Summary: Researchers have successfully induced the placebo effect in rats and mapped the neural circuitry that enables belief in pain relief to reduce pain perception. Through Pavlovian conditioning, rats learned to associate injections with analgesia; when later given saline, many showed reduced pain responses. Neuroimaging and targeted manipulations revealed that endogenous opioid signaling in the medial prefrontal cortex engages descending pain-inhibition pathways to produce placebo analgesia.
This work clarifies the brain mechanisms underlying placebo analgesia in an animal model, offering a foundation for strategies that could enhance therapeutic outcomes while minimizing drug use and adverse effects.
Key findings:
- Opioid signaling in mPFC: Placebo responses were tied to μ-opioid (MOR) activity in the medial prefrontal cortex.
- Descending pain inhibition: mPFC opioid activity activated the brain’s descending pain-suppression network, producing measurable analgesia.
- Conditioned placebo response: After conditioning with an active analgesic, around one-third of rats displayed a robust placebo effect, another third showed partial effects, and the rest showed little or no response.
Source: RIKEN
The placebo effect—where expectation of benefit produces real symptom relief—has long been documented in people, but identifying the precise neurons and circuits involved has been limited by ethical and technical constraints. By inducing placebo analgesia in rodents, the research team could use invasive neuroimaging and circuit-specific manipulations to reveal mechanisms that are otherwise inaccessible in humans.
The study began with a conditioning protocol: over several days, rats received injections of an active painkiller, leading them to associate the injection procedure with relief. When later injected with a saline solution, many animals exhibited decreased pain responses consistent with placebo analgesia. Approximately one-third of animals showed a full placebo response, facilitating detailed follow-up experiments on their brain activity.
Using imaging and chemogenetic tools, the researchers identified μ-opioid receptor–positive (MOR+) neurons in the medial prefrontal cortex (mPFC) as key mediators. Activation of MOR-related signaling in mPFC produced disinhibition of excitatory output to the ventrolateral periaqueductal gray (vlPAG), a critical node of the descending pain-inhibitory system. Specifically, MOR+ neurons in mPFC form monosynaptic connections and inhibit layer V pyramidal neurons projecting to the vlPAG via GABAA receptors; intrinsic opioid signaling suppresses these MOR+ neurons, thereby releasing excitatory drive to the vlPAG and triggering descending analgesia.
Chemogenetic experiments further demonstrated causality: manipulating MOR+ neurons or the mPFC–vlPAG circuit altered the placebo response. Suppressing the mPFC–vlPAG pathway or selectively activating MOR+ neurons blocked placebo analgesia, supporting a model in which local opioid signaling in mPFC gates descending pain inhibition to generate the placebo effect.
The patterns of brain activation observed in the conditioned rats resembled those reported in human imaging studies of placebo analgesia, suggesting that similar mechanisms operate across species. This convergence strengthens the translational relevance of the findings and suggests potential pathways to amplify beneficial expectancy effects in clinical settings.
Clinically, harnessing placebo mechanisms could reduce required doses of analgesic drugs, lowering the risk of side effects and dependence. The new insights into mPFC opioid signaling and its engagement of descending inhibitory circuits provide concrete targets for future research aimed at safely leveraging expectation-driven relief.
About this neuropharmacology research news
Author: Yilong Cui
Source: RIKEN
Contact: Yilong Cui – RIKEN
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Opioidergic activation of the descending pain inhibitory system underlies placebo analgesia” by Yilong Cui et al., Science Advances
Abstract
Opioidergic activation of the descending pain inhibitory system underlies placebo analgesia
Placebo analgesia results from an inactive treatment producing real pain relief through endogenous brain mechanisms, yet the neurobiological basis is incompletely understood. In neuropathic pain rats, μ-opioid signals in the medial prefrontal cortex (mPFC) activate the descending pain inhibitory system to initiate placebo analgesia.
Chemogenetic manipulation showed that activating μ-opioid receptor–positive (MOR+) neurons in mPFC or suppressing the mPFC–vlPAG circuit interfered with placebo analgesia. MOR+ neurons in the mPFC connect monosynaptically and inhibit layer V pyramidal neurons projecting to the vlPAG through GABAA receptors. Intrinsic opioid signaling in the mPFC disinhibits excitatory outflow to the vlPAG by suppressing MOR+ neurons, thus initiating descending pain inhibition and producing placebo analgesia.
These results illuminate a detailed neurobiological mechanism for the placebo effect, with implications for maximizing therapeutic benefit and reducing adverse effects in medical practice.