Nicotine: Cigarettes' Most Addictive Poison

Penned several decades ago, those lyrics to a popular novelty song hint humorously at what we know to be a deadly serious problem: tobacco addiction and the multitude of associated health risks. The age-old question is, why -- in the face of overwhelming evidence linking tobacco to diseases like cancer, heart disease and emphysema -- millions continue to smoke. What kind of mind games is tobacco playing on the psyche of its user?

The chief culprit is nicotine, the addictive alkaloid produced by and found in the leaves of the tobacco plant. Surprisingly, the nicotine of each plant is fairly high, on average comprising about 4 percent of the weight of the plant. By the time manufacturers process the leaves, cigarettes end up containing 7 to perhaps 20 milligrams of nicotine each, with the average smoker normally taking about 1 milligram into the body per cigarette. That's highly significant, given that just 60 milligrams, the amount in only three cigarettes of some brands, would be fatal if taken at once.

Nicotine is a poison produced by certain plants as part of their self-defense strategy to "discourage" insects from nibbling away at plant tissues. Nicotine is in fact such a potent toxin that it is sold commercially as a pesticide.

Chemically, nicotine is composed of innocuous enough elements: carbon, hydrogen and oxygen atoms, the same ones that make up sugar, caffeine and thousands of other substances considered both harmless and harmful and, in some cases, necessary for good health. But as with any substance, it's the way those atoms are arranged and configured that makes the difference. Many enslaving substances work their sorcery by mimicking the action of naturally occurring biological agents, and nicotine is no exception. When brain cells communicate, the very essence of brain function, impulses travel from one cell to another or from one to many. Brain cells are separated from one another by a gap called a synapse, and if communication between cells is to occur at all, that impulse has to be able to bridge the space between cells. The impulse needs help, so chemicals called neurotransmitters are secreted into the gap.

These substances provide the conduit over which the impulses travel from cell to cell. Acetylcholine is one of these neurotransmitters. When an impulse reaches the end of one nerve fiber, acetylcholine is released into the gap. The acetylcholine molecules migrate across the space and lock into the receptor sites on the next nerve fiber. That action stimulates the second neuron fiber, causing it continue the propagation of the original impulse.

Enter nicotine. It's molecules are just the right shape to mimic acetylcholine. Nicotine molecules are so good at this, they compete quite effectively with the acetylcholine molecules for those docking site receptors. When acetylcholine locks onto a receptor site, the nerve fibers become stimulated for no reason other than the fact that nicotine has decided to take up residence there, too. This extraneous stimulation may take many forms and may cause other secondary reactions. The overstimulation may increase alertness, decrease reflex reaction time and cause a general feeling of elation or excitement.

Nicotine may also be addicting because it causes the release of another neurotransmitter, dopamine, which is associated with feelings of pleasure and well being. Glutamate is also set free by nicotine. It is thought to strengthen neural connections and thus enhance learning and memory function.

Outside the central nervous system, nicotine may cause the release of adrenaline, the "flight/fight" hormone. Increased levels of adrenaline produce even more stimulation. Also, nicotine may block the release of insulin from the pancreas, an action which increase blood sugar levels.

Of course nicotine is not the only harmful substance in tobacco smoke. It contains dozens of toxins, various tars, cyanide and carbon monoxide. Nicotine, though, is the constituent that seems to reinforce the habitual behavior.