With the help of PET scans, the researchers have shown that the actions of the serotonin transporter-involved in regulating the mood-altering neurotransmitter serotonin-vary by season.
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", according to the authors.
During the study, Nicole Praschak-Rieder, M.D., and Matthaeus Willeit, M.D., of the Centre for Addiction and Mental Health and the University of Toronto, Ontario, Canada, and colleagues analysed 88 healthy individuals with an average age 33 between 1999 and 2003.
The participants underwent one positron emission tomography (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, 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," wrote the authors.
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," said 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."
"These findings have important implications for understanding seasonal mood change in healthy individuals, vulnerability to seasonal affective disorder and the relationship of light exposure to mood," they conclude.
""This offers a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals," they added.
The findings are reported in Archives of General Psychiatry, one of the JAMA/Archives journals.