Imaging the opioidergic reward functions with positron emission tomography
The endogenous opioid system modulates approach behaviour and reward functions in both humans and animals. Here I review our recent work on the opioidergic basis the human reward system using in vivo positron emission tomography (PET) with mu opioid receptor (MOR) specific ligand [11C]carfentanil combined with functional magnetic resonance imaging (fMRI). Our results highlight that first, individual differences in MOR availability are associated with sensitivity to rewards. Second, consumption of palatable foods engages the MOR system, and perpetual overstimulation of the MOR system due to overeating and obesity may lead to chronic MOR downregulation. These adverse changes however recover following surgery-induced weight loss. Third, opioid system also plays a key role in modulating social rewards. Prosocial behavior such as social touching (grooming) and group laughter engages the MOR system, and striatal MOR availability predicts empathetic responses (as measured by BOLD-fMRI) when seeing others in pain. Altogether these data highlight the centrality of MOR system for processing a wide array of rewards, and suggest that dysfunctions of the MOR system may have strong impact on somatic and psychological well being.