Cichlasoma dimerus as most teleost fish species present

Cichlasoma dimerus, as most teleost fish species, present neurons expressing GnRH3 (Pandolfi et al., 2005). As in most PF-03814735 species, the neurons expressing GnRH3 are mainly located in the NOR, although few neurons were detected in the olfactory bulb, ventral telencephalon and the POA (González-Martínez et al., 2002, Somoza et al., 2002, Kah et al., 2007). In Cichlasoma dimerus, fibers from these neurons are mainly located in the forebrain and contribute to pituitary innervation (Pandolfi et al., 2005). In those fish where the three GnRHs are present, the function of GnRH3 is not completely understood. There are some reports where this variant is considered as a neuromodulator of reproductive behavior (Yamamoto et al., 1997, Ogawa et al., 2006). In addition, in C. dimerus females, the somatic and nuclear area of GnRH3 neurons varies according the reproductive phase suggesting that they are also related to reproduction (Tubert et al., 2012). In previous studies, we observed that GnIH-ir neurons were localized in the NOR and the morphology of these cells was similar to GnRH3 neurons located in this area (Di Yorio et al., 2016). In the present study, we corroborated that both peptides were co-expressed in the same neurons. Similar results were previously obtained in larvae of the same species (Di Yorio et al., 2018). It is interesting to note that in the forebrain some fibers co-expressed both peptides whereas some other fibers only express GnRH3 or GnIH, suggesting that these fibers correspond to neurons located in other brain areas like the NPP (for GnIH) or from the OB, ventral TEL and POA (for GnRH3). On the other hand, at the NPP no contacts between GnRH3 fibers and GnIH neurons were observed, but co-expression was observed in fibers in the NLT and NPP. Although the probability of a cross-reaction was very low, we decided to perform preadsortion tests where each antiserum was preadsorbed with the respective and the other peptide and the obtained results discarded this possibility. Despite that GnIH neurons in the forebrain were described in many fish species (Muñoz-Cueto et al., 2017), we cannot rule out the possibility that the antiserum used recognized other LPXRFamide peptide as suggested by Sawada et al., 2002, Ogawa et al., 2016. The present results provide new clues to investigate the possible cooperation or interaction between both peptides in the modulation of the reproductive network in teleost fish. In summary, we described here the neuroanatomical relationship between GnIH and GnRH neurons in C. dimerus. To our knowledge, this is the first report analyzing such relationship considering neurons expressing the three GnRH variants in one species. Thus, we can highlight the following facts: 1) GnIH-ir neurons co-expressed GnRH3 variant in the NOR; 2) As fiber-to-fiber contacts between GnIH-ir and cGAP2-ir were detected, a direct modulation between them can be proposed; 3) Whether GnIH regulates the expression of GnRH1 and/or its secretion in this species, an indirect modulation or neuronal plasticity can be proposed since no contacts between GnIH-ir and sbGAP-ir cell and processes were observed.
Acknowledgments We thank Dr Julieta Sztarker for kindly providing the streptavidin-Alexa 647-conjugate, and Dr. Ariadna Battista and Roberto Fernandez for their technical assistance. Also, we thank Mr. Andrés Bonaldo for providing the commercial pellets to feed the adult fish. This work was supported by Agencia Nacional de Promoción Científica y Tecnológica (grants numbers: Préstamo BID PICT 2014-2405 to MPDY and Préstamo BID PICT 2015-2783 to GMS), Universidad de Buenos Aires (grant number: 20020160100110BA to PV) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIP 2014–2016: 11220130100501CO to PV).
Introduction Gonadotropin-releasing hormone (GnRH) has an important role in reproduction of vertebrates. GnRH is synthesized in the brain and stimulates the secretion of pituitary gonadotropins (GTHs). GTHs, then, stimulate the secretion of steroid hormones from the gonads. To date, a numerous forms of GnRHs have been identified in vertebrates. Two or three molecular forms of GnRH exist even within the same species (see Okubo and Nagahama, 2008).