Drug cravings can be brought on by many factors, such as the sight of drugs, drug availability and lack of self-control. Now, researchers have uncovered some of the neural mechanisms involved in cigarette craving. Two brain areas, the orbitofrontal cortex and the prefrontal cortex, interact to turn cravings on or off depending on whether drugs are available, the study reports today (Jan. 28) in the journal the Proceedings of the National Academy of Sciences.

The researchers scanned the brains of 10 moderate-to-heavy smokers using functional magnetic resonance imaging (fMRI), which measures brain activity by changes in blood flow. Researchers measured activity while the participants watched video clips of people smoking as well as neutral videos. Before viewing, some subjects were told cigarettes would be available immediately after the experiment, while others were told they would have to wait 4 hours before lighting up.

When participants watched the smoking videos, their brains showed increased activity in the medial orbitofrontal cortex, a brain area that assigns value to a behavior. When the cigarettes were available immediately as opposed to hours later, smokers reported greater cravings and their brains showed more activity in the dorsolateral prefrontal cortex. The researchers hypothesize that this area modulates value. In other words, it can turns up or down the "value level" of cigarettes (or other rewards) in the first area, the medial orbitofrontal cortex. The results show that addiction involves a brain circuit important for self-control and decision-making.

Prior to some of the scans, study participants were exposed to transcranial magnetic stimulation, or TMS. This non-invasive method excites or blocks neural activity by inducing weak electrical currents in a particular region of the brain. When the dorsolateral prefrontal cortex was deactivated using TMS, there was no difference in brain activity between those who watched the smoking clips and those who watched neutral videos; those two groups also reported similarly low cravings for cigarettes.

The blocking of this brain region cut off the link between craving and awareness of cigarette availability, suggesting that suppressing the area could reduce cravings brought on by impending access to the drug.

"This is something that we've all been working on, trying to find the target in the brain that you could hit and cause somebody to stop smoking," study researcher Antoine Bechara, a neuroscientist at the University of Southern California, told LiveScience.

Scientists will quibble over the exact brain areas that are the most important targets, Bechara said, but he thinks transcranial magnetic stimulation is a useful approach. "It gives hope to be able, in a noninvasive manner, to help people quit smoking," Bechara added.

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