Neural circuit basis of placebo pain relief [SMART-seq]

For over a millennium, mind-body interactions have fascinated scientists and doctors for their abilities to shape human perceptions of the external world 1,2. Placebo effects are striking demonstrations of mind-body interactions in which, in the absence of any treatment, a positive expectation of pain relief can reduce or even abolish the experience of pain 3–6. However, despite widespread recognition of the strength of placebo effects and their impact on everyday human experience and clinical trials for new analgesics, the neural circuit basis of the placebo effect has remained a mystery. Here, we show that analgesia from the expectation of pain relief is mediated by a distinct population of rostral anterior cingulate cortex (rACC) neurons that project to the pontine nuclei (rACC→Pn), a pair of brainstem pre-cerebellar nuclei with no established function in pain processing. To do this, we created a behavioral assay that models placebo analgesia by conditioning mice to expect pain relief when moving from a chamber with a heated floor to a second chamber. In this assay, an expectation of pain relief induces an analgesic effect that, like placebo analgesia in humans, is mediated by endogenous opioids. Calcium imaging of neural activity in freely moving mice and electrophysiological studies in cingulate cortical brain slices showed that expectations of pain relief boost the activity of rACC→Pn neurons and potentiate neurotransmission in this pathway. Transcriptomic studies of Pn neurons revealed an unusual abundance of opioid receptors in these cells, further suggesting a role in pain modulation. Selective inhibition of either the rACC→Pn pathway or of opioid-receptor-expressing Pn neurons disrupted placebo analgesia and decreased pain thresholds. Finally, a subset of cerebellar Purkinje cells exhibits activity patterns resembling those of rACC→Pn neurons during pain relief expectation, providing cellular-level evidence of a role for the cerebellum in cognitive pain modulation. Altogether, these findings elucidate longstanding mysteries surrounding the placebo effect by identifying a specific neural pathway that mediates expectation-based pain relief. This discovery opens the possibility of targeting this novel pathway with drugs or neurostimulation methods to treat pain. More broadly, our studies provide a framework for investigating the neural circuit basis of other mind-body interactions beyond those involving pain, and point to prefrontocortical-cerebellar communication as a potential basis for such effects. For SMART-seq v2 data, pontine nucleus cell from 8-week-old Snap25-IRES2-Cre;Ai14 mice (1 male and 1 female) were sorted for tdTomato into PCR tubes for library generation.