Daytime clocks influence opioid receptor levels in brown fat

Daytime clocks influence opioid receptor levels in brown fat

image: Positron emission tomography/computed tomography (PET/CT) detects mu-opioid receptors in rat brown fat and brain.
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Credit: Photographer/Author Lihua Sun /

As the season gets darker and colder, the animals’ brown fat begins to grow. The tissue produces heat efficiently and quickly and regulates appetite. Brown fat is also present in humans.

In a new study conducted at the Turku PET Center in Finland, researchers observed that shorter daylight hours affect opioid receptor signaling in animals’ brown fat. When the amount of light decreases, opioid receptor levels increase. Observations were made on rats living in an artificial environment mimicking seasonal changes in daylight.

“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 emotional brain circuits in humans and rats.” says Senior Researcher. Lihua Sun from the Turku PET Center of the University of Turku.

He states that brown fat and brain opioid receptor activity are two separate phenomena. However, they share the same goal of helping a mammal, human or animal to adapt physiologically and emotionally to the change of seasons.

“Opioid receptor levels in the brain and brown fat may be linked, for example by reinforcing each other’s activity, but more research is needed to confirm,” Sun points out.

New conquests in opioid receptor research

Professor Anne Roivainenz Turku PET Center 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 influence 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 opioid hormones can affect 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 have a central role in both pain and mood and emotion. Abnormalities of receptor function are associated with psychiatric disorders such as depression and anxiety and eating disorders. Opioid receptor levels may also be important for seasonal affective changes such as seasonal affective disorder. Its symptoms include winter blues and overeating.

According to Roivainen and Sun, whether seasonal variations in mu-opioid receptor levels in the brain and brown fat underlie seasonal affective changes still requires more scientific evidence.

The research results were published in European Journal of Nuclear Medicine and Molecular Imaging.

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