How many times in our life have we wished that we could stop a shooting pain by the flick of a switch? It need not remain wishful thinking any more as scientists have now created a molecule that can be turned on to shut out the sensation of pain, by using light.

Scientists in the US and Germany have synthesised a chemical substance that cuts off the sensation of pain when a light is shone on it. More importantly, it leaves other senses and faculties intact as it selectively targets pain-sensing neurons in the brain.

The molecule, which goes by the code name QAQ, actually mimics the activity of a new derivative of lidocaine, a local anaesthetic that dentists commonly use to numb the nerves before a tooth extraction. The lidocaine derivative, which is known as QX-314, was first announced in 2007 by a team of researchers from the Harvard Medical School in Boston. Unlike the parent drug, QX-314, which is still in development, has the ability to exclusively target neurons involved in pain sensation.

However, the scientists, led by Dirk Trauner of the University of Munich and Richard H. Kramer of the University of California, Berkeley, who developed QAQ, have gone one step further by synthesising a photosensitive pain-blocker. The moment the molecule is exposed to light, it becomes active and shuts off the pain sensation instantaneously.

“It is conceptually a new way to control neural activity, and particularly pain sensation. Nothing surpasses the temporal and spatial precision of light,” says Trauner, who along with Kramer directed the research which appeared online in the journal Nature Methods yesterday. Trauner and Kramer have been working on neurobiology of vision for a while and have in fact been exploring the possibility of restoring vision in blind people by manipulating specific neurons.

Most commonly used anaesthetics suppress pain sensation by blocking the activity of neurons involved. But these compounds also do harm. While they eliminate the pathological pain, they also impair thinking, alertness, and motor coordination as they shut down all types of neurons.

Such complete shutdown is not a problem if patients undergoing a surgery have been given general anaesthesia. But it can be problematic in local anaesthesia, when patients remain conscious or require relief from chronic pain. The attendant numbness, loss of touch, movement, co-ordination or mental alertness can be distressing or medically damaging.

Medical scientists have been aware of these shortcomings for a while. There have been serious efforts to address these issues. Some of the modern anaesthetics do preferentially target pain-sensing neurons, but their action can persist for many hours. One of the latest candidate drugs to hit the scene is QX-314. This new-generation anaesthetic is a drug cocktail made of a lidocaine derivative and a chemical — capsaicin — that gives green chilli its characteristic kick. The chilli chemical helps the active ingredient (lidocaine derivative) to get inside the pain-sensing neurons.

“One of the ways in which anaesthetic molecules like lidocaine act is by blocking the voltage gated sodium channels. These channels are present in all neurons and therefore it would block the activity of all neurons,” says S.K. Sikdar, professor of biophysics at the Indian Institute of Science, Bangalore.

An example of this would be the injection of a local anaesthetic for a tooth extraction. Although it is done to stop the sensation of pain from the tooth that is extracted, the anaesthetic also blocks other sensations giving rise to a feeling of numbness in the cheeks. There is a need to target neurons that are specifically carrying the pain, he says. Sikdar thinks that conceptually the new work is a clever idea.

Neurons that transmit pain have some capsaicin sensitive channels. It is because of these channels that the “hotness” of chilli is felt. When there is a painful stimulus, the channels open, the molecule QAQ goes through it to the inside of the neuron and remains there in an inactive form. It starts working only where there is a flash of light, and then is able to stop the propagation of the pain signal.

“Because the molecules have been specifically loaded in the neurons that form part of the pain pathway…, only the pain sensation carrying neurons will be selectively blocked. Other neurons that carry other sensations will be spared,” explains Sikdar.

However, the new substance is years away from hitting the market. “It would take years of testing, first in animals, before the molecule could be deemed safe for use in humans. We are currently not thinking much about market, would rather intend to use it as a basic research tool,” says Trauner.

For millions of pain victims, the wait may well be worth it.