First In Vivo PET Evidence Reveals Widespread M1 Receptor Deficits in Living Schizophrenia Patients
Key Insights
A landmark PET imaging study provides the first in vivo evidence of 13% to 19% lower muscarinic M1 receptor availability across multiple brain regions in living schizophrenia (search) patients compared to healthy controls.
M1 receptor reductions were more strongly associated with cognitive impairment than with psychotic symptom severity, implicating M1 dysfunction as a key driver of disabling cognitive deficits.
The findings bolster the biological rationale for recently approved non-dopaminergic therapies such as xanomeline–trospium (search) (COBENFY™), the first novel antipsychotic mechanism in over 70 years.
A pioneering neuroimaging study has delivered the first direct in vivo evidence that patients with schizophrenia (search) exhibit widespread deficits in muscarinic acetylcholine M1 receptor (search) availability, a finding that fundamentally reshapes understanding of the disorder's neurobiology and reinforces the rationale for a new class of non-dopaminergic antipsychotic therapies.
Using a novel, highly selective PET radiotracer—¹¹C-LSN3172176—researchers at Yale University School of Medicine (search) compared M1 receptor availability in 16 schizophrenia (search) patients against 16 age- and sex-matched healthy controls. The results, published in Biological Psychiatry, revealed significant reductions in M1 receptor availability ranging from approximately 13% to 19% across multiple cortical and subcortical regions.
"The development of a novel PET radiotracer for the M1 receptor provided a unique opportunity to directly measure M1 receptor availability in the living brain," explained co-lead investigator Deepak C. D'Souza, MBBS, MD, of the Department of Psychiatry at Yale University School of Medicine (search). "Although receptor availability is not identical to receptor density, it is widely accepted as a useful proxy for the brain's functional M1 receptor system. This allowed us, for the first time, to confirm that muscarinic dysfunction is a feature of schizophrenia (search) in living patients."
Regional Deficits and Effect Sizes
The study documented M1 receptor reductions with large effect sizes across an extensive set of brain regions. Using distribution volume ratio relative to the centrum semiovale (DVRCS), the frontal cortex showed a 13% reduction, temporal cortex 15%, parietal cortex 14%, occipital cortex 16%, caudate 19%, putamen 19%, hippocampus 13%, and amygdala 19%. Distribution volume (VT) measurements corroborated these findings, with reductions ranging from 11% to 17% across the same regions.
Notably, a subgroup of patients—44% as measured by DVRCS and 27% by VT—demonstrated whole-brain M1 deficits exceeding 20% from the mean of healthy controls, suggesting meaningful biological heterogeneity within the patient population.
"The study's findings were robust across multiple methods of PET quantification and remained significant after accounting for potential confounding factors, including gray matter differences and partial-volume effects," said co-first author Tommaso Volpi, MD, PhD, Associate Research Scientist in Radiology and Biomedical Imaging at Yale.
Cognitive Impairment Over Psychotic Symptoms
A particularly striking finding was that M1 receptor availability showed a stronger association with measures of cognition than with the severity of positive psychotic symptoms. This suggests that M1 dysfunction may be especially relevant to the cognitive impairments—affecting memory, learning, and executive function—that are among the most disabling aspects of schizophrenia (search) and are poorly addressed by existing dopamine-targeting treatments.
Bridging the Postmortem Gap
For decades, evidence linking muscarinic acetylcholine system abnormalities to schizophrenia (search) came almost exclusively from postmortem tissue studies. This limitation left critical questions unresolved: whether the deficits existed during life or were artifacts of chronic medication exposure, and how they related to clinical symptoms. The current study definitively bridges that gap by providing in vivo confirmation.
Validating a New Therapeutic Era
The findings arrive at what experts describe as a historic inflection point in psychiatric pharmacotherapy. John Krystal, MD, Editor of Biological Psychiatry, commented: "This study is particularly interesting in light of the emergence of M1 and M4 muscarinic agonist drugs as pharmacotherapies in schizophrenia (search). These findings are particularly timely given the recent approval of xanomeline–trospium (search) (COBENFY™), the first antipsychotic medication in more than 70 years to treat schizophrenia through a primarily non-dopaminergic mechanism of action."
The pharmacological treatment of schizophrenia (search) has long been dominated by dopamine D2 receptor antagonists, which show limited efficacy for negative and cognitive symptoms and carry significant side effect burdens. M1 receptors, which are G-protein-coupled receptors distributed throughout the cortex and subcortical regions, are now considered an important focus of both the underlying neurobiology and treatment of schizophrenia.
Toward Precision Psychiatry
Although the study did not evaluate treatment response directly, the investigators see a path toward clinical application. "Although our study did not evaluate treatment response, it strengthens the biological rationale for developing muscarinic-based therapies and raises the possibility that M1 receptor imaging could eventually help identify biologically distinct subgroups of patients and inform future precision medicine approaches," concluded co-lead investigator Rajiv Radhakrishnan, MBBS, MD, of the Department of Radiology and Biomedical Engineering and Department of Psychiatry at Yale.
The research was supported by the National Institute of Mental Health (R21MH123870 and R01MH113557), the National Center for Homelessness among Veterans, and the Schizophrenia (search) Neuropharmacology Research Group at Yale (SNRGY).