BETHESDA, MD. (FRONTLINE MEDICAL NEWS)Nascent habitual pot-smoking youths still early in their cannabis-using careers already may have damaged their ability to learn from either failure or success when challenged with a series of risk/reward tasks, according to a poster presentation at this year’s National Institute on Drug Abuse Frontiers in Addiction Research Mini-Convention.

Brain scans of marijuana smokers aged 13-22 years who smoked at least two times a week, taken during their performance of a risk/reward task, showed they had the same dopaminergic response regardless of whether they failed or succeeded at the task. Meanwhile, similar brain scans of 20 healthy, age-matched controls showed that the dopaminergic response in the ventral striatum wall was particularly strong if the person had failed the task. The ventral striatum is implicated in reward processing, motivation, and pleasure.

“We found that the healthy individuals who didn’t use marijuana tend to engage the reward regions, maybe because it motivates them to do better,” Dr. Gregory Tau of Columbia University, New York, said in an interview.

Brain scans taken just before the study subjects were given cash and told it was theirs to increase, lose, or keep according to how well they were able to push buttons in reaction to flashing lights, also showed a difference in how the two groups prepared for the challenge.

According to Dr. Tau, the control group recruited the motor aspects of the frontal striatal network, “which is normal if you want to actually do something.” However, the study group engaged more of the frontal parietal network, which requires a higher level of attention.

“The study group’s preparation to respond is a lot more effortful,” Dr. Tau said. “For this group to have the same amount of money and perform the same as the control group, they have to work a lot harder and use a lot more of their brain.”

The study was evenly spread across the genders, and both the study and the control groups were approximately a third white, black, and Hispanic.

The research adds to a growing body of data that indicate the sooner in life one begins to smoke pot, the more deleterious cognitive effects one risks experiencing. The Dunedin Multidisciplinary Health and Development Study of New Zealand, published in 2012, found that individuals who began smoking marijuana after the age of 13 years and persisted beyond age 18 years had up to an 8% decline in their IQ scores, and a range of cognitive difficulties. Even after cessation of cannabis use, the neuropsychological functioning among those who had begun smoking in adolescence was not found to recover fully, even 1 year after they’d quit using.

“It’s not exactly accurate to say that if you keep smoking pot, you’re going to lose 8 IQ points,” Dr. Tau said. “That’s too simplistic. But, the later you start, the less of an effect it has on you. So the message is, if you’re thinking of doing it, then delay it.”

The average age of the marijuana smokers in Dr. Tau’s study was 17 years, and the individuals did not necessarily smoke more than twice per week; still, their scans showed impairment in their ability to process reward, Dr. Tau said. “The direction in the marijuana users was always, without exception, negative.

“The most important message is bigger: Marijuana use has a negative impact on the brain, even at a low dose,” he added.

Dr. Tau did not state any relevant disclosures. This study was supported in part by an award from the National Institute on Drug Abuse–American Academy of Child and Adolescent Psychiatry Physician Scientist Program.

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