STRESS INTENSITY–DEPENDENT SEROTONERGIC CONTROL OF ANXIETY IN ZEBRAFISH

The serotonergic theory of anxiety has been both supported and contradicted by numerous studies. Despite inconsistencies in the reported role of the serotonergic dorsal raphe nucleus (DRN) in anxiety regulation, there has been no systematic analysis of the environmental factors that determine its involvement.
Here, we assessed whether the intensity of a previous and/or a current stress experience influences the extent to which the DRN mediates anxiety in zebrafish. Prior stress (Ps) intensity was manipulated using different concentrations of hydrocortisone (PsLow, PsHigh). Current stress (Cs) intensity was manipulated by varying the aversiveness of the anxiety assay (CsLow, CsHigh). The role of the DRN was tested via chemogenetic ablation performed between Ps and Cs. We found that PsLow produced a general anxiogenic effect across both CsLow and CsHigh test conditions, whereas PsHigh was anxiogenic only under CsHigh. Strikingly, anxiety induced by PsLow – but not PsHigh – was rescued by DRN ablation across all environments.
To elucidate the neural basis of these distinct anxiety states marked by serotonergic activity, we generated pERK/tERK-based, post-mortem, whole-brain activity maps from groups of fish exhibiting either DRN-dependent or DRN-independent anxiety. Whole-brain maps revealed that the activity of a midbrain dopaminergic cluster (4/5) specifically tracked both stress- and ablation-induced changes in anxiety. Finally, we confirmed the involvement of DRN in an etiological model of long-term lower intensity perturbation: chronic unpredictable mild stress. Our results provide a new framework for understanding reported inconsistencies in anxiety mechanisms and cases of treatment-resistant disorders.