Nicotine is a psychoactive alkaloid, meaning it's a nitrogen-containing chemical compound that activates parts of the brain. There are several chemical reactions that take place in the body because of nicotine---some of them are due to the activity of the compound on the brain, and others happen as the body attempts to rid itself of nicotine.
Two Groups of Chemical Reactions
The chemical reactions that take place because of nicotine fall into two groups. The first reactions, which take place at receptors in the nervous system, are the direct result of nicotine binding to proteins on nerve cells, notes Dr. Neil Carlson in his book, "Foundations of Physiological Psychology." The body also recognizes nicotine as a toxin and attempts to remove it. This leads to a second series of reactions, in which nicotine is broken into smaller molecules.
Significance of Binding Reactions
The reactions that take place as a direct result of nicotine acting on the brain start with the reaction whereby nicotine binds to a neuronal receptor called the nicotinic acetylcholine receptor, which also binds the common neurotransmitter acetylcholine. Notes Dr. Carlson, the nicotinic acetylcholine receptor is named for its ability to bind nicotine in addition to acetylcholine. In response to nicotine binding, the receptor causes neural excitation, or firing of the nerve, which produces the effects associated with nicotine, including mental stimulation.
Significance of Breakdown Reactions
Nicotine, like many toxins, is metabolized in the liver. Liver enzymes break down most of the nicotine into another chemical called cotinine. While there are many compounds that the liver can make from cotinine, the primary one produced by human livers is called 3'-hydroxycotinine, which is a water-soluble compound that the kidney can allow to pass into the urine, and which is then excreted. The Pharmacogenomics Knowledge Base notes that smokers typically have significant levels of 3'-hydroxycotinine in their urine.
Misconceptions
While it's certainly true that smoking and tobacco use are bad for the health, many of the chemical reactions that lead to cancer, lung disease and other effects of smoking don't have to do with nicotine-related chemical reactions as much as they have to do with chemical reactions caused by other substances in tobacco. Cigarette smoke, for instance, contains high levels of the pollutants carbon monoxide, and also contains additives such as ammonia. These, note Drs. Mary Campbell and Shawn Farrell, have significant effects on biochemistry.
Expert Insight
The reactions caused by nicotine at the nerve cells tend to decrease in frequency and strength with repeated exposure to nicotine. This, notes Dr. Carlson, is called "desensitization" or "habituation," and is observed frequently in the nervous system. When a nerve cell is repeatedly stimulated by the same compound, it tends to decrease its response to that compound. As such, habitual nicotine users often find that it takes more and more nicotine to achieve the desired mental effect over time.
References
- "Foundations of Physiological Psychology"; Neil Carlson, Ph.D.; 2004
- Pharmacogenomics Knowledge Base: Nicotine Metabolism
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
