Addiction: Why do some people become addicted, while others do not?

Guest Blog by Dr. Helen Nasser / Postdoctoral Research Fellow at the National Institute of Health

Drinking a glass of scotch or eating a piece of cake — these are the types of behaviors many of us take enjoyment in. The vast majority indulge in these behaviors on occasion. For some of us, these behaviors become more than casual indulgences and can develop into alcoholism or overeating.

Addiction itself is a chronic, often relapsing brain disease that causes the compulsion of seeking, despite harmful consequences. We are still don’t understand why and how addiction develops, and why some of us become addicted while others don’t.

But We Are Starting to Get an Idea of Why Addiction Happens More to Some Than Others

Scientists have identified subpopulations of rats that exhibit distinct motivational behaviors that might shed light on why some of us find ourselves on the slippery slope of addiction.

Within the rodent population, there are animals that become powerfully attracted to cues that signal reward, whilst for others the cue seems to have no special influence. The identification of these subpopulations might have substantial influence on how we understand and treat addiction disorders.

Hooked on cues

Addiction therapists frequently encounter patients who desperately want to quit using drugs, alcohol, or overeating but experience a triggered relapse. What do I mean by that? If you think about it, you will realize there are cues in the environment that predict rewards, and therefore can drive our behavior.

Seeing the golden arches of the MacDonald’s sign can trigger cravings for food. Smelling cigarette smoke can trigger cravings for cigarettes.

Severely overweight people are more attracted to food signals than people of healthy weights, whilst smokers who are at high risk of relapse have increased brain activity in the presence of smoking cues than neutral cues when compared to those who are less likely to relapse.

It’s not clear why some people might be more focused on cues than others, nor how that difference might contribute to overeating or addiction.

A Clinical Example of Addiction

Susceptibility to cues is identified in a simple Pavlovian task, where a metal lever inserted into the rats’ cage signals the arrival of a tasty food treat.

The animals quickly learn the lever signals reward, and upon its insertion the animals will run over to where the food will be delivered. Some sort of serious interaction with this lever is not instrumental to the delivery of reward – they will get the tasty treat regardless.

After several days of this type of training, a new pattern of behavior forms. Some rats stop running to where the food will be delivered and start engaging with the lever. These rats will lick and gnaw at the metal lever as if it were food itself.

Psychologists named this “maladaptive behavior” in 1961, and they have demonstrated it across an array of species, including rats, birds, fish, monkeys and humans.

In more recent times, they have called this behavior “sign tracking,” whereby the cue itself becomes attractive and the individual is willing to work simply for the presentation of that cue alone, without the delivery of food (Robinson & Flagel, 2009). This observation has serious implications pertaining to the relapse of addiction.

If particular people can be triggered by cues — rather than the reward or drug itself — then they are more likely to relapse. This means traditional models of therapy such as cue exposure therapy are less likely to be effective.

Terry Robinson and his group over at the University of Michigan have demonstrated that sign-tracking rats, otherwise known as sign-trackers, are more sensitive to cues that predict drugs. Sign-trackers will work harder to receive presentation of the cue (evidence of a possible reward) even when the drug (reward) is not available. This research might be relevant to human addiction.

In my current lab, we have been able to identify that sign-tracking animals still engage with the cue even when the food it was associated with is paired with gastric malaise, an unpleasant sensations. This inability to adapt to changes in the value of the reward, in spite of potential negative consequences, suggests why relapse occurs more readily in sign-tracking animals than the rest.

The Brain’s Obsession

The brain is activated by cues that signal the availability of drugs and rewards. We are currently investigating whether sign-trackers use different brain pathways when presented with rewarding cues. Terry Robinson’s group has evidence that dopamine release in the nucleus accumbens, a region that is critical in responding to rewards, is higher when a cue is presented to sign-tracking rats compared to those who are not. This suggests the brain’s sensitivity to rewarding cues might differ in sign-tracker compared to others.

Researchers are only beginning to translate the findings to humans. This research could open up new avenues for preventing and treating addiction, but the link between this procedure and human addiction has yet to be established.

This article was prepared by Dr. Helen Nasser in her personal capacity. The opinions expressed in this article are the author’s own and do not reflect the view of the National Institute of Health, the Department of Health and Human Services or the United States government.

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