Psychedelics and schizophrenia

This article is a narrative synthesis of preclinical and clinical research rather than a systematic review; the extracted text does not report a formal search strategy, inclusion criteria, or methods for study selection. The authors integrate evidence from multiple experimental approaches, including human pharmacology, postmortem and PET studies, electrophysiology, genetic mouse models (including receptor knockout lines), transcriptome fingerprinting, microdialysis, and behavioural assays in rodents. Key experimental paradigms discussed include pharmacological challenge models using LSD-like (5-HT2A agonists) and PCP-like (NMDA antagonists) drugs in humans and animals, genetic manipulations such as 5-HT2A and mGlu knockout mice, and systems-pharmacology methods such as the transcriptome fingerprint (TFP) to compare signalling patterns induced by hallucinogenic and non-hallucinogenic ligands. Clinical trial data for mGlu2/3 agonists (e.g. LY404039) and postmortem and PET receptor-binding studies are also summarised. Where available, the review draws on mechanistic experiments that examine receptor heteromerisation and downstream G-protein signalling to link cellular effects with behavioural outcomes.

Psychotomimetic drug models: The review outlines how PCP-like drugs (ketamine, MK801) and LSD-like psychedelics produce overlapping but distinct schizophrenia-like phenomena. PCP-like NMDA antagonists are used to model both positive and negative symptoms and cognitive deficits and are considered a robust pharmacological model of psychosis in healthy volunteers. LSD-like 5-HT2A agonists produce perceptual disturbances and neuropsychological responses more akin to paranoid-type features; psychedelics tend to elicit visual disturbances whereas schizophrenia more commonly involves auditory hallucinations, although early-stage schizophrenia can include visual phenomena. 5-HT2A receptor mediation of psychedelic effects: Convergent evidence indicates that cortical postsynaptic 5-HT2A receptors on pyramidal neurons are necessary and sufficient for cellular and behavioural responses to LSD-like drugs. Knockout mice lacking 5-HT2A receptors are insensitive to hallucinogen-induced behaviours. The authors describe a signalling distinction between hallucinogenic and non-hallucinogenic 5-HT2A agonists: hallucinogens activate both Gq/11 and Gi/o G-protein subtypes, whereas non-hallucinogens activate predominantly Gq/11. This supports an "agonist trafficking" model in which different ligands stabilise distinct active receptor conformations that couple preferentially to different signalling pathways. The transcriptome fingerprint (TFP) approach revealed cortex gene-expression patterns shared across 5-HT2A agonists and a hallucinogen-specific signature linked to Gi/o activation. The head-twitch behavioural response in mice (rapid lateral head movements) is presented as a reliable bioassay of hallucinogen-specific 5-HT2A activity. Serotonin–glutamate interactions and receptor complex: Electrophysiological and anatomical studies reveal functional crosstalk between cortical 5-HT2A and metabotropic glutamate receptors mGlu2/3. 5-HT2A and mGlu2 receptors are co-expressed in cortical pyramidal neurons and form a functional heterocomplex in mouse and human brain. Activation of presynaptic mGlu2/3 receptors can suppress glutamate release, and activation of mGlu2 receptors inhibits hallucinogen-specific signalling downstream of the 5-HT2A–mGlu2 complex. Postmortem analyses reported higher cortical 5-HT2A expression and lower mGlu2 expression in young, untreated schizophrenia subjects in at least some studies, although results across studies are heterogeneous and may be affected by demographic and clinical confounds (notably that 11 of 13 cases in one study died by suicide). mGlu2/3 agonists in clinical research: Preclinical work showed that mGlu2/3 activation reduces behaviours induced by NMDA antagonists (e.g. hyperlocomotion, stereotypy, working memory deficits). Clinical trials of the mGlu2/3 agonist LY404039 reportedly produced significant improvements in both positive and negative schizophrenia symptoms, suggesting therapeutic potential; genetic knockout work indicates that mGlu2, rather than mGlu3, mediates these effects in mouse psychosis models. Interactions with NMDA-receptor antagonists and antipsychotic pharmacology: PCP-like drugs increase spontaneous discharge rates of medial prefrontal cortical neurons and elevate extracellular serotonin and glutamate in cortex; their behavioural effects appear driven by excitatory inputs from subcortical regions. NMDA antagonists can enhance 5-HT2A-mediated head-twitch responses, and several atypical antipsychotics (clozapine, ritanserin, amesergide, ketanserin) block PCP-induced locomotion and head-twitch responses. Although atypical antipsychotics share high 5-HT2A affinity and modest D2 affinity, some selective 5-HT2A antagonists (M100907, eplivanserin) showed limited antipsychotic efficacy in clinical trials, indicating that simple blockade of 5-HT2A alone may be insufficient. Genetics: The review summarises genetic findings relevant to schizophrenia, noting heritability estimates of 73-90% and the probable contribution of rare, heterogeneous mutations under negative selection. Candidate genes with potentially pathogenic mutations include DISC1, PDE4B and NPAS3; copy-number variation studies have implicated deletions at 22q11.2, 1q21.1 and 15q13.3. The authors highlight gene–environment interactions and epistasis, citing an empirical family study that found interactions between obstetrical complications and polymorphisms in AKT1, BDNF, GRM3 and DTNBP. The authors propose that molecular insight gained from psychotomimetic drug studies could inform the generation of mutant mouse models to investigate disease mechanisms.

González-Maeso and Sealfon interpret the assembled evidence as supporting a model in which serotonergic and glutamatergic systems are functionally coupled through a 5-HT2A–mGlu2 receptor complex expressed on cortical pyramidal neurons. In this formulation, the 5-HT2A moiety could be the site of action for LSD-like hallucinogens and atypical antipsychotics, whereas the mGlu2 moiety could be the molecular target for emerging glutamate-based antipsychotics. The agonist-trafficking concept is used to explain how structurally related 5-HT2A ligands can produce divergent signalling patterns and behavioural outcomes, thereby linking receptor conformations to psychotomimetic versus therapeutic effects. The authors position these conclusions relative to prior hypotheses by proposing a unification of the serotonin and glutamate models of schizophrenia; they note that mechanistic work on psychotomimetic drugs has already informed antipsychotic drug discovery (for example, screening for compounds that block LSD effects). At the same time, several important uncertainties and limitations are acknowledged: postmortem and PET studies of 5-HT2A expression have produced inconsistent results that can be influenced by demographic and clinical variables (including medication status, age and suicide), there has historically been a lack of highly selective mGlu2 versus mGlu3 ligands complicating interpretation, and animal models remain imperfect because schizophrenia is uniquely human. Clinical and translational implications discussed by the authors include the potential to design novel antipsychotic agents that target specific receptor components or signalling states within the 5-HT2A–mGlu2 complex, and the suggestion that further work is required to integrate the dopamine hypothesis with the emerging serotonin–glutamate framework. They emphasise that additional experiments are needed to test the heterocomplex hypothesis directly and to clarify the contribution of presynaptic mGlu3 receptors projecting to cortex.

The review concludes that research on psychedelics and PCP-like drugs has yielded convergent insights into receptor, signalling and circuit mechanisms that bear on schizophrenia. Understanding hallucinogen-specific 5-HT2A signalling and the functional interaction between 5-HT2A and mGlu2 receptors may help unify serotonin- and glutamate-based hypotheses of psychosis and guide rational development of new antipsychotic therapies. The authors note the ongoing challenge of relating these findings to the dopamine hypothesis and stress that continued study of psychotomimetic drugs is likely to facilitate development of better treatments for schizophrenia.