Brain scans taken at different times of year using PET suggest that the actions of the serotonin transporter vary by season, according to a report in the September issue of Archives of General Psychiatry.
The serotonin transporter, a protein that binds to serotonin and clears it from the spaces between brain cells, "is a key element in regulating intensity and spread of the serotonin signal,” the authors wrote as background information in the article. These fluctuations may potentially explain seasonal affective disorder and related mood changes.
Nicole Praschak-Rieder, MD, and Matthaeus Willeit, MD, of the Centre for Addiction and Mental Health and the University of Toronto in Ontario, and colleagues studied 88 healthy individuals (average age 33) between 1999 and 2003. Participants underwent one PET scan to assess serotonin transporter binding potential value, an index of serotonin transporter density. The higher the binding potential value, the less serotonin circulates in the brain. For the analysis, the researchers said that individual scans were grouped according to the season of the scan—fall and winter, or spring and summer.
"Serotonin transporter binding potential values were significantly higher in all investigated brain regions in individuals investigated in the fall and winter compared with those investigated in the spring and summer," the authors wrote. When they matched binding potential values to meteorological data, the researchers found that higher values occurred during times when there were fewer hours of sunlight per day.
"An implication of greater serotonin transporter binding in winter is that this may facilitate extracellular serotonin loss during winter, leading to lower mood," according to the authors. "Higher regional serotonin transporter binding potential values in fall and winter may explain hyposerotonergic [related to low serotonin levels] symptoms, such as lack of energy, fatigue, overeating and increased duration of sleep during the dark season."
The authors concluded that the findings have implications for understanding seasonal mood change in healthy individuals, vulnerability to seasonal affective disorder and the relationship of light exposure to mood, offering a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals.