[Placebo effect: clinical, biological and therapeutical involvements in depression].

The placebo effect is an excellent model for understanding the mechanisms underlying the interaction between a subjective and complex mental activity (beliefs, expectations, hopes, learning, patient-physician relationship, socio-cultural context .) with different neural and biological systems. Initially, research on the placebo effect has focused on the mechanisms of pain and analgesia. The cognitive processes of conditioning and reward anticipation (hope of a relief) were highlighted. The involvement of different neurobiological pathways has been clearly shown: endogenous opioids, CCK, dopaminergic pathways, endocannabinoids, immunological factors... More recently, the field has open towards new perspectives: depression and anxiety, motor disorders, immune system, endocrine system. Intensive research in the field emerges because of its fundamental implications in neuroscience research but also because of the ethical, clinical and therapeutical issues. Moreover, the placebo effect is considered as a main methodological mean issue in clinical trials that allows the demonstration of the efficacy and tolerance of new drugs. In the field of psychiatry, depression is a placebo highly-sensitive disorder: placebo response rates in clinical trials are of the order of 30 % to 40 %. The identification of biological markers of placebo response, such as neuroimaging and quantitative electroencephalography may lead to develop more efficient models in clinical research.