Recent advances in the neuropsychopharmacology of serotonergic hallucinogens

Serotonergic hallucinogens — encompassing two main structural classes, the phenylalkylamines and the indoleamines — produce profound alterations in perception, thought, and mood and have been used by humans for thousands of years, though scientific investigation began only in the late nineteenth century. Defining hallucinogens as a pharmacological class proved contentious: standard definitions based on altered consciousness without delirium or addiction were insufficiently specific, leading to the proposal that, in addition to these properties, hallucinogens should bind to the 5-HT2A receptor and produce full substitution for the prototypical agent DOM in drug discrimination paradigms. Despite their structural heterogeneity, phenylalkylamine and indoleamine hallucinogens produce virtually indistinguishable subjective effects, show cross-tolerance with one another, and share a unifying pharmacological mechanism through 5-HT2A receptor activation. This review aimed to synthesise advances in understanding the neuropsychopharmacology of serotonergic hallucinogens, covering receptor pharmacology, validated animal behavioural models, tolerance and cross-tolerance, evidence from human experimental studies, and the role of the prefrontal cortex and its interactions with subcortical structures as a primary site of hallucinogenic drug action.

This is a comprehensive narrative review that synthesised the preclinical and human experimental literature on serotonergic hallucinogens without formal inclusion/exclusion criteria or quantitative meta-analytic methods. The author covered receptor binding and functional pharmacology — including 5-HT2A and 5-HT2C subtype selectivity — drug discrimination studies using operant conditioning paradigms in rodents, the head twitch response (HTR) assay, prepulse inhibition of startle (PPI), interval timing paradigms, and locomotor activity measures. Human experimental evidence was drawn from challenge studies examining cross-tolerance and subjective effects across compounds. Neurobiological coverage focused on 5-HT2A receptor expression patterns across cortical layers and brain regions, electrophysiological recordings from prefrontal cortex pyramidal neurons in vitro and in vivo, and the organisation of cortico-striato-thalamo-cortical (CSTC) loop circuitry as a substrate for hallucinogen action.

Cross-tolerance studies confirmed that phenylalkylamine and indoleamine hallucinogens mutually substitute in drug discrimination paradigms and produce equivalent blunting of subjective effects following repeated dosing — evidence consistent with a shared receptor mechanism. Multiple converging lines of evidence from in vitro and in vivo work identified 5-HT2A receptor activation as the unitary mechanism responsible for hallucinogenesis: selective 5-HT2A antagonists block hallucinogenic behavioural effects in animals and subjective effects in humans, and highly selective phenylisopropylamine hallucinogens such as DOM produce full substitution for indoleamines in drug discrimination. Electrophysiological investigation identified the prefrontal cortex as a primary site of hallucinogen action. The 5-HT2A receptor is expressed densely in lamina V of the PFC on pyramidal neurons and parvalbumin-positive interneurons. Systemic hallucinogen administration in rats produced a net excitatory effect on PFC pyramidal neurons, with individual cells either excited (38–53%), inhibited (27–35%), or unresponsive. These PFC effects are thought to propagate through CSTC loops involving thalamo-cortical and striato-thalamic circuits to generate the broader pattern of perceptual and cognitive alterations characteristic of hallucinogen action.

Convergence of pharmacological, electrophysiological, and human experimental evidence leaves little scientific doubt that 5-HT2A receptor activation is the primary mechanism of classical hallucinogenesis, with the prefrontal cortex — and its dense laminar expression of the receptor on pyramidal neurons — constituting a critical neuroanatomical site of drug action. The net excitatory effect of hallucinogens on PFC pyramidal cells, propagated through CSTC circuitry, offers a plausible neural basis for the disorganised and amplified pattern of cortical activity associated with altered consciousness and perceptual distortion. The resumption of human research with hallucinogens — including brain imaging studies using fMRI, PET, and MEG — has provided additional support for animal and in vitro evidence, and is beginning to elucidate how receptor-level pharmacology translates into circuit-level and behavioural effects. The author identifies understanding of functional selectivity at the 5-HT2A receptor — specifically the relative contributions of G-protein and β-arrestin signalling pathways — as an important avenue for dissociating perceptual from therapeutic effects and for the rational design of functionally selective compounds.

The neuropsychopharmacology of serotonergic hallucinogens is increasingly well characterised: both phenylalkylamine and indoleamine classes produce their subjective and behavioural effects primarily through 5-HT2A receptor activation, with the prefrontal cortex and cortico-striato-thalamo-cortical circuits serving as the principal neuroanatomical substrates. Validated animal models — including drug discrimination, head twitch response, and prepulse inhibition paradigms — have provided essential tools for dissecting receptor mechanisms, and the resumption of controlled human studies is beginning to translate these preclinical insights into neuroimaging and clinical findings. Continued investigation of functional selectivity at the 5-HT2A receptor and the circuit-level consequences of PFC activation holds promise for developing a more complete account of hallucinogen pharmacology and informing the design of novel therapeutic agents.