Reviewing the ketamine model for schizophrenia

Schizophrenia is described as a disorder of positive symptoms (for example, delusions and hallucinations), negative symptoms (for example, avolition and affective flattening) and cognitive and social dysfunction, with a lifetime prevalence reported here of 0.30-0.66% and a shortened life expectancy of about 12–15 years. Historically the dopamine hypothesis dominated explanations of psychosis, in part because D2 antagonists reduce positive symptoms; however, that hypothesis does not explain negative and cognitive symptoms well and has difficulty accounting for the pattern of cortical hypodopaminergia alongside subcortical hyperdopaminergia. In the past two decades the glutamate hypothesis—rooted in observations that N-methyl-D-aspartate receptor (NMDAR) antagonists such as PCP and ketamine transiently induce schizophrenia-like symptoms—has emerged as an alternative or complementary model that may better address negative and cognitive features and could also account for downstream dopamine abnormalities. Frohlich and Van Horn set out to review the pharmacology and psychotomimetic effects of ketamine, with emphasis on its value as a human experimental model of aspects of schizophrenia. The review examines NMDAR physiology, ketamine’s receptor pharmacology (including non-NMDAR targets), convergent genetic and molecular evidence for NMDAR hypofunction in schizophrenia, and how NMDAR hypofunction could account for dopaminergic and GABAergic dysfunction, abnormal connectivity, altered cortical oscillations and the typical adolescent/early-adult age of onset. The authors frame the ketamine challenge as a tool for studying positive, negative and cognitive symptoms and related circuit and oscillatory abnormalities in acute schizophrenia.

The extracted text does not present a dedicated Methods section describing search strategy, inclusion criteria or systematic review procedures; instead, the paper is a narrative review that organises and synthesises evidence across multiple domains. Frohlich organises the material by first outlining molecular physiology of NMDAR and ketamine’s pharmacology, then reviewing empirical findings relevant to NMDAR hypofunction and ketamine challenge in humans and animals, followed by sections on neurotransmitter interactions and cognition, connectivity/dysconnectivity, cortical oscillations, and dynamical systems perspectives. Evidence types brought together include pharmacological challenge studies in healthy human volunteers and animals, in vivo imaging (for example SPECT and PET), magnetic resonance spectroscopy and microdialysis studies of neurotransmitter release, postmortem molecular and gene-association data, electrophysiological measures including EEG/ASSR and evoked potentials such as mismatch negativity (MMN), functional MRI connectivity analyses, and intervention trials supplementing antipsychotic medication with NMDAR co-agonists (for example D-serine). Where available the review reports numerical values from primary studies (for instance correlation coefficients, ligand affinity Ki values and clinical effect sizes) embedded in the narrative.

