National Geographic

Impulsive minds are primed for drug addiction

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Blogging on Peer-Reviewed ResearchWe’ve all acted impulsively before, and we have the horrendous clothes, echoing bank accounts and hilarious memories to show for it. But science is beginning to show that impulsive people may be particularly vulnerable to drug addiction, and there is little funny or harmless about that.

img_0046_o.jpgAccording to Government statistics, half a million people in the UK are addicted to class A drugs like cocaine, heroin and amphetamines. All too often, drug addiction and other compulsive disorders like obesity are dismissed as issues of ‘willpower’ and those who succumb to temptation are labelled as ‘weak’. But this attitude is, at best, wrong and, at worst, stigmatising and self-righteous. And it provides no clues for ways of helping people with these problems.

In fact, the evidence suggests that drug addiction is linked to certain personality traits. Being impulsive is one of them, and a tendency to seek out new sensations (often described as “living life to the full”) is another. But do these traits drive people towards drug addiction, or are they a result of the drugs themselves?

Some scientists have suggested that long-term drug use impairs our prefrontal cortex, a part of our brain that helps to suppress our basic urges and is essential for appropriate social behaviour. It’s the mental equivalent of the angel on our shoulder. By relieving us of this restraining effect, drug use could lead to impulsive, reckless and anti-social behaviour.

113626048131.jpgBut a new animal study suggests that it works the other way round too. Innate differences in personality traits can predispose individuals to addictive drugs, even before a single molecule has entered their system.

Jeffrey Dalley, Trevor Robbins and colleagues at the University of Cambridge studied a dozen rats, who were cocaine virgins at the start of the experiment. The rats were trained to associate a light with the arrival of food; when it came on, the animals received food if they waited and pressed a button.

Some of the rats were twice as likely to prematurely press the button than others, allowing Dalley to separate them into an ‘impulsive’ and a ‘non-impulsive’ group. A brain scan revealed key differences in the brains of the two groups, particularly in the ventral striatum, a part of the brain involved in reward, pleasure and addiction. This region contains receptor molecules that respond to dopamine, a multi-purpose signalling molecule involved in feelings of pleasure. And impulsive rats had lower levels of two of these dopamine receptors – D2 and D3.

Dalley’s rats were given access to cocaine using a set-up that allowed them to give themselves a hit via a catheter. Sure enough, the impulsive rats gave themselves many more drug infusions than the non-impulsive ones. So in rats at least, an individual’s impulsiveness, as indicated by the levels of D2 and D3 receptors in their ventral striatum, signposts an inbuilt vulnerability to drug abuse.

Dalley’s work support earlier studies in which mutant rats that lacked D2 receptors altogether became quickly and strongly addicted to cocaine. In rats, these receptors may act as a molecular conscience, limiting excessive reactions to addictive substances.

And once drug use begins, it triggers other changes in the way the striatum reacts to dopamine. Studies in monkeys have found that as the animals start getting hooked on cocaine, the levels of D2 receptors in their dorsal striatum – next to the ventral part – start to fall, and stay that way for years after they come off the drug.

These results provide a behind-the-scenes look at the molecular changes that turn an impulsive urge to take a drug to a compulsive urge to continue taking it. It is likely that the receptors play a similar role in our own brains, and their levels are probably controlled by genetic variations.

Finding the genes responsible could provide valuable clues for identifying ways of treating addiction, or preventing successful quitters for relapse. And while cocaine was the drug of choice in this study, Dalley’s results could have implications for other drugs like nicotine (which is equally addictive), or possibly even more abstract addictions like gambling.

Reference: J. W. Dalley, T. D. Fryer, L. Brichard, E. S. J. Robinson, D. E. H. Theobald, K. Laane, Y. Pena, E. R. Murphy, Y. Shah, K. Probst, I. Abakumova, F. I. Aigbirhio, H. K. Richards, Y. Hong, J.-C. Baron, B. J. Everitt, T. W. Robbins (2007). Nucleus Accumbens D2/3 Receptors Predict Trait Impulsivity and Cocaine Reinforcement Science, 315 (5816), 1267-1270 DOI: 10.1126/science.1137073

There are 5 Comments. Add Yours.

