Placebo analgesia and the heart

Placebo-activated endogenous opioids act on pain mechanisms inducing analgesia, as well as on the respiratory centers inducing respiratory depression. Here, we show that placebo analgesia is accompanied by a reduced beta-adrenergic activity of the heart. We measured heart rate during placebo-induced expectation of analgesia, both in the clinical and the laboratory setting. In the clinical setting, we found that the placebo analgesic response to an electrical noxious stimulus was accompanied by a reduced heart rate response. In order to investigate this effect from a pharmacological viewpoint, we reproduced the same effect in the laboratory setting by using experimental ischemic arm pain. We found that the opioid antagonist naloxone completely antagonized both placebo analgesia and the concomitant reduced heart rate response, whereas the beta-blocker propranolol antagonized the placebo heart rate reduction, but not placebo analgesia. By contrast, both placebo responses were present during muscarinic blockade with atropine, indicating no involvement of the parasympathetic system. In order to better understand the effects of naloxone and propranolol, we performed a spectral analysis of the heart rate variability for the identification of the sympathetic and parasympathetic components, and found that the beta-adrenergic low frequency (0.15 Hz) spectral component was reduced during placebo analgesia, an effect that was reversed by naloxone. These findings indicate that placebo analgesia is accompanied by a complex cascade of events which affect the cardiovascular system.