Scientists get fired up over chilli anaesthetic

"An anesthetic that can block pain without impairing movement, touch or mental awareness has been developed using the chemical that gives chilli peppers their kick," reported The Times and other newspapers today.

They describe the new drug as one that targets only those nerves that send pain signals to the brain, which in theory will make pain relief safer than existing anaesthetics. Among other situations, it could theoretically be capable of leaving people with less numbness and drooling after dental anaesthetic or less weakness in the legs after epidurals for caesarean section.

This laboratory study in six rats was designed to test the action of a chemical extracted from chilli, when injected in combination with a type of local anaesthetic. The excitement generated by this study is based on the potential value for the technique but, as one of the lead researchers Professor Woolf suggested in The Daily Telegraph, we will have to wait until about “2010 for proof of concept trials in humans”.

Where did the story come from?

Dr Alexander Binshtok and colleagues from the Department of Anaesthesia and Critical care at the Massachusetts General Hospital and Harvard Medical School carried out the research. No funding source is acknowledged. The study was published in the peer-reviewed journal Nature.

What kind of scientific study was this?

This was an animal experiment in rats and was published as a letter to the editors of the journal.

In the study, researchers tried to selectively block the pain-sensing nerves in six live rats by injecting a combination of two chemicals around their sciatic (leg) nerves or into their foot nerves. One of the chemicals was QX-314, an anaesthetic derived from the drug lidocaine. This chemical blocks electric currents in nerve cells. The other chemical was capsaicin, which is the chemical that makes chillies hot.

The theory was that capsaicin would enable the anaesthetic to enter the pain sensing nerves. This is because capsaicin acts as a “gatekeeper”, opening pores in cell membranes which would, they hypothesised, allow the anaesthetic agent QX-314 to pass through and block the pain messages. QX-314 is different from conventional local anaesthetics in that it is only active if it manages to get into the nerve cells themselves.

The nerves relating to the sensation of touch and the motor nerves are different to pain nerves and do not contain this channel, therefore the researchers expected that only the pain sensing nerves would be blocked by the action of the anaesthetic.

What were the results of the study?

The researchers report that when the rats’ back paws were injected with the new chemical combination, and then exposed to painful stimuli such as radiant heat or a mechanical stimulus, they did not respond by withdrawing the paw. These anaesthetic effects were maintained for several hours following the injections. However, this combination did not lead to the muscle paralysis seen following lidocaine injection.

What interpretations did the researchers draw from these results?

The researchers claim to have discovered a possible mechanism by which pain can be relieved without impairing thinking, alertness, coordination, or other vital functions of the nervous system.

They point to potential advantages to this strategy in situations where it is important to preserve movement and touch sensations. They think the technique, if shown to be safe and effective in human studies, will be particularly useful in anaesthesia during childbirth, dental procedures and for reducing the long-term pain that some people get following shingles.

What does the NHS Knowledge Service make of this study?

This is an interesting animal study where an original idea has been tested in an animal model. The researchers do not alert us to any reason why this technique could not be applied in humans, or give any indication of how long further studies into safety and efficacy in humans will take.

News sources have also spoken to other commentators who support the potential of the technique. Papers extensively quote Story Landis, director of the National Institute of Neurological Disorders and Stroke, in Bethesda, Maryland.  He said: “The Holy Grail in pain science is to eliminate pathologic pain without impairing thinking, alertness, coordination, or other vital functions of the nervous system. This finding shows that a specific combination of two molecules can block only pain-related neurons. It holds the promise of major future breakthroughs for the millions of persons who suffer with disabling pain.”

Unfortunately, as one of the lead researchers, Professor Clifford Woolf, suggested in The Daily Telegraph, we will have to wait until about “2010 for proof of concept trials in humans”.

NHS Attribution