Socially induced plasticity of the posterior tuberculum and motor behavior in zebrafish (<i>Danio rerio</i>)

Social dominance is prevalent throughout the animal kingdom. It facilitates the stabilization of social relationships and allows animals to divide resources according to social rank. Zebrafish form stable dominance relationships that consist of dominants and subordinates. Although social status-dependent differences in behavior must arise as a result of neural plasticity, mechanisms by which neural circuits are reconfigured to cope with social dominance are poorly described. Here, we describe how the posterior tuberculum nucleus (PTN), which integrates sensory social information to modulate spinal motor circuits, is morphologically and functionally influenced by social status. We combined non-invasive behavioral monitoring of motor activity (startle escape and swim) and histological approaches to investigate how social dominance affects the morphological structure, axosomatic synaptic connectivity and functional activity of the PTN in relation to changes in motor behavior. We show that dopaminergic cell number significantly increases in dominants compared with subordinates, while PTN synaptic interconnectivity, demonstrated with PSD-95 expression, is higher in subordinates than in dominants. Secondly, these socially induced morphological differences emerge after 1 week of dominance formation and correlate with differences in cellular activities illustrated with higher phosphor-S6 ribosomal protein expression in dominants compared with subordinates. Thirdly, these morphological differences are reversible as the social environment evolves and correlate with adaptations in startle escape and swim behaviors. Our results provide new insights into the neural bases of social behavior that may be applicable to other social species with similar structural and functional organization.