Propofol, ketamine, and dexmedetomidine knock you out differently - but the brain crashes the same way

Propofol binds to GABA receptors. Ketamine blocks NMDA receptors. Dexmedetomidine shuts down norepinephrine release. Three drugs, three different molecular mechanisms, three different ways of interfering with brain chemistry. Yet all three put you to sleep.

How? According to a new study from MIT's Picower Institute for Learning and Memory, published in Cell Reports, the answer is that despite their different entry points, all three drugs converge on the same downstream effect: they destabilize the brain's balance between stability and excitability until consciousness collapses.

The knife's edge that keeps you conscious

When you are awake, your brain operates in a state neuroscientists call dynamic stability. It is responsive - sensory input, a sudden noise, a change in light - causes neural circuits to react. But after responding, the brain returns to a stable baseline. It is excitable enough to process information, but stable enough not to spiral into chaos.

"The nervous system has to operate on a knife's edge in this narrow range of excitability," says Earl Miller, the Picower Professor of Neuroscience at MIT. "It has to be excitable enough so different parts can influence one another, but if it gets too excited it goes off into chaotic activity."

In a 2024 study, Miller's lab and that of Ila Fiete, director of the K. Lisa Yang Integrative Computational Neuroscience Center, showed that propofol works by knocking the brain out of this dynamically stable state. As drug doses increased, the brain took progressively longer to return to baseline after responding to an auditory tone - a measurable degradation of stability that deepened until consciousness was lost.

One signature across three drugs

The new study, led by MIT graduate student Adam Eisen, asked whether the same destabilization pattern appears with ketamine and dexmedetomidine. The researchers administered each drug to animals while recording brain activity and measuring responses to auditory tones.

The result was unambiguous. All three drugs produced the same progressive destabilization. As doses increased, the brain's recovery time lengthened in the same characteristic pattern, regardless of which drug was being used. The destabilization measure was so consistent that researchers could not distinguish which drug was being administered based on the neural signal alone.

This is surprising because the three drugs operate through fundamentally different molecular pathways. Propofol enhances inhibitory signaling via GABA receptors. Dexmedetomidine blocks norepinephrine, a neuromodulator involved in arousal and attention. Ketamine suppresses excitatory signaling through NMDA receptors. The researchers hypothesize that each pathway disrupts the brain's stability-excitability balance through a different route, but all routes lead to the same functional cliff edge.

Toward a universal anesthesia monitor

The practical stakes are significant. Currently, anesthesiologists monitor heart rate, blood pressure, and other vital signs during surgery, but these provide only indirect indicators of how deeply a patient is unconscious. EEG readings offer more direct brain data, but interpreting them varies by drug. If all anesthetics produce the same destabilization signature, a single monitoring system could track depth of unconsciousness regardless of which drug is in use.

Miller and Emery Brown, the Edward Hood Taplin Professor of Medical Engineering and Computational Neuroscience (and a practicing anesthesiologist at Massachusetts General Hospital), are developing exactly that: a prototype device that reads EEG signals, measures brain stability in real time, and automatically adjusts drug dosing. A small clinical trial with surgical patients is planned in collaboration with Brown University.

The clinical motivation is clear. While anesthesia is generally safe, risks increase for patients over 65, very young children, and those with dementia or neuropsychiatric conditions. Excessive depth of anesthesia - a state called burst suppression - can worsen cognitive decline in dementia patients and exacerbate depression. Delivering just enough anesthesia and no more could reduce these complications.

What remains to work out

The study was conducted in animal models, and the three drugs tested, while commonly used, do not represent the full pharmacological arsenal available to anesthesiologists. Volatile anesthetics like sevoflurane and isoflurane, which are inhaled rather than injected, were not tested. Whether they produce the same destabilization signature is an open question.

The mechanistic details also remain incomplete. The team has a computational model that explains how propofol causes destabilization, but extending that model to ketamine and dexmedetomidine - whose biophysical effects on neural circuits are more complex - is an active area of work.

And translating a laboratory measurement of brain stability into a clinical device that works reliably in the noisy, high-stakes environment of an operating room is a nontrivial engineering challenge. EEG signals are susceptible to electrical interference from surgical equipment, patient movement, and other artifacts.

The research was funded by the U.S. Office of Naval Research, the National Institute of Mental Health, the Simons Center for the Social Brain, the Picower Institute, the National Science Foundation, and the National Institutes of Health.