ONE BY ONE, THE "WONDERFULL vertues of tobacco" (as a 1659 treatise put it) have gone up in, well, smoke. The belief that tobacco soothes the throat, cures colds and quenches thirst has been replaced with evidence that cigarettes can instead cause lung cancer, heart disease and early death. But one claimed benefit of tobacco is still standing: that tobacco-or, more precisely, nicotine-improves memory. Nicotine, at the levels circulating in a smoker's bloodstream after only a single cigarette, has long been known to increase recall in simple psychology tests. But no one knew exactly what nicotine was doing in the brain. Last week, John Dani and colleagues at Baylor College of Medicine in Houston claimed to have solved the mystery. Nicotine, they report in the journal Nature, strengthens communications between neurons in the hippocampus, a structure in the brain involved in learning and memory.
Nicotine seems to work by increasing the strength of messages zipping around the brain. These messages take the form of electrical impulses. As an impulse travels along a neuron, it eventually reaches the end. The next neuron lies across a gap, called a synapse. In order for the message to leap the synapse, it has to send an understudy: molecules that can diffuse across the gap. If enough of these molecules, called neurotransmitters, reach the neuron on the other side, they spark an electrical impulse in it, and the message continues on through the brain circuit. Nicotine, the Baylor team found, increases the amount of neurotransmitters released (diagram). That greatly increases the odds that the message will reach the neuron on the other side. The more messages that get through, the more the neurons in a circuit change, becoming the physical embodiments of a memory. In fact, a 1991 study found that the risk of Alzheimer's disease is lower in smokers. Another study reported last week, by neurologists at Case Western Reserve University, found that nicotine seems to inhibit, in the test tube, formation of the plaques that gum up the brains of Alzheimer's patients.
The problem with using nicotine as a memory aid, of course, is that "the delivery system contains 400 known carcinogens," as biologist Daniel McGehee of the University of Chicago puts it. But people at risk for Alzheimer's, or just forgetfulness, may not have to choose between cancer and senility. Last year a study found that nicotine delivered by a transdermal patch improves the performance of Alzheimer's patients on learning tests.
Both the hippocampus research and the Alzheimer's study were funded in part by tobacco money, from the Smokeless Tobacco Research Council and Philip Morris. At first blush, it seems that the tobacco industry would reap the benefits of reports about "good" nicotine. But in fact the discoveries, by pinpointing how nicotine acts in the brain, strengthen the argument that nicotine is a drug, and an addictive one. That is the conclusion the industry is working madly to avoid, since it threatens ever tighter government regulation.
Nicotine has long been known to improve recall. Now scientists have discovered how it actually works in the brain: it strengthens nerve signals and thus improves memory.
1. Nicotine binds to receptors on a transmitting neuron. This lets more calcium ions rush into the neuron.
2. The calcium triggers release of more neurotransmitters. They dock with receptors on the receiving neuron.
3. The docking makes the signal travel down the neuron, strengthening links between neurons-and memories.