The July 2014 edition of Science Magazine reports on a study by NIDA director Nora Volkow and colleagues about the impact of heavy marijuana use (about 5 joints a day, 5 days a week for 10 years) on the dopamine systems in the brain. It is well-known that drugs of abuse flood the brain’s rewards regions with dopamine and eventually impair the ability of the brain to produce dopamine in response to other, biological rewards. Over time, this results in drug rewards becoming more important to users than anything else. As the Science Magazine article explains, “past studies had hinted that the same might not be true for those who abuse marijuana.”
In an attempt to clarify matters, Volkow et al gave gave methylphenidate to 24 marijuana abusers and 34 control subjects, since methylphenidate, or Ritalin, is known to increase the amount of dopamine in the brain. Subsequent neuroimaging showed that both groups produced more dopamine after taking methylphenidate. However, the control subjects demonstrated expected signs of arousal afterwards, including higher heart rates, blood pressure readings and feelings of being restless and high. The marijuana abusers did not. Science Magazine notes that, in fact, “Volkow had to double-check that the methylphenidate she was giving them hadn’t passed its expiration date.”
Volkow’s team concluded that the lack of a physical response to a powerful psychostimulant among marijuana users may indicate that the drug has damaged the reward circuitry in their brains, and that even though these individuals appear to produce the same amount of dopamine as non-abusers, their brains, as Science Magazine puts it, “don’t know what to do with it.” Therefore, like other drug abusers, cannabis users probably experience less pleasure in response to other ordinarily rewarding events, and, like other drug abusers, come to increasingly rely on marijuana for feelings of pleasure, or even normalcy.
The article concludes with remarks by Paul Stokes, a psychiatrist at Imperial College London who wasn’t involved in the research. Stokes said the study “probably tells you more about cannabis dependence than about recreational use.” However, when Stokes did a similar brain imaging study of people who smoked marijuana no more than once a week, he observed “similar themes” when it came to dopamine.