The role of dopamine in the perceptual modulation of nociceptive stimuli by monetary wins or losses.

Dopamine has been suggested to have direct antinociceptive effects. However, effects on the motivation to endure or to avoid nociceptive stimulation would be more in line with dopamine's well-established role in the motivation to obtain reward. Thus, dopamine might either inhibit or facilitate the perception of nociceptive stimuli to bias an organism towards endurance or avoidance depending on the relative importance of the nociceptive input. To test this hypothesis, we conducted two psychophysical experiments in human volunteers. In Experiment 1, the respective antinociceptive and pro-nociceptive effects of monetary wins and losses were assessed by administering thermal stimuli (three intensities, within-subject factor) while participants simultaneously won, lost, or neither won nor lost (neutral condition) money (within-subject factor) in a wheel-of-fortune task. In Experiment 2, we tested the effect of low-dose sulpiride (a centrally-acting D2-receptor antagonist increasing the synaptic availability of dopamine via predominant pre-synaptic blockade) on the same task as in Experiment 1 using a placebo-controlled, cross-over design. Monetary wins decreased and losses enhanced the perception of nociceptive stimuli, which was highly reproducible. Sulpiride augmented perceptual modulation by monetary outcomes. This augmentation was driven by increased effects of monetary losses on the perception of nociceptive stimuli. The perception of nociceptive stimuli in the absence of monetary wins and losses was not affected by sulpiride. Based on these findings, we propose a new role of dopamine in the context of nociception: biasing the organism towards a decision in situations with conflicting motivations, depending on the relative importance of the nociceptive input.