Why do treatments for the symptoms of the common cold make us drowsy? How does coffee work? This chapter touches briefl y on neurotransmitters whose actions in the brain affect our sleep–wake cycle and on a few well-known substances that block these effects. HISTAMINE AND OREXIN One such neurotransmitter is histamine, whose neurons infl u-ence our level of arousal throughout the day. Lying next to these neurons is another group of neurons that release orexin, which is a neurotransmitter that infl uences both our level of arousal and craving for food. Take a moment to appreciate how the anatomical organization of this system optimizes your daily .
" ADENOSINE " This neurotransmitter has diverse functions throughout the brain that are also related to our sleep–wake cycles. We know a lot about it because of the ready availability of a very safe, highly effective adenosine receptor antagonist that is served hot or cold, with or without cream, throughout the world — caffeinated coffee! Caffeine is also commonly found with theophylline (a molecule that is very similar to caffeine) in tea. Indeed, although caffeine is found in at least 63 plant species, 54% of the world’s SLEEPING VERSUS WAKING s 125consumption derives from just two different beans, Coffea arabicaand Coffea robusta , and 43% derives from the tea plant Camellia sinensis . Coffee is rich in biologically active substances such as trigo-nelline, quinolinic acid, tannic acid, and pyrogallic acid. The vitamin niacin is formed in great amounts from trigonelline during the coffee bean roasting process. Coffee is also a rich source of the antioxidants caffeic, chlorogenic, coumaric, fer-rulic, and sinapic acids and silverskin. Various ingredients in coffee beans contribute to aspects of the drink — for example, its bitterness — that people fi nd either appealing or unpleasant. Recently, some entrepreneurs have found a way to remove the bitterness by “fi ltering” coffee beans through the gastrointesti-nal tract of the Asian Palm Civet, Paradoxurus hermaphroditus . The civets, nocturnal omnivores that are about the size of a cat, eat the beans, which then pass through the animals’ gastrointestinal systems undigested but presumably not unaffected. The beans are then extracted from the animals’ stool, cleaned up, and sold. It’s hardly an enticing process, but the claim is that the animal’s digestive enzymes metabolize the proteins that cause the bitter taste of the coffee bean. Although this is certainly possible, the novel fl avor of the beans is just as likely a result of the bean’s absorption of some of the less appealing contents of the animals’ gut. Coffee drinking (or consuming caffeine from non-coffee sources) has been associated with a signifi cantly lowered risk of developing Parkinson’s disease. The neuroprotective effect 126 S YOUR BRAIN ON FOODrequires about fi ve to six cups of coffee per day for many years and appears to be mostly benefi cial only to males. Women ben-efi t from coffee-drinking in other ways, particularly with regard to a reduced incidence of type-2 diabetes. Overall, people who drink substantial amounts of coffee daily tend to live longer than people who do not. In addition, recent evidence suggests that moderate coffee-drinking of about two to three cups each day might reduce your chance of developing Alzheimer’s dis-ease. What is the connection among coffee, diabetes, and dis-eases of the brain? No one is sure, but elevated insulin levels in the blood may be a critical link because type-2 diabetes makes both men and women more likely to develop both Parkinson’s and Alzheimer’s disease. Many people drink coffee to reduce drowsiness. How does caffeine achieve this effect in the brain? The answer begins with a consideration of the function of the acetylcholine neurons that control your ability to pay attention. Adenosine negatively controls the activity of these neurons, meaning that when ade-nosine binds to its receptor on acetylcholine neurons, their activity slows. The production and release of adenosine in your brain is linked to metabolic activity while you are awake. Therefore, the concentration of adenosine in the neighborhood of acetylcholine neurons increases constantly while your brain is active during the day. As the levels of adenosine increase, they steadily inhibit your acetylcholine neurons, your brain’s activity gradually slows, and you begin to feel drowsy and ultimately fall asleep. Caffeine comes to the rescue because it, like theophylline SLEEPING VERSUS WAKING s 127from tea, is a potent blocker of adenosine receptors and, there-fore, of the adenosine-driven drowsiness and sleep. One can take this too far, however. One of my students decided to test these caffeine effects by ingesting a packet of instant coffee, right out of the box. He reported that he enjoyed eating it so much that he decided to fi nish off the entire container of 32 packets! Three days later, he stopped having explosive diar-rhea and fi nally fell asleep completely exhausted. Given everything that you’ve read about drugs that produce a rewarding and euphoric feeling, you might suspect that coffee also somehow affects dopamine neurons. You would be correct. Caffeine sets free the activity of dopamine neurons to bring euphoria and bliss to every cup of coffee or every glass of cola. Most cans of cola contain about 40 milligrams of caffeine; therefore, most teenagers consume as much caffeine as their parents — the only thing that differs is the vehicle for the drug. The widespread availability of foods containing caffeine has led experts to suggest that 80% of all people in North America have measureable levels of caffeine in their brains from embryo to death. Caffeine and theophylline are not the only drugs we regu-larly consume to block our adenosine receptors. There is also theobromine, which is found in chocolate. Chocolate is as addicting as coffee — if not more so — possibly because it con-tains an array of other psychoactive compounds that may con-tribute to the pleasurable sensation of eating it. Chocolate contains fats that may induce the release of endogenous opiates
128 S YOUR BRAIN ON FOOD( see Chapter 8) and produce a feeling of euphoria. It contains phenethylamine, a molecule that resembles amphetamine and some of the other psychoactive stimulants discussed earlier. It contains a small amount of the marijuana-like neurotransmitter anandamide. It contains some estrogen-like compounds, a fact that may explain a recent series of reports showing that men who eat chocolate live longer than men who do not eat choco-late (the effect was not seen for women who have an ample supply of their own estrogen until menopause). Chocolate also contains magnesium salts, the absence of which in elderly females may be responsible for the common post-menopausal condition known as chocoholism. And fi nally, a standard bar of chocolate contains as many anti-oxidants as a glass of red wine. Clearly, there are many good reasons for men and women to eat chocolate to obtain its indescribably soothing, mellow, and yet euphoric effect, with or without the addition of caffeine. My fear, of course, is that one day the Food and Drug Administration may take notice of the many psychoactive compounds present in chocolate and regulate its sale.
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