Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

Abstract

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Agouti-Signalling Protein Overexpression Reduces Aggressiveness in Zebrafish

“…In fact, a true subordinate phenotype, as observed in real-opponent fights, is never exhibited in this case. The outcome of the fight does not occur, and the expression of aggressiveness is uncoupled from the experience of the contest result [35]. Nevertheless, the total level of aggressiveness and the stereotyped components of the aggression between the males exposed to real and mirror-image opponents were similar in Astatotilapia burtoni [28] and zebrafish [37].…”

“…On the whole, results from the mirror-image stimulus corroborate the submissive phenotype of asip1-Tg fish. While the mirror-image test is a standardised assay for quantification of aggressive behaviour, the physiological underpinnings are not completely reproduced, particularly at the level of the central transcriptome and/or endocrine system and behavioural responses [28,35,36]. In fact, a true subordinate phenotype, as observed in real-opponent fights, is never exhibited in this case.…”