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

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.