Chilli-based anaesthetic blocks pain without numbness or paralysis

Scientists in the US have developed an anaesthetic that can completely block pain without causing numbness or paralysis. The technique uses capsaicin - the active ingredient in chilli peppers - to target only pain-sensing neurons, and not those associated with feeling or movement.

Traditional local anaesthetics block all neurons from sending messages to the brain, leading to numbness or paralysis that can be a disadvantage during, for example, childbirth, when it is helpful if the mother is able to push. ’We’re offering a targeted approach to pain management that avoids these problems,’ said Clifford Woolf, the author of the study at Harvard Medical School and Massachusetts General Hospital.

The secret of the technique lies in capsaicin - the chemical that makes even cold food containing chillies feel hot on the tongue. This illusion arises because capsaicin molecules interact uniquely with pain-sensing neurons, tricking them into reacting as if they were hot.

Importantly, when pain-sensing neurons are stimulated in this way (by heat or capsaicin) an ion channel is opened, allowing ions to enter the cells. Woolf’s team exploited this route into cells by using a derivative of the anaesthetic drug lidocaine called QX-314.

Whereas lidocaine and other anaesthetics are able to pass straight through the cell membranes of neurons, QX-314 cannot. But when used in combination with capsaicin, QX-314 is able to travel through the open ion channels and into the cell. Once there, it is able to block pain signals like other anaesthetics. 

So far the therapy has only been tested on rats, but Woolf and colleagues are confident that it can be extended to humans. 

Commenting on the findings, Istvan Nagy, who studies pain medicine and anesthetics at Imperial College London, told Chemistry World, ’This research is definitely important. I can see this treatment working very well in humans.’

But there are still obstacles to overcome, such as ensuring that no sensation of pain or burning is felt before the QX-314 takes effect and that the pain relief can be sustained over time. 

Lewis Brindley