Is Smoking Cigarettes Addictive?
50 million Americans smoke cigarettes. In the face of clear evidence that cigarettes cause lung cancer, outlined in a previous article in this series, why dont smokers quit? Many of these individuals say they would like to, but cant; they simply find it too difficult to overcome the habit. Anyone who has ever smoked cigarettes knows how difficult it is to quit. Most studies indicate a quitting success rate -- at least two years abstinence -- of about 20 percent. Despite this widespread experience, cigarette manufacturers have steadfastly refused to admit that smoking cigarettes is addictive, even swearing so under oath before congress last year. However, the maker of Chesterfield cigarettes, Liggett, has recently broken ranks with the other tobacco companies and in a legal settlement agreed that cigarettes cause cancer and are addictive. A tobacco spokesperson, addressing the question in the wake of the Liggett settlement, quibbled that the question has no answer, as addiction means different things to different people. Who is right? Is nicotine, the key ingredient, an addictive drug?
How Psychoactive Drugs Affect the Brain
To understand the nature of addiction, you must focus your attention on how nerves in the brain communicate with one another, because it is in altering this process that chemicals like nicotine have their effect. Unlike electrical wires in a house, the nerve cells in the brain are not physically connected to one another. They are separated from one another by tiny gaps. For a signal to pass from one nerve cell to another within the brain, it must cross the space that separates the two cells. How is this achieved? By shooting chemicals across the gap! Called neurotransmitters, these chemicals bind to specific receptor proteins embedded within the cell membrane on the far side of the gap. The binding of a neurotransmitter to a matching stimulatory receptor promotes the generation of a new signal in the receiving nerve cell, thus successfully transferring the signal across the gap.
Investigators studying mind-altering drugs soon learned that mood, pleasure, and other mental states are determined by particular groups of nerves in the brain that use special sets of neurotransmitters and receptors. Much of the early work was driven by attempts to understand and treat depression. Researchers found that the mood-elevating nerve pathways of depressed individuals appeared to have too little of the neurotransmitter seratonin to function effectively. With too little seratonin in the gaps between nerve cells, the target receptors on the receiving nerve cells dont fire enough to keep the mood-elevating pathway active, and depression results. Attempts to treat depression by administering extra seratonin failed -- there were too many side effects.
Success was finally achieved with drugs that magnify the effects of the seratonin molecules already present. After each seratonin molecule has had a chance to transmit the signal across the gap by hitting a target receptor, it is either destroyed or reabsorbed by the nerve cell that released it. Antidepressant drugs like Prozac (c) block the reabsorption of seratonin. If it is not removed from the gap, the seratonin neurotransmitter molecules just keep smashing into receptors on the far side, triggering them to fire the nerve cell receiving the signal again and again.
From this research into depression a general rule emerged: Mind-altering drugs often work by prolonging the time the neurotransmitter persists in the gaps between nerves. They increase the number of hits of target receptors by simply allowing the neurotransmitter molecules to keep on shooting. Just as in a basketball game, the score increases if the game goes into overtime.
Search for the Chemical Nature of Addiction
The deep lesson learned from the studies of depression is that it is possible to understand mind-altering events in the brain at a molecular level. Part of a wave of research into the chemistry of the brain in recent decades, it sparked new investigations into many problems, one of them the chemical nature of drug addiction.
An immediate focus of research was the highly addictive drug cocaine. Cocaine affects nerve cells of the brains pleasure pathways (the so-called limbic system). These cells transmit pleasure messages using the neurotransmitter dopamine. Each cell communicates with the next by releasing dopamine into the gap, like pellets from a shotgun blast; the cell receiving the signal possess targets (the receptor proteins) that the pellets hit. The more receptor targets present on the surface of the receiving cell, the more likely a hit will occur, passing the signal to the receiving cell.
Investigators soon learned how cocaine stimulates the pleasure pathways to increase their rate of firing. Using radioactively-labelled cocaine molecules, they found that cocaine binds tightly to the carrier proteins in the gap between nerves that normally removes the neurotransmitter dopamine after it has acted. Like a game of musical chairs in which all the chairs become occupied, there are no unoccupied carrier proteins available to the dopamine molecules, so they stay in the gap, firing the receptors again and again. As new signals arrive, more and more dopamine is added, firing the pleasure pathway more and more often.
When the cells of your body are exposed to chemical signals for a prolonged period of time, they tend to loose their ability to respond to the stimulus with its original intensity. When you put on a wristwatch, how long are you aware you are wearing it? Nerve cells are