Daylight hours impact opioid receptor levels in brown fat: Study

The length of daylight hours has been found to influence opioid receptor levels in brown fat by researchers. When the daylight hours shorten, receptor activity levels rise. A similar event occurs in the brain. Both phenomena aid in the adaptation of humans and animals to seasonal variations.

In a new study conducted at the Turku PET Centre in Finland, researchers discovered that shorter daylight hours affect opioid receptor signalling in animal brown fat.

When the amount of light diminishes, the opioid receptor levels increase. The observation was done in rats living in an artificial environment imitating seasonal daylight changes. “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.

He states 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,” Sun emphasises.

Professor Anne Roivainen from the Turku PET Centre tells that 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,” says Roivainen.

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.

As a result, opioid receptor functions in the brain play an important part in both pain and mood and emotions. Receptor dysfunction has been linked to psychiatric diseases such as sadness and anxiety, as well as eating disorders. Seasonal affective changes, such as seasonal affective disorder, may potentially be influenced by opioid receptor levels. Winter blues and overeating are two of its symptoms.

More research is needed, according to Roivainen and Sun, to determine whether seasonal variations in mu-opioid receptor numbers in the brain and brown fat are underlying seasonal affective changes.

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