Effects of morning compared with evening bright light administration to ameliorate short-photoperiod induced depression- and anxiety-like behaviors in a diurnal rodent model.

The lack of appropriate animal models for affective disorders is a major factor hindering better understanding of the underlying pathologies and the development of more efficacious treatments. Because circadian rhythms play an important role in affective disorders, we recently suggested that diurnal rodents can be advantageous as model animals. We found that in diurnal rodents, short photoperiod induces depression- and anxiety-like behaviors, with similarities to human seasonal affective disorder. In a pilot study we also found that these behaviors are ameliorated by morning bright light administration. In the present study we further evaluated the effects of morning and evening bright light administration on short photoperiod-induced depression- and anxiety-like behaviors in diurnal fat sand rats. Animals were maintained under short (5L:19D) or neutral (12L:12D) photoperiod and treated with morning or evening bright light or red dim light as control. Morning bright light ameliorated the behavioral deficits in the elevated plus maze and social interaction tests whereas evening bright light was effective only in the social interaction test. This is the first detailed presentation of the effects of bright light treatment in an animal model and a clear demonstration to the advantages of utilizing diurnal rodents to study interactions between circadian rhythms and affect.