Hallucinogens and Serotonin 5-HT2A Receptor-Mediated Signaling Pathways

López-Giménez and colleagues frame the chapter within a long-standing interest in how hallucinogenic drugs — including mescaline, psilocybin and LSD — produce profound alterations in perception, emotion and cognition. Earlier pharmacological and clinical observations linked the actions of hallucinogens to the serotonergic system, and subsequent work identified the serotonin 5-HT2A receptor as a principal mediator of many hallucinogen effects. The Introduction therefore sets out the conceptual background of receptor pharmacology (especially G protein-coupled receptors, GPCRs) and the specific relevance of 5-HT2A receptors to the neuropsychological actions of hallucinogens. This chapter aims to review recent advances in understanding hallucinogen drug action by characterising the structure, neuroanatomical localisation and signalling functions of the 5-HT2A receptor. The authors focus on molecular and cellular signalling pathways, receptor–protein interactions and receptor complexes (notably with metabotropic glutamate receptors), and how these mechanisms may differentiate hallucinogenic from non‑hallucinogenic 5-HT2A agonists. The review emphasises translational links from cellular assays and rodent models to human neuropharmacology.

The extracted text presents this work as a narrative, mechanistic review synthesising molecular, cellular, anatomical and behavioural studies rather than a systematic review with a specified search strategy. The authors integrate data from radioligand autoradiography, heterologous expression systems, gene‑expression profiling, phosphoproteomics, knockout and transgenic mouse models, electrophysiology, and human pharmacology studies. The chapter draws on both in vitro assays (for example HEK293 and CHO‑K1 cell systems) and in vivo rodent experiments, as well as postmortem human brain autoradiography. The extracted material does not clearly report a formal methods section describing literature search criteria, inclusion/exclusion rules, or quality assessment procedures. Instead, the approach is to combine historic pharmacological findings with recent experimental results to build mechanistic models explaining how distinct ligands acting at the same 5-HT2A receptor can produce different cellular and behavioural outcomes. Where relevant, the authors report specific experimental manipulations (e.g. genetic knockouts, pertussis toxin treatment, viral overexpression and use of selective radioligands) that underpin the mechanistic conclusions.

The chapter summarises multiple lines of evidence about 5-HT2A receptor biology and signalling. At the anatomical level, autoradiography with selective antagonist radioligands such as [3H]MDL100,907 shows a heterogeneous distribution of 5-HT2A binding sites across mammalian brains, with the highest density in neocortex concentrated in pyramidal neurons. The corpus striatum exhibits a species‑dependent, compartmentalised pattern in humans (patchy labelling corresponding to striosomes), whereas other species (cow, monkey) show different distributions; the authors stress that these cross‑species differences complicate extrapolation from common laboratory animals to humans. Pharmacologically, both hallucinogenic and non‑hallucinogenic compounds can bind and activate 5-HT2A receptors, but closely related agonists differ in their behavioural effects: classical hallucinogens (LSD, mescaline, psilocin, DOI, DOB, DOM) produce psychosis‑like effects that are blocked by 5-HT2A antagonists, whereas some ergot derivatives (lisuride, ergotamine) are non‑hallucinogenic despite acting at the same receptor. Genetic evidence supports a central role for 5-HT2A: hallucinogen‑induced head‑twitch behaviour in rodents is absent in 5-HT2A knockout mice. Mechanistic studies support the concept of biased agonism at 5-HT2A. Although Gq/11‑mediated activation of phospholipase C (PLC) and subsequent inositol phosphate/Ca2+ signalling is a well characterised pathway, its efficacy does not correlate directly with hallucinogenic behavioural effects: hallucinogens stimulate PLC‑IP signalling with relatively low efficacy and Gαq knockout only slightly reduces DOI‑induced head‑twitch. By contrast, hallucinogen‑specific signalling involves additional pathways, notably pertussis toxin (PTX)‑sensitive Gi/o proteins and downstream ERK1/2 activation. Transcriptomic profiling demonstrated reproducible, ligand‑specific gene‑expression fingerprints: c‑Fos and IκBα were induced by both hallucinogenic and non‑hallucinogenic agonists, whereas immediate early genes Egr‑1 and Egr‑2 were consistently induced only by hallucinogens (DOI, DOM, DOB, mescaline, LSD, psilocin). These hallucinogen‑specific transcriptomic signatures were abolished in 5-HT2A knockout somatosensory cortex, supporting receptor dependence. Complementary phosphoproteomic work reported distinct phosphorylation patterns elicited by hallucinogenic versus non‑hallucinogenic 5-HT2A agonists in HEK293 cells, and PTX pre‑treatment abolished ERK1/2 phosphorylation induced by DOI and LSD but not responses to non‑hallucinogenic agonists. Together these data indicate that hallucinogens selectively engage Gi/o‑dependent signalling in addition to Gq pathways. The authors also detail protein interaction networks that shape 5-HT2A function. The extreme C‑terminal tetrapeptide (VSCV) is a canonical Type I PDZ‑binding motif that mediates interaction with PDZ proteins such as PSD‑95; PSD‑95 association enhances Gq/11 coupling, inhibits agonist‑induced internalisation and is required for normal 5-HT2A‑dependent head‑twitch behaviour and induction of genes including c‑Fos and Egr‑1 in mice. Other interacting partners and pathways include JAK2–STAT3 signalling (STAT3 co‑precipitates with 5-HT2A and JAK2), ARF1‑dependent activation of phospholipase D (PLD) that requires an NPxxY motif, and PKC‑dependent receptor internalisation that differs between human and rat receptor isoforms (serine 457 in human influences recycling kinetics). Downstream intracellular effectors and modulators of 5-HT2A signalling covered include DARPP‑32 phosphorylation state changes (multiple phosphorylation sites affect PP1 and PKA activity), and regulation of neurotrophin expression: DOI induces regionally divergent BDNF mRNA changes (down in hippocampus, up in neocortex), an effect blocked by selective 5-HT2A antagonists and modulated by mGlu2/3 ligands and stress. A prominent theme is functional cross‑talk between 5-HT2A and metabotropic glutamate receptor 2 (mGlu2). Electrophysiological studies show that the mGlu2/3 agonist LY354740 suppresses 5-HT2A‑induced excitatory postsynaptic potentials/currents and blocks DOI‑induced head‑twitch behaviour; similar effects are seen with other mGlu2/3 agonists. The authors describe biochemical and ultrastructural evidence that 5-HT2A and mGlu2 receptors form a heteromeric GPCR complex in mouse and human frontal cortex. Functionally, mGlu2 knockout mice show markedly reduced head‑twitch responses to DOI and LSD; viral overexpression of wild‑type mGlu2 in frontal cortex rescues DOI‑induced head‑twitch in mGlu2 knockouts, whereas expression of an mGlu2/mGlu3 chimera that fails to form the heteromer (mGlu2∆TM4N) does not rescue the behaviour. These observations indicate that the 5-HT2A–mGlu2 heteromeric complex is critical for hallucinogen‑like behaviours in mice. The chapter notes that expression alterations of 5-HT2A and mGlu2 have been observed in postmortem frontal cortex of schizophrenic subjects, suggesting translational relevance, but also emphasises that GPCR heteromerisation remains a debated topic. Finally, the authors report practical experimental findings that support these mechanistic claims: antagonist blockade of psychosis‑like effects in humans (ketanserin blocks LSD/psilocybin effects), PDZ‑protein effects on receptor signalling and behaviour in knockout mice, and differential effects of kinase inhibitors and ARF mutants on vascular and cellular responses to 5-HT2A activation.

