Regular marijuana use takes a worse toll on the teenage brain than thought, say researchers in Montreal who studied the effects of cannabis in rats.

The findings suggest daily marijuana use by teens can cause depression and anxiety, and have an irreversible effect on the brain.

The study, which was published in the journal Neurobiology of Disease, looked at 18 adolescent and adult rats that were exposed to cannabis.

Those given the drug had decreased levels of serotonin, which affects mood.

The animals also showed higher levels of norepinephrine that can increase susceptibility to long-term stress, said Dr. Gabriella Gobbi, a psychiatric researcher from the Research Institute of the McGill University Health Centre.

"These permanent changes in the brain are also linked to certain mental illnesses, like schizophrenia," Gobbi said Thursday.

"And we showed that even if we stopped the cannabis use at the end of adolescence, the changes were still detectable in adulthood."

Previous population studies have looked at how smoking marijuana can affect behavior in some teenagers. But Gobbi said this was one of the first to focus on how the drug affects depression and anxiety in adolescence compared with adulthood.

The researchers also plan to observe a group of young human marijuana smokers to investigate the idea further.

The study was funded by a grant from The Canadian Psychiatric Research Foundation.

With files from The Canadian Press