Figure 1.

Pharmacological evidences. As reported by different pharmacological studies, placebo administration activates endogenous opioid, cannabinoid, and dopamine systems (A). Participants in the placebo groups experienced analgesic effects, namely pain reduction, compared to participants that received no treatments (natural history group). This analgesic effect is enhanced by proglumide, oxytocin, and vasopressine ((B), upper graph) while it is disrupted by rimonabant, naloxone, and pentagastrin ((B), middle graph). Nocebo effects exacerbate pain perception compared no treatment groups (natural history group). This effect is partially reversed by CCK antagonist proglumide ((B), bottom graph).