Coordinated action of CRH and cortisol shapes acute stress-induced behavioural response in zebrafish

background: The stress response mediated by the hypothalamus-pituitary-adrenal (HPA) axis activation is highly conserved in vertebrates. Hyperactivity is one such established acute stress response and corticotropin-releasing hormone (CRH), the primary step in HPA activation, signalling has been implicated in this stressor-mediated behaviour. However, whether CRH mediates the acute behavioural effects either alone or in conjunction with glucocorticoids (GCs) are far from clear. We hypothesized that the CRH receptor 1 (CRHR1)-mediated rise in GCs post-stress is necessary for the initiation and maintenance of acute stress-related behaviour. methods: We first generated zebrafish (Danio rerio) with a mutation in the crhr1 gene (CRHR1-KO) to assess the function of CRH. The behavioural readout utilized for this study was the locomotor activity of larval zebrafish in response to acute light exposure, a protocol that freezes the larvae in response to the light stimulus. To test whether cortisol signalling is involved in the stress-mediated hyperactivity, we treated wildtype fish with metyrapone, an inhibitor of 11β-hydroxylase, to suppress cortisol production. The temporal role for cortisol signalling in the stress-related hyperactivity was tested using the glucocorticoid receptor (GRKO) and mineralocorticoid receptor (MRKO) knockout zebrafish mutants. results: CRHR1-KO larvae did not increase cortisol, the principal GC in teleosts, post-stress, confirming a functional knockout. An acute stress resulted in the hyperactivity of the larvae in light at 15, 60 and 240 min post-stress and this was absent in CRHR1-KO larvae. Addition of metyrapone effectively blocked the attendant rise in cortisol post-stress; however, the stress-mediated hyperactivity was inhibited only at 60 min and 240 min but not at 15 min post-stress. Addition of human CRH peptide caused hyperactivity at 15 min and this response was also abolished in the CRHR1-KO mutants. The stress-induced hyperactivity was absent in the MRKO fish, while GRKO mutants showed transient effects. conclusions: The results suggest that the stress-induced hyperactivity is induced by the CRH/CRHR1 system, while the temporal activation of cortisol production and the associated GR/MR signalling is essential for prolonging the stressor-induced hyperactivity. This study underscores the importance of systems-level analysis to assess stress responsivity.