The Endocannabinoid System's Contribution to Placebo Analgesia
Placebo analgesia--pain reduction from inert treatments--varies widely across individuals, yet its neurochemical basis remains poorly understood. While endogenous opioids, specifically {beta}-endorphin, contribute to placebo effects, mu-opioid receptor blockade does not fully abolish analgesia, implicating additional systems. Here, we investigate the endogenous cannabinoid (eCB) systems contribution to placebo analgesia in 48 healthy adults using a validated placebo paradigm with blood sampling. We quantified circulating levels of eCB ligands and {beta}-endorphin at baseline, as well as before and after placebo and control conditions to determine condition-related changes. Individual differences in placebo analgesia were associated with increases in fatty acid amide hydrolase (FAAH) substrates--a composite of anandamide, palmitoylethanolamide, and oleoylethanolamide--but not 2-arachidonoylglycerol or {beta}-endorphin alone. Critically, {beta}-endorphin moderated this relationship: FAAH substrates were strongly associated with pain reduction only when {beta}-endorphin levels were low. These findings provide evidence that eCB and opioid systems interact in a state-dependent manner during placebo analgesia in humans, with implications for understanding individual variability in treatment responses.