Optogenetic induction of chronic glucocorticoid exposure in early-life impairs stress-response in larval zebrafish

Organisms respond to stressors through a coordinated set of physiological and behavioural responses. Zebrafish provides an opportunity to study conserved mechanisms underlying the stress-response that is regulated largely by the neuroendocrine Hypothalamus-Pituitary-Adrenal/Interrenal (HPA) axis, with glucocorticoids (GC) as the final effector. In this study, we evaluated the effect of chronically active GC signalling in early life on the baseline and stress evoked GC(cortisol) levels in larval zebrafish. To this end, we employed an optogenetic actuator, Beggiatoa photoactivated adenylyl cyclase, expressed in the interrenal cells of zebrafish and demonstrate that its chronic activation leads to hypercortisolaemia and dampens the acute-stress evoked cortisol levels, across a variety of stressor modalities during early life. This blunting of stress-response, a phenotype reported by many studies to be observed in human subjects exposed to early-life trauma, was conserved in ontogeny at a later developmental stage. Furthermore, we observe a strong reduction of proopiomelanocortin (POMC)-expressing cells in the pituitary as well as global upregulation of FKBP5 gene expression, impinging on the negative feedback regulation elicited by elevated cortisol levels. Going forward, we propose that this model can be leveraged to tease apart the mechanisms underlying developmental programming of HPA axis by early-life stress and its implications for vulnerability and resilience to stress in adulthood. ### Competing Interest Statement SR holds a patent, European patent number 2928288 and US patent number 10,080,355: A novel inducible model of stress. The remaining authors declare that they have no known competing financial interests that could have appeared to influence the work reported in this paper.