Effects of dexmedetomidine on conditioned pain modulation in humans

Background: Systemic administration of dexmedetomidine (DEX; selective alpha(2)-adrenoceptor agonist) is found to inhibit diffuse noxious inhibitory control in rats, now referred to as conditioned pain modulation (CPM) in humans. The present study was designed to investigate the effect of intravenous administration of DEX on CPM in humans. Methods: There were two sequential sessions in this double blind, randomized study. The first session was the control with normal saline infusion (N-1st, L-1st, H-1st). During the second session, three types of agents were infused: normal saline (N-2nd); a low plasma concentration of DEX (0.04 ng/mL; L-2nd); and a high plasma concentration of DEX (0.08 ng/mL; H-2nd). The amplitude of somatosensory evoked potentials (ampSEP)s and the visual analogue scale of tooth pain (VASt) induced by electrical tooth stimulation were evaluated with and without conditioning CO2 laser stimulation of the hand. The inhibition rate (% inhibition) was calculated [= (1-[ampSEP or VASt with conditioning stimuli]/[ampSEP or VASt without conditioning stimuli]) x 100] to compare the magnitude of the DEX effects on CPM. Results: The inhibition rates of ampSEPs and VASt in Types N, L and H varied significantly, demonstrating a dose-dependent reduction of CPM effects of ampSEP and VASt during randomized DEX administration, consistent with results from animal studies. Conclusions: The present study shows that systemic administration of an alpha(2)-adrenoceptor agonist (DEX), less than the clinical dose, inhibited CPM in humans. These results may provide some mechanistic insight into why many chronic pain patients show impaired CPM.