If you Google muscarinic receptors and schizophrenia, you'll find a blizzard of drug names, receptor subtypes, and diagrams that look like a squid tried to solve electrical engineering. That search result pile can make the story feel settled: new muscarinic drugs are here, dopamine is old news, everyone clap. The new PET study by Volpi and colleagues is more interesting, and more unsettling, than that. It asks whether some living brains with schizophrenia actually have fewer available M1 receptors in the first place, not just whether a drug can poke that system and get a useful response.

The Deep-Sea Switchboard

M1 receptors are one flavor of muscarinic acetylcholine receptor. Acetylcholine is one of the brain's chemical messengers, a tiny courier that helps regulate attention, learning, memory, and the general business of keeping thought from becoming soup. M1 receptors are especially common in brain regions you would not want to run on bargain-bin wiring: cortex, hippocampus, striatum, amygdala.

Schizophrenia is not just hallucinations, though that is usually where the public conversation parks itself and leaves the hazards blinking. It can involve delusions, disorganized thinking, social withdrawal, blunted motivation, and cognitive problems. Traditional antipsychotics mostly work by blocking dopamine D2 receptors. That can help psychosis, but it often leaves cognition and negative symptoms bobbing in the dark water like a flashlight with dying batteries.

So the cholinergic system has become a tempting place to look. Not because dopamine stopped mattering. Dopamine still has main-character syndrome. But the brain is less like a single broken faucet and more like an abyssal control room where every lever is mislabeled and one of them probably controls childhood memories.

A PET Scan Goes Fishing

The new study used PET imaging with a radiotracer called carbon-11 LSN3172176, which binds to M1 receptors. PET is basically a way to follow a radioactive molecular breadcrumb trail inside the living brain. Glamorous? Not exactly. Astonishing? Absolutely. Humans built a machine that watches chemistry whisper through tissue. Then we used it to ask why consciousness sometimes betrays itself. Normal Tuesday.

Volpi's team compared 16 people with schizophrenia with 16 age- and sex-matched healthy controls. They measured M1 receptor availability using two PET metrics, including a reference-tissue measure called DVR relative to the centrum semiovale and, in a subset, distribution volume. They also adjusted for gray matter fraction, because brain imaging studies must constantly ask: are we seeing receptor biology, tissue differences, or statistics wearing a fake mustache?

The result: people with schizophrenia showed lower M1 availability across several cortical and subcortical regions. The reductions were not tiny polite coughs. Depending on the region and measure, they ranged roughly from 11% to 19%, including frontal, temporal, parietal, and occipital cortex, plus caudate, putamen, hippocampus, and amygdala. Even more intriguing, 44% of patients by one whole-brain measure showed more than a 20% reduction compared with the control mean. By another measure, 27% did.

That suggests a subgroup. Not "all schizophrenia is M1 deficiency." More like: in this vast diagnostic ocean, there may be a current with a distinct chemical signature.

Why This Lands Differently Now

The timing matters because Cobenfy, a xanomeline-trospium combination, won FDA approval in 2024 as the first schizophrenia treatment approved in the United States that targets cholinergic receptors rather than dopamine receptors directly. Xanomeline activates M1 and M4 receptors; trospium helps block peripheral muscarinic side effects, because apparently the gut also wants a vote, and it votes loudly.

Clinical trials already showed that xanomeline-trospium can reduce schizophrenia symptoms over five weeks in acute psychosis studies, including EMERGENT-2 and EMERGENT-3. Reviews over the last few years have argued that M1 and M4 receptors may influence cortical, striatal, and hippocampal circuits involved in psychosis and cognition. The new PET finding adds a sharper question: could imaging identify who has a muscarinic-deficit form of schizophrenia, and maybe who is more likely to benefit from muscarinic treatments?

That would be real-world useful in the least glamorous, most humane way. Psychiatry still often treats by careful trial and error: prescribe, wait, adjust, repeat, try not to let the patient's life get swallowed by the process. A biomarker would not solve schizophrenia. But it might give clinicians a lantern before they step into the cave.

The Necessary Cold Water

This was a small study: 16 patients, 16 controls. PET studies are expensive, technically demanding, and not something you casually order between lunch and your 2 p.m. existential crisis. The findings need replication in larger, more diverse groups. Researchers also need to know how M1 availability relates to cognition, symptoms, medication exposure, illness stage, and response to drugs like Cobenfy or more selective M1/M4 agents.

Still, the study gives the field something rare: in vivo evidence that the M1 deficit seen in postmortem work may be visible in living people. That matters because living brains can be followed over time, linked to symptoms, and studied before and after treatment. The abyss becomes slightly less opaque. Not friendly, exactly. The brain is never friendly. But maybe it is starting to return our sonar pings.

References

1. Volpi T, Radhakrishnan R, Hird R, et al. Lower Muscarinic M1 Receptor Availability in Schizophrenia: In Vivo PET Evidence. Biological Psychiatry. 2026. doi:10.1016/j.biopsych.2026.06.002
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3. Brannan SK, Sawchak S, Miller AC, Lieberman JA, Paul SM, Breier A. Muscarinic Cholinergic Receptor Agonist and Peripheral Antagonist for Schizophrenia. New England Journal of Medicine. 2021;384(8):717-726. doi:10.1056/NEJMoa2017015, PMCID: PMC7610870
4. Kaul I, Sawchak S, Correll CU, et al. Efficacy and safety of the muscarinic receptor agonist KarXT in schizophrenia (EMERGENT-2). The Lancet. 2024;403(10422):160-170. doi:10.1016/S0140-6736(23)02190-6
5. Kaul I, Sawchak S, Walling DP, et al. Efficacy and Safety of Xanomeline-Trospium Chloride in Schizophrenia: A Randomized Clinical Trial. JAMA Psychiatry. 2024;81(8):749-756. doi:10.1001/jamapsychiatry.2024.0785, PMCID: PMC11063924
6. Paul SM, Yohn SE. Targeting muscarinic receptors for treating schizophrenia. Neurotherapeutics. 2026;23(1):e00839. doi:10.1016/j.neurot.2026.e00839, PMCID: PMC12976554

Disclaimer: The image accompanying this article is for illustrative purposes only and does not depict actual experimental results, data, or biological mechanisms.