WEDNESDAY, Aug. 3 (HealthDay News) -- Changes in brain areas that control mood may explain why some heavy smokers become depressed after they quit smoking, which increases their risk of resuming smoking.

That's the finding of Canadian researchers who conducted PET brain scans on 24 nonsmokers, 12 moderate smokers (15 to 24 cigarettes a day) and 12 heavy smokers (25 or more cigarettes a day).

The scans revealed that after quitting smoking, heavy smokers had an elevation of the enzyme monoamine oxidase A (MAO-A) in regions of the brain that regulate mood, such as the prefrontal cortex (24 percent increase) and the anterior cingulate cortex (33 percent increase).

Previous research has linked elevated MAO-A with depression.

Researchers did not find increased levels of MAO-A, which metabolizes mood-enhancing chemicals, in nonsmokers or moderate smokers after they kicked the habit.

The heavy smokers also reported experiencing a depressed mood after they quit smoking.

The study appears in the August issue of the journal Archives of General Psychiatry.

"Understanding the neurobiology of heavy cigarette smoking is important because those who smoke heavily are much more likely to have major depressive disorder and to experience medical complications resulting from cigarette smoking," wrote Ingrid Bacher and colleagues at the Centre for Addiction and Mental Health in Toronto in a journal news release.

They recommended clinical trials to test the effects of MAO-A- inhibiting drugs in heavy smokers immediately after they quit smoking.

More information

The American Cancer Society offers a guide to quitting smoking.