Androgen receptor deficiency is associated with reduced aromatase expression in the ventromedial hypothalamus of male cichlids.

Social behaviors are regulated by sex steroid hormones, such as androgens and estrogens. However, the specific molecular and neural processes modulated by steroid hormones to generate social behaviors remain to be elucidated. We investigated whether some actions of androgen signaling in the control of social behavior may occur through the regulation of estradiol synthesis in the highly social cichlid fish, Astatotilapia burtoni. Specifically, we examined the expression of cyp19a1, a brain-specific aromatase, in the brains of male A. burtoni lacking a functional ARα gene (ar1), which was recently found to be necessary for aggression in this species. We found that cyp19a1 expression is higher in wild-type males compared to ar1 mutant males in the anterior tuberal nucleus (ATn), the putative fish homolog of the mammalian ventromedial hypothalamus, a brain region that is critical for aggression across taxa. Using in situ hybridization chain reaction, we determined that cyp19a1+ cells coexpress ar1 throughout the brain, including in the ATn. We speculate that ARα may modulate cyp19a1 expression in the ATn to govern aggression in A. burtoni. These studies provide novel insights into the hormonal mechanisms of social behavior in teleosts and lay a foundation for future functional studies.

A phylogenetics-based nomenclature system for steroid receptors in teleost fishes.

Teleost fishes have emerged as tractable models for studying the neuroendocrine regulation of social behavior via molecular genetic techniques, such as CRISPR/Cas9 gene editing. Moreover, teleosts provide an opportunity to investigate the evolution of steroid receptors and their functions, as species within this lineage possess novel steroid receptor paralogs that resulted from a teleost-specific whole genome duplication. Although teleost fishes have grown in popularity as models for behavioral neuroendocrinology, there is not a consistent nomenclature system for steroid receptors and their genes, which may impede a clear understanding of steroid receptor paralogs and their functions. Here, we used a phylogenetic approach to assess the relatedness of protein sequences encoding steroid receptor paralogs in 18 species from 12 different orders of the Infraclass Teleostei. While most similarly named sequences grouped based on the established phylogeny of the teleost lineage, our analysis revealed several inconsistencies in the nomenclature of steroid receptor paralogs, particularly for sequences encoding estrogen receptor beta (ERß). Based on our results, we propose a nomenclature system for teleosts in which Greek symbols refer to proteins and numbers refer to genes encoding different subtypes of steroid receptors within the five major groups of this nuclear receptor subfamily. Collectively, our results bridge a critical gap by providing a cohesive naming system for steroid receptors in teleost fishes, which will serve to improve communication, promote collaboration, and enhance our understanding of the evolution and function of steroid receptors across vertebrates.

Social regulation of immediate early gene induction in gonadotropin releasing-hormone 1 neurons and singing behavior in canaries (Serinus canaria).

Social cues modulate the neuroendocrine control of reproduction. However, the neural systems involved in the integration of social cues are not well described. Gonadotropin-releasing hormone 1 (GnRH1) cells in the preoptic area (POA) are the final common node that links the brain with peripheral reproductive physiology. These experiments investigated whether induction of the immediate early gene, EGR1, in anatomically localized GnRH1 cell populations in Border canaries is regulated by the social environment. First, we characterized behavioral modifications in singing behavior and found males paired with a female for 2 weeks significantly reduced many aspects of singing behavior. However, paired males had a significantly higher percentage of GnRH1 cells co-labeled with EGR1. The second experiment manipulated the social environment by pairing males and females in mixed sex dyads, same sex dyads or housed birds in isolation. Only when birds are paired in mixed sex dyads was there a significantly greater percentage of GnRH1 cells expressing EGR1 cells. Increased GnRH1-EGR1 co-expression was localized to the rostral POA. These data reveal that discrete GnRH1 cells are involved in the neural integration of specific social cues and support the hypothesis that the POA exhibits functional topography related to courtship and sexual behaviors.