Optogenetic induction of chronic glucocorticoid exposure in early‐life leads to blunted stress‐response in larval zebrafish

Abstract

Early life stress (ELS) exposure alters stress susceptibility in later life and affects vulnerability to stress‐related disorders, but how ELS changes the long‐lasting responsiveness of the stress system is not well understood. Zebrafish provides an opportunity to study conserved mechanisms underlying the development and function of the stress response that is regulated largely by the neuroendocrine hypothalamus–pituitary–adrenal/interrenal (HPA/I) axis, with glucocorticoids (GC) as the final effector. In this study, we established a method to chronically elevate endogenous GC levels during early life in larval zebrafish. To this end, we employed an optogenetic actuator, beggiatoa photoactivated adenylyl cyclase, specifically 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 was conserved in ontogeny at a later developmental stage. Furthermore, we observe a strong reduction of proopiomelanocortin (pomc)‐expression in the pituitary as well as upregulation of fkbp5 gene expression. Going forward, we propose that this model can be leveraged to tease apart the mechanisms underlying developmental programming of the HPA/I axis by early‐life GC exposure and its implications for vulnerability and resilience to stress in adulthood.

Exposure to elevated glucocorticoid during development primes altered transcriptional responses to acute stress in adulthood

“…The fish interrenal gland, a homolog of the adrenal gland in mamals, is composed of two cell types: aminergic chromaffin cells and steroidogenic interrenal cells, the latter of which produce glucocorticoids (GCs). We established that Tg(star:bPAC-2A-tdTomato) uex300 raised under ambient light containing blue light (hitherto referred to as bPAC+) showed increased cortisol levels and expression of fkbp5 , one of the known GR signaling marker genes, 17 , 18 , 19 at larval stages 15 , 16 ( Figure 1 C). Elevated cortisol levels through larval and juvenile stages were detected in bPAC+, but not maintained until adulthood (after 3 months) allowing us to achieve a robust and persistent increase in endogenous cortisol levels during development ( Figures 1 D and 1E).…”

“…We developed a double-hit stress model using zebrafish which combines exposure to a high level of GC during development with acute adult stress exposure ( Figure 1 A). First, we used our previously reported optogenetic transgenic model Tg(star:bPAC-2A-tdTomato) uex300 where the elevation of endogenous cortisol (GC in fish) is induced by blue light which activates beggiatoa photoactivated adenylyl cyclase (bPAC), expressed specifically in steroidogenic interrenal cells 14 , 15 , 16 ( Figure 1 B). The fish interrenal gland, a homolog of the adrenal gland in mamals, is composed of two cell types: aminergic chromaffin cells and steroidogenic interrenal cells, the latter of which produce glucocorticoids (GCs).…”