Marijuana use among teens could trigger neurological changes in the developing brain that lead to increased anxiety and stress levels that could persist into adulthood, new animal research suggests.  Although the finding stems solely from work conducted with adolescent and adult lab rats, the work suggests that certain troublesome changes in levels of the key brain chemicals serotonin and norepinephrine may linger long after marijuana use ceases.

"Here, the goal was simply to understand the neurological mechanism that could be underlying the specific phenomenon of depression and anxiety observed in previous studies among adolescents chronically exposed to cannabis," explained study author Dr. Gabriella Gobbi, a psychiatric researcher at the Research Institute of the McGill University Health Centre in Montreal. "And what we found with the animals we worked with is that when those that were exposed to cannabis as adolescents became adults they had low serotonin activity, which is related to depressive behavior, and high norepinephrine levels, which is related to an increase in anxiety and stress," Gobbi continued.

"This means," she cautioned, "that cannabis exposure when young seems to cause changes in the adult brain. And these changes could perhaps be irreversible, even if you stop consuming cannabis."

The study appears in Neurobiology of Disease.

The authors say that the main ingredient in marijuana, delta-9-tetrahydrocannabinol (THC), has previously been identified as having an impact on receptors in the brain that regulate cognition and emotion.

In addition, they point out that the adolescent brain is perhaps particularly sensitive to both drug use and related stress, given that this is the pre-adult period during which the brain and its neurochemical composition undergoes extensive reshaping and reorganizing.

"We were a little bit surprised by our findings because we didn't expect to see such a strong effect on the adult brain from adolescent usage. It was a very significant effect," said Gobbi.

"So, in general, I think that what people should take away from this work," she advised, "is that just because it's a plant it doesn't mean that marijuana is harmless. It's a pharmacologically active drug, and it must be used with awareness."