ByÊAdi Jaffe, Ph.D.
Pleasure center activation is only part of the picture in addiction
Firstly, supporters of the notion that addiction is, at least partly, an outcome of specificÊbrain function point not only to pleasure center activation, but also to a whole host of findings showingÊgenetic variability that is either protective from, or a risk factor for, dependence onÊdrugs and likely also behavioral addiction likeÊeating disorders,Êcompulsive gambling, and maybeÊsex addiction as well (you can start out looking upÊALDH2-2 variability and alcoholism andÊcocaine addiction,ÊDRD4 and stimulant addiction, and many more).
While it is true that all those who consume addictive substance activate the brain similarly, there are considerable differences in the specific of that activation in reaction to drugs. Some release moreÊdopamine while others have more ÒactiveÓ versions of specific importantÊreceptors; neurotransmitterÊrecycling is quick in some, but not all, andÊdrug metabolism is different in different individuals in ways that have been shown to be important not just for addiction risk, but also for the probability of treatment success. Just look at theÊnicotine and CPY26 literature for an example. ItÕs right there.
Additionally an entire body of literature exists that shows differential activation, as well as structural differences, between addicts and non-addicts in regions as varied as theÊOFC,ÊPFC,ÊInsula, and more. This is not to mention a slew of evidence that shows different behavioral test performance onÊrisk-taking,Êimpulsivity, and delay-discounting, allÊpersonality variables highly associated with addiction. If one simply ignore all of this evidence, it may be easy to believe that there is no biological explanation for these phenomena, but thatÕs just wrong.
To say thatÊmesolimbic activation (what the good doctor called Òpleasure centersÓ) is the only evidence for physiological factors in addiction is dismissive at best.
Drug addiction develops in only some drug users
The notion that not everyone who takes drugs becomes addicted is nothing close to evidence against a brain explanation for addiction. EveryoneÕs motorÐcortex, striatum, and substantia nigra (the areas of the brain responsible for movement) activate in the same way during movement, but only a small group ends up suffering from ParkinsonÕs or HuntingtonÕs disorders. One fact does not preclude the other but instead may specifically point to the fact the group which develops the disorder has somewhat different neurological functioning. Researchers arenÕt concerned with explaining whyÊall individuals can become addicted to drugs, but rather why that small subgroup developsÊcompulsive behavior. A short reading of the literature makes that fact pretty clear. Additionally, while Dr. DodesÕ claims otherwise, imaging technology HAS produced evidence explaining this ÒmysteryÓ, including differences in the waysÊaddicted smokers respond to smoking-related triggers, and an increasedÊdopamine response in cocaine addicts to cues, and well as to cocaine.
As mentioned in the motor disorder section above, ingestion of chemicals is not at all necessary for brain disorders to occur or indeed develop later in life. Dr. Dodes example of shifting addiction could be used as evidence for an underlying neurological difference just as well as it would serve to make his pointÉ Or even better. If thereÕs a faulty basic mechanism attached to rewarding behaviors, it doesnÕt really matter what the behavior is, does it?ÊSex addiction,Êgambling, and more can all be explained using a similar mechanism, though drugs of abuse may just have a more direct impact. I know, IÕve written about them all.
The Vietnam vet heroin story used by Dr. Dodes as evidence that emotional, rather than physiological, factors are responsible for addiction actually fits right in line with the notion ofÊpredisposition and underlying differences, and IÕm surprised to hear a physician point to group differences as an indicator of no neurobiological basis. Indeed, when it comes to the emotional reactivity associated with drug associated cues, normal learning literature, as well as drug-specific learning research, has revealed over and over that drug-related stimuli activate brain regions associated with drug reward in the same way that natural-reward predictors do for things like food andÊsex. Once again, these facts are part of the basic understanding of the neuroscience of learning, with or without drug abuse involvement.
My own dissertation work shows that it is very likely that only a subsection of those exposed toÊnicotine will develop abnormal learning patterns associated with that drug. However, among those, learning about drug-related stimuli (as in ÒtriggersÓ) continues in an exaggerated manner long after the other ÒnormalÓ animals have stopped learning. That sort of difference can lead to a seriously problematic behavioral-selection problem whereby drug-related stimuli are attended to, and pursued, more so than other, non-drug-related ones. If that sounds familiar, it should, since drug users continuously pursue drug-associated activities and exposures in a way that seems irrational to the rest of the world. It just might be due to such a mechanism and others like it.
Some important points about science in Dr. DodesÕ article
One very true fact about mental health pointed out by Dr. Dodes is that diseases likeÊschizophrenia, which used to be explained simply as demon possession and evidence of witchcraft can now be, to a large extent, explained by the study ofÊbehavioral neuroscience andÊcognition. The same is true forÊbipolar disorder,Êdepression,ÊADHD, and a host of other such conditions. In fact, the study of psychology has only been able to rely on technological advances that allow us to ÒseeÓ brain function for a few short decades, leading to incredible advances in the field that I think will continue. The thinking that no such advances have, or will continue to be, made in the study of addiction is, in my opinion short sighted.
As I mentioned above, I donÕt for a second think that the entire explanation for drug abuse and addiction will come from neurophysiological evidence. The doctor points out that ÒIf we could take a more accurate image of addiction in the brain, it would encompass much of the history and many of the events that make us who we are.Ó I agree that we need to advance our technology as well as expand our understanding, but I think that to discount neuroscientific explanations completely is a big mistake.