The behavioral pharmacology of hallucinogens

Fantegrossi and colleagues frame this review by noting that although substances that alter perception have long histories of human use, modern scientific study of classic hallucinogens contracted after the 1960s and has only recently begun to revive. The authors restrict their focus to three prototypical classes — phenethylamines (mescaline-like), tryptamines (psilocybin-like) and ergolines (LSD-like) — and emphasise that a convergent body of preclinical and some clinical work now implicates 5-HT2A receptor agonism as a central molecular initiation point for the characteristic effects of these drugs. They also acknowledge conceptual and terminological controversies (for example over the label "hallucinogen") and stress that a range of pharmacologies exist under that rubric, so their review concentrates on compounds sharing 5-HT2A-mediated actions. The paper sets out to synthesise behavioural pharmacology, structure–activity relationships, receptor pharmacology, results from animal models (conditioned and unconditioned behaviours), and selected human findings to chart recent advances and remaining uncertainties. Fantegrossi and colleagues highlight new experimental tools — selective receptor antagonists, cloned receptor subtypes, genetic mouse models and signalling studies — that have permitted more mechanistic investigations than were possible in earlier eras of research, and they aim to show how these tools have clarified (and complicated) understanding of hallucinogen action.

The extracted text does not present a discrete Methods section; this article is a narrative review that integrates findings from animal behavioural assays, receptor binding and functional studies, genetic manipulations, and selected human psychopharmacology reports. Rather than reporting original empirical methods, the authors organise the literature around experimental paradigms and mechanistic themes: drug discrimination procedures, the drug-elicited head twitch response (HTR), intravenous self-administration paradigms, receptor binding/functional assays (including use of different radioligands), molecular signalling work in transfected cells and brain slices, and pharmacological antagonist/agonist challenge studies. Where relevant, the review summarises experimental paradigms in enough detail to interpret results. For example, drug discrimination is described as a two-response operant task in which an animal learns to report the interoceptive cue of a training drug versus vehicle; HTR is operationally defined and its dose–effect characteristics are discussed; and self-administration methods (notably intravenous catheter models in primates and rodents) are outlined to contextualise reinforcement findings. The authors also draw on receptor pharmacology studies that compare affinities and functional efficacies across receptor subtypes and that use different radioligands and expression systems, noting when methodological differences (for instance the radioligand used) affect apparent affinity estimates.

Drug discrimination and head twitch behaviour The behavioural assays yield convergent evidence that 5-HT2A receptor activity is central to the actions of classical hallucinogens. Drug discrimination studies show that phenethylamine and tryptamine hallucinogens generalise with one another across species, and that selective 5-HT2 receptor antagonists (for example ketanserin, pirenperone or the more selective M100907) block discriminative stimulus effects. The HTR in rodents is presented as a selective, operationally defined behavioural model of 5-HT2A agonist activity: many classical hallucinogens produce a rapid, rotational head jerk that is antagonised by 5-HT2A antagonists and shows a biphasic dose–response (increasing to a peak then declining at higher doses). The HTR is not specific to hallucinogens, however, and the authors caution that induction of HTR alone does not prove a compound is hallucinogenic. Self-administration Classical phenethylamines and tryptamines generally fail to maintain robust intravenous self-administration in non-human primates and rodents: early studies reported no spontaneous self-injection of mescaline or DOM in rhesus monkeys. By contrast, MDMA and related methylenedioxy phenethylamines do act as reinforcers in multiple species (baboons, rhesus monkeys, rats and mice), and NMDA-antagonist ‘‘dissociative’’ compounds (PCP, ketamine) and cannabinoid agonists have also been shown to maintain self-administration. The authors suggest that reinforcement depends strongly on schedule parameters and drug history, and that weak or sporadic self-administration of classical hallucinogens might reflect suboptimal schedules or mixed reinforcing/aversive effects. Serotonin receptor pharmacology and signalling A major empirical thread is the tight relationship between action at 5-HT2A receptors and hallucinogenic effects. A seminal correlation is cited in which affinity at 5-HT2 receptors correlated with human hallucinogenic potency with r = 0.97. Nonetheless, the authors stress methodological caveats: apparent affinity estimates vary with the radioligand used and receptor conformational state, and some putatively non-hallucinogenic compounds (notably lisuride) can show high 5-HT2A affinity while some hallucinogens show lower affinity in particular assays. The review summarises that phenethylamines typically have high affinity for 5-HT2A receptors (examples cited include DOI, DOM, DOB in low nanomolar ranges) while tryptamines tend to show lower 5-HT2A affinity in many assays (examples given include DMT and psilocin in the hundreds to thousands of nanomolar range in some reports), although reported values differ across studies. LSD is emphasised as having very high 5-HT2A affinity (reported ~0.95 nM) and high human potency (typical active doses 0.05–0.20 mg; effects lasting up to ~12 h). Functional selectivity and other receptor contributions The authors describe emerging evidence for agonist-specific signalling (functional selectivity) at 5-HT2A receptors: hallucinogenic and non-hallucinogenic 5-HT2A agonists appear to differentially activate downstream pathways and gene expression programs, and some hallucinogens may promote receptor conformations that enable coupling to Gi proteins in addition to the classical Gq/11 coupling. LSD, for instance, preferentially stimulates phospholipase A2 (PLA2) versus phospholipase C (PLC) in certain cell lines, whereas other hallucinogens have different signalling fingerprints. These observations offer a possible explanation for discrepancies between affinity and subjective/hallucinogenic effects. Tryptamine-specific actions and other targets Tryptamines often have meaningful activity at 5-HT1A receptors, and several behavioural findings support a modulatory role for 5-HT1A in tryptamine effects. For example, the discriminative stimulus effects of 5-MeO-DMT correlate more tightly with 5-HT1A affinity, and selective 5-HT1A antagonists (pindolol, WAY100635) can attenuate some tryptamine cues; conversely, 5-HT1A agonists can substitute for 5-MeO-DMT in some paradigms. The review also notes relatively high affinities of some tryptamines and LSD for trace amine-associated receptors (TAAR), but emphasises that TAAR research is nascent, has technical and interspecies issues, and their role in hallucinogen effects remains uncertain. Glutamate and dopamine interactions Stimulation of cortical 5-HT2A receptors is typically associated with increased glutamate-mediated synaptic activity. Multiple mechanistic hypotheses have been proposed — including a retrograde messenger versus direct excitation of deep-layer pyramidal cells — with recent slice and cellular work favouring direct excitation of specific pyramidal populations and increased recurrent network activity. Dopamine D1/D5 agonists may reduce extracellular glutamate in prefrontal cortex and thereby oppose hallucinogen-induced recurrent activity; in slices, D1/D5 agonists suppressed recurrent activity and could counteract DOI-induced network excitation. Tolerance and receptor regulation Repeated administration of phenethylamine hallucinogens produces rapid tolerance in humans and animals, which the authors link to downregulation of cortical 5-HT2A receptors. Chronic DOI treatment decreased both HTR and 5-HT2A receptor density in rodents, and endocrine or behavioural responses to repeated dosing were attenuated alongside measurable receptor decreases. LSD, lisuride and adverse effects LSD’s broad pharmacology and potency are reviewed alongside epidemiological and clinical remarks: although LSD produces profound perceptual and cognitive effects, there are few documented direct physical harms or fatalities attributable to overdose. Hallucinogen persisting perception disorder (HPPD) or "flashbacks" are discussed as a reported but variably estimated phenomenon, with some reports of recurrence rates as high as 33% but important confounds (polysubstance use, incomplete dosage data) limiting firm incidence estimates. Lisuride is discussed as an ergoline with high 5-HT2A affinity but debated hallucinogenicity; clinical reports of psychotomimetic side effects in some patients complicate its classification.

