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Somatostatin (SST) is a 14-amino acid peptide produced in the hypothalamus of vertebrates, including fish. It regulates many physiological processes such as growth development and metabolic processes in the animal’s body. Negative control of growth hormone in vivo and in vitro was characterized in several fish species such as salmon, goldfish, rainbow trout and tilapia. Although very important, the SST/SST-R system in Nile tilapia (Oreochromis niloticus) was not deeply characterized. The somatostatin system in tilapia possess two ligands (Somatostatin1b and Somatostatin 2), and five receptors (SST-R 1-5). Unlike mammals, in fish, FSH and LH are secreted from different cell populations in the pituitary. By performing cell specific transcriptome analysis of double-labelled transgenic tilapia expressing GFP and RFP in LH or FSH cells, respectively, we identified genes specifically enriched in each cell type. Analysis of the RNA-seq discovered 4 types of SST-Rs: sstr2, sstr3, sstr5 and sstr5x3. The specific localization of each SST-R was identified by In Situ hybridization with specific probes for each of the SST-Rs. SST-R2 and SST-R5x3 were expressed on LH and FSH cells, while SST-R5 was exclusively expressed on LH cells. Interestingly, SST-R3, which was expressed on GH secreting cells, was also expressed on both gonadotropin-secreting cells. Transactivation assays, using COS7 cell line transfected with tilapia SST-Rs together with the reporter plasmid CRE-luc, demonstrated an effect through the cAMP/PKA pathway. Signal transduction analysis demonstrated that SST agonist (Octreotide; IC50 = 0.8-60nM) decreased the cAMP/PKA pathway, while an opposite effect was found when SST antagonist (Cyclosomatostatin; EC50 = 0.1 - 188 nM) was used. To understand the physiological effects of somatostatin on gonadotropins and GH release, we examined the effect of ip injection (100 μg/kg BW) of somatostatin agonist and antagonist on plasma FSH, LH and GH levels. SST agonist decreased plasma GH and FSH levels, as fast as two hours post injection and their levels remained low until the end of the experiment. On the other hand, SST antagonist increased LH and FSH levels two hours post injection, but while FSH levels remained high during the entire experiment, LH levels went back to basal levels afterwards. Our results show - for the first time in fish - a direct effect of SST on gonadotropin release, that could serve as a bridge between the GH-axis and the GTH-axis. The research was funded by the Israel Science Foundation (ISF) no. 1540/17.

From mammals to fish, gametogenesis and sexual maturation are driven by LH and FSH, the two gonadotropic hormones temporally secreted from the pituitary. Teleost fish are an excellent model for addressing the unique regulation and function of each gonadotropin hormone since, unlike mammals; they synthesize and secrete LH and FSH from distinct cells. By performing cell specific transcriptome analysis of double-labelled transgenic Nile tilapia (Oreochromis niloticus) expressing GFP and RFP in LH or FSH cells, respectively, we identified genes specifically enriched in each cell type. Though GnRH is considered the main neuropeptide regulating LH and FSH, we found that each LH and FSH cell express unique GPCR signature that reveals the direct regulation of additional metabolic and homeostatic hormones (like cholecystokinin, somatostatin and glutamate). Moreover, some of those GPCRs were conserved also in gonadotrophs of mammals (like PACAP receptor, Adropin receptor and GABBA receptor). Next, we had exploited the unique behavior of Nile tilapia where a behavioral hierarchy is created between males, to compare the gene expression in the pituitary and brain of dominant (reproducing) males to a subordinate (non-reproducing) males. By combining the two transcriptome sets we had identified novel players in the hypothalamic regulation of the HPG axis, and revealed how brain aromatase (cyp19a1b), that is enriched specifically in LH cells, is the key factor in regulating the activity of LH and FSH cells in dominant reproducing fish. Thereby, unraveling novel mechanisms in the differential regulation of LH and FSH. The research was funded by the Israel Science Foundation (ISF) no. 1540/17.