www.medscimonit.com/pub/vol_11/no_5/7202.pdf
Summary:
Evidence suggests that the placebo response is related to the tonic
effects of constitutive nitric oxide in neural, vascular and immune
tissues. Constitutive nitric oxide levels play a role in the
modulation of dopamine outflow in the nigrostriatal movement and the
mesolimbic and mesocortical reward and motivation circuitries.
Endogenous morphine, which stimulates constitutive nitric oxide, may
be an important signal ...