Season, Light Exposure and Serotonin Transporter Binding
The serotonin transporter (5-HTT) is a protein that clears extracellular serotonin from the synapses and depletion of serotonin is associated with depressive symptoms. Previously, in healthy individuals, we discovered that 5-HTT BPNo, an index of 5-HTT levels, was greater in affect-modulating brain regions in the fall/winter compared to the spring/summer; a finding that was replicated by three independent research groups. In a pilot study, we also evaluated the effect of light therapy on 5-HTT BPNo in healthy humans and found a 15% reduction in the anterior cingulate cortex (ACC), a region which participates heavily in affective control. On the basis of these data, we propose that the winter increase in BPNo, as an index of 5-HTT levels, reflects an important mechanism underlying SAD that can be influenced by the environment (i.e. light exposure). We are investigating this phenomenon in individuals with SAD to develop strategies for prevention and treatment. Hypotheses/Methods: This project will use [1 1C] DASB positron emission tomography (PET) to measure seasonal variation in brain 5-HTT BPNo in SAD and healthy individuals. In this investigation, we will test three hypotheses; (1) in healthy humans, 5-HTT BPNo in affect-modulating brain regions will be significantly greater in winter than in summer applying a within subject design. (2) Seasonal differences in 5-HTT BPNo in affect-modulating brain regions will be greater in SAD individuals as compared to healthy subjects. (3) In SAD subjects, light therapy will be associated with a reduction in 5-HTT BPNo in affect-modulating brain regions as compared to a placebo treatment. To test the first and second hypotheses, healthy and SAD subjects will undergo two PET scans; one scan in the summer and another in the winter. To test the third hypothesis, SAD subjects will have one PET scan prior to one week of daily light therapy or placebo treatment followed by a second scan after treatment completion.
Andrea Tyrer , Centre for Addiction and Mental Health
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