Why is it that I have to keep adding more and more chilli to my curry every week in order to get the same mouth watering, face burning, eye tingling burn that you get with a really hot vindaloo?
Kat Arney: Chillies are a crucial ingredient in a fiery curry and if you don’t wash your hands carefully after handling them you will discover exactly how burning hot they can be, especially if you rub your eyes or go to the toilet. This is because chilli peppers contain capsaicin, which is the chemical that gives them their fiery heat, and in fact this burning power could be used to reduce pain. Research published in the journal PloS Biology has explained how this might be happening.
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Chris Smith: It sounds a bit counter-intuitive, you take something that gives you pain and you’re saying at the same time it can relieve pain?
Kat: Yes, this is work by Feng Qin and Jing Yao, and they’ve been investigating how capsaicin can shed light on how our bodies respond and adapt to painful stimuli, but it’s not clear in the case of pain receptors whether they truly adapt or whether they just get completely desensitised to any stimulus and shut down when they get overloaded.
Chris: But you mention that capsaicin might actually relieve pain, so what’s going on there?
Kat: Well, the pain and the heat effect happens with relatively low concentrations of capsaicin, and other scientists have discovered that if you rub large amounts of capsaicin onto the skin it also causes nerve cells to show a drop in the levels of a molecule called PIP2, and this causes desensitisation to pain in the end. In fact, this effect is so powerful that capsaicin creams are sold in pharmacies as treatment for muscle and joint pain, and even for the pain caused by arthritis and neuropathy.
Chris: But how do the researchers actually answer that question about desensitisation and what’s going on?
Kat: They used a range of techniques to measure how the pain receptors responded to capsaicin and they found that this drop in PIP2 is directly linked to the activation of the nerve cells by capsaicin, and the scientists also showed that even when the receptors are apparently desensitised by capsaicin they can still respond to new higher doses of the chemical, so this tell us that they’re not being truly desensitised, otherwise they wouldn’t respond at all, but in fact these pain receptors are adapting, they’re getting used to a certain level of stimulation from the capsaicin and that gives a sensation of pain relief, but higher doses of capsaicin or more intense pain would still leak through.
Chris: So what are the implications for this in terms of exploiting it for pain relief?
Kat: Well, it does have big implications. Effectively the intensity of the pain that you experience depends on the pain that you’ve recently felt, so at a very trivial level this suggests that if you kick someone in the leg you should kick them twice because it will hurt less the second time, but it’s also very important for doctors because finding ways to manipulate these pain-sensing nerves could develop better painkillers and anaesthetics in the future.
Listen to the whole edition of Breaking Science, originally broadcast on BBC Radio Five Live, 2009.
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good!