The world is gasping after US researchers reported the hot news that the chemical that makes peppers hot is the key to targeted pain killing.

Image showing capsaicin and the new experimental pain killer QX-314.

To understand the significance of this report, we need to step back and understand what makes hot chili peppers hot. Chili pepper have a protein called capsaicin which makes them hot. The amount of capsaicin present is measured using the Scoville scale. Pure capsaicin scores about 15000000 on the Scoville scale while cayenne pepper scores a few tens of thousands. Capsaicin causes a burning sensation of pain by opening a molecular gate found in pain neurons. This gate, called TRPV1, is usually closed and only when it is open do molecules freely enter and leave the cell. Also, TRPV1 is found only in pain neurons and not other neurons, making it a very attractive target we can selectively open up.

Now that we can open a channel into pain neurons, we need the actual pain killer. Enter QX-314. On the face of it, QX-314 is a bad pain killer: add it to nerves and nothing happens. This is because it cannot enter the nerves on its own.

So now we have two nice pieces: we have a pain killer that needs help to enter pain nerves and numb them and we have a way to selectively open up pain nerves. Surely it's not that simple... is it?

Actually, it is! The paper talks about the experiments that demonstrated this. The first test was using isolated nerve cells grown in a dish. The researchers applied capsaicin and QX-314 (separately and in combination) to isolated pain-sensing and other neurons and measured how they responded. What they found was the combination of capsaicin and QX-314 selectively blocked the excitability of pain-sensing neurons, leaving the others unaffected.

Next, the researchers injected capsaicin and QX-314 into the paws of rats and measured their ability to sense pain by placing them on an uncomfortable heat source. With the combination injected, the rates tolerated much more heat than usual. Next, capsaicin and QX-314 were injected near the sciatic nerve of the rats, and a researched pricked their paws with stiff piece of nylon. The animals ignored the provocation. Although the rats seemed immune to pain, they continued to move normally and respond to other stimuli, indicating that QX-314 failed to penetrate their motor neurons.

This last observation is crucial. Pain killers these days are not specific causing general numbness since they affect all nerves. Having a very specific pain killer is very useful indeed!

So what's catch? Well, as we talked about above, capsaicin causes a burning sensation, so the formulation must not cause this kind of pain in humans, perhaps using something else than capsaicin. Also, research is still needed to figure out the best way to make the pain killing effects last as long as possible. Regardless, it's a great first step to a pain free world in a few years time.