[Note: This research pertains to winter-onset SAD, vs. the summer-onset SAD, more prevalent at lower latitudes. See also "Contrasts between symptoms of summer depression and winter depression."
Decreased synaptic serotonin during depressive episodes is a central element of the monoamine hypothesis of depression. The serotonin transporter (5-HTT, SERT) is a key molecule for the control of synaptic serotonin levels.
Here we aimed to detect state-related alterations in the efficiency of 5-HTT-mediated inward and outward transport in platelets of drug-free depressed patients suffering from seasonal affective disorder (SAD). 5-HTT turnover rate, a measure for the number of inward transport events per minute, and tyramine-induced, 5-HTT-mediated outward transport were assessed at baseline, after 4 weeks of bright light therapy, and in summer using a case-control design in a consecutive sample of 73 drug-free depressed patients with SAD and 70 nonseasonal healthy controls.
Patients were drug-naive or medication-free for at least 6 months prior to study inclusion, female patients were studied in the follicular phase of the menstrual cycle. All participants were genotyped for a 5-HTT-promoter polymorphism (5-HTTLPR) to assess the influence of this polymorphism on 5-HTT parameters. Efficiency of 5-HTT-mediated inward (p=0.014) and outward (p=0.003) transport was enhanced in depressed patients.
Both measures normalized toward control levels after therapy and in natural summer remission. Changes in outward transport showed a clear correlation with treatment response (rho=0.421, p=0.001). Changes in inward transport were mediated by changes in 5-HTT transport efficiency rather than affinity or density. 5-HTTLPR was not associated with any of the 5-HTT parameters.
In sum, we conclude that the 5-HTT is in a hyperfunctional state during depression in SAD and normalizes after light therapy and in natural summer remission.
Source: Neuropsychopharmacology. 2007 Sep 19; PMID: 17882235, by Willeit M, Sitte HH, Thierry N, Michalek K, Praschak-Rieder N, Zill P, Winkler D, Brannath W, Fischer MB, Bondy B, Kasper S, Singer EA. Department of Biological Psychiatry, Medical University of Vienna, Vienna, Austria.