Abstract
All animals are exposed to threats, whether intrinsic or extrinsic, acute or chronic, significant or minor. Threats pose a risk to homeostasis, and thus require an integrated, whole-body response to maintain homeostasis and survival. For this reason, all animals possess stress responses. Rodents are a popular model in stress research as they have similar neural stress networks to humans and have well characterised stress behaviours. In recent years, zebrafish have become a popular model, as they too have similar neural stress networks and well characterised stress behaviours. Zebrafish have the added benefits of rapid reproduction and transparent bodies for ease of neural imaging.
We aimed to utilise these benefits, to investigate how stress and cortisol effects the behaviour and neural activity of larval zebrafish. We hypothesised that osmotic stress would increase locomotor stress behaviours, and neural activity in stress-sensitive brain regions. Further, we hypothesised that cortisol would decrease stress evoked locomotor behaviour, as well as neural activity in stress sensitive brain regions. We utilised a 2-hour exogenous cortisol treatment, and measured behaviour in an inescapable osmotic stress environment, as well as within an osmotic gradient to obtain a broad view of stress behaviour. We then stained for phosphorylated extracellular signal-related kinase (ERK) as a proxy for neural activity, and imaged the brains of larvae following cortisol treatment, and osmotic stress.
We report that cortisol and osmotic stress interact to increase acute locomotion in an inescapable environment, causing a strong effect within 15 seconds of osmotic stress, and a persistent yet weaker effect during the 15 minutes following stress. We also found that within an osmotic gradient, larvae avoid the high-salt environment, and this effect was not impacted by cortisol. However, in the absence of an osmotic gradient, cortisol treatment changed locomotion suggesting possible anxiety-like behaviour. We also report an effect of cortisol and osmotic stress on activity in multiple forebrain structures including the olfactory bulb, pallium, and habenula.
This study is the first of our knowledge to investigate the effects of a short-term cortisol treatment on stress behaviour in zebrafish larvae. Overall, our data suggest that cortisol may prime the behavioural response to stress. These results provide new insights into acute stress responses in zebrafish larvae, and the role cortisol plays in modifying these behaviours