Introduction
pain
EVENTS
One of the most powerful and universally appreciated aspects of being in pain is the desire for relief, and the positive sensation of pleasure once achieved. However, in contrast to pain itself, much less is known about how the perception of relief is modulated by various behavioral and motivational factors (Endogenous modulation of pain involves a number of different processes mediated by distinct descending signaling pathways, and involves at least two critical neurochemical systems: opioidergic and dopaminergic (The aim of the present study was therefore first to better characterize information processing aspects of relief motivation, and second to investigate the roles of dopaminergic and opioidergic signaling in pain relief perception and modulation. We expected that pain relief would be modulated by the value of information it carries, as hence enhanced by (i) active vs passive reception and thus controllability, since this reflects potential to exploit relief information; (ii) unpredictability, since this reflects the extra information carried by surprising events, and (iii) trait novelty-seeking, since this reflects individual information sensitivity. At the same time, we aimed to identify the potential role of dopamine and opioids for each of these factors, in particular to explore whether increased dopamine availability would enhance endogenous pain relief under these conditions, and whether modulation could be reduced by blocking opioid receptors. We expected increased dopamine availability to enhance phasic release of dopamine in response to rewards, and hence, to increase the effect of active compared to passive reception of pain relief. In contrast, we expected the inhibition of endogenous opioid signaling to decrease the effect of active controllability on pain relief. The latter is based on the observation that blocking of opioid receptors attenuates other types of endogenous pain inhibition such as placebo analgesia (To test these hypotheses, we employed a previously developed wheel of fortune task utilizing relief of a tonic capsaicin-sensitive thermal pain stimulus as ‘wins’, and allowing to quantify endogenous pain inhibition induced by gaining pain relief in active versus passive conditions (