A Newly Discovered Neural Circuit for Placebo Pain Relief

In a long-sought breakthrough, researchers have delineated a pathway, from the cortex to the pontine nucleus and cerebellum, that governs pain relief from positive expectations, in a mouse study.

Researchers have long known that the mere expectation of pain relief, in the absence of any active treatment – think of the familiar sugar pill of clinical trials – can have beneficial effects on pain. But an understanding of this phenomenon of placebo analgesia, particularly the neural circuits in the brain that are responsible for it, has remained elusive. A new mouse study now goes a long way toward explaining the mystery.

Researchers led by Grégory Scherrer, University of North Carolina at Chapel Hill, US, have discovered a pathway responsible for placebo analgesia that projects from a cortical region called the rostral anterior cingulate cortex (rACC) to a brainstem structure called the pontine nucleus. Furthermore, neurons in the pontine nucleus indirectly conveyed information to Purkinje cells, the key neurons of the cerebellar cortex. And, experimentally inhibiting the pathway disrupted placebo analgesia, whereas activating the pathway induced pain relief, helping to make the case for those regions as key players underlying the phenomenon.

“Finally, information about the mechanisms of placebo analgesia that was missing in human research has now been found in mice. These results are so translatable, and show how evolutionary meaningful placebo is,” said Luana Colloca, who studies the neurobiology of placebo analgesia at the University of Maryland, Baltimore, US, but who was not involved in the new work.

The authors “have certainly delivered the most in-depth neural explanation of placebo analgesia so far, using essentially all of the modern, high-resolution techniques currently available for defining neural circuits in mice,” wrote Jeffrey Mogil, a pain researcher at McGill University in Montreal, Canada, in an accompanying News & Views.

The study and News & Views appeared in the August 29, 2024, issue of the journal Nature.

An animal model of placebo pain relief
The authors began the project based on human brain imaging studies that had shown increased activity in the ACC during pain relief from placebo.

But the bigger picture behind the study, Scherrer said, was to better understand the biological mechanisms underlying the cognitive dimensions of pain, which are less well understood than the sensory and affective aspects of pain.

“This is why we focused on placebo analgesia, which is a learned phenomenon,” Scherrer told Migraine Science Collaborative in an interview.

To investigate placebo mechanisms, Scherrer and colleagues placed mice in a box with two chambers: One with a floor at an innocuous, comfortably warm temperature, and one with a floor at a painfully hot temperature. After three days of daily conditioning, the mice developed a preference for the former.

Then, after the three days, when the temperature was held at the same painfully high temperature in each chamber, the conditioned mice preferred to spend time in the chamber that had previously been at a more comfortable temperature. These results showed that the mice had learned to expect pain relief when they entered the chamber previously associated with the relatively cooler chamber – the conditioned mice even now showed fewer pain-related behaviors, such as licking of the paws.

“These experiments are interesting because they show we can study placebo without any pharmacological treatment. They show placebo analgesia can be learned purely based on environmental cues,” said Colloca, who also added one note of caution.

“However, they only tested the mice for one week. I was wondering how long the placebo effect will last. This is important to know, because we know placebo analgesia can last for extended periods in humans,” she said.

An unexpected anatomical pathway
With their animal model of placebo analgesia established, the team then traced the underlying neural circuits involved. They did so by using techniques allowing for visualization of rACC neurons and where these cells project to. (The rostral portion of the ACC is the part of the ACC toward the front of the brain.)

Scherrer said he originally thought placebo analgesia would depend on connections between the ACC and amygdala, a brain region associated with the unpleasantness of pain, with the ACC telling the amygdala to “shut up” and “this is going to be painful, but I can deal with it,” as Scherrer put it.

But, surprisingly, instead of projecting to the amygdala, rACC neurons went to the pontine nucleus (Pn), a region of the pons (the middle of the brainstem) that mediates communication between the cortex and the cerebellum.

Study first author Chong Chen said that there were breadcrumbs of evidence hiding in human brain imaging studies that implicated the cerebellum in placebo.

“People don’t normally talk about the cerebellum in placebo – it’s thought to be more involved with motor coordination – but experiments in people have shown that the cerebellum is active during placebo,” Chen said, referring to a 2004 study led by pain researcher Tor Wager.

