Humans are, essentially, sadomasochists. For more 600 years we have sought out, cultivated, eaten, applied to our skin and even weaponised a chemical capable of making us feel heat and pain.
Heat, it seems, is desirable. Whether in the kitchen or the bedroom, we have embraced the volcanic properties of the chilli pepper to enhance our food and spice up our love lives. But in The Pain Detective, published on Mosaic, chilies crop up unexpectedly. In the film, they feature as a component of a medically manufactured, super-hot patch being gingerly applied to Colin Froy's feet -- not to inflict pain but, curiously, to diminish it. What is it about chilies that allows them to be both master and servant of pain?
In an attempt to understand the heat properties of the wildly varied and abundant chilli, American pharmacist Wilbur Lincoln Scoville sought a measure of hotness that was more standardized and reproducible than the universal 'hand fanning' used to indicate too much heat. By 1912, Scoville had devised a scale to measure the sensation of chill intensity. The Scoville scale has been used ever since to rate the potency of chilies.
The bell or sweet pepper is devoid of spice, scoring a round zero on the Scoville scale. Its relatives, however, are a little more spirited. Jalapeños fall into the 2,500-8,000 Scoville Heat Unit (SHU) range, while cultivars of the diminutive Scotch Bonnet pepper span 100,000 -- 350,000 SHU.
The real firestarters include the bhut jolokia, or ghost chilli, a pepper so incendiary that its roughly 900,000 SHU have been harnessed by the Indian military to build a chilli grenade for incapacitating rowdy crowds. But the current world record holder is the aptly named Carolina Reaper, which is ready to inflict over 1.5 million SHU on anyone unfortunate enough to try and tame it.
The source of heat in every example above is the colourless, odourless chemical capsaicin. Capsaicin itself defines the very limit of the Scoville scale: the pure chemical form ranks north of 15 million SHU.
While undetectable to birds -- a major means of seed dispersal for chilli plants -- the hot sensation of this entirely natural, plant-based chemical acts as a deterrent to mammals who might destroy the seeds with their teeth. The capsaicin 'burn' is actually an illusion, a chemical trick fooling the brain into perceiving heat. This happens because capsaicin binds to and activates a specific channel protein (a biological gate) on the surface of nerve cells that sense pain and heat.
Normally, these proteins, known as TRPV1 receptors, remain inactive in mammals unless stimulated by temperatures above 42°C (107.6°F). When stimulated, they elicit a sensation of heat and pain, a warning to get away from a source of potential damage. Due to slight differences in TRPV1 structure between mammals and birds, capsaicin is incapable of binding to this same protein in birds.
This 'heat receptor' protein is found throughout the nervous system, including in nerve cells in our skin and the digestive system. This means that, inside and out, chilies burn. Perhaps counterintuitively, eating chilli can trigger a cascade of cooling effects, such as sweating and flushing -- one reason why the chilli is associated with the bedroom.
Applied externally, chilies cause a sensation of burning, as capsaicin activates TRPV1 in nerves in the skin. But, if exposed to capsaicin for long enough, these pain nerve cells will become 'exhausted', having depleted their internal chemical stores. The nerve cells are no longer able to respond to capsaicin (or indeed, anything that might cause pain) and so you are no longer able to perceive pain. This is why chronic exposure to capsaicin acts as an analgesic. And this is why people experiencing chronic pain, like Colin Froy, are willing to endure chilli patches. Each of which contains chemicals equivalent to around 10 million SHU.
This article first appeared on Mosaic and is republished here under Creative Commons licence. Mosaic is dedicated to exploring the science of life. Each week, it publishes a feature on an aspect of biology or medicine that affects our lives, our health or our society; it tells stories with real depth about the ideas, trends and people that drive contemporary life sciences. Mosaic is published by the Wellcome Trust, a global charitable foundation that seeks to drive extraordinary improvements in human and animal health. It covers subjects that fit with the Trust's mission and vision, but isn't limited to the research the Trust funds.
Picture: Dave Morris/Flickr