Abusing marijuana blunts the brain’s ability to respond to dopamine, the neurotransmitter that’s responsible for our feelings of pleasure, motivation, and reward. The effects of that "high" might actually lead to depression and anxiety, according to a new study published in Proceedings of the National Academy of Sciences this week.

Despite its prevalence, we know surprisingly little about the effects of marijuana abuse on the brain. Many abused substances stimulate brain dopamine signaling -- the mechanism underlying the rewarding effects of drugs, food, and sex. But while studies have shown that cocaine and alcohol, for example, increase dopamine release in the brain’s reward region, this relationship hasn’t been demonstrated consistently for marijuana.

So, to investigate marijuana’s impact on the human brain, a team led by Nora Volkow from the U.S. National Institute on Drug Abuse recruited 24 marijuana users who smoked on average five joints a day, five days a week for a decade. Using personality questionnaires and two types of brain imaging, the team examined how their brains reacted to the drug methylphenidate -- a stimulant that elevates dopamine, used to treat ADHD and narcolepsy -- compared with 24 control participants.

Both groups produced just as much extra dopamine after taking the drug, but the marijuana abusers had significantly blunted behavioral, cardiovascular, and brain dopamine responses to the stimulant. Their heart rate and blood pressure were lower in comparison, and they reported feeling restlessness and anxiety.

Same amount of dopamine but weaker (or lack of) physical responses suggests the reward circuitry in their brains are damaged. Unlike cocaine and alcohol abusers, marijuana abusers appear to produce the same amount of dopamine with methylphenidate as people who aren’t users, Science explains, but their brains don’t know what to do with it.

Their lower scores on positive and higher scores on negative tests of emotional activity are consistent with dampened sensitivity to reward and motivation on one hand, and with increased stress and irritability on the other hand. The researchers think that the effect of dampened dopamine responses on the brain’s reward region (called the ventral striatum) might contribute to addictive behaviors and a tendency toward depression and anxiety. But as Volkow explains to Science, not being able to tease out cause and effect “is a limitation in a study like this one.”

Image: Ian Sane via Flickr CC BY 2.0