In addition, regarding the cerebellum and pain, “although the cerebellum has previously been linked to pain (one of the long-standing ‘dirty little secrets’ of pain imaging is how often the cerebellum lights up), its role remains ‘mysterious,’” wrote Mogil in the News & Views.

Not only anatomy, but function, too
The team next asked if the rACC-Pn pathway had an active role in placebo analgesia. To do so, they looked at the activity of neurons projecting from the rACC to the Pn as the mice took part in the two-chamber experiment.

They found that rACC neurons projecting to the Pn became more active during the conditioning phase of the experiment, when the mice were developing a preference for one chamber over the other.

“These cells light up just before mice cross into the area where they get placebo analgesia. We performed all sorts of control experiments to verify this,” said Scherrer.

Further building their case, when the investigators blocked communication between the rACC and Pn, the trained animals no longer showed a pain reduction within the comfortably warm floor.

“What’s even more exciting is that if you don’t do any placebo conditioning, you can artificially increase the activity in this pathway to produce actual analgesia,” said Scherrer.

Opioids and placebo analgesia
Back in the 1980s, scientists found that naloxone, an opioid antagonist that binds to opioid receptors in the brain and thereby lessens the effects of opioids, could prevent placebo analgesia.

“That work showed a mechanism whereby opioid peptides released by cells in our body can reduce pain just based on expectation of pain relief,” said Scherrer.

So the team asked if placebo analgesia resulting from activity in the rACC-Pn pathway involved opioid signaling.

First, by examining the expression of opioid receptors and peptides in almost 5,000 neurons in the Pn, the researchers found that 65% of them contained delta- and/or mu-opioid receptors (two of the three main subtypes of opioid receptors).

“It turned out that the pontine nucleus had one of the highest rates of expression of the two receptors in the nervous system,” said Chen.

Further, inhibiting opioid signaling in the rACC-Pn pathway affected placebo analgesia: Selectively blocking mu- or delta-opioid receptors in the rACC prevented mice from learning the placebo analgesia task.

The group next turned to the cerebellum, considering that this brain structure is the main target of Pn neurons. Cell tracing experiments revealed that opioid receptor-expressing cells in the Pn projected to the cerebellum. And, the activity of the Purkinje cells in the cerebellum was similar to that of neurons projecting from the rACC to the Pn during pain relief expectation, in the animal model of placebo analgesia.

“These findings do make some sense if you think about pain expectations and motor planning. We often adjust our movements and posture to minimize pain such as, when we have an injury on one leg, we put more weight on the other leg, or when we have low back pain, we are going to anticipate and avoid movements that put strain on our lower back,” Scherrer said.

Relevance to migraine
Since placebo analgesic effects have also been described in migraine, the new paper has bearing on it, Colloca said.

 “The study could help us understand why some people do or don’t respond to placebo. This information could be used for treatment or even prevention of migraine attacks,” she said.

She added that placebo effects might not just be limited to improving headache pain but also to alleviating other symptoms, like the photophobia that those with migraine often experience. Scherrer and Chen agreed but said studies in humans and animals are needed to understand whether the newly identified pathway plays a role in placebo effects beyond those of pain relief.

Chen, who is also a physician, said he hopes the study will help people recognize that the placebo effect is real.

“I’ve found that many physicians and patients don’t trust the placebo effect to work. Its effect relies on what you believe – if you don’t believe in it, it won’t work. We provide biological evidence that placebo is real,” said Chen.

Chen said doctors and patients should take advantage of this natural method of pain relief, which doesn’t require medical intervention.

“I hope clinicians might feel more confident in offering therapies like yoga and cognitive treatments to patients, which potentially leverage the benefits of the placebo effect, among other mechanisms,” he said.

In the future, Chen added, it could be possible to target the ACC-to-Pn pathway with medications to bolster the benefit from placebo, not only for pain conditions but for other disorders like migraine as well.

Fred Schwaller, PhD, is a freelance science writer based in Germany. Follow him on Twitter @SchwallerFred

Image credit: 123RF Stock Photo.

References
Neural circuit basis of placebo pain relief.
Chen et al.
Nature. 2024 Aug;632(8027):1092-1100.

Placebo effect involves unexpected brain regions.
Mogil JS.
Nature. 2024 Aug;632(8027):990-991.