What Chillies have in common with Taratulas
Chris - Now talking of pain, David Julius is on the line. He's not a pain but he works on pain at the University of California in San Francisco. You've made some very interesting discoveries regarding the relationship between what comes out of a chilli and what comes out of a tarantula. Tell us about that.
David - Well what we found several years ago is that chillies have their hot and spicy bite, which has been known for many years that that's caused by a main pungent in chemical or ingredient in the chilli called capsaicin, and what we showed was that capsaicin acts on a specific molecule on the surface of pain sensing nerve fibres. In doing so, it activates those nerve fibres and gives the sensation of burning pain.
Chris - So that's why if you eat a curry you get this burning sensation in your mouth, because you're fooling the brain or the nervous system's pain pathways into thinking you're being burned.
David - Exactly.
Chris - So why should there be this chemical overlap between the real burning, so someone holding a cigarette lighter to your finger, and say eating a chilli?
David - Well I think in the case of the chilli and what I think we'll be coming to in a minute, the tarantula, all of these organisms have usurped this pathway and taken it over as a way of conveniently using this system as an anti-predatory mechanism.
Chris - So the chilli wants to stop itself being eaten?
David - Yes by something like a ground squirrel or a predatory mammal. So presumably nature has endowed it with the ability to make these compounds that have evolved to activate our pain pathway as a way of saying keep away. They have done this by targeting a specific molecule that we believe is involved in sensing heat. And so the psychophysical sensation is similar because it's activating the same neural pathways. Both of those stimuli are activating the same neural pathways that tell our brain that we've touched something hot or experienced something that we have come to know as being hot or of that ilk.
Chris - Now you mentioned something interesting when you were talking just know. You said mammals and ground squirrels. Are you saying that not all animals are sensitive to the effects of chilli or capsaicin?
David - That's right. Not all animals are, and in particular birds are relatively insensitive to capsaicin.
Chris - Why?
David - I think it's because they have again found their ecological niche as vectors for dispersal of the pepper seeds. In addition to not being sensitive in terms of pain lines being sensitive to capsaicin and this burning sensation, their digestive tract does also not destroy the seeds, so they're well suited to eat the pepper plant and disperse the seeds as a way of carrying out germination of the plant.
Chris - So when people say that a good way to stop rats from nicking your chicken's food or to keep squirrels off your bird feeder would be to cover the nuts with a healthy lashing of chilli.
David - Yes, well people do that and can buy seeds that are laced with capsicum dust. I have trouble with squirrels in my bird feeder too and I have to try that. I'm told that eventually the squirrels will adapt and ignore the hot pepper on there but I have to try it myself.
Chris - Because there's some evidence that people who regularly ingest very hot curry become a bit less sensitive to its effects. Is this because it's damaging the receptor or is it damaging the nerves? Are their mouths becoming less sensitive to pain, or is it just that the target for the chilli is being reduced in density on the surface of the nerve fibre?
David - I think it's probably all of those things. Capsaicin, when applied to a sensory nerve fibre, will cause some destruction or temporary desensitisation of the nerve fibre ending. You can see this in rodents as well as in man, so there is some what we call functional desensitisation of the nerve fibre and in some cases even anatomical damage. Of course, those fibres will regenerate and grow back. I think in the case of people there's also a psychophysical desensitisation in a way that if you've been eating kimchi or something spicy ever since you were three or four years old, not only do you come to appreciate it, you also get used to the burning sensation.
Chris - Now one thing that does produce a burning sensation though is the tarantulas you've been looking at. Tell us about the link between chillies and those.
David - We asked a little while ago why is it that bites and stings from some venomous creatures are associated with acute and intense pain. Not much is known about that, so what we did was to carry out biochemical fractionation of venom from a number of spiders and asked whether there was something in those venoms that was particularly potent in activating the receptors the we and other labs have identified as being important players in initiating pain in nerve fibres. We identified this one spider from Trinidad and Tabago called the Trinidad Chevron, and in its venom is a small protein, three of them actually that we identified.
Chris - And they activate the receptor.
David - They target the same receptor that the hot chilli pepper targets.
Chris - Now we're very short for time, but could you tell us in about twenty seconds why it is that when I suck on a polo mint it makes my breath feel cold?
David - For the same logic that hot peppers make you feel heat. So that is that the mint acts on a receptor that's activated by cold and so it tickles the same signalling pathway through which cold tells your nerve fibres that you've experienced something thermally cold.