A new study conducted at the Turku PET Centre, Finland has suggested that the availability of opioid receptors in the brain may change with the length of daylight time, which may explain seasonal changes in human mood and behavior. It's known that rates of seasonal affective disorder, a type of depression, rise during the winter months while negative emotions are more muted during summer months.
The research, published in the Journal of Neuroscience, looked at brain scans of humans and rats, analyzing the availability of the µ-opioid receptor (MOR) system. Opioid receptors have a role in regulating mood, and MORs have a high affinity for types of endorphin, as well as binding to morphine. According to the authors, this is the first in vivo (in living humans) evidence of the MOR system in humans fluctuating with seasons.
“We observed that the number of opioid receptors was dependent on the time of the year the brain was imaged,” said postdoctoral researcher Lihua Sun from the Turku PET Centre in a statement. “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."
The researchers first looked at Positron Emission Tomography (PET) data retrieved from the AIVO database on 204 human subjects. The data was gathered between 2003 and 2018 and was from 11 different research projects. The researchers then calculated the length of daylight time on the days the scans were taken.
A statistically significant effect of day length on MOR availability was observed in a large brain cluster. The season with the highest MOR availability was spring (defined as March, April, and May), with MOR activity peaking when days were between 15 and 19 hours long. This seasonal variation occurred in parts of the brain tied to emotional and social processing, such as the amygdala, cingulate cortex, and posterior superior temporal cortices.
In PET data from nine rats, where day length was either constant or in a cycle mimicking seasons, the same relationship between day length and MOR availability was shown.
The authors cite previous studies that show an “intimate” relationship between opioid neurotransmission and seasonal fluctuations in mood. Previous research has also shown that MORs affect feeding, with humans eating more fats in winter and fall. Suicide rates are the highest in the spring, and studies show that attempted suicides with prescription opioids peak in the spring and fall. Post-mortem examinations of people who have died by suicide show increased MOR densities in their brains.
The researchers theorize that seasonal changes in MOR availability could be behind these effects, as well as seasonal affective disorder.