The role of endogenous opioids in the placebo effect in post-traumatic stress disorder.

The concept of the placebo effect has received a considerable attention over the past several decades. The placebo effect has been observed in different psychiatric disorders, including post-traumatic stress disorder (PTSD), a chronic and severe disorder precipitated by exposure to a psychologically distressing event. The placebo response rates in patients with PTSD range from 19% to 62%. A considerable number of research publications suggest that endogenous opioids are involved in the mechanisms of the placebo effect. Endogenous opioid peptides play an important role in stress response and in the pathophysiology of PTSD. Therefore, endogenous opioids may be involved in the neurobiology of the placebo effect in PTSD. Possibly, the endogenous opioid system mediates the effect of placebo on all 3 PTSD symptom clusters (re-experiencing symptoms, avoidance and numbing, and physiologic arousal). The placebo effect-related activation of the endogenous opioid system may result in an improvement in intrusive symptomatology and symptoms of increased arousal because the administration of exogenous opioids improve these symptoms. The placebo effect-related activation of the endogenous opioid system may have a mood-enhancing effect, and, consequently, diminish avoidance and numbing. Multiple neurotransmitter and neuroendocrine pathways may be involved in the mechanisms of the placebo effect in PTSD. Further studies of the neurobiology of the placebo effect on patients with PTSD and other psychiatric disorders may produce interesting and important results.