Expectations of analgesia do not affect spinal nociceptive R-III reflex activity: an experimental study into the mechanism of placebo-induced analgesia.
The purpose of this study was to investigate whether placebo analgesia is mediated by the release of beta-endorphin. In addition to subjective pain reports, we included an objective physiological parameter of nociception reflected by the opioid sensitive nociceptive R-III reflex. Placebo consisted of strong suggestions of pain relief and an intravenous injection of saline. Forty minutes after placebo, either the opioid antagonist naloxone or saline was administered intravenously without subjects noticing (hidden). Sixty healthy males, aged 18-30 years, voluntarily participated in this study. Subjects were randomized into one of four groups: group 1 received placebo and hidden naloxone, group 2 received hidden naloxone only, group 3 received placebo and hidden saline and group 4 received hidden saline only. Pain was induced by electrical stimulation of the sural nerve and evaluated with a visual analogue scale (VAS). In addition, changes in the magnitude of the nociceptive R-III reflex activity were assessed. We determined to what extent R-III reflex activity and subjective pain reports were decreased by placebo and we investigated whether these placebo-induced changes in reflex activity and subjective pain reports were naloxone reversible. Furthermore, we measured the degree of association between pain relief as measured on VAS and changes in R-III reflex activity. Finally, the role of beta-endorphin was assessed by measuring plasma endorphin levels before and after the administration of placebo. This study could not demonstrate a placebo effect as measured on VAS and R-III responses. The administration of placebo did not appear to have an effect on the release of beta-endorphins. Consistently, the antagonizing effects of naloxone were negligible. A subgroup analysis of those who did show a placebo response as indicated on the VAS did not support the supposition that beta-endorphin is released due to placebo suggestion. It is suggested that intensified stimuli and a more effective procedure to induce placebo analgesia (e.g. conditioning) may produce a proper placebo effect.
Roelofs
Jeffrey
J
Department of Medical, Clinical and Experimental Psychology, Maastricht University, 6200 MD Maastricht, The Netherlands Department of Epidemiology, Maastricht University, 6200 MD Maastricht, The Netherlands Department of Clinical Epidemiology and Medical Technology Assessment, Maastricht University Hospital, 6202 AZ Maastricht, The Netherlands Department of Clinical Neurophysiology, Maastricht University Hospital, 6202 AZ Maastricht, The Netherlands Department of Clinical Chemistry, Maastricht University Hospital, 6202 AZ Maastricht, The Netherlands.
Ter Riet
Gerben
G
Peters
Madelon L
ML
Kessels
Alphons G H
AGH
Reulen
Jos P H
JPH
Menheere
Paul P C A
PPCA
D000293:Adolescent; D000328:Adult; D006801:Humans; D008297:Male; D009270:Naloxone / Q000008:administration & dosage; D009292:Narcotic Antagonists / Q000008:administration & dosage; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology*; D010146:Pain / Q000188:drug therapy / Q000503:physiopathology*; D015990:Placebo Effect; D010919:Placebos / Q000008:administration & dosage*; D012018:Reflex / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*; D001615:beta-Endorphin / Q000097:blood
D009292:Narcotic Antagonists; D010919:Placebos; D009270:Naloxone; D001615:beta-Endorphin