Classic Psychedelic Drugs: Update on Biological Mechanisms

Classic psychedelics, or serotonergic hallucinogens, include indoleamines (for example DMT and psilocybin/psilocin), phenylalkylamines (for example mescaline) and ergolines (for example LSD), plus several synthetic amphetamine-like compounds. These substances produce profound, dose-dependent alterations in perception, cognition, emotion and self-consciousness and have a long history of ritual and medicinal use. Human research on their clinical potential expanded in the 1950s–1960s but largely stalled after restrictive scheduling; renewed research since the 1990s has combined modern brain imaging and cognitive neuroscience to probe mechanisms of action. Vollenweider and colleagues set out to update current knowledge about the biological mechanisms of classic psychedelics across multiple levels of analysis. The review first describes the phenomenology and psychometric measurement of psychedelic states and potential state and trait predictors of acute responses. It then summarises molecular and cellular pharmacology, neuroplastic effects, and circuit- and whole-brain models (including thalamic gating and entropy-based accounts), and relates neural correlates of altered self- and emotion-processing to the therapeutic potential for affective disorders. The authors frame this synthesis as a foundation for developing targeted interventions and prognostic tools in psychedelic-assisted therapies.

The extracted text presents a narrative review rather than a report of original experimental methods or a systematic review with an explicit search strategy. The authors integrate findings from preclinical (animal and in vitro) studies and human research, including controlled psychopharmacology trials, neuroimaging (fMRI, PET, ASL), electrophysiology (EEG, MEG), perturbational imaging approaches (TMS‑EEG, discussed as ongoing), psychometric assessments (for example the 5D-ASC, MEQ30, M-Scale), and some clinical studies in patient populations. Because no explicit literature search, inclusion criteria, or risk-of-bias assessment are described in the extracted text, the synthesis should be understood as an expert narrative that highlights converging lines of evidence from molecular receptor pharmacology, cellular signalling and neuroplasticity studies, animal behavioural paradigms, and human neurophysiological and psychometric data. The review emphasises mechanistic links across levels (receptor → cellular → circuit → network → phenomenology) and discusses both human experimental findings and animal models that bear on therapeutic mechanisms.

Phenomenology and predictors: Classic psychedelics produce multifaceted altered states characterised centrally by a dose-dependent alteration of self-consciousness (often termed ego-dissolution), alongside perceptual changes (visual phenomena, synesthesia), emotional release, and cognitive changes. Standardised instruments are commonly used to quantify these experiences, notably the 5D-ASC (with core dimensions oceanic self-boundlessness (OB), dread of ego dissolution (DED), visionary restructuralisation (VIS), auditory alterations, and vigilance reductions) and mysticism scales (MEQ30, M-Scale), which correlate with OB. A meta-analysis pooling data from 23 controlled studies (409 psilocybin administrations in 261 healthy volunteers) is cited. Positive acute mystical-type experiences are predicted by traits and state factors such as openness, absorption, optimism, recent well-being, mindful, non‑judgemental emotion regulation, prior altered-state experience, older age, pleasant environment and music; adverse responses are associated with high neuroticism, younger age and impersonal settings. The authors note limitations in existing predictor studies (small, homogenous samples) and call for more rigorous investigation of set, setting and psychological interventions. Receptor pharmacology: The review emphasises agonism at serotonin receptors—particularly 5-HT2A—as a principal mechanism mediating many psychological effects. Classic psychedelics act as partial agonists at multiple serotonergic subtypes (5-HT1, 5-HT2, 5-HT6, 5-HT7); ergolines like LSD also interact with dopaminergic and adrenergic receptors. Pharmacological blockade of 5-HT2A/2C with ketanserin abolished most subjective effects of psilocybin, LSD and DMT in humans, and subjective intensity correlated with 5-HT2A occupancy in cortical regions. Modulation by 5-HT1A receptors is reported (buspirone reduced visual effects; pindolol increased responses to DMT). Dopaminergic contributions are discussed: psilocybin increases striatal dopamine correlated with euphoria and depersonalisation, and LSD has intrinsic D2 activity that may underlie more psychotic-like effects; trace amine-associated receptor 1 (TAAR1) involvement in high-dose LSD effects is noted from animal data. Neuroplasticity: Multiple preclinical studies show that psychedelics (DOI, LSD, psilocybin, DMT) increase cortical glutamate release, activate layer 5 pyramidal neurons and drive synaptogenesis and structural plasticity via pathways involving 5-HT2A, TrkB (BDNF receptor) and mTOR signalling. DOI increased BDNF expression in rodent PFC and hippocampus. Some animal studies report that the spine-remodelling and antidepressant-like effects can be blocked by antagonism of TrkB or mTOR or by 5-HT2A antagonism, whereas other studies found neuroplastic and behavioural effects of psilocybin that were not prevented by ketanserin, suggesting possible 5-HT2A-independent mechanisms or dose/administration-route dependencies. Behaviourally, psychedelics facilitated fear-extinction, enhanced prosocial behaviour (after repeated LSD in mice) and produced fast antidepressant-like effects accompanied by strengthened hippocampal synaptic transmission. Human evidence for neuroplasticity is limited and indirect: an ayahuasca trial found correlations between plasma BDNF 48 hours post-treatment and symptom improvement, and one study reported increased plasma BDNF 6 hours after 200 µg LSD, but BDNF peripheral measures are recognised as a poor proxy for brain levels. Functional network and systems-level findings: Two main conceptual models are reviewed. The cortico-striato-thalamo-cortical (CSTC or thalamic gating) model proposes that 5-HT2A-mediated disruption of thalamic gating produces an information overload of cortex, contributing to increased sensory perception, cognitive disruption and ego-dissolution. Supporting findings include dose-dependent effects of LSD on thalamic-cortical connectivity in neuroimaging and animal data showing altered thalamic and prefrontal firing. The entropic brain / REBUS model proposes that psychedelics increase entropy or signal diversity of spontaneous cortical activity, reducing precision of high-level priors and increasing bottom-up information flow; empirical support includes increases in Lempel-Ziv complexity (MEG/EEG) after LSD, psilocybin and DMT that correlate with subjective intensity, and regional entropy changes in fMRI and modelling work linking 5-HT2A activation to non-uniform entropy reconfiguration across cortex. Network integration and oscillations: Resting-state fMRI studies report that LSD and psilocybin can increase connectivity in sensory and somatomotor networks while decreasing connectivity in associative networks including the default mode network (DMN); regional global connectivity changes correlated with HTR2A gene expression topography in some analyses. However, results vary across studies and appear sensitive to preprocessing choices (for example the use of global signal regression). Consistent electrophysiological findings include decreases in low-frequency oscillatory power (delta, theta, alpha, beta), with alpha power reduction—particularly in DMN regions like PCC and ACC—being the most consistent. Gamma-band changes are more variable across studies and regions. Decreases in DMN intra-network functional connectivity, notably mPFC–PCC decoupling, have been correlated with acute ego-dissolution and in one study predicted positive psychosocial changes months later; similar DMN changes are also observed with other serotonergic agents, complicating claims of specificity. Self- and emotion-processing correlates: Imaging and EEG studies link ego-dissolution and related phenomenology to alterations in somatomotor and associative regions (for example angular gyrus, insula), decoupling within the DMN and reduced PCC alpha power, and disruptions in medial temporal and salience network connectivity. EEG evidence shows psilocybin reduces P300 self-stimulus encoding, correlating with reported unity. Regarding emotion, psilocybin and LSD attenuate recognition of negative facial expressions and reduce amygdala responses to negative stimuli, correlating with increased positive mood; amygdala reductions were observed acutely and in some studies persisted at one week but returned to baseline by one month. The authors suggest these acute modulations of self-referential processing and negative-bias reduction may facilitate therapeutic effects, but longitudinal and mechanistic evidence in clinical populations remains limited.

