The Placebo Effect Is Real and Scientists Just Mapped the Brain Circuit Behind It

Scientists trained mice to experience placebo pain relief, then traced the effect to a brain circuit that releases the body’s own opioid-like painkillers. The findings could point toward drug-free ways to prepare the brain for pain after surgery or injury.

The placebo effect has always sat on the strange side of medicine. Give someone a sugar pill, tell them it is a painkiller, and sometimes their pain really does ease. We know it’s real, and we know it can be impactful.

Doctors know expectations can shape pain. They also know that placebo responses can make clinical trials harder to interpret. Neuroscientists know the brain has its own opioid-like chemicals. But the exact circuitry that turns an expectation of relief into actual pain relief has proven extremely tough to figure out.

Now, researchers say they have mapped a key part of that pathway in mice.

Reversing the Playbook

The study was led by Matthew Banghart at the University of California San Diego. Banghart adapted a placebo protocol from human studies and used it in mice.

Traditionally, medical research flows in one direction: from animal models to human clinical trials. Most biomedical research moves from animals to humans. Scientists discover something in mice, then hope it also matters in people.

Instead, Banghart’s team used a “reverse translation” approach, taking a placebo setup known from human research and rebuilding it for mice. The animals were placed in chambers with distinct visual patterns, such as stripes or dots, and distinct smells, such as banana or lemon. In one specific chamber, the mice received morphine before being placed on a hot surface.

Over several days, the animals learned the association: this room meant pain relief.

Then came the real test. The researchers removed the morphine and injected the mice with a saline solution that doesn’t do anything (the placebo). But when the mice returned to the morphine-paired room, they still showed signs of pain relief.

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“We took a placebo protocol from humans and worked it out in mice, and used that to deconstruct the underlying mechanisms,” said Matthew Banghart, the lead researcher of the study. “We went much further than previous studies and pinpointed a site at which endogenous opioid peptides are critical, which previously had not been done.”

The Chemical Flashlight

Then, once the team knew the mice were having a placebo, they tried to see what was going on in their brains.

To figure this out what was happening, the researchers focused on the cortex, the outer layer of the brain involved in prediction, evaluation, and decision-making.

To map how a simple expectation physically blocked pain, the researchers looked to the cortex—the thinking, evaluating outer layer of the brain. They found that two higher-order regions, the medial prefrontal cortex and the anterior cingulate cortex, were sending active signals downward. These regions act as the architects of belief, beaming instructions to a primitive structure deep in the brainstem called the ventrolateral periaqueductal gray, or vlPAG.

The vlPAG is a known hub for pain signaling. The team used custom-built, glowing molecular sensors to watch this region in real time. The moment the mice entered the placebo room, the sensors lit up. The brainstem was flooded with endogenous opioids—the body’s homegrown endorphins.

However, the team needed absolute proof that these specific endorphins were doing the heavy lifting. They turned to a light-activated drug called photoactivable naloxone, or PhNX, a caged version of the overdose-reversal drug naloxone. By threading microscopic optical fibers into the mice, they could fire a burst of ultraviolet light directly into the vlPAG. The light instantly uncaged the PhNX, blocking the brain’s opioid receptors in that exact spot.

The instant the light fired, the placebo effect vanished. The animals felt the heat again.

“We essentially trained a mouse brain to create its own broad-spectrum painkillers on demand, precisely where they are needed to treat pain, without the off-target effects of opioid-based painkillers,” said Janie Chang-Weinberg, a PhD student and co-first author of the study.

“Is It Possible to Learn This Power?”

The most intriguing part came next.

The mice had been trained using heat pain. But when the researchers tested them with a different kind of pain, a mechanical pinprick, the placebo effect still helped. The relief generalized.

That matters because real pain is messy. Surgery, inflammation, injury, and chronic pain do not all work the same way. If expectation-based pain relief can generalize across different kinds of pain, it could be more clinically useful than a narrow response tied to one stimulus. The implications are striking.

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“This finding has direct implications for how placebo training in humans might be used to produce resilience to future pain that results from injury, whether anticipated—such as an upcoming surgery—or unanticipated pain, such as a broken bone from a fall,” Banghart said.

We have good reasons to suspect this could translate to humans, but we do not know that yet. Humans have similar pain-modulating brain systems to mice, including cortical regions involved in expectation and brainstem pathways that use endogenous opioids. Still, this study only proves the mechanism in mice. But this is definitely something we should be looking into, the researchers conclude.

“This is something that can be very powerful,” Banghart said. “We should be tapping into it intentionally in order to reduce pain and suffering.”

The study was published in the journal Neuron.