Molecular identification and reproductive function of spexin in the big-belly seahorse (Hippocampus abdominalis)

Limited data are available regarding reproductive endocrinology of seahorse. Here, we reported the potential function of spexin (SPX1) in the reproduction of seahorse. SPX1, also known as neuropeptide Q (NPQ), is a novel neuropeptide that coevolved with galanin and kisspeptin. In the current study, the entire open reading frame (ORF) sequence of spx1 of the big-belly seahorse (Hippocampus abdominalis) was cloned and characterized, which is 360 base pairs in length, encoding a 119-amino acid precursor peptide, with a 26-aa signaling peptide and a 14-aa C-terminal amidated mature peptide. Tissue distribution expression profiles of spx1 transcripts were analyzed and revealed that spx1 mRNA could be detected in a variety of tissues, with the highest abundance in the mixture tissues of brain and pituitary. The expression profile of spx1 mRNA in the mixture of brain and pituitary during the first reproductive cycle of the big-belly seahorse was evaluated, which showed that the expression level of spx1 mRNA was highest during the critical period of ovarian and testicular transition from stage II to stage III at 3.5-month old, and high in females with IV-stage, V-stage and VI-stage, as well as in males with the early-stage brood pouch, suggesting that SPX1 may play a crucial role at these stages of gonadal and brood pouch development. Intraperitoneal injection of SPX1 can suppress the mRNA expressions of galr2b and fshβ in female seahorses, however, it promoted the mRNA expressions of spx1, gal, kiss2, gnrh2, kiss2r, galr1a, galr2b, gnrh3 and gnihr in males, besides, low doses of SPX1 enhanced lhβ mRNA expression, while high dose of SPX1 suppressed it. Intraperitoneal injection of SPX1 did not alter the mRNA levels of spx1, gal, kiss2, galr1a, galr2a, gnihr or plasma 17β-E2 in females, nor did it change the mRNA levels of galr2a, gthα, fshβ or plasma 11-KT in males. These results revealed that SPX1 may participate in regulating reproduction of the big-belly seahorse by controlling production of GnRH2, GnRH3, FSH and LH of HPG axis, as well as some key hypothalamic neuropeptides including Kiss and GAL. Above all, our results indicate the presence of a functional SPX1 system in the big-belly seahorse, as well as reveal its potential significance in the neuroendocrine regulation of reproduction in this species, which also lay a foundation for future research on optimizing fish reproductive performance through the regulation of SPX1.