Abstract
Seasonal Affective Disorder (SAD) is one of the most common mood disorders with depressive symptoms recurring in winter when there is less sunlight. The fact that light is the most salient factor entraining circadian rhythms leads to the phase-shifting hypothesis, which suggests that the depressive episodes of SAD are caused by misalignments between the circadian rhythms and the habitual sleep times. However, how changes in environmental lighting conditions lead to the fluctuations in mood is largely unknown. The objective of this study is to develop an animal model for some of the features/symptoms of SAD using the diurnal grass rats Arvichantis niloticus and to explore the neural mechanisms underlying the light associated mood changes. Animals were housed in either a 12∶12 hr bright light∶dark (1000lux, BLD) or dim light∶dark (50lux, DLD) condition. The depression-like behaviors were assessed by sweet-taste Saccharin solution preference (SSP) and forced swimming test (FST). Animals in the DLD group showed higher levels of depression-like behaviors compared to those in BLD. The anxiety-like behaviors were assessed in open field and light/dark box test, however no significant differences were observed between the two groups. The involvement of the circadian system on depression-like behaviors was investigated as well. Analysis of locomotor activity revealed no major differences in daily rhythms that could possibly contribute to the depression-like behaviors. To explore the neural substrates associated with the depression-like behaviors, the brain tissues from these animals were analyzed using immunocytochemistry. Attenuated indices of 5-HT signaling were observed in DLD compared to the BLD group. The results lay the groundwork for establishing a novel animal model and a novel experimental paradigm for SAD. The results also provide insights into the neural mechanisms underlying light-dependent mood changes.
Highlights
Seasonal Affective Disorder (SAD) is a recurrent major depressive disorder, in which episodes of depression occur in fall and winter followed by full remissions in spring and summer [1]
Using a diurnal rodent model, the present study demonstrates that decreasing daytime light intensity is effective in producing depression-like behaviors, but without apparent disruption in daily rhythms
Consistent results were observed in the two separate cohorts of animals with the DLD group showing a higher level of depression-like behaviors compared to BLD group
Summary
Seasonal Affective Disorder (SAD) is a recurrent major depressive disorder, in which episodes of depression occur in fall and winter followed by full remissions in spring and summer [1]. Light therapy is one of the most effective treatments for SAD, further supporting the association between mood and light [1,2]. The current theories on SAD and light therapy have been focusing on the effects of light on circadian rhythms [2,4,5]. The objectives of the present study are to develop an animal model for some of the features/symptoms of SAD using the diurnal grass rats Arvicanthis niloticus and to explore the potential underlying neural substrates mediating the light-dependent mood changes. Given the role that light and circadian rhythms play in SAD, a diurnal animal model has advantages over its nocturnal counterparts in elucidating the neurological responses that could be occurring in diurnal humans [8,9,10]. The grass rat shows depression-like behaviors when housed in short photoperiods, suggesting its potential use as an animal model for SAD [8,11,12]
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