The Neuroscience of Being Human

The Neuroscience of Cannabis

What THC and CBD do to the endocannabinoid system, why the adolescent brain is uniquely vulnerable, and what the evidence actually shows about psychosis risk, cognitive decline, and medical use

1,580-word article with 8 Harvard references.

The system the plant did not create

Cannabis did not give the brain its cannabinoid receptors. The brain built them for itself. The endocannabinoid system, identified in the early 1990s following the discovery of the CB1 receptor, is one of the most widely distributed signalling systems in the central nervous system. CB1 receptors are found at high density in the hippocampus, the cerebral cortex, the basal ganglia, the cerebellum, and the amygdala. CB2 receptors, initially thought to be confined to immune cells, are now known to be present in the brainstem and other neural structures. The system's endogenous ligands, anandamide and 2-AG, are synthesised on demand from membrane lipids and act as retrograde messengers, travelling backwards across the synapse to modulate the release of other neurotransmitters including glutamate, GABA, serotonin, and dopamine (Mechoulam and Parker, 2013).

The endocannabinoid system is not a curiosity. It is a master regulator. It modulates appetite, pain perception, mood, memory consolidation, immune response, and the stress axis. When you feel calm after exercise, part of that calm is endocannabinoid-mediated. When appetite increases after fasting, endocannabinoid signalling in the hypothalamus is involved (Di Marzo et al., 1998). When a stressful memory begins to fade over time, endocannabinoid-dependent synaptic depression in the amygdala is one of the mechanisms. The system exists to maintain homeostasis, to keep neural signalling within a functional range. Cannabis works because the plant produces compounds that fit the locks the brain already built. The question is what happens when those locks are opened with a key the brain did not design.

THC and CBD: the same plant, opposite pharmacology

The cannabis plant contains more than one hundred phytocannabinoids, but two dominate the pharmacological story. Delta-9-tetrahydrocannabinol, THC, is a partial agonist at CB1 receptors. It mimics anandamide but with greater potency, longer duration of action, and none of the enzymatic regulation that keeps endogenous signalling in check. When THC binds to CB1 receptors in the hippocampus, short-term memory is impaired. When it binds in the amygdala, anxiety may increase or decrease depending on dose. When it binds in the ventral tegmental area, it stimulates dopamine release in the nucleus accumbens, producing the reward signal that underlies the drug's reinforcing properties (Curran et al., 2016).

Cannabidiol, CBD, does something fundamentally different. It has negligible binding affinity for CB1 receptors. Instead, it modulates the endocannabinoid system indirectly, most notably by inhibiting fatty acid amide hydrolase, the enzyme responsible for breaking down anandamide. The result is that CBD raises anandamide levels without flooding receptors with an exogenous agonist. Leweke et al. (2012), publishing in Translational Psychiatry, demonstrated that CBD produced antipsychotic effects comparable to the standard antipsychotic amisulpride in patients with schizophrenia, and that these effects were correlated with increased anandamide levels in cerebrospinal fluid. The plant that can trigger psychosis through one compound appears to contain its own antidote in another. The ratio between them matters enormously, and the modern cannabis market has systematically shifted that ratio towards THC.

The adolescent brain and why timing changes everything

The adult brain and the adolescent brain are not the same organ in the same state of readiness. The adolescent brain is undergoing extensive remodelling, including synaptic pruning, myelination of long-range axonal tracts, and maturation of the prefrontal cortex, processes that continue into the mid-twenties. The endocannabinoid system plays a direct role in guiding these developmental processes. When exogenous cannabinoids disrupt endocannabinoid signalling during this critical window, the consequences can be lasting.

The Dunedin Multidisciplinary Health and Development Study, a longitudinal cohort study following more than one thousand individuals from birth, provided the most striking evidence. Meier et al. (2012) reported that participants who began using cannabis persistently in adolescence showed an average decline of eight IQ points between childhood and age thirty-eight, a decline that did not recover fully with cessation. Participants who began persistent use in adulthood did not show comparable decline. The finding is not that cannabis makes everyone less intelligent. The finding is that the adolescent brain, because it is still under construction, is uniquely vulnerable to disruption by a substance that interferes with the signalling system guiding that construction. The adult brain, whose architecture is largely complete, absorbs the same chemical insult with less structural consequence.

Cannabis, dopamine, and the psychosis question

The relationship between cannabis and psychosis is not a myth, and it is not simple. Murray et al. (2017) reviewed the accumulated evidence and identified a dose-response relationship: the risk of psychotic disorders increases with frequency of use, potency of the product, and age of onset. Daily use of high-potency cannabis, defined as products with more than ten per cent THC and negligible CBD, is associated with a fivefold increase in the risk of a first psychotic episode compared with non-use. Occasional use of lower-potency cannabis carries a much smaller risk. The mechanism involves THC-induced dopamine release in the striatum. In individuals whose dopamine system is already dysregulated, or who carry genetic variants that impair dopamine clearance in the prefrontal cortex, this additional dopaminergic activity can push the system beyond the threshold at which psychotic symptoms emerge.

The COMT gene, which encodes an enzyme responsible for metabolising dopamine in the prefrontal cortex, is one of the most studied genetic moderators of cannabis-psychosis risk. Individuals carrying the Val/Val variant of COMT metabolise prefrontal dopamine more rapidly and appear to be more vulnerable to the psychotogenic effects of THC than those carrying the Met/Met variant (Murray et al., 2017). This is not a rare genetic configuration. Roughly twenty-five per cent of the population carries the Val/Val genotype. The implication is that a substantial minority of people are at meaningfully elevated risk from high-THC cannabis, and that this risk is invisible without genetic testing. The person does not know they are vulnerable until symptoms appear.

Medical cannabis: what the evidence supports and what it does not

The medical cannabis debate has been conducted largely without reference to the medical cannabis evidence. Whiting et al. (2015), publishing in JAMA, conducted the most comprehensive systematic review and meta-analysis of cannabinoid medicines to date, analysing seventy-nine randomised controlled trials involving more than six thousand participants. The results showed moderate-quality evidence supporting the use of cannabinoids for chronic neuropathic pain, chemotherapy-induced nausea and vomiting, and spasticity in multiple sclerosis. For every other condition examined, including depression, anxiety, sleep disorders, Tourette syndrome, and psychosis, the evidence was rated as low quality or insufficient. This does not mean cannabinoids are ineffective for these conditions. It means the evidence is not yet strong enough to support clinical recommendation.

The distinction between what the evidence supports and what the public believes the evidence supports is substantial. Cannabis has become a cultural remedy projected upon by both proponents and opponents, each selecting data that confirms their position. The neuroscience offers neither vindication nor condemnation. It offers a more precise account. The endocannabinoid system is real and clinically important. THC is a psychoactive compound with genuine risks that are dose-dependent, age-dependent, and genotype-dependent. CBD is a pharmacologically distinct compound with a different risk profile and emerging therapeutic applications. Medical cannabis is a legitimate field of research producing results that are promising in some areas and inconclusive in many others. The most honest summary of the evidence is that cannabis is neither the harmless herb its advocates describe nor the unmitigated danger its opponents portray. It is a pharmacologically complex plant that interacts with a neurobiologically complex system, and the outcome depends on which compounds, at what dose, at what age, in which brain (Curran et al., 2016).