June 25, 2008

Science Pinpoints Why General Anaesthetics Worsen Pain in Recovery

General anaesthetic can make the pain of operations worse for patients recovering after surgery by activating the body's "mustard receptors", researchers have found.

Many of the drugs that send surgical patients to sleep are known to make them more sensitive to pain when they wake up. Scientists now believe they have discovered the reason for the side effect. The findings may help researchers develop new anaesthetics that are kinder to recovering patients.

General anaesthetics all suppress the central nervous system to cause unconsciousness and paralysis.

But they can also activate "pain-sensing" cells in the peripheral nervous system, leading to post-operative discomfort.

The new research focused on two of these pain receptors, TRPV1 and TRPA1, which often appear together and also react to irritants such as the chilli chemical capsaicin and mustard.

Study leader Dr Gerard Ahern, from Georgetown University Medical Centre in Washington DC, said: "Plants produce chemicals such as capsaicin, mustard and garlic that were meant to stop animals from eating them. When they are eaten, the two main receptors that react to them are TRPV1 and TRPA1. In fact, TRPA1 is more commonly known as the mustard oil receptor, and is a principal receptor in the pain pathway."

Experiments showed that general anaesthetics had a direct effect on TRPA1 activity.

Mice without TRPA1 genes appeared to feel no pain when they were exposed to the drugs.

"Most anaesthetics activate the mustard oil receptor, and animals that don't have the receptor don't have irritation, " said Dr Ahern.

The findings are reported in the journal Proceedings of the National Academy of Sciences.

They suggest that sensory nerve stimulation throughout the body caused by anaesthetic adds to the pain felt by a patient waking up after surgery.

General anaesthetics exist that do not activate the mustard oil receptor, but they may lack effectiveness, said Dr Ahern.

He added: "This tells us that there is room for improvement in these drugs."

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