Scientists have known for decades that opioids relieve pain by binding to molecular switches in the brain called mu-opioid (pronounced "mew-opioid") receptors. What they didn't know — until now — was exactly what happens next.
A team led by biologists at the USC Dornsife College of Letters, Arts and Sciences, in collaboration with the Keck School of Medicine of USC , has captured those receptors mid-action, creating the molecular equivalent of a slow-motion movie. Their discovery , published this week in Nature and supported by the National Institutes of Health, could help scientists design painkillers that aren't as addictive and develop longer-acting overdose antidotes like naloxone, better known by its brand name Narcan.
"It's a little like watching an engine run in super slow motion," said study corresponding author Cornelius Gati , assistant professor of biological sciences, chemistry, and quantitative and computational biology at USC Dornsife. "We can finally see which parts move when an opioid drug binds to the receptor — and how Narcan literally jams the mechanism before the process can start."
An opioid molecular movie in motion
To capture these fleeting molecular events, Gati's team used cryo-electron microscopy (cryo-EM), a technique that flash-freezes molecules and then images them at near-atomic resolution. The method allowed the researchers to see how the receptor and its partner molecule, a "G protein" that transmits signals inside the cell, change shape when an opioid binds and activates the receptor.
The team conducted their experiments using USC's in-house cryo-EM facility housed in the USC Michelson Center for Convergent Bioscience .
"Before this, scientists only had two still images of this receptor — one off and one on," said Saif Khan, the study's first author and a PhD student in Gati's lab. "Now we can see everything that happens in between. It's like going from two snapshots to a flipbook that finally reveals the full motion."
Within a set of eight unique 3D models and 16 cryo-EM 3D images, the team captured six different receptor states, each representing a distinct step in how opioid drugs and their antidotes affect the function of their receptor.
How opioids turn on — and Narcan turns off — the signal
The mu-opioid receptor is part of a large family of proteins called G protein-coupled receptors (GPCRs), which help regulate everything from pain to mood, heartrate and metabolism. When an opioid drug binds to the receptor, it triggers the release of a small molecule called GDP from its G protein, setting off a cascade of signals that activate the body's pain-relief pathways.
But when that signaling goes too far, it can slow breathing and produce euphoria — effects that underlie both overdose and addiction.
The researchers found that different drugs influence this process in distinct ways. Loperamide, a powerful opioid that doesn't cross the blood-brain barrier, shifts the receptor into a structural shape that quickly releases GDP — essentially flipping the "on" switch. In contrast, Narcan locks the receptor in what Gati's team calls a "latent" state, like pressing a molecular pause button before GDP can be released.
"Scientists thought that drugs like Narcan stop the receptor from talking to its G protein," said Khan. "We now see that the conversation starts — it just never finishes because Narcan interrupts it."
Opioid receptor breakthrough points to new drug discovery paths
Despite growing awareness of their dangerous side effects, opioids remain widely prescribed, with about 125 million prescriptions filled in the U.S. in 2023, according to the U.S. Centers for Disease Control and Prevention. More than 80,000 Americans died from opioid overdoses that year.
Narcan is the standard lifesaving antidote used by first responders, but with new synthetic opioids like fentanyl — hundreds of times more potent than morphine — patients may not always respond quickly enough to it. Because current treatments wear off faster than the opioids they counteract, patients often require multiple doses.
Knowing precisely how Narcan interacts with the receptor could guide chemists in designing longer-lasting or faster-acting antidotes, while understanding the receptor's step-by-step mechanics could help fine-tune opioid painkillers so they relieve pain without causing dangerous side effects such as difficulty breathing.
"If we can design drugs that activate only part of this molecular machinery," Gati said, "we might be able to keep the good — pain relief — and lose the bad, like addiction and respiratory depression."
Gati added that the implications extend far beyond opioids. The mu-opioid receptor belongs to one of the largest families of drug targets in the human body, and roughly a third of all prescription medicines, involved in everything from mood regulation to metabolism, act on these GPCRs.
"This is a template for understanding how an entire class of receptors works," he said. "If we can map these molecular movements for opioids, we can apply the same principles to designing better drugs for heart disease, depression and diabetes."
The images Gati's team produced are among the most detailed views ever obtained for an opioid receptor complex. Each snapshot revealed subtle shifts in how the receptor grips the G protein, releases GDP, and opens the pathway that triggers activation.
The researchers also used computer simulations to watch the molecules move in real time, confirming that the transitions captured in frozen form match the receptor's natural behavior.
"Proteins are like tiny molecular machines," said Khan. "And the best way to understand how a machine works is to watch it in motion. That's what we've finally been able to do — see this receptor operating at the nanoscale, in real time."
A new chapter in the opioid story
While the findings won't solve the opioid crisis overnight, they offer something the field has long lacked: a detailed roadmap of how opioid drugs and antidotes work from the inside out.
"This is basic science, but it's the kind of basic science that can transform medicine," said Gati, who also holds a joint appointment in pharmacology and pharmaceutical sciences at the USC Alfred E. Mann School of Pharmacy and Pharmaceutical Sciences . "By understanding these receptors in such detail, we can finally start designing drugs that are as smart as the molecules they target."
About the study
In addition to Gati and Khan, study authors include Aaliyah Tyson, Mohsen Ranjbar, Jaskaran Singh and Gye Won Han of USC Dornsife, and Zixin Zhang of Keck School of Medicine of USC.
The research was supported by National Institutes of Health grant R01AT012075.
