A chemical from chilli peppers may be able to kill pain without affecting touch or movement.

This might in theory mean a woman in labour could have an epidural without losing the ability to move her legs, or the sensation of her baby being born.

Conventional local anaesthetics affect all nerve cells.

But the researchers Harvard team, writing in Nature, said that with capsaicin, the chilli chemical, they can target just pain receptors.

However, a UK expert said it might be difficult to inject it safely.

The key chemical is capsaicin

Numbness is actually a side-effect of the pain-killing properties of local anaesthetics - caused when the drug blocks signals not only from the nerve endings which cause pain, but other nerve endings which simply detect the sensation of touch.

And when anaesthetic "blocks", are injected into the spine, they can interfere with other nerves, causing temporary paralysis - such as that felt in the lower limbs after an epidural injection.

Cell door

The Harvard team used a molecule - QX-314 - which interferes with nerve signals in the same way as any other conventional anaesthetic, but which is too big to get into any nerve cell on its own.

Capsaicin, which is the substance that makes chilli peppers taste "hot", has an unique property - it can open a channel in the cell wall of nerve cells big enough for QX-314 to get in.

However, it can only do this in the cell walls of pain receptor neurons, meaning that these are the only nerve cells affected by the anaesthetic.

In rats, an injection of QX-314 and capsaicin didn't just kill the pain, but caused no other effects. When injected near the nerve controlling the hind leg, there was no paralysis.

The researchers said that this had the potential to "profoundly change pain treatment" before and during the millions of operations carried out under local anaesthetic every year.

Dr Story Landis, the director of the National Institute of Neurological Disorders and Stroke in the US, said that patients in chronic pain might also benefit.

"The Holy Grail in pain science is to eliminate pathologic pain without impairing thinking, alertness, coordination or other vital functions of the nervous system.

"It holds the promise of major future breakthroughs for the millions of people who suffer with disabling pain."

Burning feeling

Dr Joan Hester, the president of the British Pain Society, said that while capsaicin had been used for many years to reduce skin sensitivity linked to chronic pain, it caused an unpleasant burning sensation that was too much for some patients.

This might be even more of a problem if the chemical was injected below the skin, she said.

"The technique has not yet been tried on humans, and it is hard to see how capsaicin could be used in this situation."

However, she said that the study broke new ground: "Selective block of pain nerve fibres without numbness or motor block would be of great benefit in local anaesthesia by injection, for example in epidural anaesthesia"