Nicotine (cont.)

With each puff of a cigarette, a smoker pulls nicotine and other harmful substances into the lungs, where it is absorbed into the blood. It takes just 8 seconds for nicotine to hit the brain. Nicotine happens to be shaped like the natural brain chemical acetylcholine. Acetylcholine is one of many chemicals called neurotransmitters that carry messages between brain cells. Neurons (brain cells) have specialized proteins called receptors, into which specific neurotransmitters can fit, like a key fitting into a lock. Nicotine locks into acetylcholine receptors, rapidly causing changes in the brain and body. For instance, nicotine increases blood pressure, heart rate, and respiration (breathing).

Nicotine also attaches to acetylcholine receptors on neurons that release a neurotransmitter called dopamine. Dopamine is released normally when you experience something pleasurable like good food, your favorite activity, or the company of people you love. But smoking cigarettes causes neurons to release excess dopamine, which is responsible for the feelings of pleasure. However, this effect wears off rapidly, causing people who smoke to get the urge to light up again for another dose of the drug.

Nicotine may be the primary addictive component in tobacco but it's not the only ingredient that is biologically important. Using advanced neuroimaging technology, scientists have found that people who smoke have a significant reduction in the levels of an enzyme called monoamine oxidase (MAO) in the brain and throughout the body. This enzyme is responsible for the breakdown of dopamine, other neurotransmitters involved in mood regulation, and in a variety of bodily functions. Having lower amounts of MAO in the brain may lead to higher dopamine levels and be another reason that people who smoke continue to do so -- to sustain the pleasurable feelings that high dopamine levels create.

Also, researchers have recently shown in animals that acetaldehyde, another chemical constituent of tobacco smoke, dramatically increases the rewarding properties of nicotine -- particularly in adolescent animals -- which may be one reason why teens are more vulnerable to becoming addicted to tobacco than adults.

Source: MedicineNet.com
http://www.medicinenet.com/nicotine/article.htm