Photoperiod modulates mu-opioid receptor availability in brown adipose tissue

Photoperiod drives metabolic activity of brown adipose tissue (BAT), and affects food intake and weight gain in mammals. Sympathetic innervation in BAT controls thermogenesis and facilitates physiological adaption to seasons, but the exact mechanism remains elusive. Previous studies show that the central opioid signaling tunes BAT heating and the brain mu-opioid receptor (MOR) levels have seasonal patterns. It is hence intriguing to know whether the peripheral MOR signaling shows seasonal variation. Here, we examined the effect of photoperiod on BAT MOR availability using [11C]carfentanil positron emission topography (PET). Adult rats (n = 9) were repeatedly imaged under changing photoperiods which simulates the local seasons. Long photoperiod downregulated MOR availability in BAT, while MOR availability in the muscles was unaffected. We confirmed the expression of MOR in BAT and muscle using immunofluorescence imaging. We conclude that photoperiod causally affects MOR availability in BAT, and sympathetic innervation of BAT may influence thermogenesis via the peripheral MOR system. ### Competing Interest Statement The authors have declared no competing interest.