Fish have particularly high levels of adult neurogenesis, and this high neurogenic capacity may contribute to behavioural plasticity. While it is known that adult-born cells can differentiate into neurons and incorporate into neural circuits, it is unclear whether they are responsive to external stimuli and thereby capable of contributing to behavioural change. We tested whether cells born in the telencephalon of adult zebrafish are activated by social stimuli. We marked cell birth with BrdU and, 40 d later, exposed fish to brief (15 min) visual social stimuli and assayed cellular activity through immunolocalization of phospho-S6-ribosomal protein (pS6). BrdU + /pS6 + colabeled cells were found in six brain regions, and, in four regions (D, Dl, Dm and POA), the number of colabelled cells and fraction of BrdU + cells that labeled pS6 + increased during social stimulation. These results are consistent with the hypothesis that adult-born neurons play a role in regulating social behaviour.
Group-living animals must adjust the expression of their social behaviour to changes in their social environment and to transitions between life-history stages, and this social plasticity can be seen as an adaptive trait that can be under positive selection when changes in the environment outpace the rate of genetic evolutionary change. Here, we propose a conceptual framework for understanding the neuromolecular mechanisms of social plasticity. According to this framework, social plasticity is achieved by rewiring or by biochemically switching nodes of a neural network underlying social behaviour in response to perceived social information. Therefore, at the molecular level, it depends on the social regulation of gene expression, so that different genomic and epigenetic states of this brain network correspond to different behavioural states, and the switches between states are orchestrated by signalling pathways that interface the social environment and the genotype. Different types of social plasticity can be recognized based on the observed patterns of inter- versus intra-individual occurrence, time scale and reversibility. It is proposed that these different types of social plasticity rely on different proximate mechanisms at the physiological, neural and genomic level.