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Nicotine is an alkaloid found in the nightshade family of plants (Solanaceae), predominantly in tobacco, and in lower quantities in tomato, potato, eggplant (aubergine), and green pepper. Nicotine alkaloids are also found in the leaves of the coca plant. Nicotine constitutes 0.3 to 5% of the tobacco plant by dry weight, with biosynthesis taking place in the roots, and accumulating in the leaves. It is a potent neurotoxin with particular specificity to insects; therefore nicotine was widely used as an insecticide in the past, and currently nicotine derivatives such as imidacloprid continue to be widely used. In lower concentrations (an average cigarette yields about 1mg of absorbed nicotine), the substance acts as a stimulant in mammals and is one of the main factors responsible for the dependence-forming properties of tobacco smoking. According to the American Heart Association, "Nicotine addiction has historically been one of the hardest addictions to break."[1]
History and nameNicotine is named after the tobacco plant Nicotiana tabacum, which in turn is named after Jean Nicot, a French ambassador, who sent tobacco and seeds from Portugal to Paris in 1550 and promoted their medicinal use. Nicotine was first isolated from the tobacco plant in 1828 by German chemists, Posselt & Reimann. Its chemical empirical formula was described by Melsens in 1843, and it was first synthesized by A. Pictet and Crepieux in 1893. Chemistry
PharmacologyPharmacokineticsAs nicotine enters the body, it is distributed quickly through the bloodstream and can cross the blood-brain barrier. On average it takes about seven seconds for the substance to reach the brain when inhaled. The half life of nicotine in the body is around two hours[2]. The amount of nicotine inhaled with tobacco smoke is a fraction of the amount contained in the tobacco leaves (most of the substance is destroyed by the heat). The amount of nicotine absorbed by the body from smoking depends on many factors, including the type of tobacco, whether the smoke is inhaled, and whether a filter is used. For chewing tobacco, often called dip, snuff, or snus, which is held in the mouth between the lip and gum, the amount released into the body tends to be much greater than smoked tobacco. Nicotine is metabolized in the liver by cytochrome P450 enzymes (mostly CYP2A6, and also by CYP2B6). A major metabolite is cotinine. PharmacodynamicsNicotine acts on the nicotinic acetylcholine receptors. In small concentrations it increases the activity of these receptors, among other things leading to an increased flow of adrenaline (epinephrine), a stimulating hormone. The release of adrenaline causes an increase in heart rate, blood pressure and respiration, as well as higher glucose levels in the blood. The sympathetic nervous system, acting via splanchnic nerves to the adrenal medulla, stimulates the release of epinephrine. Acetylcholine released by preganglionic sympathetic fibers of these nerves acts on nicotinic acetylcholine receptors, causing cell depolarization and an influx of calcium through voltage-gated calcium channels. Calcium triggers the exocytosis of chromaffin granules and thus the release of epinephrine (and norepinephrine) into the bloodstream[citation needed]. Cotinine is a byproduct of the metabolism of nicotine which remains in the blood for up to 48 hours and can be used as an indicator of a person's exposure to smoke. In high doses, nicotine will cause a blocking of the nicotinic acetylcholine receptor, which is the reason for its toxicity and its effectiveness as an insecticide.[citation needed] In addition, nicotine increases dopamine levels in the reward circuits of the brain. Studies have shown that smoking tobacco inhibits monoamine oxidase (MAO), an enzyme responsible for breaking down monoaminergic neurotransmitters such as dopamine, in the brain. It is currently believed that nicotine by itself does not inhibit the production of monoamine oxidase (MAO), but that other ingredients in inhaled tobacco smoke are believed to be responsible for this activity. In this way, it generates feelings of pleasure, similar to that caused by cocaine and heroin, thus causing the addiction associated with the need to sustain high dopamine levels. Psychoactive EffectsNicotine's mood-altering effects are different by report. First causing a release of glucose from the liver and epinephrine (adrenaline) from the adrenal medulla, it causes stimulation. It also causes a feeling of relaxation, calmness, and alertness that most who use tobacco find appealing. It is even reported to produce a mildly euphoric state. By reducing the appetite and raising the metabolism, some smokers may lose weight as a consequence. It also allows the mouth to be stimulated without food and the taste of tobacco smoke may curb the appetite. When a cigarette is smoked, nicotine-rich blood passes from the lungs to the brain within seven seconds and immediately stimulates the release of many chemical messengers including acetylcholine, norepinephrine, dopamine, and beta-endorphin. This results in enhanced pleasure, decreased anxiety, and a state of alert relaxation. Nicotine enhances concentration and memory due to the increase of acetylcholine. The effects of nicotine last from five minutes to two hours. Most people who use nicotine do so several times a day (the average smoker smokes about 20 cigarettes in a 24-hour period). They do this in an effort to keep the pleasant effects of nicotine throughout the day and to avoid withdrawal. Most cigarettes contain 0.1 to 2.8 milligrams of nicotine. Most studies have been conducted with smoked cigarettes, not nicotine by itself. Nicotine gum and patches are now available, usually in 2 mg or 4 mg doses of gum, that do not have all the other ingredients in smoked tobacco. They appear to be not as addictive or as pleasurable, and perhaps have fewer side effects. Whether all the other psychoactive effects also occur has not been well studied. DependenceModern research shows that nicotine acts on the brain to produce a number of effects. Specifically, its addictive nature has been found to show that nicotine activates reward pathways—the circuitry within the brain that regulates feelings of pleasure and euphoria. [3] Dopamine is one of the key brain chemicals actively involved in the desire to consume drugs. Research shows