Vollenweider and colleagues interpret the converging evidence as indicating that classic psychedelics exert their acute psychological effects primarily via agonism at cortical 5-HT2A receptors, with important modulatory contributions from other serotonergic subtypes and non-serotonergic systems (dopaminergic, glutamatergic). They argue that downstream consequences include altered excitation–inhibition balance, increased cortical entropy and signal diversity, network-level reconfiguration (reduced associative network integrity and altered thalamic gating), and promotion of neuroplastic processes that could plausibly underpin enduring therapeutic benefits. The review situates these findings relative to earlier work by noting that modern neuroimaging, electrophysiology and molecular techniques have allowed more precise mapping from receptor activation to circuit and phenomenological effects than was possible in mid‑20th-century studies. Nevertheless, the authors acknowledge substantial uncertainties and methodological limitations: many human studies have small, homogenous samples and variable doses and analytic pipelines; BDNF peripheral measures do not directly index brain plasticity; results depend on preprocessing choices in imaging (for example global signal regression); and causal links between observed neural changes and therapeutic outcomes are not yet definitively established. They also note theoretical challenges in mapping abstract Bayesian constructs (for example priors and precision) onto neuronal population dynamics. Implications highlighted by the authors include the potential to refine therapeutic interventions by better understanding non‑pharmacological moderators (set and setting) and by developing mechanistically informed biomarkers and modelling approaches (for example DCM, perturbational imaging) to predict individual responses. The review suggests that integrating biophysical models with empirical imaging can help unravel how receptor-level effects scale to whole-brain changes, and that such advances may aid development of novel compounds and prognostic tools for psychedelic-assisted therapies. Future research priorities specified by the authors include larger, better-powered human studies with diverse samples, systematic investigation of dose–response and non‑pharmacological moderators, longitudinal clinical trials to link acute neurophenomenology with lasting outcomes, and more causal probing of circuits using perturbational methods.

Elucidating the biological mechanisms of classic psychedelic drugs remains a promising endeavour for neuropsychopharmacology despite historical and socio-political complications. Standardised psychometrics allow quantification of psychedelic phenomenology and correlation with biomarkers. The receptor targets are well characterised—principally serotonergic subtypes—and are situated within circuits for cortico-thalamic and cortico-cortical feedback. By modulating excitatory–inhibitory balance and engaging neuroplastic signalling, psychedelics can induce multi-level changes that affect sensation, cognition, emotion and the narrative of self. Multiple complementary theoretical frameworks (for example thalamic gating, entropy/REBUS) have been developed to explain these effects, and modern causal imaging approaches promise to convert mechanistic insights into prognostic and therapeutic tools. Neurophenomenological metrics that predict clinical outcomes continue to emerge and the field shows considerable promise for informing targeted clinical applications.