Identification of environmental stressors and validation of light preference as a measure of anxiety in larval zebrafish

Background: Larval zebrafish, with a simple and transparent vertebrate brain composed of similar to 100 K neurons, is well suited for deciphering entire neural circuit activity underlying behavior. Moreover, their small body size (similar to 4-5 mm in length) is compatible with 96-well plates, making larval zebrafish amenable to high content screening. Despite these attractive features, there is a scarcity of behavioral characterizations in larval zebrafish compared to other model organisms as well as adult zebrafish. Results: In this study, we have characterized the physiological and behavioral responses of larval zebrafish to several easily amenable stimuli, including heat, cold, UV, mechanical disturbance (MD), and social isolation (SI). These stimuli are selected based on their perceived aversive nature to larval zebrafish. Using a light/dark choice paradigm, in which larval zebrafish display an innate dark avoidance behavior (i.e. scotophobia), we find that heat, cold and UV stimuli significantly enhance their dark avoidance with heat having the most striking effect, whereas MD and SI have little influence on the behavior. Surprisingly, using the cortisol assay, a physiological measure of stress, we uncover that all stimuli but heat and SI significantly increase the whole body cortisol levels. Conclusion: These results identify a series of stressors that can be easily administered to larval zebrafish. Those stimuli that elicit differential responses at behavioral and physiological levels warrant further studies at circuit levels to understand the underlying mechanisms. The findings that various stressors enhance while anxiolytics attenuate dark avoidance further reinforce that the light/dark preference behavior in larval zebrafish is fear/anxiety-associated.