Nicotine is a potent parasympathomimetic alkaloid found in the nightshade family of plants (Solanaceae) and a stimulant drug. Nicotine is a nicotinic acetylcholine receptor (nAChR) agonist, except at nAChRα9 and nAChRα10 where it acts as an antagonista. It is made in the roots of and accumulates in the leaves of the nightshade family of plants. It constitutes approximately 0.6–3.0% of the dry weight of tobacco and is present in the range of 2–7 µg/kg of various edible plants. It functions as an antiherbivore chemical; consequently, nicotine was widely used as an insecticide in the past and nicotine analogs such as imidacloprid are currently widely used.In lesser doses (an average cigarette yields about 2 mg of absorbed nicotine), the substance acts as a stimulant in mammals, while high amounts (50–100 mg) can be harmful. This stimulant effect is a major contributing factor to the addictive properties of tobacco smoking.icotine's mood-altering effects are different by report: in particular it is both a stimulant and a relaxant. First causing a release of glucose from the liver and epinephrine (adrenaline) from the adrenal medulla, it causes stimulation. Users report feelings of relaxation, sharpness, calmness, and alertness. Like any stimulant, it may very rarely cause the often uncomfortable neuropsychiatric effect of akathisia. By reducing the appetite and raising the metabolism, some smokers may lose weight as a consequence.
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 such as acetylcholine, norepinephrine, epinephrine, arginine vasopressin, serotonin, dopamine, and beta-endorphin. This release of neurotransmitters and hormones is responsible for most of nicotine's psychoactive effects. Nicotine appears to enhance concentration and memory due to the increase of acetylcholine. It also appears to enhance alertness due to the increases of acetylcholine and norepinephrine. Arousal is increased by the increase of norepinephrine. Pain is reduced by the increases of acetylcholine and beta-endorphin. Anxiety is reduced by the increase of beta-endorphin. Nicotine also extends the duration of positive effects of dopamine and increases sensitivity in brain reward systems. Most cigarettes (in the smoke inhaled) contain 1 to 3 milligrams of nicotine.
Studies suggest that when smokers wish to achieve a stimulating effect, they take short quick puffs, which produce a low level of blood nicotine. This stimulates nerve transmission. When they wish to relax, they take deep puffs, which produce a higher level of blood nicotine, which depresses the passage of nerve impulses, producing a mild sedative effect. At low doses, nicotine potently enhances the actions of norepinephrine and dopamine in the brain, causing a drug effect typical of those of psychostimulants. At higher doses, nicotine enhances the effect of serotonin and opiate activity, producing a calming, pain-killing effect. Nicotine is unique in comparison to most drugs, as its profile changes from stimulant to sedative/pain killer in increasing dosages and use, a phenomenon described by Paul Nesbitt in his doctoral dissertation and subsequently referred to as "Nesbitt's Paradox".
By binding to nicotinic acetylcholine receptors, nicotine increases the levels of several neurotransmitters – acting as a sort of "volume control". It is thought that increased levels of dopamine in the reward circuits of the brain are a major contributor to the apparent euphoria and relaxation, and addiction caused by nicotine consumption. Nicotine-induced dopamine release occurs via the cholinergic–dopaminergic reward link, which is mediated by the neuropeptide ghrelin in the ventral tegmentum. Nicotine has a higher affinity for acetylcholine receptors in the brain than those in skeletal muscle, though at toxic doses it can induce contractions and respiratory paralysis. Nicotine's selectivity is thought to be due to a particular amino acid difference on these receptor subtypes.
Tobacco smoke contains anabasine, anatabine, and nornicotine. It also contains the monoamine oxidase inhibitors harman and norharman. These beta-carboline compounds significantly decrease MAO activity in smokers. MAO enzymes break down monoaminergic neurotransmitters such as dopamine, norepinephrine, and serotonin. It is thought that the powerful interaction between the MAOIs and the nicotine is responsible for most of the addictive properties of tobacco smoking. The addition of five minor tobacco alkaloids increases nicotine-induced hyperactivity, sensitization and intravenous self-administration in rats.
Chronic nicotine exposure via tobacco smoking up-regulates alpha4beta2* nAChR in cerebellum and brainstem regions but not habenulopeduncular structures. Alpha4beta2 and alpha6beta2 receptors, present in the ventral tegmental area, play a crucial role in mediating the reinforcement effects of nicotine.
Research published in 2011 found that nicotine inhibits class I and II histone deacetylases, chromatin-modifying enzymes involved in epigenetics. This inhibition has been shown to increase susceptibility to cocaine addiction in rodents
Nicotine also activates the sympathetic nervous system, acting via splanchnic nerves to the adrenal medulla, stimulating the release of epinephrine. Acetylcholine released by preganglionic sympathetic fibers of these nerves acts on nicotinic acetylcholine receptors, causing the release of epinephrine (and noradrenaline) into the bloodstream. Nicotine also has an affinity for melanin-containing tissues due to its precursor function in melanin synthesis or due to the irreversible binding of melanin and nicotine. This has been suggested to underlie the increased nicotine dependence and lower smoking cessation rates in darker pigmented individuals. However, further research is warranted before a definite conclusive link can be inferred.