Capsaicin

Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact. Capsaicin and several related compounds are called capsaicinoids and are produced as a secondary metabolite by chilli peppers, probably as deterrents against herbivores. Pure capsaicin is a hydrophobic, colorless, odorless, crystalline to waxy compound.

History
The molecule was first isolated in 1816 in crystalline form by P. A. Bucholz and again 30 years later by L. T. Thresh, who gave it the name capsaicin. In 1878, the Hungarian doctor Endre Hogyes (calling it capsicol) isolated it and proved that it not only caused the burning feeling when in contact with mucous membranes but also increased secretion of gastric juice. Capsaicin was first synthesized in 1930 by E. Spath and F. S. Darling. In 1964, similar substances were isolated from chili peppers by the Japanese chemists S. Kosuge and Y. Inagaki, who named them capsaicinoids.

Capsaicinoids
Capsaicin is the main capsaicinoid in chilli peppers, followed by dihydrocapsaicin. These two compounds are also about twice as potent to the taste and nerves as the minor capsaicinoids nordihydrocapsaicin, homodihydrocapsaicin, and homocapsaicin. Dilute solutions of pure capsaicinoids produced different types of pungency; however, these differences were not noted using more concentrated solutions.

Capsaicin is believed to be synthesized in the interlocular septa of chili peppers by addition of a branched-chain fatty acid to vanillylamine. Biosynthesis depends on the gene AT3, which resides at the pun1 locus, and which encodes a putative acyltransferase.

Besides six natural capsaicinoids also one synthetic member capsaicinoid exists. It is vanillylamide of n-nonanoic acid (VNA) and it is used as a reference substance for determining capsaicinoids relative pungency.

Natural function
Capsaicin is present in large quantities in the placental tissue which holds the seeds, the internal membranes and to a lesser extent in the other fleshy parts of the fruits of plants in the genus Capsicum. Despite popular belief, the seeds do not produce any capsaicin. Unlike many such fruits which have evolved to aid in seed dispersal by attracting animals, which consume the fruits and swallow the seeds, which pass through the digestive tract and are subsequently deposited elsewhere, the seeds of Capsicum plants are predominantly dispersed by birds, in which capsaicin has analgesic properties rather than acting as an irritant. Chilli pepper seeds consumed by birds pass through the digestive tract unharmed, whereas those consumed by mammals do not germinate at all. The presence of capsaicin in the fruits therefore protects them from being consumed by mammals.

It was recently discovered that tarantula venom activates the same pathway of pain as is activated by capsaicin, the first demonstrated case of such a shared pathway in both plant and animal anti-mammal defense.

Food
Because of the burning sensation caused by capsaicin when it comes in contact with human mucous membranes as found in the eye or the mouth, it is commonly used in food products to give them added spice or "heat" (pungency). The degree of heat found within a food is often measured on the Scoville scale, although the high performance liquid chromatography method is now preferred. Typically the capsaicin is obtained from chilli peppers. Hot sauce is an example of a product customarily containing large amounts of capsaicin and may contain chilli peppers or pure capsaicin.

Cooling and mechanical stimulation are the only proven methods to relieve the pain, however many questionable tips are widely perpetuated, including oil and oily food (probably because capsaicin in its pure state is poorly soluble in water but soluble in oil). However, the burning sensation will slowly fade away without any measure taken. Milk products are one of the most effective forms of relief; casein, a phosphoprotein found in milk, acts as a detergent to dissociate the capsaicin from the nerve receptors, allowing it to wash away. (Dustrophsky, 2006).

Medical
Capsaicin is currently used in topical ointments to relieve the pain of peripheral neuropathy such as post-herpetic neuralgia caused by shingles. It may be used in concentrations of between 0.025% and 0.075%.

It may also be used as a cream for the temporary relief of minor aches and pains of muscles and joints associated with arthritis, simple backache, strains and sprains. The treatment typically involves the application of a topical anesthetic until the area is numb. Then the capsaicin is applied by a therapist wearing rubber gloves and a face mask. The capsaicin remains on the skin until the patient starts to feel the "heat", at which point it is promptly removed. Capsaicin is also available in large adhesive bandages that can be applied to the back.

The result appears to be that the nerves are overwhelmed from the burning sensation and are unable to report pain for an extended period of time. With chronic exposure to capsaicin, neurons are depleted of neurotransmitters and it leads to reduction in sensation of pain and blockade of neurogenic inflammation. If capsaicin is removed, the neurons recover.

Recently, capsaicin is being tested for the prevention of pain post surgery. "David Julius, a physiology professor at the University of California, San Francisco, recently discovered that capsaicin selectively binds to a protein known as TRPV1 that resides on the membranes of pain-sensing neurons. When capsaicin binds to TRPV1, it causes the protein to open a gate leading to a small channel in the nerve cell's membrane. (Neurons that do not contain TRPV1 are unaffected.)". This causes extended numbness following surgery, and the patient does not feel pain as the capsaicin is applied under anesthesia.

Capsaicin is being explored as a possible cure for diabetes by researchers in Toronto, Canada; capsaicin was injected into pancreatic sensory nerves of mice with Type 1 diabetes because of a suspected link between the nerves and diabetes.

The American Association for Cancer Research reports studies suggesting capsaicin is able to kill prostate cancer cells by causing them to undergo apoptosis. The studies were performed on tumors formed by human prostate cancer cell cultures grown in mouse models, and showed tumors treated with capsaicin were about one-fifth the size of the untreated tumors. It has long been noted that in Thailand, where lots of spicy food is consumed, there is very low incidence of gastrointestinal cancers, including colorectal and stomach cancers, compared to the rest of Asia, including Japan and China. Mexico also has low rates of the same cancers compared to the USA. Several recent studies have shown that capsaicin may actually prevent the growth of certain types of cancer. In particular, there have been several clinical studies conducted in Japan and China that showed natural capsaicin directly inhibits the growth of leukemic cells. Although these studies used pure capsaicin directly injected into isolated diseased cells in a laboratory setting, scientists have also concluded that daily consumption of hot peppers (thus capsaicin) may actually prevent certain types of cancer. Throughout South America, intestinal, stomach, and colon cancer rates are very low compared to the United States.

