Light-At-Night and Mood: Examining The Role Of Sex In C57Bl/6 Mice

Rates of major depression have increased substantially in recent years, although it is not currently clear what are the factors behind such increases. Environmental factors may be important, and it has recently been postulated that dim nocturnal light may contribute to depression symptoms in humans and in rodents. Sex is also a very important factor in affective disorders, with prevalence rates of major depression twice as high in females than in males. We set out to test the hypothesis that dim-light would interfere with the circadian rhythm of C57Bl/6 mice and induce both depressive-like and anxiety-like behaviours and that there would be sex-specific differences. Animals were either singly or group housed for a three week period where locomotor activity was measured. After this period they were tested on a range of tests of emotional behaviours. Animals were subsequently placed into either 12 h light: 12h dim nocturnal light (~5 lux) cycle or a 12:12 light/dark condition and retested on the behavioural battery after three weeks. Brains of the same animals were used to measure stem cell proliferation in the dentate gyrus using the biomarker Ki-67. Exposure to dim light-at-night did not lead to significant circadian disruption nor to significant changes in any of the parameters examined. Apart from the Forced Swim Test no sex-dependent effects were detected. Levels of neural progenitor cell as measured by Ki-67 were significantly decreased in the dentate gyrus of light-at-night animals. In the context of previous research in this area our results indicate that species and strain differences may be important in assessing the potential impact of dim nocturnal light on circadian and affective systems in rodents