Seasonal Variation in Daylight Influences Brain Function

Summary: The seasonal duration of daylight influences the number of opioid receptors in the brain. The findings shed new light on a potential mechanism behind seasonal affective disorder.

Source: University of Turku

Seasons have an impact on our emotions and social life. 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.

In the study conducted at the Turku PET Centre, Finland, researchers compared how the length of daylight hours affected the opioid receptors in humans and rats.

“In the study, we 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,” says Postdoctoral Researcher Lihua Sun from the Turku PET Centre and the University of Turku.

Animal studies confirm the significance of daylight

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.

“On the basis of the results, the duration of daylight is a particularly critical factor in the seasonal variation of opioid receptors. These results help us to understand the brain mechanisms behind seasonal affective disorder,” says Professor Lauri Nummenmaa from the Turku PET Centre.

This shows brain scans from the study
Brain opioid receptors measured with positron emission tomography (A) and regions, where opioid receptor density varied seasonally. Credit: University of Turku

The study was conducted with Positron Emission Tomography (PET) and altogether 204 volunteers participated as 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 is 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, University of Turku, with the genuine support of Professor Anne Roivainen and Dr Emrah Yatkin.

About this neuroscience research news

Source: University of Turku
Contact: Lihua Sun – University of Turku
Image: The image is credited to University of Turku

Original Research: Closed access.
Seasonal Variation in the Brain μ-Opioid Receptor Availability” by Lihua Sun, Jing Tang, Heidi Liljenbäck, Aake Honkaniemi, Jenni Virta, Janne Isojärvi, Tomi Karjalainen, Tatu Kantonen, Pirjo Nuutila, Jarmo Hietala, Valtteri Kaasinen, Kari Kalliokoski, Jussi Hirvonen, Harry Scheinin, Semi Helin, Kim Eerola, Eriika Savontaus, Emrah Yatkin, Juha O. Rinne, Anne Roivainen and Lauri Nummenmaa. Journal of Neuroscience


Seasonal Variation in the Brain μ-Opioid Receptor Availability

Seasonal rhythms influence mood and sociability. The brain μ-opioid receptor (MOR) system modulates a multitude of seasonally varying socioemotional functions, but its seasonal variation remains elusive with no previously reported in vivo evidence.

Here, we first conducted a cross-sectional study with previously acquired human [11C]carfentanil PET imaging data (132 male and 72 female healthy subjects) to test whether there is seasonal variation in MOR availability. We then investigated experimentally whether seasonal variation in daylength causally influences brain MOR availability in rats. Rats (six male and three female rats) underwent daylength cycle simulating seasonal changes; control animals (two male and one female rats) were kept under constant daylength. Animals were scanned repeatedly with [11C]carfentanil PET imaging.

Seasonally varying daylength had an inverted U-shaped functional relationship with brain MOR availability in humans. Brain regions sensitive to daylength spanned the socioemotional brain circuits, where MOR availability peaked during spring. In rats, MOR availabilities in the brain neocortex, thalamus, and striatum peaked at intermediate daylength. Varying daylength also affected the weight gain and stress hormone levels. We conclude that cerebral MOR availability in humans and rats shows significant seasonal variation, which is predominately associated with seasonal photoperiodic variation.

Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.


Seasonal rhythms influence emotion and sociability. The central μ-opioid receptor (MOR) system modulates numerous seasonally varying socioemotional functions, but its seasonal variation remains elusive.

Here we used positron emission tomography to show that MOR levels in both human and rat brains show daylength-dependent seasonal variation. The highest MOR availability was observed at intermediate daylengths.

Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.

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