Placebo and nocebo effects are defined by opposite opioid and dopaminergic responses.
BACKGROUND
Placebo and nocebo effects, the therapeutic and adverse effects, respectively, of inert substances or sham procedures, represent serious confounds in the evaluation of therapeutic interventions. They are also an example of cognitive processes, particularly expectations, capable of influencing physiology.


OBJECTIVE
To examine the contribution of 2 different neurotransmitters, the endogenous opioid and the dopaminergic (DA) systems, to the development of placebo and nocebo effects.


METHODS
Using a within-subject design, subjects twice underwent a 20-minute standardized pain challenge, in the absence and presence of a placebo with expected analgesic properties. Studies were conducted in a university hospital setting.


METHODS
Twenty healthy men and women aged 20 to 30 years recruited by advertisement.


METHODS
Activation of DA and opioid neurotransmission by a pain stressor with and without placebo (changes in the binding potential of carbon 11 [11C]-labeled raclopride and [11C] carfentanil with positron emission tomography) and ratings of pain, affective state, and anticipation and perception of analgesia.


RESULTS
Placebo-induced activation of opioid neurotransmission was detected in the anterior cingulate, orbitofrontal and insular cortices, nucleus accumbens, amygdala, and periaqueductal gray matter. Dopaminergic activation was observed in the ventral basal ganglia, including the nucleus accumbens. Regional DA and opioid activity were associated with the anticipated and subjectively perceived effectiveness of the placebo and reductions in continuous pain ratings. High placebo responses were associated with greater DA and opioid activity in the nucleus accumbens. Nocebo responses were associated with a deactivation of DA and opioid release. Nucleus accumbens DA release accounted for 25% of the variance in placebo analgesic effects.


CONCLUSIONS
Placebo and nocebo effects are associated with opposite responses of DA and endogenous opioid neurotransmission in a distributed network of regions. The brain areas involved in these phenomena form part of the circuit typically implicated in reward responses and motivated behavior.
Scott
David J
DJ
Department of Psychiatry and Molecular and Behavioral Neuroscience Institute, University of Michigan, Ann Arbor 48109-0720, USA.
Stohler
Christian S
CS
Egnatuk
Christine M
CM
Wang
Heng
H
Koeppe
Robert A
RA
Zubieta
Jon-Kar
JK
D000328:Adult; D000700:Analgesics / Q000627:therapeutic use*; D001921:Brain / Q000000981:diagnostic imaging / Q000503:physiopathology*; D001931:Brain Mapping; D002250:Carbon Radioisotopes; D003469:Culture; D005260:Female; D005283:Fentanyl / Q000031:analogs & derivatives; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D010146:Pain / Q000188:drug therapy* / Q000503:physiopathology*; D010147:Pain Measurement; D015990:Placebo Effect; D049268:Positron-Emission Tomography; D020891:Raclopride; D011954:Receptors, Dopamine / Q000502:physiology*; D017448:Receptors, Dopamine D2 / Q000502:physiology; D050637:Receptors, Dopamine D3 / Q000502:physiology; D011957:Receptors, Opioid / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology
D000700:Analgesics; D002250:Carbon Radioisotopes; D011954:Receptors, Dopamine; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D011957:Receptors, Opioid; D017450:Receptors, Opioid, mu; D020891:Raclopride; C017114:carfentanil; D005283:Fentanyl