  1. speedwell
    October 19, 2008

    That’s interesting. I’m sitting here with two recent root canals, for which the dentist gave me a prescription for Vicodin. Four pills into a 20-pill scrip, I switched over to regular Tylenol, as I have a horror of becoming dependent on a drug. This has happened multiple times; after having a kidney removed two years ago, I still have half the Vicodin given to me for post-op pain. I’m particularly pain sensitive, so it is not that I simply didn’t feel the pain. Vicodin works great on me (in fact, I can’t seem to properly tolerate Percocet), so I didn’t have a problem with it.
    I’m pretty impulsive, all in all. I can be objective enough to give in to mostly GOOD impulses, but there have been times… you know. I have also been clinically depressed most of my life. These two things, I understand, predispose me to being an addict of SOME sort, but I don’t drink, or smoke, or anything… the best I can do in that respect is that I eat too much, like the majority (I was about to say the bulk) of the population.
    My own case, where I actually take fewer doses of the addictive medication than other people despite being, arguably, an “addict” personality, leads me to reflect that the main difference between me and the rats is that I’m a human being with the power of choosing my behavior based on the knowledge of what I’m taking into my body. Addicts, once addicted, lose some of that power of choice, and I’m not blaming them for that. After somebody is addicted, there is only so much you can do. But before they’re addicted… that’s where we can really work on what is unique about us as human beings.
    It’s common to harp on “prevention” as if admonishing people that “drugs are bad” is anything but crude and condescending. If we are to agree that addictions are horrible and we should take steps to prevent them, we need to approach things respectfully and in a more nuanced way. I’m not entirely sure how your rat experiments will help us do this, but I’m hoping that you will come up with something unusual and good. Thanks for posting.

  2. outofocus
    October 19, 2008

    speedwell – You might as well have been describing me.
    I severely broke my leg when I was 12. They gave me codeine to take home. I have in my stack of journals, several pages where I documented when I took the codeine and how much. I tapered myself off and switched to tylenol within a few days of getting out of the hospital.
    I’ve collected a nice stack of pain killers over the years for a variety of things (falling out of a tree, crashing while skateboarding, childbirth, motorcycle accidents, surgery) and most of them remain unopened. Unlike you, I do have a high pain tolerance but that doesn’t mean I enjoy it.
    I grew up around drug users, alcoholics, etc and was lucky to have parents who were honest with me about their experiences. The idea of being addicted terrified me and since that is the only consistent and obvious genetic weakness in my family, I’ve rearranged that part of me so that I’m basically addicted to controlling myself so that it’s never an issue. I make fun of myself but I have spreadsheets keeping tabs on my calorie intake, my exercise regime, etc etc.
    I know that the reason that I declined drugs when I was a teenager was because I was educated about drugs and really understood the long term risks. I opted for other sorts of sensation seeking behavior that had more short term risks. I know some people believe they are impervious to the risks and I think that’s a secondary issue, a disconnect with reality.
    I am a huge supporter of educating children as they come into appropriate ages for it. My siblings grew up with my dad and his wife. They were secretive about their behavior when asked by their children (I’m 10 yrs older than my sister, I can remember my dad doing coke) so my siblings were not given the opportunity to learn. My sister spent pretty much all of her teenage years under the influence of drugs, moving from THC to speed to heroine. She kicked the heroine but hasn’t learned to kick the people from the lifestyle and so she is always in danger of backsliding (which is another major issue for addicts).

  3. speedwell
    October 19, 2008

    Indeed. I learned when I was in middle school that my grandfather was an alcoholic (something carefully kept from his grandchildren for many years). Since he was a beer drinker, something in my mind associated beer with alcoholism, and I never cultivated a taste for beer… something that made me really stand out in my recent business trip to Germany, I can assure you. :)

  4. Kapitano
    October 23, 2008

    Innate differences in personality traits can predispose individuals to addictive drugs
    Innate? As in “determined solely by genetics, inborn and forever unchangeable”…or as in “pre-existing for unspecified reasons, cultural or natural, beyond the scope of this paper”?
    I ask as one who was raised to be quiet and accepting, and learned that impulsive and skeptical is more interesting.

  5. Michael K Moriarty
    April 16, 2009

    This question of Self regulation is essential to all of our lives and yet presents such complexity. What is impulsivity? Why do we all act impulsively at times. Since the capacity to make sudden, ill thought out actions, is present in all, then we must assume that this capacity has been selected by natural selection and confers advantage. It would seem so. While the addage”look before you leap” is sound generally, there are circumstances such as danger
    where leaping first is life saving. Leaping first when we are angered seems more prone to negative consequences.
    but even with anger surely there are moments when delaying action to weigh my options could be risky.
    Then too, impulsivity may be advantageous when we need to try new actions, or respond with novelty in situations where habitual actions brook nothing new or have lost their effectiveness. Men believe that women are attracted to guys who are edgy and take risks. Young men will behave impulsively in front of women to gain their admiration and attention.
    Use of drugs or alcohol to reduce pain and to promote pleasure seem to involve us in risky behavior DDrugs/alc may beused impulsively, or when overused to the point of
    habituation or dependence, taken compulsively to stave off the physical discomforts of withdrawl., or as a self comforting or self rewarding behavior.
    Their is more to the notion of impulsivity
    than just insufficiently delayed action, unregulated by higher brain functions as appraisal and better judgement.
    we are designed with this capacity/ Like so many of our capacities they serve us well in many instances and bring harm in others. MIke

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