Seasonal daylight variation influences brain function, research finds

New research from the University of Turku compared how the length of daylight hours affected the opioid receptors in humans and rats. The study was published in the Journal of Neuroscience.

Seasons have an impact on our emotions and social life, the researchers said. Negative emotions are more subdued in the summer, whereas seasonal affective disorder rates peak during the darker winter months. Opioids regulate both mood and sociability in the brain, they said.

For the study, the researchers wanted to ensure that the changes in brain function were caused by the amount of daylight and not some other factor. To achieve this, they measured the opioid receptors in rats when the animals were kept in standard conditions where only the length of daylight hours was changed. The results were similar to those observed in humans, according to the study.

The study was conducted with Positron Emission Tomography (PET) and included 204 volunteer subjects. A small dose of radioactive tracer that binds to the brain's opioid receptors was injected in the subjects' blood circulation. The decay of the tracers was measured with a PET scanner. The study was based on the AIVO database hosted by Turku University Hospital and Turku PET Centre. The database contains different in vivo molecular brain scans for extensive analyses. Furthermore, the amount of opioid receptors was studied with PET imaging of rats. Animal studies were conducted at the Central Animal Laboratory at the University of Turku.

The researchers observed that the number of opioid receptors was dependent on the time of the year the brain was imaged. The changes were most prominent in the brain regions that control emotions and sociability. The changes in the opioid receptors caused by the variation in the amount of daylight could be an important factor in seasonal affective disorder.

"On the basis of the results, the duration of daylight is a particularly critical factor in the seasonal variation of opioid receptors,” said Lauri Nummenmaa, PhD, lead author of the study, in a statement. “These results help us to understand the brain mechanisms behind seasonal affective disorder.”