Molecular physiology and ketamine pharmacology: The NMDAR is an ionotropic glutamate receptor requiring glutamate and a glycine-site co-agonist (glycine or D‑serine) for activation; NMDARs are heteromeric complexes with NR1 and NR2 (A–D) subunits, and subunit composition influences biophysics and pharmacology. Ketamine is an uncompetitive NMDAR antagonist that binds within the channel (PCP site) and, despite primary action at NMDAR, interacts with multiple other targets. Reported affinities in the extracted text include a contentious D2 receptor Ki of 0.5 ± 0.2 μM (some work equating ketamine’s affinity at D2 and NMDAR, though this finding lacks consistent replication), 5‑HT2A Ki ≈ 15 ± 5 μM, sigma receptor Ki ≈ 66 ± 10 μM, and kappa opioid receptor Ki ≈ 85.2 ± 26 μM. The two enantiomers differ: (S)-ketamine has 4–5× higher affinity at the PCP site and produces more psychiatric symptom ratings and increased regional glucose metabolism than (R)-ketamine, which often shows the opposite metabolic pattern. Evidence for NMDAR hypofunction in schizophrenia: Convergent lines of evidence support NMDAR involvement. Challenge studies show that NMDAR antagonists induce schizophrenia-like symptoms in healthy humans and that ketamine increases cortical glutamate release; anterior cingulate glutamate release under ketamine correlates with positive symptom scores (r = 0.72). A SPECT study using an intrachannel NMDAR tracer found reduced binding in the left hippocampus of unmedicated patients. Genetic and molecular findings reported include associations of schizophrenia with polymorphisms in genes affecting NMDAR function or co-agonist metabolism (for example DTNBP1/dysbindin, GRIN2B, DAAO and G72/DAOA). Postmortem reductions in NMDAR-related mRNAs (for example GRIN1) and in proteins linked to NMDAR function are described. Clinical supplementation with D‑serine showed symptom reductions at six weeks (examples reported: PANSS positive −17.4%, p = 0.004; SANS −20.6%, p = 0.0004; PANSS cognitive −11.6%, p = 0.004; CGI −45.8%, p = 0.004), and meta-analyses report pooled improvements for negative (p = 0.02; ES = 0.48), cognitive (p = 0.007; ES = 0.42) and total symptoms (p = 0.02; ES = 0.4) when D‑serine is added to antipsychotics. Neurotransmitter interactions and cognition: Although ketamine increases glutamate release (and sometimes acetylcholine), it can reproduce positive, negative and cognitive symptoms. The review highlights data linking NMDAR dysfunction to downstream dopaminergic abnormalities: PET studies show correlations between ketamine-induced symptoms and regional dopamine release, and animal NMDAR antagonist exposure potentiates amphetamine-induced dopamine release—effects that can be mitigated by glycine-site agonists or glycine transport inhibitors. NMDAR dysfunction offers a mechanistic account for learning and memory impairment because NMDAR are essential for long-term potentiation (LTP; the synaptic mechanism supporting learning). Ketamine reduces MMN generation, mirroring reduced MMN amplitudes in schizophrenia, and lamotrigine (which limits glutamate release) attenuates ketamine-induced frontal BOLD changes, linking glutamatergic over-release to functional imaging findings. Connectivity and development: The authors relate NMDAR hypofunction to dysconnectivity and the typical adolescent onset of schizophrenia. They discuss synaptic pruning in adolescence and propose that prenatal NMDAR weakness may render networks vulnerable to pruning, precipitating clinical onset. Imaging and electrophysiological studies show altered anatomical and functional network topology in patients (reduced clustering, fewer hubs, altered small-world properties) alongside some regionally increased connectivity such as within the default mode network (DMN). Ketamine challenge recapitulates many connectivity changes: in animals and humans it produces both increases and decreases of connectivity depending on region and measure; human resting-state fMRI under ketamine increases global brain connectivity in clusters associated with positive symptoms while striatal and thalamic connectivity may decrease with negative symptoms. Cortical oscillations and E/I balance: Schizophrenia is associated with abnormalities in gamma-band (30–80 Hz) oscillations, which are generated by parvalbumin-positive fast-spiking GABAergic interneurons. Findings reviewed include both reductions and increases in gamma power and coherence, with decreases often linked to negative symptoms and increases to positive symptoms. Ketamine and other NMDAR antagonists alter gamma activity in a context- and region-dependent manner—in some preparations increasing gamma power, in others decreasing it—consistent with heterogeneous gamma findings in patients. The review emphasises the excitation/inhibition (E/I) balance as a key control parameter: NMDAR hypofunction preferentially reduces excitation onto inhibitory interneurons, producing interneuron dysfunction, disinhibition of pyramidal cells, altered synchrony and large-scale dysconnectivity. The authors note parallels between these effects and reductions in interneuron markers (for example GAD67, parvalbumin) in postmortem schizophrenia tissue. Dynamics and complexity: Drawing on nonlinear systems ideas, the review suggests that cortical networks may operate near critical points and that tuning of parameters such as E/I balance could produce phase transitions (bifurcations) that manifest as acute pathological states. Evidence of nonlinearity in EEG time series of schizophrenia patients is mentioned and the authors call for dynamical analyses of ketamine effects on EEG to look for bifurcation-like transitions.

Frohlich and colleagues interpret the assembled evidence as supporting a ketamine-based model in which NMDAR hypofunction, possibly present from development, can account for many features of schizophrenia: positive and negative symptoms, cognitive deficits, downstream dopaminergic abnormalities, interneuron/GABAergic dysfunction, altered connectivity, abnormal cortical oscillations and the characteristic adolescent onset. The review emphasises that ketamine’s effects are multi-transmitter in nature: although NMDAR blockade is central, ketamine interacts with dopamine, opioid, sigma and serotonergic systems among others, and these interactions may contribute to its psychotomimetic profile. At the same time, the authors acknowledge important limitations and uncertainties. Ketamine is pharmacologically promiscuous, and some psychotomimetic features of ketamine differ from those typical of chronic schizophrenia—for example, ketamine tends to induce visual disturbances while chronic patients more commonly report auditory verbal hallucinations, and auditory steady-state responses under ketamine do not match those observed in patients. Context dependence of ketamine’s effects is highlighted: its impact on gamma oscillations and connectivity varies with dose, brain region and experimental paradigm, complicating direct mapping between ketamine challenge and patient pathology. Methodological issues are also noted, such as pitfalls in analysing phase synchronization in sensor space; the authors recommend source-space analyses for future studies. For future research the review recommends comparing multiple pharmacological challenges within the same subjects (for example ketamine versus amphetamine or salvinorin A) to test glutamate–dopamine and other multi-transmitter hypotheses directly. The authors also call for more work on receptor-specific actions (including the roles of sigma and kappa receptors and enantiomer-specific effects), further integration of the ketamine model with findings on cannabis and other agents, and explicit investigation of nonlinear dynamical properties—searching for nonlinearity and bifurcations in EEG following ketamine infusion—to better characterise transitions underlying acute psychotic states.

The review concludes that the ketamine model provides a useful framework for studying many aspects of acute schizophrenia by linking NMDAR hypofunction to symptoms, neurotransmitter interactions, interneuron pathology, dysconnectivity and abnormal cortical oscillations. Nevertheless, ketamine’s actions at multiple receptor systems and the context-dependent nature of its electrophysiological and connectivity effects limit a simple one-to-one mapping onto chronic schizophrenia. The authors therefore urge further comparative pharmacological work, more precise receptor- and enantiomer-focused studies, methodological improvements in electrophysiological analysis, and application of nonlinear dynamical approaches to better understand how tuning of excitation–inhibition balance and related control parameters might precipitate psychotic transitions.