Serotonin, psychedelics and psychiatry

Introduction

Serotonin is presented as a central but complex neuromodulator implicated in development, perception, cognition and mood, yet lacking a single unifying functional theory comparable to dopamine. The introduction notes the exceptional complexity of the serotonergic system, with more than 14 receptor subtypes, and situates serotonin within contemporary debates in psychiatry: despite widespread use of selective serotonin reuptake inhibitors (SSRIs) for depression, population prevalence of depressive disorders has not fallen, and there is renewed questioning of a purely pharmacological model. Earlier research is described as increasingly indicating that serotonergic processes interact with genetic and environmental factors to determine mental-health outcomes, arguing for a hybrid model that combines pharmacological tools with psychological/contextual approaches rather than relying on chronic pharmacotherapy alone. Susser and colleagues set out to synthesise evidence linking serotonin, classical serotonergic psychedelics (for example LSD, psilocybin, DMT), and psychiatric treatment, and to propose a mechanistic model to explain how psychedelics might exert therapeutic effects. The authors emphasise that classic psychedelics are 5-HT2A receptor agonists, that 5-HT2A signalling is implicated in neural plasticity and heightened environmental sensitivity, and that both the subjective quality of the psychedelic experience and an increase in neural complexity during the acute state predict longer-term outcomes. On this basis they advance the thesis that serotonin differentially encodes responses to uncertainty: 5-HT1A receptor activity provides baseline emotional moderation and ‘patience’, while 5-HT2A receptor engagement facilitates a relaxation of prior beliefs across perceptual, emotional and cognitive domains, increasing context sensitivity and creating an optimal condition for change. The paper functions as a conceptual synthesis drawing on small-scale clinical pilot studies, neurobiological evidence and emerging clinical trials. It frames a testable hypothesis about receptor-specific roles in uncertainty processing and situates that hypothesis within ongoing clinical developments, noting that larger randomised controlled trials of psilocybin for depression are beginning to be conducted.