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, eta = 0.054), and significant effect for the 'session' (F = 5.719, p = 0.019, eta = 0.006), but not for the 'time' (F = 0.586, p = 0.446, eta = 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. (C) 2013 Elsevier Ireland Ltd. All rights reserved.