An experimental gene therapy treatment has given rats relief from chronic pain for three months, say US researchers.

Dr Andreas Beutler and colleagues at the Mount Sinai School of Medicine in New York report their findings in this week's issue of the Proceedings of the National Academy of Sciences.

The rats were injected with a gene that tricks the body into releasing endorphins, a natural painkiller, in the nerve cells surrounding the spinal cord.

The treatment simulates the effect of painkilling drugs but is much narrower in scope, targeting nerve cells along the spinal cord, but not in the brain or in other parts of the central nervous system.

Researchers are hopeful that the therapy could potentially be used to treat people with severe or chronic pain sparing them some of the debilitating side effects of the opiod drugs currently on the market which act systemically, rather than locally.

The rats, which had an animal version of neuropathic pain, were symptom free for over three months.