Photoperiodic Regulation of Affective Responses and Hippocampal Cell Morphology in Siberian Hamsters

Seasonal affective disorder (SAD) is characterized by depressive episodes during winter that are alleviated during summer and by morning bright light treatment. Currently, there is no animal model of SAD. However, it may be possible to use rodents that respond to day length to understand how day length can shape brain and behavior in humans. For instance, Siberian hamsters use day length to time seasonal cycles of reproduction and also exhibit changes in nonreproductive behaviors dependent on day length. Specifically, short-day Siberian hamsters increase floating in the forced swim test (a behavioral test used to screen antidepressant compounds). Current research in depression and animal models of depression suggests that hippocampal atrophy may underlie the symptoms of depression and depressive-like behaviors, respectively. The goal of this study was to determine whether altered depressive-like responses after exposure to short days are associated with photoperiod-mediated plasticity within the hippocampus of Siberian hamsters. Hamsters were housed in either short (8:16 LD) or long days (16:8 LD) for 10 weeks. At the end of 10 weeks hamsters were tested in the forced swim test and 48 h later, brains were removed and stained using the Golgi impregnation method. Brains were processed for hippocampal dendritic length, branching, and spines, as well as cell body size. Short days significantly reduced cell body size and dendritic complexity in the CA1 region of the hippocampus. This suggests that altered depressive-like behavior induced by exposure to short days may be a consequence of reduced complexity (and perhaps connectivity) in the hippocampus