Brain and circulating steroids in an electric fish: relevance for non-breeding aggression

Steroids play a crucial role in modulating brain and behavior. While traditionally it is considered that the brain is a target of peripheral hormones produced in endocrine glands, it has been discovered that the brain itself produces steroids, known as neurosteroids. Neurosteroids can be produced in brain regions involved in the regulation of social behaviors and can act locally regulating behaviors like reproduction and aggression. Here, for the first time in a teleost fish, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify a panel of 8 steroids in both plasma and brain. We use the weakly electric fish Gymnotus omarorum, a species which shows non-breeding aggression in both sexes, to characterize these hormonal profiles in wild non-breeding adults. We show that: 1) systemic steroids in the non-breeding season are similar in both sexes, although only males have circulating 11-KT, 2) brain steroid levels are sexually dimorphic, as females display higher levels of AE, T and E1, and only males had 11-KT, 3) systemic androgens such as AE and T in the non-breeding season are potential precursors for neuroestrogen synthesis, and 4) estrogens, which play a key role in non-breeding aggression, are detectable in the brain (but not the plasma) in both sexes. These data fall in line with previous reports in G. omarorum which show that non-breeding aggression is dependent on the estrogenic pathway, as has also been shown in bird and mammal models. Overall, our results constitute a fundamental groundwork to understanding the complexity of hormonal modulation, its potential sex differences, the role of neurosteroids and the interplay between central and peripheral hormones in the regulation of behaviors.