The study was carried out by Dr. Peter Manza, Dr. Dardo Tomasi, and Dr. Nora Volkow, of the National Institute on Alcohol Abuse and Alcoholism in Bethesda, MD.
The findings have been published in the journal Biological Psychiatry: Cognitive Neuroscience and Neuroimaging.
As Dr. Manza and his colleagues explain in their paper, heavy cannabis use has been linked with a higher risk of psychosis, depression, and schizophrenia.
Additionally, here at Medical News Today, we have reported on studies pointing to a link between marijuana use and bipolar disorder.
That being said, the precise mechanisms underlying such associations have remained unclear. So, the new research helps to illuminate these links, as the study examines what goes on inside the brains of heavy cannabis users.
Hyperconnectivity and feelings of alienation
For their study, the researchers used functional MRI to examine the “resting-state brain function” of the subcortical brain regions of 441 adults, all aged between 22 and 35.
The researchers also recruited 30 cannabis users who met the criteria for substance abuse, as laid out in the Diagnostic and Statistical Manual of Mental Disorders IV. They then compared their brain activity with that of a group of 30 healthy controls.
The study revealed that those who abused cannabis had remarkably high connectivity in areas associated with “habit formation and reward processing.”
Specifically, these areas were the ventral striatum (which hosts the nucleus accumbens, or an area associated with reward behavior and which is involved in addiction and drug abuse), the midbrain (which hosts the substantia nigra that has dopamine-containing neurons), the brainstem, and the lateral thalamus.
Importantly, the authors note, this brain hyperconnectivity was “most pronounced in individuals who began cannabis use earliest in life and who reported high levels of negative emotionality.”
In particular, the researchers found that feelings of alienation were strongly associated with high subcortical connectivity.
The researchers focused on feelings of alienation — that is, the feeling that friends betray you and others reject you or wish to harm you — because their previous research had shown that people who abused cannabis reported very high levels of this feeling.
The authors conclude, “Together, these findings suggest that chronic [cannabis use] is associated with changes in resting-state brain function, particularly in dopaminergic nuclei implicated in psychosis but that are also critical for habit formation and reward processing.”
The psychopathology of cannabis use
“These results shed light on neurobiological differences that may be relevant to psychopathology associated with cannabis use,” write Dr. Manza and his colleagues.
They explain that examining resting-state brain function is a noninvasive procedure that could be easily used to assess the development and evolution of psychiatric symptoms in cannabis users.
Dr. Cameron Carter, editor of the journal that published the paper, comments on these findings, saying, “These brain imaging data provide a link between changes in brain systems involved in reward and psychopathology and chronic cannabis abuse.”
“[The findings suggest] a mechanism by which heavy use of this popular drug may lead to depression and other even more severe forms of mental illness.”
Dr. Cameron Carter