Inconsistent effects of photoperiod manipulations in tests for affective-like changes in mice: implications for the selection of appropriate model animals.

Deficiencies in appropriate animal models are a significant factor hindering the research of affective disorders. Significant data suggest that systems related to circadian rhythms are strongly linked to affective changes, but study with animal models in this context had unclear and inconsistent results. Circadian physiology is significantly different in diurnal and nocturnal animals and a recent project showed that in diurnal rodents, short photoperiods induce depression and anxiety-like phenotypes. This study was designed to evaluate the possibility that using a similar methodology would also result in behavioral changes in nocturnal mice. Mice from two strains were maintained in either short photoperiod, neutral photoperiod or long photoperiod for 3 weeks and tested for depression or anxiety-related behaviors, as done earlier with the diurnal rodents. Tests included activity levels, sweet solution preference, elevated plus-maze, resident-intruder aggression, and forced swim test. Tests were conducted either during the light phase or during the dark phase of the mice. In contrast to the clear phenotype in diurnal rodents, the effects of photoperiod manipulations in nocturnal mice were inconsistent. These results suggest that diurnal rodents may be advantageous compared with nocturnal species for studies exploring the relationship between circadian rhythms and affective disorders.