that by increasing the levels of dopamine within the reward circuits in the brain, nicotine acts as a chemical with intense addictive qualities. In many studies it has been shown to be more addictive than cocaine, and even heroin. Like other physically addictive drugs, nicotine causes pathological down-regulation of the production of dopamine and other stimulatory neurotransmitters as the brain attempts to compensate for artificial stimulation. When nicotine exposure ceases, the neural changes it creates in the brain and body are unpleasant. Also like other highly addictive drugs, nicotine is addictive to many animals besides humans. Mice will self-administer nicotine and suffer from behavioral changes when its administration is stopped. Gorillas have learned to smoke cigarettes by watching humans, and have similar difficulty quitting. [4] A study found that nicotine exposure in adolescent mice retards the growth of the dopamine system, thus increasing the risk of substance abuse during adulthood [5]. There is only anecdotal evidence about abuse or addiction with nicotine gum or nicotine patches. ToxicologyThe LD50 of nicotine is 50 mg/kg for rats and 3 mg/kg for mice. 40–60 mg can be a lethal dosage for adult human beings.[6] This makes it an extremely deadly poison. It is more toxic than many other alkaloids such as cocaine, which has a lethal dose of 1000 mg.[citation needed] The carcinogenic properties of nicotine in standalone form, separate from tobacco smoke, have not been evaluated by the IARC, and it has not been assigned to an official carcinogen group. The currently available literature indicates that nicotine, on its own, does not promote the development of cancer in healthy tissue and has no mutagenic properties. Its teratogenic properties have not yet been adequately researched, and while the likelihood of birth defects caused by nicotine is believed to be very small or nonexistent, nicotine replacement product manufacturers recommend consultation with a physician before using a nicotine patch or nicotine gum while pregnant or nursing. However, nicotine and the increased cholinergic activity it causes have been shown to impede apoptosis, which is one of the methods by which the body destroys unwanted cells (programmed cell death). Since apoptosis helps to remove mutated or damaged cells that may eventually become cancerous, the inhibitory actions of nicotine creates a more favourable environment for cancer to develop. Thus nicotine plays an indirect role in carcinogenesis. It is also important to note that its addictive properties are often the primary motivating factor for tobacco smoking, contributing to the proliferation of cancer. At least one study has concluded that exposure to nicotine alone, not simply as a component of cigarette smoke, could be responsible for some of the neuropathological changes observed in infants dying from Sudden Infant Death Syndrome (SIDS).[7] It has been noted that the majority of people diagnosed with schizophrenia smoke tobacco. Estimates for the number of schizophrenics that smoke range from 75% to 90%. It was recently argued that the increased level of smoking in schizophrenia may be due to a desire to self-medicate with nicotine. [8] [9] More recent research has found the reverse, that it is a risk factor without long-term benefit, used only for its short term effects. [10]However, research on nicotine as administered through a patch or gum is ongoing. Therapeutic usesThe primary therapeutic use of nicotine is in treating nicotine dependence in order to eliminate smoking with its risks to health. Controlled levels of nicotine are given to patients through gums, dermal patches, lozenges, or nasal sprays in an effort to wean them off their dependence. However, in a few situations, smoking has been observed to apparently be of therapeutic value to patients. These are often referred to as "Smoker’s Paradoxes"[11]. Although in most cases the actual mechanism is understood only poorly or not at all, it is generally believed that the principal beneficial action is due to the nicotine administered, and that administration of nicotine without smoking may be as beneficial as smoking, without the high risk to health. For instance, recent studies suggest that smokers require less frequent repeated revascularization after percutaneous coronary intervention (PCI).[11] Risk of ulcerative colitis has been frequently shown to be reduced by smokers on a dose-dependent basis; the effect is eliminated if the individual stops smoking.[12][13] Smoking also appears to interfere with development of Kaposi's sarcoma,[14] breast cancer among women carrying the very high risk BRCA gene,[15] preeclampsia,[16] and atopic disorders such as allergic asthma.[17] A plausible mechanism of action in these cases may be nicotine acting as an anti-inflammatory agent, and interfering with the inflammation-related disease process, as nicotine has vasoconstrictive effects.[18] With regard to neurological diseases, a large body of evidence suggests that the risks of Parkinson's disease or Alzheimer's disease might be twice as high for non-smokers than for smokers.[19] Many such papers regarding Alzheimer's disease[20] and Parkinson's Disease[21] have been published. A plausible mechanism of action in these cases may be the effect of nicotine, a cholinergic stimulant, in decreasing the levels of acetylcholine in the smoker's brain; Parkinson's disease occurs when the effect of dopamine is less than that of acetylcholine. Recent studies have indicated that nicotine can be used to help adults suffering from Autosomal dominant nocturnal frontal lobe epilepsy. The same areas that cause seizures in that form of epilepsy are also responsible for processing nicotine in the brain.[22] Nicotine and its metabolites are being researched for the treatment of a number of disorders, including ADHD and Parkinson's Disease. [23] The therapeutic use of nicotine as a means of appetite-control and to promote weight loss is anecdotally supported by many ex-smokers who claim to put on weight after quitting. However studies of nicotine in mice [24] suggests it may play a role in weight-loss that is independent of appetite. And studies involving the elderly suggest that nicotine affects not only weight loss, but also prevents some weight gain. [25] See also
References
Further reading
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