Effect of photoperiod on the anxiety of zebrafish

Photoperiod takes part in various physiological and ecological processes in animals; however, whether varied photoperiods interfere with fish anxiety and the mechanisms possibly involved remains largely unknown. In this study, adult zebrafish (Danio rerio) males and females were exposed to short day (SD, 6-h light/18-h dark) and long day (LD, 18-h light/6-h dark) for 28 days. After exposure, the fish were performed to a novel tank diving test, in which the anxiety parameters, including the latency to enter the upper layer, the total duration spent in the lower layer, and the freezing duration, were measured. Our data showed that SD significantly shortened the total duration spent in the lower layer in males, whereas LD dramatically prolonged the latency to enter the upper layer and extended the freezing duration in females. Consistently, LD elevated the cortisol level in plasma while decreasing the concentrations of melatonin (MT) and serotonin (5-HT) in the brains of females. In contrast, declined cortisol levels in plasma and increased levels of MT, 5-HT, and DA in the male brains were also observed. In addition, the expressions of genes involved in the synthesis and actions of MT, 5-HT, DA, and cortisol in the brains of both males and females altered according to SD or LD exposure. These data strongly suggest a possible involvement of MT-5-HT/DA and MT-cortisol systems regulating photoperiodic anxiety response in zebrafish. The present study shed light on the regulatory role and the mode(s) of action of photoperiod on mood control of fish.