The authors explain seasonality as it relates to mood: "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." It has been suggested that this is related to serotonin levels in the brain, as this neurotransmitter is integral to functions such as mating, feeding, balance of energy, and sleep. The serotonin transporter binds to serotonin, helping it exit the space between brain cells when the communicate. This protein "is a key element in regulating intensity and spread of the serotonin signal," according to the authors.
To investigate this relationship, 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 examined 88 healthy adults, with an average age of 33 years, between 1999 and 2003. One positron emission tomography (PET) scan was performed on each subject to assess the potential binding of serotonin transporter using a measure of density of the transporter. If this value is higher, serotonin circulates less in the brain. the individual scans were grouped according to the season taken, into either fall and winter or spring and summer.
In the fall and winter scans, the measured value was higher. The authors summarize: "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." Matching the scans to meteorological data, higher values generally occurred when fewer hours of sunlight were available in the day.
The authors note that this correlation may have some bearing on mood: "An implication of greater serotonin transporter binding in winter is that this may facilitate extracellular serotonin loss during winter, leading to lower mood," they say. They continue, "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."
In conclusion, they state, the implications of such results could be very important for an understanding of Seasonal Affective Disorder and other related diseases: "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... This offers a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals."
Seasonal Variation in Human Brain Serotonin Transporter Binding
Nicole Praschak-Rieder, MD; Matthaeus Willeit, MD; Alan A. Wilson, PhD; Sylvain Houle, MD, PhD; Jeffrey H. Meyer, MD, PhD
Arch Gen Psychiatry. 2008;65(9):1072-1078.
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