Smoking Pot

Hardcore pot smoking could damage the brain's pleasure center

Lizzie is Science's Latin America correspondent, based in Mexico City.

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Lizzie Wade 

14 July 2014 3:00 pm



It probably won’t come as a surprise that smoking a joint now and then will leave you feeling … pretty good, man. But smoking a lot of marijuana over a long time might do just the opposite. Scientists have found that the brains of pot abusers react less strongly to the chemical dopamine, which is responsible for creating feelings of pleasure and reward. Their blunted dopamine responses could leave heavy marijuana users living in a fog—and not the good kind.

After high-profile legalizations in Colorado, Washington, and Uruguay, marijuana is becoming more and more available in many parts of the world. Still, scientific research on the drug has lagged. Pot contains lots of different chemicals, and scientists don’t fully understand how those components interact to produce the unique effects of different strains. Its illicit status in most of the world has also thrown up barriers to research. In the United States, for example, any study involving marijuana requires approval from four different federal agencies, including the Drug Enforcement Administration.

One of the unanswered questions about the drug is what, exactly, it does to our brains, both during the high and afterward. Of particular interest to scientists is marijuana’s effect on dopamine, a main ingredient in the brain’s reward system. Pleasurable activities such as eating, sex, and some drugs all trigger bursts of dopamine, essentially telling the brain, “Hey, that was great—let’s do it again soon.”

Scientists know that drug abuse can wreak havoc on the dopamine system. Cocaine and alcohol abusers, for example, are known to produce far less dopamine in their brains than people who aren’t addicted to those drugs. But past studies had hinted that the same might not be true for those who abuse marijuana.

Nora Volkow, the director of the National Institute on Drug Abuse in Bethesda, Maryland, decided to take a closer look at the brains of marijuana abusers. For help, she and her team turned to another drug: methylphenidate (aka Ritalin), a stimulant known to increase the amount of dopamine in the brain. The researchers gave methylphenidate to 24 marijuana abusers (who had smoked a median of about five joints a day, 5 days a week, for 10 years) and 24 controls.

Brain imaging revealed that both groups produced just as much extra dopamine after taking the drug. But whereas the controls experienced increased heart rates and blood pressure readings and reported feeling restless and high, the marijuana abusers didn’t. Their responses were so weak that Volkow had to double-check that the methylphenidate she was giving them hadn’t passed its expiration date.

This lack of a physical response suggests that marijuana abusers might have damaged reward circuitry in their brains, Volkow and her team report online today in the Proceedings of the National Academy of Sciences. Unlike cocaine and alcohol abusers, marijuana abusers appear to produce the same amount of dopamine as people who don’t abuse the drug. But their brains don’t know what to do with it. This disconnect could be “a key mechanism underlying cannabis addiction,” says Raul Gonzalez, a neuropsychologist at Florida International University in Miami who was not involved with the research. The study “suggests that cannabis users may experience less reward from things others generally find pleasurable and, contrary to popular stereotypes, that they generally feel more irritable, stressed, and just plain crummy. This may contribute to ongoing and escalating cannabis use among such individuals.”

But do marijuana abusers smoke a lot because they feel crummy, or do they feel crummy because they smoke a lot? Volkow doesn’t know. Not being able to tease out cause and effect “is a limitation in a study like this one,” she says. Perhaps the abusers already had less reactive dopamine systems and started smoking a ton of pot to cope with their general malaise. Or maybe prolonged marijuana abuse is actually damaging their brains’ reward circuitry, leading to the apathy and social withdrawal that marijuana abusers often experience.

The lessons for recreational users of marijuana, if any, are unclear. This study used “hardcore volunteer[s]” who were “using quite a lot of cannabis,” says Paul Stokes, a psychiatrist at Imperial College London who wasn’t involved in the research. As such, “it probably tells you more about cannabis dependence than about recreational use.” But when he 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.

All of these are important questions to answer, Volkow says. As availability of the drug increases, she says, it’s something “we all need to know.”

Posted in Brain & Behavior, Health

14 Replies

  • And an earlier study:

    July 2013

    New research discovers long-term cannabis users tend to produce less dopamine, a chemical in the brain linked to motivation.

    UK investigators found that dopamine levels in a part of the brain called the striatum were lower in people who smoke more cannabis and those who began using the drug at a younger age.

    They suggest this finding could explain why some cannabis users appear to lack motivation to work or pursue their normal interests.

