People suffering chronic pain from bad backs may not get relief from conventional painkillers, but new drugs are being developed to help them.

Normal analgesics stop the brain from receiving pain signals, but have little or no effect on many types of long-lasting pain.

But scientists believe they may have found a way to block the brain from sensing this kind of chronic pain.

The latest research centres on a compound called A-317491.

This appears to block chemical receptors on nerves which are believed to play a vital role in the way the brain perceives pain.

If these receptors are blocked, then the intensity of pain experienced should be less.

Not only this, but this kind of receptor is generally found only on this kind of sensory nerve cell.

Bigger doses

This means that higher doses can be given, with greater effect, but without the drug interfering with some other important cell function by blocking receptors on other types of cell.

All this adds up to fewer side effects for the patient, even at higher doses.

The researchers, based at the University of Iowa, tested their compound on rats.

They found, that when injected, it reduced certain types of chronic pain linked to inflammation and nerve damage.

This is similar to the long-lasting, debilitating pain suffered by people with serious back problems.

Falling effect

In addition, there were few side-effects compared with other analgesics, and giving the drug repeatedly did not lead to a reduced effect per dose.

Morphine based painkillers suffer from this "tolerance" effect.

Dr Charles Kennedy, who researches the mechanisms of pain at Strathclyde University, said that conventional painkillers did not make much impression on chronic "neuropathic" pain such as this.

He told BBC News Online: "There are a number of research projects looking at ways to treat this kind of pain.

"This certainly looks promising, and if it works, it would be very helpful to these patients.

"It is certainly a potentially big market for pharmaceutical firms."

The research results were published in the Proceedings of the National Academy of Sciences.