The dopamine agonist apomorphine enhances conditioned pain modulation in healthy humans

Although cumulative evidence suggests that dopamine plays a role in pain processing, the mechanisms by which dopaminergic transmission affects pain remain elusive. Conditioned pain modulation (CPM) is a psychophysical paradigm based on endogenous descending inhibitory pain modulation. The current study was aimed to test the effects of apomorphine, a non-specific dopamine agonist, on the magnitude of CPM in healthy subjects. One hundred and five healthy subjects participated in this randomized, double-blind study. CPM was assessed by subtracting the response to a phasic painful heat stimulus administered simultaneously with a conditioning cold pain stimulus from the response to the same heat stimulus administered alone. CPM was tested prior to and 25 min following a subcutaneous injection of either apomorphine (1.5 mg) or a placebo. CPM following apomorphine administration increased by 27.3% and by only 4% following placebo administration. RM-ANOVA revealed a significant interaction between 'session' and 'time' factors (F=5.316, p=0.023, η=0.054), and significant effect for the 'session' (F=5.719, p=0.019, η=0.006), but not for the 'time' (F=0.586, p=0.446, η=0.057). These results suggest that dopaminergic pathways both participate in and enhance pain modulation, represented by CPM. The role of dopamine in pain processing should be further studied.