we investigated the effects of rearing temperature during the thermosensitive period (5-60days post-fertilization, dpf) on the expression of the GnIH gene (gnih) and its receptor (gnihr). Animals were maintained under two different conditions, low temperature (LT, 15°C) or high temperature (HT, 21°C), throughout the thermosensitive period and sampled from 5 to 360dpf at mid-light (ML) and mid-dark (MD). Our results showed significant effects of temperature on gnih and gnihr expression during the thermosensitive period, with higher transcript levels under LT condition. Some differences were also evident after the completion of the sex differentiation process. Moreover, we revealed daily variations in the developmental expression of gnih and gnihr, with higher diurnal mRNA levels at early stages (until 25dpf), and a shift to higher nocturnal expression levels at 300-360dpf, which corresponded with the beginning of the winter (reproductive season). To the best of our knowledge, this work represents the first study reporting the effects of rearing temperature on the transcription of gnih system genes, as well as its daily variations during the development of a fish species.
Neuropeptide FF (NPFF) and gonadotropin-inhibitory hormone (GnIH) are thought to be paralogous, and a recent study has revealed that both NPFF and GnIH peptides can activate the GnIH receptor (GnIHR, ...also called NPFFR1) in the European sea bass (
Dicentrarchus labrax
). However, whether GnIH can bind to the NPFF receptor (NPFFR2) is still yet unknown in this species. Accordingly, we further investigated the potential interactions between GnIH and NPFFR2 (two NPFFR2 forms present in sea bass, namely NPFFR2-1 and NPFFR2-2) on the intracellular signaling pathways. Neither GnIH1 nor GnIH2 had any effect on basal CRE-luc activity, while forskolin-stimulated CRE-luc activity was significantly reduced when COS-7 cells expressing sea bass NPFFR2-1 and NPFFR2-2 were challenged with these two GnIH peptides. NPFF and NPAF also inhibited forskolin-induced CRE-luc activity via their cognate receptors. An evident stimulation of SRE-luc activity was observed when COS-7 cells transfected with NPFFR2-1 and NPFFR2-2 were treated with NPFF and NPAF, whereas GnIH peptides had no effect, except a slight but significant increase elicited by 1000 nM of GnIH1 in COS-7 cells expressing NPFFR2-2. Moreover, only GnIH2 exerted an inhibitory action on NFAT-RE-luc activity in COS-7 cells expressing NPFFR2-1. None of GnIH or NPFF peptides altered ERK phosphorylation levels via NPFFR2 receptors. Our results provide new evidence that sea bass GnIH peptides may exert their functions partially via NPFFR2, and PKA, PKC and Ca
2+
routes are potential mediators.
The seasonally changing photoperiod controls the timing of reproduction in most fish species, however, the transduction of this photoperiodic information to the reproductive axis is still unclear. ...This study explored the potential role of two candidate neuropeptide systems, gonadotropin-inhibitory hormone (Gnih) and kisspeptin, as mediators between the pineal organ (a principle transducer of photoperiodic information) and reproductive axis in male European sea bass, Dicentrarchus labrax. Two seven-day experiments of pinealectomy (Px) were performed, in March (end of reproductive season) and August (resting season). Effects of Px and season on the brain expression of gnih (sbgnih) and its receptor (sbgnihr), kisspeptins (kiss1, kiss2) and their receptors (kissr2, kissr3) and gonadotropin-releasing hormone (gnrh1, gnrh2, gnrh3) and the main brain receptor (gnrhr-II-2b) genes, plasma melatonin levels and locomotor activity rhythms were examined. Results showed that Px reduced night-time plasma melatonin levels. Gene expression analyses demonstrated a sensitivity of the Gnih system to Px in March, with a reduction in sbgnih in the mid-hindbrain, a region with bilateral connections to the pineal organ. In August, kiss2 levels increased in Px animals but not in controls. Significant differences in expression were observed for diencephalic sbgnih, sbgnihr, kissr3 and tegmental gnrh2 between seasons. Recordings of locomotor activity following surgery revealed a change from light-synchronised to free-running rhythmic behavior. Altogether, the Gnih and Kiss2 sensitivity to Px and seasonal differences observed for Gnih and its receptor, Gnrh2, and the receptor for Kiss2 (Kissr3), suggested they could be mediators involved in the relay between environment and seasonal reproduction.
Visual photoreceptors in fish are usually adjusted to the light environment to ensure the highest efficiency and best adaptation. In the Senegalese sole, metamorphosis determines migration from ...pelagic to benthic habitats, with marked differences in both light intensity and spectrum. Here, we analysed the ontogeny of six visual photopigments, namely, rod opsin (
rh1
), short wavelength-sensitive (
sws1
and
sws2
), medium wavelength-sensitive (
rh2.3
and
rh2.4
), and long wavelength-sensitive (
lws
) cone opsins, in sole specimens maintained in light-dark cycles of white (LDW), blue (LDB), red (LDR), and continuous white (LL) lights by using RT-qPCR and
in situ
hybridisation. Most of the opsins displayed a similar developmental expression pattern under all tested conditions. However, lower transcripts were detected under LDR and LL compared to LDW and LDB. A significant increase in gene expression was detected before and after metamorphosis, reaching minimum transcript levels at hatching and during metamorphosis. Interestingly, green opsins (
rh2.3
and
rh2.4
) displayed a significant increase only before metamorphosis, with their expression remaining low during and after metamorphosis. The rod opsin and short-, medium-, and long-wavelength sensitive cone opsins were detected in retinal photoreceptors of the sole from pre-metamorphic to adult stages by
in situ
hybridisation. In adults, the short-wavelength cone opsins (
sws1
and
sws2
) were found in single cones, whereas the medium- (
rh2.4
) and long-wavelength (
lws
) cone opsins were present in double cones. The results obtained by
in situ
hybridisation in the retina of developing sole, in terms of number of positive cells and/or intensity of labelling, were consistent with the ontogenetic transcript patterns found by RT-qPCR, suggesting that most of the visual opsin expressions detected in the whole specimens could correspond to retinal expression. Taken together, our results pointed out that the ontogeny of the Senegalese sole is accompanied by remodelling in opsin gene expression, with the green-cone opsins being the most abundant photopigments in pre-metamorphosis and rod opsin the dominant visual photopigment from the completion of metamorphosis onwards. These results enlarge our knowledge of flatfish metamorphosis and ecology and provide useful information to develop light protocols adapted to different ontogenetic stages that could improve welfare and production in sole aquaculture.
PDF
