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% increase) and the anterior cingulate cortex (33% increase).
Previous research has linked elevated MAO-A with depression.
Depression and smoking
Researchers did not find increased levels of MAO-A, which metabolises 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 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.
They recommended clinical trials to test the effects of MAO-A- inhibiting drugs in heavy smokers immediately after they quit smoking.
The American Cancer Society offers a guide to quitting smoking.
(Copyright © 2010 HealthDay. All rights reserved.)