Modulation of Simulated Microgravity-Induced Behavioral Responses by Caffeine in Zebrafish: A Pilot Study

This pilot study evaluates the effects of acute caffeine administration on adult zebrafish behavioral responses under simulated microgravity, aiming to understand how common dietary stimulants modulate neurobehavioral adaptations in altered-gravity environments. Adult zebrafish were immersed in 25 mg/L caffeine for 30 min, followed by a 1 h exposure to simulated microgravity using a clinostat system, before immediate assessment in a standard locomotor test and a vertical Novel Tank Test tracked via EthoVision XT 14 software. While independent exposures to caffeine or microgravity yielded minimal modifications to baseline swimming kinetics, their simultaneous application resulted in an increased hyperlocomotor response, characterized by increased distance moved, average velocity, and moving duration. Co-exposure also altered stereotypic motor coordination, significantly enhancing clockwise rotations. Furthermore, dual-view heat maps in the Novel Tank Test showed that the simultaneous exposure to caffeine and microgravity showed altered wall-associated spatial behavior (thigmotaxis) and bottom-dwelling avoidance, shifting behavioral output toward active vertical exploration and central arena habituation. These results suggest that simulated microgravity may influence the psychopharmacological expression of caffeine, with the combined exposure being associated with altered vertical exploration and motor activity, thereby supporting the potential utility of adult zebrafish as an experimental model for space biomedical research and highlighting the complex role of dietary countermeasures in altered-gravity environments.