Seasonal profiles of circulating steroids and brain steroid‐related gene expression in <i>Gymnotus omarorum</i> , a wild fish with year‐round territorial aggression

Gymnotus omarorum is a teleost fish in which both males and females maintain territorial aggression year-round, despite exhibiting a seasonal reproductive cycle. This provides a unique opportunity to examine seasonal neuroendocrine mechanisms across reproductive states. As a teleost, G. omarorum extends our understanding of these neuroendocrine patterns beyond birds and mammals, offering a comparative framework for investigating seasonal neuroendocrine reorganization across the vertebrate reproductive cycle. We quantified eight circulating steroids by LC-MS/MS and measured the expression of steroid-related genes (aromatase, estrogen receptors, and an androgen receptor) by qRT-PCR in three nuclei of the social behavior network in wild breeding and non-breeding males and females. Circulating 11-ketotestosterone in males and estrogens in females showed no marked seasonality. In contrast, androstenedione, a steroid precursor for both androgens and estrogens, was elevated during the non-breeding season in both sexes. This increase was accompanied by upregulation of brain aromatase and estrogen receptor expression, suggesting an enhanced capacity for local estrogen synthesis and signaling outside the breeding period. These results reveal coordinated seasonal shifts in circulating steroid precursors and neural estrogenic pathways that covary with reproductive state, shedding light on the neuroendocrine mechanisms underlying seasonal transitions in vertebrates.