The behavioral paradigm to induce repeated social defeats in zebrafish.

Social subordination, which causes severe stress in animals, can affect animal's behaviors, homeostasis, and mental health. In rodents, experiences of repeated social defeats, but not a single defeat, induce a depression-like state. However, it is unclear whether such experiences similarly affect behaviors of other model animals than rodents. Here, we established a behavioral paradigm for repeated social defeats with zebrafish, an emerging model for behavioral neuroscience and pharmacological analysis. We put fish into repeated social subordination for 6 consecutive days. Using behaviors during fighting as indicators, we observed that experiencing repeated social defeats led to a reduction in fight frequency and duration. The continuously-defeated zebrafish failed to win even against the transgenic fish with an impaired winning-associated neural pathway. These results suggest that repeated social defeats led to demotivation to fight and to win against opponents. Moreover, they showed strong activity in the ventral habenula, an evolutionary homolog of the mammalian lateral habenula. However, unlike the mice model, the continuously-defeated zebrafish showed no change in anxiety level and sociability. Our established behavioral paradigm will be a new tool to investigate neural mechanisms underlying social defeats.