Another study carried out at the University of Nottingham suggests capsaicin is able to trigger apoptosis in human lung cancer cells as well.

Capsaicin is also the key ingredient in the experimental drug Adlea, which is in Phase 2 trials as a long-acting analgesic to treat post-surgical and osteoarthritis pain for weeks to months after a single injection to the site of pain.

Non-lethal force
Capsaicin is also the active ingredient in the chemical riot control agent pepper spray. When the spray comes in contact with skin, especially eyes or mucous membranes, it is very painful. Refer to the Scoville scale for a comparison of pepper spray to other sources of capsaicin.

In large quantities, capsaicin can cause death. Symptoms of overdose include difficulty breathing, blue skin, and convulsions. The large amount needed to kill an adult human and the low concentration of capsaicin in chilis make accidental poisoning by chili consumption exceedingly unlikely.

Possible drug abuse deterrent
Clifford Woolf, the Richard J. Kitz Professor of Anesthesia Research at Harvard Medical School, has suggested using capsaicin to deter abuse of certain extended-release drugs such as OxyContin and Ritalin. When taken as prescribed, opioid prescription drugs such as OxyContin or stimulant drugs such as Adderall XR release their active chemical over time, but when crushed and snorted, taken as a suppository, chewed, or injected, the larger than normal dosage is absorbed all at once and a much stronger effect is produced that can be highly habit forming and dangerous due to the higher risk of overdose. Woolf has argued that adding capsaicin into the capsules would be a safe way to deter abuse. A person taking the capsule in the prescribed way (i.e., swallowing it whole) would suffer no ill effects from the additive. However, a person crushing it would expose the irritant. Anyone then swallowing it, snorting it, or injecting it would be exposed to the full power of the chemical. "Imagine snorting an extract of 50 jalapeño peppers and you get the idea," Woolf said in an interview with the Harvard University Gazette. As of 2006, Woolf's proposal is still in the preliminary stages of development and the additive has not yet entered the production stage.

Pest deterrent
Capsaicin is also used to deter pests. A common example is the use of ground-up or crushed dried chilli pods in birdseed to deter squirrels, since birds are unaffected by capsaicin. Insects that feed on pepper and related plants are also unaffected.

Mechanism of action
The burning and painful sensations associated with capsaicin result from its chemical interaction with sensory neurons. Capsaicin, as a member of the vanilloid family, binds to a receptor called the vanilloid receptor subtype 1 (VR1). First cloned in 1997, VR1 is an ion channel-type receptor. VR1, which can also be stimulated with heat and physical abrasion, permits cations to pass through the cell membrane and into the cell when activated. The resulting depolarization of the neuron stimulates it to signal the brain. By binding to the VR1 receptor, the capsaicin molecule produces the same sensation that excessive heat or abrasive damage would cause, explaining why the spiciness of capsaicin is described as a burning sensation.

The VR1 ion channel has subsequently been shown to be a member of the superfamily of TRP ion channels, and as such is now referred to as. There are a number of different TRP ion channels that have been shown to be sensitive to different ranges of temperature and probably are responsible for our range of temperature sensation. Thus, capsaicin does not actually cause a chemical burn, or indeed any damage to tissue at all; it causes only the sensation of one.

Acute health effects
Capsaicin is a highly irritant material requiring proper protective goggles, respirators, and proper hazmat handling procedures. It is hazardous in cases of skin contact (irritant, sensitizer), of eye contact (irritant), of ingestion, of inhalation (lung irritant, lung sensitizer). Severe over-exposure can result in death. Painful exposures to capsaicin-containing peppers are among the most common plant-related exposures presented to poison centers. They cause burning or stinging pain to the skin, and if ingested in large amounts by adults or small amounts by children, can produce nausea, vomiting, abdominal pain, and burning diarrhea. Eye exposure produces intense tearing, pain, conjunctivitis, and blepharospasm.

Treatment after exposure
The primary treatment is removal from exposure. Contaminated clothing should be removed and placed in airtight bags to prevent secondary exposure. Capsaicin could be washed off the skin using soap, shampoo, or other detergents, or rubbed off with oily compounds such as vegetable oil, paraffin oil, vaseline, cremes, or polyethylene glycol. Plain water as well as home remedies such as vinegar, bleach, sodium metabisulfite, or topical antacid suspensions are ineffective in removing capsaicin.

Burning and pain symptoms can be effectively relieved by cooling, e.g. from ice, cold water, cold bottles, cold surfaces, or a flow of air from wind or a fan. In severe cases, eye burn might be treated symptomatically with topical ophthalmic anesthetics; mucous membrane burn with lidocaine gel. Capsaicin-induced asthma might be treated with nebulized bronchodilators or oral antihistamines or corticosteroids.

Effects of dietary consumption
The effects of ingestion of small quantities has given different experimental results. One study showed no mucosal erosions or other abnormalities after eating ground jalapeño peppers. Whereas another showed mucosal microbleeding and even one case of grossly visible gastric bleeding after eating red and black peppers. This study also showed "no spice was significantly different from aspirin in any parameter studied." Other studies have shown an association between chronic consumption of capsaicin-rich foods and stomach cancer, yet one such study was not adjusted to any other factors in diet. Another of the said studies also showed a similar risk to stomach cancer with higher consumption of rice.