Active behaviour during early development shapes glucocorticoid reactivity

Abstract

12Glucocorticoids are the final effectors of the stress axis, with numerous targets in the central 13 nervous system and the periphery. They are essential for adaptation, yet currently it is unclear 14 how early life events program the glucocorticoid response to stress. Here we provide evidence 15 that involuntary swimming at early developmental stages can reconfigure the cortisol 16 response to homotypic and heterotypic stress in larval zebrafish (Danio rerio), also reducing 17 startle reactivity and increasing spontaneous activity as well as energy efficiency during 18 active behaviour. Collectively, these data identify a role of the genetically malleable zebrafish 19 for linking early life stress with glucocorticoid function in later life. 20 21 22The increased secretion of glucocorticoids like cortisol after the onset of stress (a.k.a.) 23 glucocorticoid reactivity (GCR) plays a pivotal role in the response to challenge. It is critical 24 for adaptation and central to an organism's resilience 1 . GCR is a tightly regulated 25 phenomenon, a response of the hypothalamic-pituitary-adrenal (HPA) axis to exogenous or 26 endogenous stressors. Altered functionality of the HPA axis and of GCR have been associated 27 with detrimental and beneficial consequences for health. They have been linked to stress-28 evoked disorders including mental disorders as well as increased resilience 2,3,4,5,6,7,8 . 29 Glucocorticoid secretion has been investigated extensively under steady-state and stress 30 conditions 9 , and there is ample evidence that HPA axis functionality is susceptible to 31 disturbance by early life stress. Early adversity can, for example, alter glucocorticoid 32 regulation and coping capacities later in life 10,11 . However, it is still unclear how active 33 responses to early life stress can reconfigure HPA axis function, pending a detailed functional 34 evaluation of developmental programming of GCR. Larval zebrafish are excellent to address 35 this knowledge gap due to their external development, their hypothalamic-pituitary-interrenal 36 (HPI) axis, homologous to the mammalian HPA axis 12 , their translucent body, ideal for non-37 invasive brain imaging and optogenetics 13,14,15 , their small size, highly suitable for high-38 throughput screens with full environmental control, and the availability of tools and methods 39 for identifying genetic and epigenetic modulators, including proteomic technology. Therefore, 40 as a first step, we set out to determine the effect of early life stress on GCR and coping 41 capacity in larval zebrafish. Taken together our results introduce a high-throughput forced 42 swim test for developing zebrafish and demonstrate that mild early life stress can at least 43 transiently reconfigure GCR and elicit modulatory adjustments in spontaneous activity and 44 startle reactivity. 45 46 48 Firstly, we exposed groups of larvae to water vortex flows of varying strength, expressed in 49 revolutions per minute (rpm) (for details, see the 'Methods' section). To comp...