Dopaminergic and opioidergic regulation of implicit and explicit wanting and liking of social and nonsocial rewards

Rewards can be parsed into a motivational component (9wanting9), which relies mainly on the brain9s dopaminergic system, and a hedonic component (9liking9), which relies on the opioidergic system. The observation of animal facial and behavioral reactions to rewards (e.g. pleasant tastes) has played a crucial role for our understanding of the neurochemical bases of reward processing. In adult humans, however, implicit facial reactions to reward anticipation and consumption are rarely reported, and the role that the dopaminergic and opioidergic systems play in human facial reactions to rewards remains largely unknown. It is also unclear, if human facial reactions to different types of rewards have the same neurochemical basis. To answer these questions, we conducted a study using a randomized, double-blind, between-subject design in which 131 volunteers (88 females) received orally either the D2/D3 receptor antagonist amisulpride (400 mg), the non-selective opioid receptor antagonist naltrexone (50 mg), or placebo. Explicit (ratings and physical effort) and implicit (facial EMG) reactions to matched primary social and nonsocial rewards were assessed on a trial-by-trail basis. Sweet milk with different concentrations of chocolate flavor served as nonsocial food reward. Gentle caresses to the forearm, delivered by the same-sex experimenter at different speeds, served as social reward. Results suggested 1) reduced wanting of rewards after administration of both dopamine and opioid receptor antagonists, compared to placebo, as indicated by less physical effort produced to obtain the announced reward and increased negative facial reactions during reward anticipation; 2) reduced liking of rewards only after administration of the opioid receptors antagonist, compared to placebo, as indicated by reduced positive and increased negative facial reactions during and following reward consumption. Most drug effects were either stronger or restricted to food trials, suggesting that wanting and liking of both social and nonsocial rewards may only partially share the same neurochemical brain substrates. The results are in line with the distinction of wanting and liking by current theories of reward, and underline the importance of assessing implicit facial reactions when conducting research on reward processing in adult human participants.