Seasonal variation in brain mu-opioid receptor availability

Seasonal rhythms influence mood and sociability. The mu-opioid receptor (MOR) system modulates a multitude of seasonally varying socioemotional functions, but seasonal variation in the MOR system remains elusive. We used positron emission tomography (PET) imaging with [11C]carfentanil, a high affinity MOR radioligand, to repeatedly measure in vivo brain receptor availability of adult rats (n=12) under seasonal photoperiodic simulation. Seasonal effects of human MOR availability were addressed retrospectively using a large historical dataset (n=204) of [11C]carfentanil scans. In rats daylength had a causal, inverted U-shaped functional relationship with MOR availability. Similar pattern of changes was also found in human brains. We conclude that brain MOR availability in both rats and humans shows significant seasonal variation that aligns with expected seasonal variation in mood.