Cold Pain Threshold and Role of NMDA Receptor Antagonists

Abstract

Clonidine, the α2-adrenergic agonist, is usually used as an antihypertensive drug. Dextromethorphan is a non-competitive NMDA antagonist which is routinely prescribed to suppress cough. However, there are not confidential documents regarding their analgesic effects. Due to the controversies over the analgesic properties of these two drugs, this study was designed to evaluate cold pain threshold changes following their administration. This study was conducted to assess the impact of oral clonidine and dextromethorphan on ice-water immersion tolerance. Four closed sachets labeled with codes were dedicated to each participant. Each of these four sachets contained placebo, 0.3 mg/kg dextromethorphan, 0.2 mg clonidine or both of the previous drugs randomly. The cold pain threshold was measured five times for each participant, once before taking any drug (T1) and the next four times (T2-T5) after taking each of the four sachets. 35 volunteers (15 men and 20 women) participated in the study. The study showed that cold pain threshold was higher in men than women (P=0.004) and also in participants above 30 than those under 30-year-old (P=0.007). Moreover, the pain threshold did not change significantly after the administration of clonidine (P=0.33) or dextromethorphan (P=0.21), but the threshold significantly increased after receiving a combination of dextromethorphan and clonidine compared with placebo overall (P=0.001). Cold pain threshold was higher in men and individuals over than 30-year-old and decreased significantly after administration of clonidine and dextromethorphan conjointly. Body mass index has no relation with changes in cold pain threshold by taking mentioned medications.

Keywords

• Dextromethorphan
• Clonidine
• Cold pain threshold