The brain's addiction centre

It's mid-October. For most of us, our New Year's resolutions have long been forgotten and our bad habits remain frustratingly habitual. The things that are bad for us often feel strongly compelling, be they high-fat foods, gambling or alcohol. And nowhere is the problem of addiction more widespread, serious and dangerous than the case of cigarette smoking.

Smoking is the leading preventable cause of death in the developed world, and in the UK, it kills five times more people than all non-medical causes combined. The dangers of smokers are both well-established and well-known, and surveys repeatedly show that the majority of smokers want to quit. But weaning oneself off a substance as addictive as nicotine is not easy.

People often view quitting smoking as a question of willpower - a problem of the mental world. But like all mental processes, addiction eventually boils down to physical matter, to our brains and the chemicals that reside within. Neurological studies have found that smoking causes long-term changes to various parts of the brain including the dopamine system involved in feelings of pleasure, and the amygdala, involved in emotional responses. Even cues associated with smoking such as the smell of smoke or the sight of a cigarette, can trigger distinctive patterns of activity in these areas, and are likely to contribute to the urges that smokers feel.

Now, Nasir Naqvi and colleagues from the University of Iowa have tracked down the neurons that control the addictive urges of smokers to a part of the brain called the insula. Located deep inside the brain, the insula is involved in emotion. It collects and processes sensory information from the rest of the body, and translates them into conscious emotional experiences, such as cravings, hunger or pain. And in doing this, the insula could control cravings for cigarettes in response to smoking-related cues.

Naqvi found compelling evidence for this by looking at several smokers who had suffered brain damage, often because of a stroke. Many of these smokers successfully kicked their habits, but in those with damage to their insulas, something more unusual happened.

While most people find quitting a long and difficult process, those with insula damage quit easily and immediately. They never touched a cigarette again, and most importantly, never felt the urge to do so. They completely lost their addiction to smoking and were 22 times more likely to do so than smokers with other types of brain damage.

Naqvi believes that becoming addicted to nicotine causes the insula to change, making smoking just as necessary a bodily need as hunger or thirst. The insula processes information about sights, smells and feelings that relate to smoking and anticipates both the pleasurable effects of nicotine and the negative effects of nicotine withdrawal.  The end result is a strong and conscious urge to smoke, that disappears when the insula is damaged. As one patient said, his body just "forgot the urge to smoke".

But can this knowledge be used to help smokers to quit? Directly changing the insula seems an unlikely step. The major worry is the direct treatments could have knock-on effects other aspects of a patient's life.

However, Naqvi promisingly found that none of the patients with insula damage lost their urge to eat, or the pleasure of doing so. He suspects that urges like hunger, that are essential for our survival, are controlled by multiple brain networks that act as fail-safes should any one fail. Alternatively, the insula may only control bodily cravings that people develop over time, while other brain regions deal with more instinctive drives.

Nonetheless, the knowledge that the insula is the brain's nicotine addiction centre could help in indirect ways. Scientists could look for drugs that target neurotransmitters active in the insula. And by monitoring brain activity in the insula, doctors could compare how effective some quitting methods are against others. In the mean time, smokers should bear in mind that giving up can be difficult but millions of people have successfully done it. And in doing so, they have reduced their risk of cancer, heart disease and many other illnesses.

Reference: N. H. Naqvi, D. Rudrauf, H. Damasio, A. Bechara (2007). Damage to the Insula Disrupts Addiction to Cigarette Smoking Science, 315 (5811), 531-534 DOI: 10.1126/science.1135926