The effects of Δ9-tetrahydrocannabinol on the dopamine system
Michael A P Bloomfield, Abhishekh H Ashok, Nora D Volkow, and Oliver D Howes
Nature, 2016, 539, 7629, 369–377.
doi : 10.1038/nature20153.
Δ9-tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, is a pressing concern to global mental health. Patterns of use are changing drastically due to legalisation, availability of synthetic analogues (‘spice’), cannavaping and aggrandizements in the purported therapeutic effects of cannabis. Many of THC’s reinforcing effects are mediated by the dopamine system. Due to complex cannabinoid-dopamine interactions there is conflicting evidence from human and animal research fields. Acute THC causes increased dopamine release and neuron activity, whilst long-term use is associated with blunting of the dopamine system. Future research must examine the long-term and developmental dopaminergic effects of the drug.
Cannabis is a widely used recreational drug. Over half of young Americans have used the drug1. In Europe cannabis has now overtaken heroin as the most widely reported illegal drug used amongst people entering specialist addiction services2. At the same time, political debates about changes to the legal status of the drug continue internationally. Although causality has not been conclusively demonstrated, heavy cannabis use is associated with increased risk of mental disorder3 including psychosis4, addiction5, depression6, suicidality7, cognitive impairment8 and amotivation9.
Δ9-Tetrahydrocannabinol (THC), cannabis’ main psychoactive component10, elicits its acute psychoactive effects via the endocannabinoid type 1 (CB1) receptor (CB1R)11. THC has been linked to the rewarding aspects of cannabis and the induction of symptoms of mental illnesses and cognitive impairment. Lately the THC content of cannabis has been increasing12, synthetic THC analogues (potent cannabinoid agonists; termed ‘spice’) are now widely used13. The future consumption of cannabinoids through electronic cigarettes (‘cannavaping’) and edible products14 changes the landscape further15. Given the widespread use of cannabinoids, and the links between THC exposure and adverse outcomes, it is imperative to understand the neurobiological effects of THC. Recently, we and others have found that heavy cannabis use is associated with reductions in dopaminergic function. Since the rewarding and psychotogenic effects of THC and its analogues are thought to be mediated by the dopaminergic system, demonstrating dopaminergic alterations in vivo in human users is of clinical relevance for the prevention and treatment of cannabis use disorders and psychoses. Therefore, we review the animal and human literature on the complex effects of acute and longer-term THC on dopamine synthesis, release, and its receptors, critically analysing the factors that contribute to effects, and variations between studies, before finally providing a framework for future research including pharmacologically dissecting these effects, especially in the developing brain.
THC receptor binding in the brain
THC is a CB1R and endocannabinoid type 2 receptor (CB2R) partial agonist11. The psychoactive effects of THC are blocked by the CB1R antagonist rimonabant16,17 indicating that these are mediated through activating G-protein-coupled CB1R receptors which reduce cyclic adenosine monophosphate (cAMP) levels by inhibiting adenylate cyclase18. THC disrupts finely-tuned endocannabinoid retrograde signalling systems due to the temporal and neuronal specificity of endocannabinoids over THC. Under conditions of low CB1R density, THC antagonises endogenous agonists possessing greater receptor efficacy than THC19. THC also allosterically modulates opioid receptors20, which may provide additional indirect routes for altering dopamine transmission21. Furthermore, THC has psychoactive metabolites with CB1R affinity, further complicating the analyses of receptor binding studies22.
CB1 receptors and dopamine
Early animal studies described the interactions of amphetamine, which increases dopamine release, and THC23. These reported that amphetamine’s behavioural effects were potentiated or antagonised depending on the dose of THC leading researchers24 to propose that dopamine was “a prime candidate for…the mode of action of Δ9-tetrahydrocannabinol”. Indeed, THC produces complex effects on the dopamine system, contributing to the drug’s recreational and harmful effects. However, there are inconsistencies between the preclinical and clinical findings which challenge the field. It is thus timely to review the evidence and provide a framework for understanding the inconsistencies between the preclinical and clinical findings.
Dopaminergic neurons are modulated by the endocannabinoid system (eCBS)25. CB1Rs and the endocannabinoid ligands anandamide and 2-arachidonoylglycerol (2-AG) are abundant in dopaminergic pathways including the striatum26 where they act as a retrograde feedback system on presynaptic glutamatergic and γ-aminobutyric acid (GABA) nerve terminals (Fig. 1) to modulate dopamine transmission. Anandamide27 and 2-AG28 stimulate dopamine release in the nucleus accumbens (NAc) shell. This effect is blocked by the CB1 antagonist rimonabant, indicating that dopaminergic effects of endocannabinoids involve CB1 receptors. The rewarding properties of THC via increased dopamine release and dopaminergic neuron firing are underpinned by biased signal transduction mechanisms from the CB1R16. There is evidence of differential effects of acute vs. chronic THC exposure on the dopaminergic system. Therefore, we will treat these separately and describe the effects on different neurobiological components of the dopaminergic system including neuron firing, synthesis, release, reuptake and receptors.