Facts of lifeWhat is nicotine? [Archives:2005/871/Health]

August 25 2005

By Dr. Khaled alNsour
For the Yemen Times

Nicotine is a naturally occurring liquid alkaloid. An alkaloid is an organic compound made out of carbon, hydrogen, nitrogen and sometimes oxygen. These chemicals have potent effects on the human body. For example, many people regularly enjoy the stimulating effects of another alkaloid, caffeine, as they quaff a cup or two of coffee in the morning.

Nicotine normally makes up about 5 percent of a tobacco plant, by weight. Cigarettes contain 8 to 20 milligrams (mg) of nicotine (depending on the brand), but only approximately 1 mg is actually absorbed by your body when you smoke a cigarette.

Effects of Nicotine

Nicotine changes how your brain and your body function. The net results are somewhat of a paradox: Nicotine can both invigorate and relax a smoker, depending on how much and how often they smoke. This biphasic effect is not uncommon. Although the actions of nicotine and ethanol in the body are quite different, you also see dose-dependent effects when you drink alcoholic beverages. Your first drink may loosen your inhibitions and fire you up, but after several drinks, you're usually pretty sedate.

Nicotine and the Body

Nicotine initially causes a rapid release of adrenaline, the “fight-or-flight” hormone. If you've ever jumped in fright at a scary movie or rushed around the office trying to finish a project by your deadline, you may be familiar with adrenaline's effects:

– Rapid heartbeat

– Increased blood pressure

– Rapid, shallow breathing

Adrenaline also tells your body to dump some of its glucose stores into your blood. This makes sense if you remind yourself that the “fight-or-flight” response is meant to help you either defend yourself from a hungry predator or hightail it out of a dangerous situation ) running or brawling both require plenty of energy to fuel your muscles.

Nicotine itself may also block the release of the hormone insulin. Insulin tells your cells to take up excess glucose from your blood. This means that nicotine makes people somewhat hyperglycemic, having more sugar than usual in their blood. Some people think that nicotine also curbs their appetite so that they eat less. This hyperglycemia could be one explanation why: Their bodies and brain may see the excess sugar and down-regulate the hormones and other signals that are perceived as hunger.

Nicotine may also increase your basal metabolic rate slightly. This means that you burn more calories than you usually would when you are just sitting around. However, losing weight by smoking doesn't give you any of the health benefits that you'd get if you were losing weight by exercising ) it actually does the opposite! Over the long haul, nicotine can increase the level of the “bad” cholesterol, LDL, that damages your arteries. This makes it more likely that you could have a heart attack or a stroke.

Nicotine and the Brain

Your brain is the key player in nicotine's action. Like a computer, your brain processes, stores and uses information. In a computer, information travels in the form of electricity moving through wires; information transfer is a binary process, with switches being either “on” or “off.” In your brain, neurons are the cells that transfer and integrate information. Each neuron has thousands of inputs from other neurons throughout the brain. Each of these signals is included in the calculation of whether or not the neuron will pass the signal it receives on to other neurons in the pathway. While signals are conducted through individual neurons as electric current, communication between neurons is mediated by chemical messengers, called neurotransmitters. Neurotransmitters traverse the physical space between two neurons and bind to special protein receptors on the postsynaptic cell. Once bound, these receptors set in motion physiological changes within the neuron that allow it to send the signal on down the line.

Each neurotransmitter has its own specific family of receptors. Nicotine works by docking to a subset of receptors that bind the neurotransmitter acetylcholine. Acetylcholine is the neurotransmitter that (depending on what region of the brain a neuron is in):

– Delivers signals from your brain to your muscles

– Controls basic functions like your energy level, the beating of your heart and how you breathe

– Acts as a “traffic cop” overseeing the flow of information in your brain

– Plays a role in learning and memory

Like acetylcholine, nicotine leads to a burst of receptor activity. However, unlike acetylcholine, nicotine is not regulated by your body. While neurons typically release small amounts of acetylcholine in a regulated manner, nicotine activates cholinergic neurons (which mainly use acetylcholine to communicate to other neurons) in many different regions throughout your brain simultaneously. This stimulation leads to:

– Increased release of acetylcholine from the neurons, leading to heightened activity in cholinergic pathways throughout your brain. This cholinergic activity calls your body and brain to action, and this is the wake-up call that many smokers use to re-energize themselves throughout the day. Through these pathways, nicotine improves your reaction time and your ability to pay attention, making you feel like you can work better.

