A narrative review examined the neurobiological mechanisms that may explain the rapid antidepressant effects of serotonergic psychedelics such as psilocybin, LSD, and ayahuasca. The drugs act as agonists or partial agonists at serotonin 5HT2A receptors, and their rapid effects may involve downregulation of these receptors. They also influence brain-derived neurotrophic factor and immune responses. Neuroimaging studies suggest that psychedelics may disrupt the default mode network, a brain system involved in self-referential thinking that is overactive in major depressive disorder. The review concludes that multiple competing theories are being investigated and more research is needed to identify the most robust evidence.
Up to 60% of people with obsessive–compulsive disorder (OCD) do not respond to standard treatments such as selective serotonin reuptake inhibitors, antipsychotic augmentation, or cognitive–behavioural therapy. This open-label pilot trial will test whether a single 25 mg dose of psilocybin combined with psychological support is feasible, tolerable, and safe for ten adults with treatment-resistant OCD. Clinical improvement will be measured with the Yale–Brown Obsessive–Compulsive Scale. Exploratory brain imaging, electroencephalogram, and transcranial magnetic stimulation-electroencephalogram measures will examine changes in dynamic connectivity and brain dynamics before, during, and up to one week after dosing. Results will inform the design of larger randomized trials and help clarify neurobiological mechanisms of psilocybin-assisted therapy.