López‑Giménez and colleagues interpret the assembled evidence to propose that hallucinogens are distinctive because they stabilise particular active conformations of the 5-HT2A receptor that engage a characteristic subset of downstream signalling pathways. This biased agonism model explains how chemically related agonists acting at the same receptor can produce different cellular transcriptomic and phosphoproteomic fingerprints and thereby distinct behavioural outcomes. In particular, hallucinogens appear to recruit both Gq/11 and Gi/o signalling (the latter being PTX‑sensitive) as well as non‑G protein mechanisms involving scaffold proteins such as PSD‑95, β‑arrestins and ARF1‑dependent pathways; these combined signalling events correlate with hallucinogen‑specific gene induction (Egr‑1, Egr‑2) and behavioural readouts such as the rodent head‑twitch. The authors position these findings relative to earlier research by highlighting genetic and pharmacological converging evidence: 5-HT2A knockout animals lack hallucinogen‑induced behaviours, 5-HT2A antagonists block psychosis‑like effects in human challenge studies, and mGlu2 deletion or disruption of the 5-HT2A–mGlu2 heteromer markedly reduces hallucinogen behavioural effects. They further caution that substantial cross‑species differences in 5-HT2A neuroanatomy (for example striatal compartmentalisation patterns) limit straightforward extrapolation from commonly used laboratory species to humans. The authors also note that the functional significance of GPCR homo‑ and heteromerisation is still controversial and requires further validation. Key limitations and uncertainties acknowledged in the text include the incomplete understanding of how specific intracellular cascades translate into complex neuropsychological states, unresolved issues about the physiological relevance of some protein–protein interactions observed in vitro, and the debated generalisability of animal models to human subjective effects. The authors call for additional basic and translational research — including more refined molecular, circuit, and behavioural studies — to define which signalling and neuronal circuit mechanisms are necessary and sufficient for hallucinogen action. In terms of implications, the chapter suggests that elucidating biased signalling at 5-HT2A has potential therapeutic relevance: understanding which pathways mediate desirable versus adverse effects could guide the design of novel drugs that retain therapeutic efficacy without unwanted psychoactive properties. The authors also point to emerging clinical evidence that under medical supervision some hallucinogens may have therapeutic utility for conditions such as alcoholism, obsessive–compulsive disorder and cluster headache, arguing for continued research into both mechanisms and clinical applications.