Loss of function in somatostatin receptor 5 has no impact on the growth of medaka fish due to compensation by the other paralogs.

Somatic growth in vertebrates is regulated endocrinologically by the somatotropic axis, headed by the growth hormone (GH) and the insulin growth factor-I (IGF-I). Somatostatin (Sst), a peptide hormone synthesized in the hypothalamus, modulates GH actions through its receptors (Sstr). Four Sstr subtypes (Sstr 1-3 and 5) have been identified in teleosts. However, little is known about whether they have a specific function or tissue expression. The aim of this study was to determine the role of sstr2 and sstr5 in the growth of the medaka (Oryzias latipes). The assessed expression pattern across diverse tissues highlighted greater prevalence of sstr1 and sstr3 in brain, intestine and muscle than in pituitary or liver. The expression of sstr2 was high in all the tissues tested, while sstr5 was predominantly expressed in the pituitary gland. A CRISPR/Cas9 sstr5 mutant with loss of function (sstr5-/-) was produced. Assessment of sstr5-/- indicated no significant difference with the wild type regarding growth parameters such as standard length, body depth, or peduncle depth. Furthermore, the functional loss of sstr5 had no impact on the response to a nutritional challenge. The fact that several sstr subtypes were upregulated in different tissues in sstr5-/- medaka suggests that in the mutant fish, there may be a compensatory effect on the different tissues, predominantly by sstr1 in the liver, brain and pituitary, with sstr2 being upregulated in pituitary and liver, and sstr3 only presenting differential expression in the brain. Analysis of the sstr subtype and the sstr5-/- fish showed that sstr5 was not the only somatostatin receptor responsible for Sst-mediated Gh regulation.

Gonadotrophin-Inhibitory Hormone in the Cichlid Fish Cichlasoma dimerus: Structure, Brain Distribution and Differential Effects on the Secretion of Gonadotrophins and Growth Hormone.

The role of gonadotrophin-inhibitory hormone (GnIH) in the inhibition of the reproductive axis has been well-established in birds and mammals. However, its role in other vertebrates, such as the teleost fish, remains controversial. In this context, the present study aimed to evaluate whether GnIH modulates the release of gonadotrophins and growth hormone (GH) in the cichlid fish Cichlasoma dimerus. First, we partially sequenced the precursor polypeptide for GnIH and identified three putative GnIH peptides. Next, we analysed the expression of this precursor polypeptide via a polymerase chain reaction in the reproductive axis of both sexes. We found a high expression of the polypeptide in the hypothalamus and gonads of males. Immunocytochemistry allowed the observation of GnIH-immunoreactive somata in the nucleus posterioris periventricularis and the nucleus olfacto-retinalis, with no differences between the sexes. GnIH-immunoreactive fibres were present in all brain regions, with a high density in the nucleus lateralis tuberis and at both sides of the third ventricle. Finally, we performed in vitro studies on intact pituitary cultures to evaluate the effect of two doses (10(-6)  m and 10(-8)  m) of synthetic C. dimerus (cd-) LPQRFa-1 and LPQRFa-2 on the release of gonadotrophins and GH. We observed that cd-LPQRFa-1 decreased ß-luteinising hormone (LH) and ß-follicle-stimulating hormone (FSH) and also increased GH release to the culture medium. The release of ß-FSH was increased only when it was stimulated with the higher cd-LPQRFa-2 dose. The results of the present study indicate that cd-LPQRFa-1, the cichlid fish GnIH, inhibits ß-LH and ß-FSH release and stimulates GH release in intact pituitary cultures of C. dimerus. The results also show that cd-LPQRF-2 could act as an ß-FSH-releasing factor in this fish species.

Growth hormone, luteinizing hormone, and follicle-stimulating hormone regulation by neuropeptide Y in both sexes of the cichlid fish, Cichlasoma dimerus.

Neuropeptide Y (NPY) is considered the most potent orexigenic peptide, increasing before meal time and during fasting. In teleost, most studies on NPY action upon growth hormone (GH) and luteinizing hormone (LH) were conducted in females or group of animals without sex discrimination. The aim of this study was to evaluate whether NPY modulates the expression and release of GH and gonadotropins in both sexes of Cichlasoma dimerus. By double-label immunofluorescence, we first determined the association between NPY fibers and pituitary cells. In addition, we performed in vitro studies to evaluate the effect of NPY on GH and gonadotropins expression by real-time PCR, and release by Western blot, in males and females separately. Contacts between NPY fibers and GH and follicle-stimulating hormone (FSH)-producing cells were detected, indicating possible functional relationships. We observed an increase in GH release in the culture medium at 2 nM for males (p = 0.043) and 20 nM for females (p = 0.028). Pituitary FSH release was stimulated at 20 nM (p = 0.026) and 200 nM (p = 0.033) for males and females, respectively. Finally, NPY only increased ß-LH mRNA expression at 20 nM in females (p = 0.028) and its release at 2 nM (p = 0.049) and 200 nM for males (p = 0.005) and 200 nM for females (p = 0.018). In conclusion, NPY acts as a GH-, LH- and FSH-releasing factor, in a dose- and sex-dependent way.