(365) Pain modulation and autonomic function: the effect of clonidine

Abstract

The α2-agonist clonidine is an analgesic agent, whose yet uncertain action may involve either increase in pain modulation efficiency, change in autonomic function and/or decrease in anxiety level. We aimed to examine the effect of oral clonidine on pain perception in healthy subjects, in order to reveal its mode of action. This randomized, double blind, placebo controlled study included 40 healthy subjects. Subjects received either 0.15 mg oral clonidine or placebo. We measured: 1. measured pain parameters of heat pain thresholds, tonic heat stimulus, mechanical temporal summation, offset analgesia (OA) and conditioned pain modulation (CPM); 2. autonomic parameters of deep breathing ratio and heart rate variability indices obtained before, during and after tonic heat stimulus; and 3. psychological parameters of anxiety and pain catastrophizing. Clonidine decreased systolic blood pressure (p=0.022) and heart rate (p=0.004) and increased rMSSD (p=0.020), though no effect was observed on pain perception, pain modulation and psychological parameters. Autonomic changes were correlated with pain modulation capacity; for OA, the separate slope model was significant (p=0.008) such that in the clonidine group more efficient OA was associated with lower heart rate (r=0.633, p=0.005) unlike the placebo group. A similar trend was found for CPM. The change in autonomic function which was related to the increase in pain modulation capacity, and the lack of change in anxiety suggest a combined modulatory-autonomic mode of analgesic action for clonidine.