The story of O--is oxytocin the mediator of the placebo response?

While the placebo responses in various medical conditions have been shown to follow a few basic principles such as expectancies, reward learning and Pavlovian conditioning, the underlying neurobiology and the mediating hormonal and/or neuromodulating processing have remained obscure. We here report the collected evidence that oxytocin (OXT), a 389-amino acid polypeptide located on chromosome 3p25 that is released in the brain (hypothalamus) and in peripheral tissue, is the central mediator of the placebo response: we hypothesize that exogenous OXT via an OXT agonist will enhance the placebo response, while exogenous OXT blockade by an antagonist will reduce the placebo response in placebo analgesia and other placebo models. It is furthermore proposed that the placebo response in trials may be predicted by circulating plasma OXT levels, the OXT receptor density in the brain and/or the presence of one or more of the single nucleotide polymorphisms of the OXT promoter gene.