THE SPECIFIC EFFECTS OF PRIOR OPIOID EXPOSURE ON PLACEBO ANALGESIA AND PLACEBO RESPIRATORY DEPRESSION

Although in most of the cases the placebo response appears to be unpre dictable, several factors have been considered in order to explain the placebo analgesic effect. For example, it is widely recognized, albei t with little empirical evidence, that placebo analgesia is more likel y to occur after a successful analgesic therapy. On the basis of this assumption, we tested the placebo response in a population of patients who were treated with buprenorphine the day before for relieving post operative pain. However, due to the high variability of opioid respons iveness, buprenorphine was effective in some patients and poorly effec tive in some others. Similarly, buprenorphine produced respiratory dep ression with a large variability, ranging from mild depression to no e ffect. We found that the placebo analgesic response depended on the bu prenorphine analgesic effectiveness of the previous day. Analogously, we found that a placebo respiratory depressant response was more prono unced in those patients with a respiratory depressant response to bupr enorphine on the day before, irrespective of the analgesic effectivene ss. These specific effects suggest that (1) the placebo effect is expe rience-dependent; (2) the mechanisms underlying placebo analgesia and placebo respiratory depression are independent from each other and, by considering the role of endogenous opioids in placebo analgesia, migh t involve different subpopulations of opioid receptors. (C) 1998 Inter national Association for the Study of Pain. Published by Elsevier Scie nce B.V.