Social Isolation of Larval Zebrafish Alters Baseline Locomotion and Behavioral Plasticity in a Passive Coping Assay

Animals respond to aversive or stressful experiences by selecting actions to escape or avoid them. However, when escape fails to counteract the aversive event, animals suppress it as an adaptive strategy to reduce energy expenditure and minimize harm. Repeated failure to escape can reduce the perceived value of escape. This discounting becomes maladaptive when expressed as hopelessness. Early-life social isolation can alter the evaluation of negative events and contribute to depression. We aimed to determine whether hopelessness is affected by early-life social isolation in zebrafish (Danio rerio). We tested whether one week of social isolation altered their response to sustained inescapable aversive stimulation. We implemented a passive coping assay (PAS) in which 100 larvae (17–23 days old), raised socially or in isolation, were exposed to mild inescapable shock to study the transition from active coping, characterized by escape attempts, to passive coping, characterized by reduced mobility. We found that social isolation increased baseline locomotor activity, reflected in higher mean and instantaneous swimming velocity during acclimation, and reduced wall-hugging behavior. During the PAS, social animals shifted from active to passive coping, reducing swimming velocity during shock, whereas isolated animals failed to adjust and maintained high velocity. Thus, social isolation increases baseline activity and reduces behavioral plasticity during sustained stress.