Effects of bright light treatment on depression- and anxiety-like behaviors of diurnal rodents maintained on a short daylight schedule

A possible relationship between circadian rhythms and affective disorders has been strongly implicated, but understanding of the biological basis of such a relationship demands the utilization of appropriate animal models. Most research is performed with nocturnal rodents while some of the effects of daylight cycles or melatonin levels in nocturnal animals may differ greatly from effects in diurnal species (including humans). Recent studies suggested the diurnal fat Sand rat as an appropriate model animal to study circadian mechanisms involvement in mood and anxiety disorders, especially seasonal affective disorder (SAD). These studies demonstrated that Sand rats chronically exposed to short daylight (SD), or to melatonin regimen mimicing short daylight, show anxiety- and depression-like behaviors. These findings established face and construct validity for the model. The present study evaluated predictive validity by testing the effects of bright light treatment in Sand rats exposed to chronic SD. Sand rats maintained on SD for 3 weeks were treated with 1 h daily 3000 lx light for 3 weeks, 1 h after "lights on" (during the light phase of the light/dark cycle), and their behavior tested in the sweet solution preference test (SSP), elevated plus-maze (EPM) and forced swim test (FST) and compared with control animals without treatment. Results indicate that bright light treatment reduced anxiety-like behavior in the EPM and depression-like behavior in the FST but not SSP. It is suggested that the results support the possibility that the diurnal Sand rat might be a preferred model animal for the study of SAD.