The Neuroscience of Being Human

The Neuroscience of Psilocybin

How a tryptamine from a mushroom disrupts the default mode network, why reduced brain activity produces expanded consciousness, and what clinical trials reveal about psilocybin's capacity to treat depression, anxiety, and the fear of death

1,580-word article with 8 Harvard references.

5-HT2A agonism: what psilocybin does at the receptor

Psilocybin itself is pharmacologically inactive. It is a phosphorylated prodrug that is rapidly converted to psilocin by alkaline phosphatase in the gut, liver, and kidneys. Psilocin is the active compound, and its primary target is the serotonin 5-HT2A receptor, a G-protein-coupled receptor concentrated on the apical dendrites of layer V pyramidal neurons in the prefrontal cortex, the anterior cingulate cortex, and other association areas. When psilocin binds to these receptors, it activates intracellular signalling cascades involving phospholipase C, protein kinase C, and downstream transcription factors that alter neuronal excitability and gene expression (Nichols, 2004).

The consequence of this activation is not straightforward stimulation. It is a disruption of the cortical filtering mechanisms that normally constrain perception, cognition, and self-representation. Vollenweider and Kometer (2010), writing in Nature Reviews Neuroscience, proposed that 5-HT2A activation disrupts thalamic gating, the process by which the thalamus filters sensory information before it reaches the cortex. Under normal conditions, this gating ensures that only relevant information reaches conscious awareness. Under psilocybin, the gate opens wider, allowing a flood of sensory, emotional, and associative information to reach cortical processing areas simultaneously. The subjective result is the psychedelic experience: altered perception, synaesthesia, emotional intensification, dissolution of the boundary between self and environment, and the sense that ordinary categories of thought have become fluid.

The default mode network paradox

The most counterintuitive finding in psychedelic neuroscience emerged from Carhart-Harris et al. (2012), who used functional magnetic resonance imaging to measure brain activity in volunteers who had received intravenous psilocybin. The expectation, based on the richness and intensity of the subjective experience, was that psilocybin would increase brain activity. It did not. The most significant changes were decreases in blood flow and BOLD signal in the default mode network, a set of midline cortical structures including the medial prefrontal cortex and the posterior cingulate cortex that are most active during rest, self-reflection, and mind-wandering.

The default mode network is sometimes described as the brain's narrative engine. It constructs and maintains the autobiographical self, the continuous sense of being a person with a history, a set of concerns, and a predictable future. In depression, the default mode network is hyperactive, generating the repetitive, self-critical, ruminative thought patterns that characterise the disorder. Psilocybin reduces this activity. The self-referential narrative quietens. The rigid patterns of self-criticism loosen. The correlation between subjective reports of ego dissolution and measured reductions in default mode network connectivity was significant, meaning that the people who experienced the most profound alteration of self-experience were the same people whose default mode networks showed the greatest disruption. The paradox is that reduced neural activity in these regions produces an experience that feels like more, not less. The brain is doing less of the thing it normally does compulsively, and the result feels like liberation.

Depression, cancer anxiety, and the clinical evidence

Griffiths et al. (2016), at Johns Hopkins University, conducted a randomised controlled trial in which patients with life-threatening cancer diagnoses and clinically significant anxiety or depression received either a high dose or a low dose of psilocybin in a carefully controlled setting with trained psychological support. The high-dose group showed immediate and substantial reductions in depression and anxiety that were sustained at six-month follow-up. Approximately eighty per cent of participants showed clinically significant improvement, and sixty-seven per cent rated the experience among the five most personally meaningful experiences of their entire lives. The low-dose group, when crossed over to receive the high dose, showed equivalent responses.

Carhart-Harris et al. (2016), at Imperial College London, tested psilocybin in a different population: patients with treatment-resistant depression who had failed to respond to at least two conventional antidepressant medications. Two psilocybin sessions, a lower dose followed by a higher dose one week later, produced rapid reductions in depressive symptoms. At one week, all twelve patients showed reduced depression scores. At three months, seven of twelve remained in remission. The effect was not merely symptomatic relief. Patients described qualitative shifts in their relationship to their own thought patterns, a loosening of the rigid, self-critical narratives that had characterised their depression. The default mode network findings provide a neurobiological correlate for this clinical observation: the network that maintains those narratives was the network most disrupted by the drug.

Neuroplasticity: dendrites, synapses, and lasting structural change

One of the most pressing questions in psychedelic research has been how a single dose of a drug that is pharmacologically active for only four to six hours can produce therapeutic effects lasting months. Ly et al. (2018), publishing in Cell Reports, provided part of the answer. Using both in vitro and in vivo models, they demonstrated that psychedelics including psilocybin, LSD, and DMT promote structural neural plasticity. Cortical neurons exposed to these compounds showed increased dendritic arbor complexity, meaning the branching trees of their dendrites became more elaborate. They showed increased dendritic spine density, meaning more protrusions available for synaptic contact. And they showed increased synaptogenesis, meaning more functional synaptic connections were formed.

These structural changes are mediated through the same 5-HT2A receptor activation that produces the acute psychedelic experience, but they persist long after the drug has been cleared. The implication is that psilocybin does not merely alter consciousness temporarily. It promotes physical remodelling of the neural architecture, increasing the brain's capacity for new patterns of connectivity. In the context of depression, where chronic stress and elevated cortisol have been shown to reduce dendritic complexity and spine density in the prefrontal cortex, psilocybin's neuroplastic effects may represent a direct structural reversal of stress-induced neural damage. The brain is not merely thinking differently during the experience. It is growing differently afterwards.

Abuse potential and why psilocybin is not addictive

Johnson et al. (2018) conducted a systematic analysis of psilocybin's abuse potential using the eight factors specified by the United States Controlled Substances Act: actual and relative potential for abuse, scientific evidence of pharmacological effect, current scientific knowledge, history and current pattern of abuse, scope and significance of abuse, risk to public health, psychic or physiological dependence liability, and whether it is a precursor of a known controlled substance. Their conclusion was that psilocybin's scheduling as a Schedule I substance, which designates high abuse potential and no accepted medical use, is inconsistent with the evidence across all eight factors.

Psilocybin produces rapid tolerance, meaning that taking the same dose on consecutive days produces markedly diminished effects. This pharmacological property discourages the compulsive, escalating use patterns that characterise addictive substances. It produces no physical dependence and no withdrawal syndrome. Animal self-administration studies, the standard preclinical measure of abuse potential, show that animals do not reliably self-administer psilocybin the way they self-administer cocaine, heroin, or alcohol. The experience itself, while often profoundly meaningful, is also demanding, effortful, and sometimes frightening, which further discourages casual or recreational repetition. The molecule that produces one of the most intense experiences available in pharmacology is also one of the least likely to be abused.