Nicotine and caffeine belong to a group of chemicals called alkaloids. Alkaloids have a cyclic nitrogen-containing structure, although no defined chemical classification exists. The alkaloid grouping contain poisons, neurotoxins, psychedelic and some social drugs. Of the social drugs caffeine and nicotine are two of the most widely known and used. In total around 20% of plant species contain alkaloids and there are over 12,000 currently identified and catalogued. Plants use alkaloids to deter predators which may include animals such as herbivores as well as insects. The alkaloids interact directly with the nervous systems of the organisms consuming the plant material resulting in modulation of their central nervous system. Caffeine and nicotine are effective insecticides and are toxic in high doses in humans. Some herbivores are able to metabolise alkaloids to prevent poisoning, but they remain an important defence mechanism for plants.
Caffeine belongs to a sub-group of alkaloids called the methylxanthines and is chemically a purine alkaloid. Other methylxanthines includes theobromine and theophylline, which are found along with caffeine in tea, coffee, cocoa, guarana and maté. In humans, methylxanthines are competitive antagonists of the inhibitory adenosine A1 and A2 receptors. Binding of a methylxanthine to the receptor leads to activation and subsequent increases in dopaminergic and glutamatergic neuronal activity. This causes the increase in alertness associated with caffeine, or in most habitual consumers, decreases the withdrawal symptoms to maintain normal functioning. High doses of methylxanthine cause anxiety, insomnia, psychomotor agitation and death. In addition, methylxanthines can antagonise vascular adenosine A2α receptors which leads to vasoconstriction. Consumption of caffeine in the long-term causes adenosine receptor up regulation which causes withdrawal symptoms when caffeine intake is attenuated.
Nicotine, a pyridine alkaloid, is used by the plant Nicotiana tabacum as an insecticide against predators, and is highly toxic to both insects and mammals. Nicotine toxicity is difficult to attain through smoking because of the small amounts released during burning the tobacco, but by ingesting nicotine it is much easier to achieve a lethal dose. In mammals nicotine is an acetylcholine receptor agonist and increases the release of acetylcholine mainly in the amagdala and hippocampus. Nicotine can also cause the release of glutamate and serotonin. The addictive properties of nicotine stem from its ability to release dopamine from the ventral tegmental area of the brain. Nicotine is known to increase cognitive performance, by improving episodic and working memory as well as attention. Nicotine, like caffeine, is an important social drug and addiction of both is common throughout the World.
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