A new study in Molecular Psychiatry reports that cannabis dependence results in decreased striatal dopamine release, the same effect seen in substance abuse of heavy drugs like amphetamine and cocaine abuse
With the growing popularity and legality of cannabis, its use is increasing and many scientists are focusing their research on the side effects of chronic cannabis use. Cannabis dependence (now known as cannabis use disorder) is prevalent in 8.3% of young adults in the US alone. Its use has already been linked to decreasing the brain’s ability to identify and empathize with human emotions and increases “neural noise” in the brain, similar to those seen in schizophrenia. As well, cannabis dependence has been associated with psychosis and depression, and shown to alter brain function and structure.
Researchers believe cannabis exerts its effects on the brain through the psychoactive component THC. THC is a partial agonist at the endocannabinoid receptor, which is found throughout the brain, especially at the basal ganglia (responsible for the coordination of movement) and the substantia nigra pars reticulata (which transmits signals from the basal ganglia to other brain areas and inhibits dopamine activity in the basal ganglia). THC induces firing of mesolimbic dopamine neurons and increases the levels of dopamine in the striatum. The mesolimibic system is a dopaminergic pathway and is known as the “reward pathway” and has been implicated in a number of drug addictions. With initial drug use, there is an increase in dopamine levels as the drug binds and activates dopamine release. Once dependence on a drug has been established, the brain produces less dopamine or decreases the amount of available dopamine receptors to compensate; because of this, dependence on dopamine-enhancing drugs is associated with blunted striatal dopamine transmission.
Other substances of abuse, like amphetamines, have been implicated in reduced dopamine release, and researchers now use this relationship as a hallmark for substance addiction and dependence. Long-term cannabis use has not yet been implicated in the reduction of striatal dopamine release.
A new study published in Molecular Psychiatry has now confirmed that cannabis dependence also results in stunted dopamine release in the brains of users, during the phase of dependence. In this study, 11 cannabis-dependent and 12 non-drug users completed PET scans in order to observe the level of dopamine release in both of these groups. The cannabis-dependent users were not comorbid for any other disease or substance abuse. Researchers looked at the level of dopamine release within and outside of the striatum, which is part of the “reward pathway” and is responsible for receiving dopamine from other brain areas and conveying it to the basal ganglia. Researchers induced the release of dopamine in the participants through the administration of d-amphetamine.
Researchers determined that cannabis dependence resulted in a decreased release of dopamine from the striatum. Researchers also discovered that the severity of dependence (indicated by frequency of use) dictated the level of stunted striatal dopamine transmission. As well, cannabis dependence was associated with psychiatric disorder-related symptoms, like inattention and poor memory.
The results of this study provide strong evidence that severe cannabis dependence (without any comorbidty) is associated with deficits in striatal dopamine release and an increase in psychosis-related symptoms. This shows that heavy cannabis use results in brain changes that mimic the same changes seen in abuse of other drugs, like amphetamines and cocaine. As well, it results in psychotic symptoms similar to those seen in schizophrenia.
Written By: Alexandra Lostun, BSc