The dopamine agonist apomorphine enhances conditioned pain modulation in healthy humans

- Pain modulation in healthy subjects is affected by dopamine based intervention.
- Specifically, there was an enhancement in CPM after dopamine agonist administration.
- CPM was not changed following placebo administration.

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.