Daylight Duration Impacts Brown Fat Opioid Receptor Levels

Researchers in Finland have observed that shorter daylight hours increase opioid receptor signaling in the brown fat of animals—findings that could have implications for treating several psychiatric disorders.

Opioid receptors are parts of the cell through which the opioid hormones can impact the cell. An example of such hormones is endorphin, which promotes pleasure and relieves pain in the body. Consequently, the functions of opioid receptors in the brain have a central role in both pain and mood and emotions. Abnormalities of receptor function have been linked to psychiatric disorders such as depression and anxiety and eating disorders. Opioid receptor levels may also be important for the seasonal affective changes such as seasonal affective disorder, which is marked by “winter blues” and overeating.

The study evaluated rats living in an artificial environment imitating seasonal daylight changes. When the environment turned darker and colder, the rats’ brown fat—which produces heat and regulates appetite—started to grow.

“In the study, we observed that the number of mu-opioid receptors in brown fat was dependent on the length of daylight the rat was exposed to. This complements our previous findings that day length modulates opioid receptor levels in the brain emotional circuits in humans and rats,” says Senior Researcher Lihua Sun from the Turku PET Centre of the University of Turku.

Sun adds that the opioid receptor activity of brown fat and brain are two separate phenomena. However, they share the same goal of helping a mammal, a person, or an animal, adapt both physiologically and also emotionally to the change of seasons. “Opioid receptor levels in the brain and brown fat might be interconnected, for example strengthening each other’s activity, but more research is needed to confirm this.”

Professor Anne Roivainen, also from Turku PET Centre, says this is the first time mu-opioid receptor levels have been assessed in peripheral regions using positron emission tomography (PET) imaging. “The finding highlights that mu-opioid receptors affect the seasonality of brown fat activity. Future studies should further investigate whether mu-opioid receptors in brown fat are directly related to tissue energy consumption,” she says.

According to Roivainen and Sun, whether the seasonal variations in mu-opioid receptor levels in the brain and brown fat are underlying the seasonal affective changes still requires more scientific evidence. The findings were published recently in European Journal of Nuclear Medicine and Molecular Imaging.