The role of the µ-opioid system in touch reward

C-tactile (CT) afferents are activated by caress-like pleasant touch. This type of social interaction is associated with increased β-endorphin release and social bonding in non-human animals. Studies in non-human primates and rodents suggest a state-dependent role of µ-opioids in motivation and preference for social touch. In a state of comfort where appetitive motivation dominates, µ-opioid agonism consistently promotes while antagonism diminishes approach of social and non-social rewards. An opposite pattern has been reported for approach of social rewards of a comforting nature in animals during states of distress and aversive motivation. Here, we investigated of pharmacological µ-opioid effects on hedonic responses to CT-optimal touch in humans not subjected to experimental stress manipulations. We hypothesized that opioid agonism would increase, and opioid antagonism decrease ‘liking’ and ‘wanting’ of pleasant touch as measured in two separate tasks. 49 healthy males received single oral doses of a µ-opioid agonist (morphine 10 mg), a non-selective opioid antagonist (naltrexone 50 mg) or placebo on three separate days in a randomized double blind cross-over study. As part of a larger reward task battery, participants received soft brush strokes on their forearms with three different speeds: 0.3 cm/s, 3 cm/s (optimal speed for CT activation) and 30 cm/s. In the liking task, participants rated touch pleasantness on a visual analog scale after each 15-second trial. In the wanting task, they instead manipulated the duration of each trial through key presses. Pleasantness ratings and ratio durations were analyzed using a linear mixed model approach with brushing speed and drug condition as the main fixed factors. Session, stimulus orders and other variables related to the experimental design, recruitment and counterbalancing were also included as fixed factors, as were measures of subjective drug effects. The results demonstrated the expected preference for CT-optimal brushing in both tasks, but provided no evidence for a role of opioids in modulating touch pleasantness or ratio durations under the current experimental conditions. Effects of mood variables and interactions between mood, drug and brushing speed were examined in a separate set of explorative analyses, and significant interactions between state anxiety and drug were found at 3 cm/s brushing. Task design issues and potential avenues for future investigations of state-dependent opioid modulation of motivation for social touch are discussed