Scientists find overlapping dopamine activity in cannabis use disorder and psychosis

People with cannabis use disorder show signs of altered dopamine activity in the brain that mirror patterns seen in psychosis, according to a new brain imaging study published in JAMA Psychiatry. Using a specialized form of magnetic resonance imaging, researchers found that individuals who used cannabis heavily over time had increased dopamine-related signals in a midbrain region previously associated with psychotic symptoms.

The research aimed to understand how chronic cannabis use might influence brain systems that are also implicated in psychotic disorders. Past studies have shown a connection between frequent cannabis use and an increased risk of psychosis, but the underlying biological mechanisms have remained unclear. This study focused on the dopamine system, a network of brain structures and chemicals long linked to psychosis, particularly the positive symptoms of schizophrenia such as hallucinations and delusions.

“I’m personally interested in the neurobiological basis of schizophrenia, and there is a well-established link between cannabis use and psychosis; however, the mechanism of this relationship is unclear. Since both psychosis and cannabis use are linked to changes in dopamine activity, we were interested in investigating whether they share the same dopamine pathway,” said study author Jessica Ahrens, a PhD student in the Integrated Program in Neuroscience at McGill University.

To better understand this connection, researchers in Canada recruited 61 individuals aged 18 to 35, including people with and without cannabis use disorder. Some participants were also experiencing a first episode of schizophrenia, allowing the researchers to explore how cannabis use might relate to psychosis.

All participants completed clinical interviews, cognitive testing, and provided saliva samples to measure THC levels. Brain imaging was conducted using a technique known as neuromelanin-sensitive MRI, which indirectly reflects dopamine activity in the substantia nigra and ventral tegmental area—regions critical for motivation, reward, and the processing of salient stimuli.

Neuromelanin is a byproduct of dopamine metabolism and accumulates in these brain regions over time. The more dopamine a person produces in this part of the brain, the stronger the neuromelanin signal on the scan. In disorders like schizophrenia, higher neuromelanin signal has been observed in these same areas, suggesting long-term increases in dopamine activity. By comparing the MRI results of people with and without cannabis use disorder, the researchers were able to examine whether cannabis use is associated with similar dopamine-related changes.

The study found that individuals with cannabis use disorder had significantly higher neuromelanin-MRI signal in specific parts of the midbrain, particularly in regions previously linked to the severity of psychotic symptoms. This pattern was not observed in individuals who did not meet criteria for cannabis use disorder, even though many of them had used cannabis in the past.

Notably, the increase in signal was strongest among participants with more severe cannabis use disorder, suggesting a dose-related pattern. The findings were independent of whether a person had a diagnosis of schizophrenia, although the effect was numerically stronger among participants with both conditions.

“We found that individuals with cannabis use disorder exhibited increased neuromelanin-MRI signal in a brain region previously associated with psychosis symptoms,” Ahrens told PsyPost. “Since neuromelanin-MRI signal is considered a potential marker of dopamine function, our findings suggest that people with cannabis use disorder may have elevated dopamine activity in a brain area linked to psychosis risk. We hope this research helps inform young people about the potential risks associated with harmful cannabis use — especially those with a family history of psychosis or other factors that may increase their vulnerability.”

Interestingly, the researchers did not find any significant interaction between cannabis use disorder and schizophrenia diagnosis, meaning that having both conditions did not amplify the neuromelanin signal beyond the effect of cannabis use alone. Over the course of one year, the elevated neuromelanin signal remained stable in those with cannabis use disorder, suggesting that the change may reflect long-lasting alterations in dopamine function. This is notable because increased dopamine activity in this brain region has been consistently linked to psychotic symptoms in other studies.

The researchers also explored whether neuromelanin signal decreased over time with continued cannabis use but found no evidence of this in the one-year follow-up. One explanation is that any reduction in dopamine function from long-term cannabis use may have already occurred before the first scan and remained stable. Another possibility is that increased dopamine activity is a pre-existing vulnerability that contributes to both psychosis and problematic cannabis use, though the study was not designed to test this idea directly.

An unexpected finding was that individuals with cannabis use disorder reported fewer personal problems related to their cannabis use, despite meeting clinical criteria for the disorder and showing biological evidence of its effects on the brain.

“Something that surprised me is that when asked about their cannabis use patterns, we did not find a significant difference in self-endorsed problems (when asked the question: ‘Have you ever had problems because of your use of cannabis?’); those with a cannabis use disorder reported less frequent personal harm due to cannabis,” Ahrens explained. “This indicates that individuals with a cannabis use disorder had lower perception of harm despite the higher measured severity.”

The researchers highlighted several strengths of their approach, including the use of multiple cannabis-related measures (clinical diagnosis, THC levels, and self-report), a well-matched control group, and detailed brain imaging protocols. However, the study also had limitations. The sample size was relatively small, particularly for the follow-up scans. Women were underrepresented, limiting the generalizability of the results. Additionally, the study focused exclusively on the substantia nigra and ventral tegmental area and did not examine other neuromelanin-rich regions like the locus coeruleus, which may also play a role in psychosis.

Importantly, the study design does not allow conclusions about cause and effect. It remains unclear whether higher neuromelanin signals reflect a preexisting vulnerability to cannabis use disorder, a consequence of cannabis exposure, or some combination of both. Longitudinal studies that begin before cannabis use starts would be needed to clarify this.

“More research is needed to explore the neurobiological changes that occur before and after the development of cannabis use disorder, as well as the effects of quitting cannabis in those with a history of use,” Ahrens said. “Moving forward, our group plans to examine another neuromelanin-rich brain region, where the neuromelanin signal may be linked to a different neurotransmitter. Additionally, we aim to employ more sensitive MRI techniques to improve our understanding of neuromelanin dynamics.”

The study, “Convergence of Cannabis and Psychosis on the Dopamine System,” was authored by Jessica Ahrens, Sabrina D. Ford, Betsy Schaefer, David Reese, Ali R. Khan, Philip Tibbo, Rachel Rabin, Clifford M. Cassidy, and Lena Palaniyappan.