– Stimulation of cholinergic neurons promotes the release of the neurotransmitter dopamine in the reward pathways of your brain. This neural circuitry is supposed to reinforce behaviors that are essential to your survival, like eating when you're hungry. Stimulating neurons in these areas of the brain brings on pleasant, happy feelings that encourage you to do these things again and again. When drugs like cocaine or nicotine activate the reward pathways, it reinforces your desire to use them again because you feel so at peace and happy afterwards.

– Release of glutamate, a neurotransmitter involved in learning and memory – Glutamate enhances the connections between sets of neurons. These stronger connections may be the physical basis of what we know as memory. When you use nicotine, glutamate may create a memory loop of the good feelings you get and further drive the desire to use nicotine.

Nicotine also increases the level of other neurotransmitters and chemicals that modulate how your brain works. For example, your brain makes more endorphins in response to nicotine. Endorphins are small proteins that are often called the body's natural pain killer. It turns out that the chemical structure of endorphins is very similar to that of heavy-duty synthetic painkillers like morphine. Endorphins can lead to feelings of euphoria also. If you're familiar with the runner's high that kicks in during a rigorous race, you've experienced the “endorphin rush.” This outpouring of chemicals gives you a mental edge to finish the race while temporarily masking the nagging pains you might otherwise feel.

Nicotine in the Body

As with most addictive substances, humans have devised a number of ways of delivering nicotine to their bodies. Nicotine readily diffuses through:

– Skin

– Lungs

– Mucous membranes (such as the lining of your nose or your gums)

Nicotine moves right into the small blood vessels that line the tissues listed above. From there, nicotine travels through your bloodstream to the brain, and then is delivered to the rest of your body.

The most common (and the most expedient way) to get nicotine and other drugs into your bloodstream is through inhalation ) by smoking it. Your lungs are lined by millions of alveoli, the tiny air sacs where gas exchange occurs. These alveoli provide an enormous surface area ) 90 times greater than that of your skin ) and thus provide ample access for nicotine and other compounds. Once in your bloodstream, nicotine flows almost immediately to your brain. Although nicotine takes a lot of different actions throughout your body, what it does in the brain is responsible for both the good feelings you get from smoking, as well as the irritability you feel if you try to quit. Within 10 to 15 seconds of inhaling, most smokers are in the throes of nicotine's effects.

Nicotine doesn't stick around your body for too long. It has a half-life of about 60 minutes, meaning that six hours after a cigarette, only about 0.031 mg of the 1 mg of nicotine you inhaled remains in your body.

How does your body get rid of nicotine? Here's the process:

– About 80 percent of nicotine is broken down to cotinine by enzymes in your liver.

– Nicotine is also metabolized in your lungs to cotinine and nicotine oxide.

– Cotinine and other metabolites are excreted in your urine. Cotinine has a 24-hour half-life, so you can test whether or not someone has been smoking in the past day or two by screening his or her urine for cotinine.

– The remaining nicotine is filtered from the blood by your kidneys and excreted in the urine.

Different people metabolize nicotine at different rates. Some people even have a genetic defect in the enzymes in their liver that break down nicotine, whereby the mutant enzyme is much less effective at metabolizing nicotine than the normal variant. If a person has this gene, their blood and brain nicotine levels stay higher for longer after smoking a cigarette. Normally, people keep smoking cigarettes throughout the day to maintain a steady level of nicotine in their bodies. Smokers with this gene usually end up smoking many fewer cigarettes, because they don't constantly need more nicotine.

After knowing all this, do you still want to smoke? Think again!