Fantegrossi and colleagues interpret the assembled evidence as supporting a central, but not necessarily exclusive, role for 5-HT2A receptor agonism in the behavioural and subjective effects associated with classical hallucinogens. They position the 5-HT2A receptor as a "final common mechanism" while recognising that modulatory contributions from 5-HT2C and 5-HT1A receptors, differential downstream signalling (functional selectivity), glutamatergic network effects and potential actions at other targets (for example TAAR) all complicate a simple one-receptor explanation. The authors contrast older hypotheses — such as dorsal raphe suppression as the primary mechanism — with more recent, receptor- and signalling-based accounts. They emphasise methodological caveats that affect interpretation: different radioligands and assay conditions produce disparate affinity estimates; species differences and the receptor conformational state matter; the HTR has biphasic dose–response properties that limit the informativeness of single-dose tests; and animal models, while informative about direct pharmacology, cannot fully capture the strong influence of set and setting on human subjective effects. Key uncertainties and limitations are explicitly acknowledged. Agonist-specific signalling at 5-HT2A is an attractive explanatory hypothesis but remains early in development; TAAR involvement is plausible but hampered by low receptor densities, numerous receptor subtypes and interspecies differences; and human clinical data are limited by ethical and practical constraints. The ambiguous hallucinogenic status of lisuride and the variable, confounded reports of HPPD highlight gaps in clinical characterisation. The authors also note the sparse literature on reinforcement/self-administration for many classical hallucinogens and call for further studies that manipulate drug history and reinforcement schedules. In terms of implications, the review suggests that modern molecular, genetic and pharmacological tools have substantially advanced mechanistic understanding and reduced the field’s earlier "mystical" reputation. This progress, they argue, provides firmer preclinical ground upon which carefully controlled human research and potential clinical applications might be reconsidered, while underscoring the need for more mechanistic and translational work.

The authors conclude that contemporary behavioural pharmacology and neuroscience have demystified many aspects of hallucinogen action. Structure–activity relationships and the development of selective ligands have enabled hypothesis-driven studies, and conditioned (drug discrimination) and unconditioned (HTR) animal assays have produced largely concordant data implicating 5-HT2A receptors as critically important to the effects of classical hallucinogens. Modulatory roles for 5-HT2C (particularly for phenethylamines) and 5-HT1A (notably for tryptamines) receptors are identified, and agonist-specific signalling at 5-HT2A represents a promising area for future work. The authors close by noting that the accumulated preclinical evidence now provides a stronger scientific foundation for renewed, controlled human research and for exploration of therapeutic potential, while acknowledging remaining uncertainties that warrant further investigation.