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Nicotine and Your Brain

How smoking may harm aspects of memory.

One day, long ago, a tobacco plant discovered humans and it has never relinquished its control over our brain. During the intervening millennia, we humans have tried to find novel uses for this plant in order to distract us from being a slave to its actions in our brain. Tobacco was once used to treat persistent headaches, colds, and abscesses or sores on the head. Tobacco enemas were used to treat flatulence, and even more surprising, the smoke was once inhaled deeply in order to lessen bad coughs.

In 1560, Jean Nicot, (then the French Ambassador to Portugal), sent some tobacco to Catherine de Medici, who was then queen to Henry II of France; she reported that it helped treat her migraines. The plant initially was given the title of herbe sainte, or holy plant, and then later, was dubbed Herba Regina, the queen's herb. Nicot got the credit for the discovery and Von Linné named the genus Nicotiana in his honor. Despite all this royal glory, in the 1890s the U.S. pharmacopeia dropped nicotine from its list of useful therapeutic agents.

A cigarette made from tobacco will contain about 1 to 2 milligrams of nicotine. Because nicotine is quite volatile and heat labile, only about 20% of it is actually inhaled into the body. However, because of its exceptional lipid solubility, at least 90% of the inhaled nicotine is absorbed into the body. Nicotine can also be rapidly absorbed by the mouth or intact skin. Once the smoke is inhaled, absorption via the lungs and transport to the brain occurs within 2 to 7 seconds. This makes smoking tobacco as efficient as an intravenous injection in terms of getting nicotine to its site of action within the brain. This speed of entry into the body may also underlie why nicotine is so toxic. Sixty milligrams is considered a lethal dose for a human; death takes only a few minutes to occur and results from a loss of control of the nicotinic receptors on the diaphragm muscles.

Nicotine affects cortical function in a complex dose-dependent fashion; low doses tend to activate the left hemisphere and produce mental stimulation and a feeling of arousal, whereas high doses tend to activate the right hemisphere more strongly and are associated with the sedative effects of nicotine. Therefore, when doing boring tasks, you could take a low dose of nicotine by, say, smoking one cigarette and could increase your subjective feelings of arousal and attention. In contrast, during anxious or stressful situations, you could take a high dose of nicotine by chain-smoking and actually reduce your stress by activating the right hemisphere and producing a bit of sedation.

These findings nicely demonstrate the competing roles of nicotine receptors in the two hemispheres and tell us something profound about how the two halves of the brain normally function to produce a balance of emotions, attention, and arousal. Moreover, 60% of adults diagnosed with attention deficit disorder smoke cigarettes as compared to less than 30% of the rest of the population, another interesting finding that indicates that acetylcholine nicotinic receptors play an important role in paying attention. People with serious mental illness often are heavy smokers. They may be using nicotine to balance the activity between their two hemispheres and thereby lessen the severity of their symptoms.

A recent study published in the journal Drug and Alcohol Dependence investigated the effect of smoking on our ability to remember to carry out activities at some future point in time; this is called prospective memory. For example, we use prospective memory to remember to meet friends for dinner, to stop by the grocer on the way home from work, or to take a medication at the correct time each day. Prospective memory plays an important role in our daily routine.

The study examined 78 people between the ages of 18 and 35 years; thus, they were relatively young and not likely to be influenced by age-related memory impairments. The "smokers" averaged 65 cigarettes per week and had been smoking an average of 9 years. The results indicated that the persistent smokers had significant impairments in prospective memory. Thus, if you're a smoker and you're young but are already experiencing memory lapses similar to your aged or demented relatives, you have yet another reason to give up smoking. However, you might wish to use your remaining cigarettes to control any bothersome flatulence!

© Gary L. Wenk, Ph.D., author of Your Brain on Food (Oxford, 2010)