Molecular Imaging of the Human Emotion Circuit
The endogenous opioid and dopamine systems support appetitive, motivational and social behaviour in humans and animals. In this talk I discuss our recent work on mapping the role of the μ-opioid receptor (MOR) and type 2 dopamine receptor system (D2R) systems in human emotions and social behaviour using fusion imaging with functional magnetic resonance imaging (fMRI) and in vivo positron emission tomography (PET) with agonist radioligands [11C]carfentanil and [11c]raclopride selective for MORs and D2Rs, respectively. First, fusion imaging with PET-fMRI confirms that MORs modulate emotional arousal during life-like audiovisual episodes. Second, reward consumption such as feeding leads to central opioid release, and individual differences in MOR expression underlie reward sensitivity as measured by questionnaires and anticipatory reward responses in BOLD-fMRI experiments. Repeated overstimulation of the MOR due to overeating may lead to receptor downregulation, promoting development of obesity. This effect is however reversible due to weight loss. Third, both activation studies and cross-sectional work show that MORs are associated with sociability. Both social grooming and social laughter modulate central opioidergic activity, and multiple aspects of prosociality measured by laboratory tasks and questionnaires are positively associated with MOR expression. Finally, MOR (but not D2R) expression is associated with BOLD-fMRI responses during vicarious pain perception confirming the contribution of MOR system in empathy. Altogether these results suggest that particularly the opioid system plays a major role in in human reward processing and sociability. Central opioid release during social interaction may act as a safety signal, promoting establishment and maintenance of social relationships. Consequently, malfunction of the opioid system may predispose individuals to developing disorders involving abnormal hedonic and socioemotional processing.