In lab, atomic snapshots show ketamine binds human opioid receptors

In a laboratory chilled below -148°F, researchers froze human opioid receptors within liquid ethane, trapping ketamine and its chemical relative phencyclidine (PCP) in place. Transmission electron microscopy then assembled those snapshots into atomic-scale 3D structures showing how the drugs sit inside the receptors.

The images showed ketamine and PCP binding directly to the human mu and kappa opioid receptors, rather than merely influencing them from a distance. Both settled into the orthosteric site, the main binding pocket also used by endorphins and classic opioids such as morphine. Their connection was looser: ketamine and PCP relied on hydrophobic interactions instead of the tight salt bridge that anchors classic opioids.

The structures also identified two molecular gatekeepers, the amino acids Y139 and I290. They have to move aside before ketamine or PCP can enter the pocket. In mice, the biological consequence was visible: naloxone blocked ketamine’s pain relief, as did the kappa-specific blocker aticaprant, indicating that opioid receptors contribute directly to the effect in living animals.

The finding adds an opioid mechanism to ketamine’s better-known action on NMDARs, or N-methyl-D-aspartate receptors, major receptors in the brain. Doctors had already observed that naltrexone, an opioid-blocking drug, could wipe out ketamine’s rapid antidepressant effects, but scientists disagreed on whether ketamine touched the main opioid binding site or acted indirectly.

So, concretely, the atomic map gives drug developers a target for preserving ketamine’s rapid effects on pain and treatment-resistant depression while reducing hallucinations, cognitive side effects and abuse potential. That possibility remains a research goal, not a delivered treatment: the evidence reported here comes from purified receptors grown in insect cells and tests in mice, while the journal publication provides the structural basis for the next round of drug design.