First author Dr. Gabriella Gobbi of McGill University said it has been long known that depletion of the neurotransmitter serotonin in the brain leads to depression, so antidepressants like Prozac and Celexa work by enhancing the available concentration of serotonin in the brain.

This study offers the first evidence that marijuana can also increase serotonin, at least at lower doses, but at higher doses the serotonin in the rats' brains dropped below the level of those in the control group.

The study, published in The Journal of Neuroscience, finds excessive marijuana use in people with depression poses high risk of psychosis.

The antidepressant and intoxicating effects of marijuana are due to its chemical similarity to natural substances in the brain known as "endo-cannabinoids," which are released under conditions of high stress or pain, Gobbi said.