Monday, August 15, 2011

Addiction and Neuroscience

One of the treasures of radio (nowadays expanded to include podcasts), even within the wonderful world of NPR, is Terry Gross’ daily Fresh Air program. To my mind, she’s the best interviewer in broadcasting. I recently listened to a podcast of her June 23 interview with Dr. David Linden, professor of neuroscience at Johns Hopkins University School of Medicine, and author of The Compass of Pleasure. He does a nice of job pulling together the exploding body of scientific research that reconfirms that addiction is a largely genetic and neurobiological condition rather than simply a matter of choice and self-control, as some opponents to the so-called “disease model” have argued for years.

There has been evidence for decades, from a large number of studies of identical twins, adoptees, etc., that addictions (most studies focusing on alcoholism) are largely genetic. (Dr. Linden quotes a figure of 40%; almost all the recent research references I have found say 50%.) Thus, a number of people, usually with genetic family histories of alcoholism/addiction, appear to be born with a predisposition to addiction. However, environment (including stress level), personality, and learning also play large roles. People who do have this predisposition but who barely use alcohol or drugs will still not become addicted to these substances. On the other hand, people without these genes (and there seem to be many genes involved, not just one) will have a lower chance of developing addictions despite heavy exposure to substance use.

Even the process of learning to drink or drug in progressively greater amounts, and increasing frequency, is heavily influenced by biology, which, for example, determines how good or relieving it feels to use a given drug and how much of the substance is needed to get an effect.

What is kind of exciting is how many of the characteristics of addictive behavior that we have observed for generations are now understood to be paralleled by neurobiological processes that can, at this point, be identified rather than merely inferred.

Anyone who took an introductory psychology course heard about both Operant (or Instrumental) and Classical (or Pavlovian) Conditioning, both of which are hugely important in the development of addictions. In operant conditioning, when a particular action of an animal (such as yourself) is followed repeatedly by a reinforcement/reward, the individual is likely to engage in more of that behavior. In classical conditioning, when a particular stimulus (e.g., a pub; white powder) becomes associated with a response (e.g., drinking, drug use), exposure to the first tends to trigger the body to prepare for the second (e.g., craving).

What neuroscientists have clarified is that underlying these behavioral experiences are chain reactions in the brain’s reward system (located in the medial forebrain and involving structures you may have heard of, such as the ventral tegmental area and nucleus accumbens, as well as chemicals, neurotransmitters, especially one called dopamine that is increased in reaction to addictive drugs). This reward system evolved because it promoted the survival of our species; it is turned on by food, water, sex, and also by exercise, altruism, and the experience of learning – all essential and all pleasurable to most humans. (And Linden notes that non-substance related behaviors, such as eating and sex, can also become addictive for predisposed individuals.)

One kind of genetic predisposition, according to Linden, is that people born with a “blunted pleasure circuit” are driven to put more energy into risk, and seeking pleasure and novelty. Clinicians have noticed for years that some people seem unusually drawn to seek excitement/sensation and that they are more prone toward substance abuse – now we have a way to understand why that may be.

As Linden also notes, the heavy and repetitive substance use of addicts seems to actually change the cellular structure of their brains, in a way that tends to be irreversible. Over time, the addictive behavior actually comes to have less to do with pleasure/reward than with simply feeling OK and functioning adequately. He calls this a transformation from “liking” to “wanting.” I think “wanting” is too weak a word, and that in the addict’s experience it is closer to “needing.”

None of this is to say that addicts are without any level of choice or responsibility. Obviously, the only way anyone moves in the direction of recovery is by acknowledging and taking responsibility for his or her addiction, and making choices, developing new behaviors, etc., in an effort to get around these established but ultimately self-defeating patterns. But let’s recognize that when we advise people to give up addictive behavior, we are typically asking them to take on a herculean task, to use their intelligence and consciousness to override what has become their natural mode of brain functioning. That’s why so often they need strong support, frequent reminders and motivators, etc. And, since changed brains don’t revert to their pre-addictive structure, it is for good reason that the term “recovering” has replaced the term “recovered.”