Acute effects of estradiol on shoaling in male and female zebrafish (Danio rerio)

The role that estrogens play in the dynamic modulation of social behaviors related to reproduction has been well established, yet whether they can acutely modulate social responses outside of reproductive contexts remains less clear. Further, while estrogens typically promote aggressive responses in competitive contexts, especially in territorial species, it is possible they enhance non-sexual, prosocial interactions in other contexts, especially in species that live in groups. We therefore tested the acute effects of two doses of estradiol (E2) and of an aromatase inhibitor, Fadrozole, on social approach/preference responses for same-sex shoals in male and female zebrafish, as well as the effects of an agonist for the membrane G-protein coupled estrogen receptor (GPER). Estradiol, added to the water at a dose of 10-6 M, was able to significantly increase approach/preference responses in both sexes in multiple experiments in <1 h, whereas Fadrozole inhibited social approach responses 1 h and 17 h after exposure in females, but not in males. A GPER agonist did not enhance social preference responses like E2 did. Neither the effects of E2 nor FAD were paralleled by influences on measures of stress/anxiety, indicating E2 rapidly increases tendencies to approach and maintain proximity to groups in this highly social species through direct actions on social brain circuits.