    Scientists at Imperial College London, UCL and King’s College London used PET brain imaging to look at dopamine production in the striatum of 19 regular cannabis users and 19 non-users of matching age and sex.

    The cannabis users in the study had all experienced psychotic-like symptoms while smoking the drug, such as experiencing strange sensations or having bizarre thoughts, like feeling as though they are being threatened by an unknown force.

    The researchers expected that dopamine production might be higher in this group, since increased dopamine production has been linked with psychosis. Instead, they found the opposite effect.

    The cannabis users in the study had their first experience with the drug between the ages of 12 and 18.

    There was a trend for lower dopamine levels in those who started earlier, and also in those who smoke more cannabis.......

  • This article needs to be debunked as it is full of falsehoods but why waste my time when I could indulge myself in an innocent pastime like smoking a joint…

  • Scotty, this is not one article it is two separate reports on two separate studies, one at Imperial College London and UCL and King’s College London (top London neuro hospitals) and a year later a study in the USA. Im not sure what falsehoods you refer to that need debunking.

    It is sad that you and Lethe here and on other Parkinson forums object to any post that you interpret as not supportive of your views with the result that now instead of discussion we have long threads on the topic with only one poster.

    Personally I have never opposed the use of marijuana, quite the opposite I support its decriminalisation and appropriate use and I have said so on this and other forum. But I also believe that with all medication there are positives and negatives and I like to make informed choices.

  • Hope you can appreciate my sense of humor, The number one thing I have to fight PD.

  • I would love to understand what it is that you have noticed in it that is innacurate. Could you tell us please?

  • It is sad that you and Lethe here and on other Parkinson forums object to any post that you interpret as not supportive of your views with the result that now instead of discussion we have long threads on the topic with only one poster.

    Give me (us) a break will ‘ya? I would LOVE more posts from other people, but not the nonsense you post with vague “studies” that prove nothing but imply everything. The truth is that you and others just HATE that MM is getting any positive press and refuse to admit that you really believe it’s the tool of the devil.

    And your attempts to shut me up by responding with one word questions that you think will keep me busy looking up stuff isn’t going to work and undermine what I do. I wasn’t even going to respond to this thread until you mentioned my name like I’m some kind of bully... What nonsense!

    In any case I’m sure people can see through your questionable studies.

  • Lethe, Scotty T. Here. How about these latest findings and who funds this specious research?

    I have communicated as you have my own experiences and there is absolutely no connection between mine and this bogus study, absolutely none. Spreading fear among the mass's. I ask myself why.

  • So it's not inaccurate, just a lie?

  • Have you ever partaken of the oldest medicine on the planet, Cannabis?

  • I have had no need to. I am not ill, just trying to understand the advantages and disadvantages of different drugs, both pharmaceutical and non pharmaceutical.

    I am afraid clarity is lack in some of the discussions.

  • Thank you for this information Anthony1162. It us difficult to get much scientific information on such an emotive subject. I appreciate it. I would welcome scientific evidence to the contrary also, then I can make up my own mind about it.

  • What I wonder now is if heavy marijuana use is shown to lower dopamine levels in the brain could it be a cause or be a contributing cause of Parkinsons later on?

    This guy asks another interesting question in a blog called Pesticides, Paraquat, Pot and Parkinsons.

    During the late 1970s (and again in 1988), a controversial program sponsored by the US government sprayed paraquat on cannabis fields in Mexico. Since much of this cannabis was subsequently smoked by Americans, the US government's "Paraquat Pot" program stirred much debate.

    Like Agent Orange and other chemicals that were deemed "safe" in the past, paraquat may yet show its darkest side when baby-boomers who smoked cannabis in their youth, often handling and rolling the tainted plant material, begin to see the effects of their paraquat exposure.

  • A very interesting link Hikoi but I wonder what the half life of such pesticides is. I suspect it may have been involved in a few cases initially but unless it survives at sufficiently high levels in the soil it will not affect the younger users of the drug.

    If the levels of pesticide persisting in the soil for the next year's crop were still that high as to have an effect on the brain of smokers, it would be more worrying for people eating non organically grown food.

    I will do some research into the persistence of pesticides and see what I come up with.

  • Thankyou, interesting.

    You mention above wanting scientific evidence. FYI a review of 105 studies.

    105 Peer-Reviewed Studies on Marijuana

    Medical Studies Involving Cannabis and Cannabis Extracts (1990 - 2012)

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