Smoking VS Non-Smoking

The illustration shows the concentration of radioactive tracer bound to monoamine oxidase B (MAO B). Red shows the highest concentration. Clearly, lower concentrations are seen in the smoker. In certain areas, such as the lungs and brain, concentrations are so low as to be virtually absent. This demonstrates decreased amounts of MAO B in the peripheral organs of smokers compared with nonsmokers.
Proceedings of the National Academy of Sciences, September 8, 2003, "Low Monoamine Oxidase B in Peripheral Organs in Smokers."

Disorders resulting from MAO dysfunction
Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction (too much/too little MAO activity) is thought to be responsible for a number of neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with depression, substance abuse, criminality, attention deficit disorder, and social phobias. Monoamine oxidase inhibitors are one of the major classes of drug prescribed for the treatment of depression.
Recent PET research has shown that MAO is also heavily depleted by tobacco use