The findings could lead to the development of new drugs that make morphine last longer and relieve pain at lower doses.

This could reduce the risk of side effects, and even minimise the potential for patients to become tolerant to morphine, and eventually addicted to the drug.

A team from Duke University Medical Center in North Carolina report in the journal Science that morphine had a greater effect when administered to mice genetically engineered to lack a chemical called beta-arrestin 2.

If this technique means that a lower dose is required to achieve pain relief, but a normal dose is still required to get side effects, then that is very exciting Professor David Rowbotham

Beta-arrestin normally comes blocks the chemical signal that morphine sends to the brain to suppress pain sensation.

Therefore, when morphine was given to the mice who lacked the chemical it remained effective for longer periods and lower doses were required to achieve pain relief.

Receptor cells in the body usually become less sensitive to chemical stimuli over time.

For example, a person who enters a coffee shop will initially notice the smell of coffee, but quickly becomes accustomed to the smell, and no longer notices it.

The stimulus has not disappeared, but the olfactory sensory cells that first responded to the smells ignore the stimulus after a time.

In this way, the body becomes accustomed to the effect of morphine after a time, so that its pain relief properties gradually wear off.

When morphine is injected into the blood stream it initially binds to a receptor on the surface of nerve cells which in turn release a chemical message that suppresses the perception of pain.

However, the same process triggers beta-arrestin to turn off the receptor so it is not being constantly stimulated by the morphine.

Without beta-arrestin, morphine's action is enhanced and lasts longer.

The scientists discovered that when morphine was given to mice that had the gene for beta-arrestin knocked out, the mice were resistant to a standard, mildly uncomfortable exposure to a warmed surface almost three times longer than were normal mice.

Surprising findings

The research was carried out on mice

The scientists were surprised at how big the effect of removing beta-arrestin appeared to be.

Researcher Marc Caron said: "We were amazed that beta-arrestin was, in fact, very important in determining how quickly the analgesic properties of morphine wear off."

Dr Caron said that if the same effect was found in humans it might be possible to develop drugs to block the action of beta-arrestin.

Such drugs might be used in combination with lower doses of morphine to extend the amount of pain relief it can provide and reduce the side effects.

Morphine is currently the "gold standard" of analgesic drugs and is used to treat moderate to severe pain following surgery and chronic pain experienced, for example, by cancer patients.

However, morphine sometimes produces nausea, and may suppress breathing if given in too great a dose.

Also, chronic administration of morphine produces tolerance, such that patients need greater and greater doses to achieve pain relief.

The researchers believe that blocking beta-arrestin might also prevent the process of tolerance and even addiction.

Potentially exciting

David Rowbotham, professor of anaesthesia and pain management at Leicester Royal Infirmary, said potentially the results were "very exciting".

He said: "Morphine is good pain killer, but its use is often limited by the risk of side effects.

"There is not much difference in dose between obtaining decent pain relief and unacceptable side effects.

"If this technique means that a lower dose is required to achieve pain relief, but a normal dose is still required to get side effects, then that is very exciting."