Characterization of Gnrh/Gnih elements in the olfacto-retinal system and ovary during zebrafish ovarian maturation

Abstract

Gonadotropin releasing hormone (GnRH) is one of the key players of brain-pituitary-gonad axis, exerting overall control over vertebrate reproduction. In zebrafish, two variants were characterized and named as Gnrh2 and Gnrh3. In this species, Gnrh3, the hypohysiotropic form, is expressed by neurons of the olfactory-retinal system, where it is related with food detection, intra/interspecific recognition, visual acuity and retinal processing modulation. Previous studies have reported the presence of Gnrh receptors in the zebrafish retina, but not yet in the zebrafish olfactory epithelium. The current study analyzed the presence of gnrh2 and gnrh3, their receptors (gnrhr 1,2,3 and 4) and gnih (gonadotropin inhibitory hormone) transcripts, as well as the Gnrh3 protein in the olfactory epithelium (OE), olfactory bulb (OB), retina and ovary during zebrafish ovarian maturation. We found an increase of gnrh receptors transcripts in the OE at the final stages of ovarian maturation. In the OE, Gnrh3 protein was detected in the olfactory receptor neurons cilia and in the olfactory nerve fibers. Interestingly, in the OB, we found an inverse expression pattern between gnih and gnrh3. In the retina, gnrhr4 mRNA was found in the nuclei of amacrine, bipolar, and ganglion cells next to Gnrh3 positive fibers. In the ovary, gnrh3, gnrhr2 and gnrhr4 transcripts were found in perinucleolar oocytes, while gnih in oocytes at the cortical alveolus stage. Our results suggested that Gnrh/Gnih elements are involved in the neuromodulation of the sensorial system particularly at the final stages of maturation, playing also a paracrine role in the ovary.

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

“…We identified 2 GnIH‐IR cell clusters during early life stages, with different spatial and temporal expression patterns: GnIH‐IR neurones in the anterior region of the brain were detected in the NOR by 3 dph, whereas those at the hypothalamic level were found in the NPP by 14 dph. The location of GnIH‐IR neurones in the NOR was similar to that previously described in adults of C. dimerus and other fish species . However, Biswas et al .…”

“…An increase in GnIH‐IR fibre density was observed in the inner plexiform layer of the retina, optic tract and optic tectum from 5 dph. A high number of fibres in these brain regions was also detected during the ontogeny of L. rohita and in adults of other fish species such as D. labrax , O. niloticus , S. senegalensis and D. rerio . These results provide anatomical evidence supporting a potential role of GnIH as a neuromodulator of photoreception and integration of different sensory modalities not only in adults, but also throughout development.…”

“…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 . By contrast, another study on the same fish species reported the presence of a cluster of GnIH‐IR cells in that region, after performing in situ hybridisation . The opposing results of both studies may reflect the different methodologies employed.…”

“…The location of GnIH-IR neurones in the NOR was similar to that previously described in adults of C. dimerus 31 and other fish species. 7,8,43,44 However, Biswas et al 27 reported the presence of GnIH-IR cells in the olfactory epithelium and bulb, which varied in immunostaining intensity during development, and observed no immunoreaction in adult L. rohita. Considering that the present study and that by Biswas et al 27 used the same antiserum, it is possible that species-specific differences exist in the expression of GnIH in the NOR.…”

“…11 By contrast, another study on the same fish species reported the presence of a cluster of GnIH-IR cells in that region, after performing in situ hybridisation. 44 The opposing results of both studies may reflect the different methodologies employed.…”