Researchers from the Centre for Addiction and Mental Health and the University of Toronto, Ontario, Canada Brain report that PET scans taken at different times of year detected changes in the actions of the serotonin transporter, which contributes in regulating the mood-altering neurotransmitter serotonin, that varied by season. The report is being published in the September issue of Archives of General Psychiatry, one of the JAMA/Archives journals. The changes in serotonin might explain mood changes related to seasonal affective disorder.

Nicole Praschak-Rieder, M.D., and Matthaeus Willeit, M.D., and colleagues studied 88 healthy individuals (average age 33) between 1999 and 2003. The participants in the study underwent a positron emission tomography (PET) scan to determine serotonin binding potential value, which indicates transporter density. The higher the value, the less serotonin circulating in the brain. The participants were grouped by the season in which they were scanned, fall and winter or spring and summer.

The authors point out, "It is a common experience in temperate zones that individuals feel happier and more energetic on bright and sunny days and many experience a decline in mood and energy during the dark winter season." The fluctuations in mood are thought to be related to variations in brain levels of serotonin, which help to regulate energy, feeding and sleep. The serotonin transporter protein "is a key element in regulating intensity and spread of the serotonin signal."
The serotonin transporter binding potential values were found to be higher in all investigated brain regions in individuals investigated in the fall and winter-when there were fewer hours of sunlight per day, compared with the spring and summer group.

According to the authors, "An implication of greater serotonin transporter binding in winter is that this may facilitate extracellular serotonin loss during winter, leading to lower mood." The authors explain that the higher transporting binder potentials in fall and winter might explain symptoms related to low serotonin values, including lack of energy, fatigue, overeating and increased duration of sleep.

"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," the report concludes. "This offers a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals."

Adapted by a Press Release from the JAMA and Archives.

Read more information at the Archives of General Psychiatry .

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