The study on reproduction in fish has been acquired great importance in last years, mainly for the benefit of threatened species. During the reproductive process, the hypothalamic neurons synthesize and release GnRH that stimulates the pituitary cells to release FSH and LH, which, in turn, promote the gonadal maturation. In fact, the morphological changes in gonads are the result of the endocrine action of the reproductive axis, in which the GnRH is the key molecule to starting the reproductive axis control. Thus, the knowledge about the GnRH, as well as about the gonadal morphological changes in the spawning might contribute to effectiveness of reproduction. Therefore, in this work, with Astyanax altiparanae as a model, we made the molecular characterization of the GnRHs, and we analyzed the gonadal morphological changes during the reproductive stimulus. In addition, we evaluated the role of injected GnRHs in vivo. As results, we obtain the cDNA complete sequence of preproGnRH2 (612 bp) and preproGnRH3 (407 bp) of A. altiparanae. Regarding the induction of reproduction by water level drawdown, the released of gametes occurred at 20 hours after stimulus in female and at 16 hours after stimulus in males, the mRNA expression of GnRH3 increased at 8 hours after stimulus in female and the mRNA expression of GnRH2 increased at 0 hours in males. Regarding the effects of injected GnRH, all of them stimulated the ²LH but not ²FSH mRNA expression, and only the GnRH2 was able to rise the MIS and stimulate the released of gametes. We conclude that the cDNAs sequences of preproGnRH2 and preproGnRH3 were conserved, although there is a change in the amino acid at the position 8 of the GnRH3 decapeptide of A. altiparanae. Furthermore, the induced reproduction by water level drawdown was effective, and in captivity, the A. altiparanae has an asynchronous development with splitted spawning during the breeding season. The analysis of the animals submitted to the reproductive stimulus allowed us to suggest that in A. altiparanae, the GnRH2 probably has a role in sexual behavior and the GnRH3 possibly is the hypophysiotropic form. Finally, analyzing the GnRH effects, we observed that only the GnRH2 was able to start the entire reproductive hormonal cascade, leading the animal to spawning.
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