Hitting pain where it hurts

A technique to selectively deactivate pain pathways from specific parts of the body has been developed by scientists in the US. For some, enduring chronic pain makes daily life almost intolerable. Analgesic drugs frequently have disabling side effects, such as sedation or mood disturbances, and cutting nerves to block pain, which is sometimes necessary, can also destroy motor pathways, leading to muscle weakness and paralysis.

Now researchers have found a way to selectively delete pain nerves from an affected body part while leaving all of the other pathways intact.

Working with pigs, which are a good model for humans, University of California scientist Jacob Brown and his colleagues have used a CT scanner to inject tiny amounts of a chemical called resiniferatoxin into sites known as the dorsal root ganglia.

A succession of these ganglia flank the backbone on each side of the body. They are the point where sensory nerves enter the spinal cord to relay sensory messages to the brain.

Each ganglion is connected to a different patch of neurological territory, which it supplies. Pain coming from a certain region of the body will therefore correspond to a specific ganglion or group of ganglia.

The purpose of injecting resiniferatoxin is that this chemical, which comes from a species of Euphorbia plant, works like capsaicin, the spicy ingredient in chilli peppers. It selectively activates a chemical receptor called TRPV1, which is found only on pain-transmitting nerve cells. When resiniferatoxin engages this receptor, it stimulates the nerve so fiercely that the cell dies, cutting off that pain pathway. Other types of nerve are left unharmed.

In pigs that underwent trials of these injections, within two weeks the patch of skin supplied by the operated ganglia showed a sustained drop in pain sensitivity. This was borne out by a microscope study of the injected ganglion, which showed a 66% drop in the number of cells carrying the TRPV1 receptor.

Critically, the animals did not show any negative side effects or skin changes on the affected part of the body.

If translated to humans, this new approach, which is described this week in Science Translational Medicine, could offer an alternative to traditional analgesia for certain chronic pain states.