A new study has revealed that general anesthesia given to patients during surgery is likely to increase post-surgical pain.
Researchers from Georgetown University Medical Centre have found that the "noxious" anesthesia drugs activate and then sensitize specific receptors on neurons in the peripheral nervous system.
Anesthetics cause irritation at the infusion site or in the airways when inhaled and can also activate so called "pain-sensing" or nociceptive nerve cells on the peripheral nervous system, leading to discomfort for patients when they wake up.
To understand the specific mechanism by which anesthetics affect sensory neurons, researchers focussed on two specific receptor on the nerves cells (TRPV1 and TRPA1), which are often expressed together and react to other irritants.
"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," said the study's lead investigator, Gerard Ahern, Ph.D., an assistant professor in the Department of Pharmacology at Georgetown University Medical Centre
"In fact, TRPA1 is more commonly known as the mustard-oil receptor, and is a principal receptor in the pain pathway," he added.
They found showed that general anesthetics appear to regulate TRPA1 in a direct fashion, and are thus responsible for the acute noxious effects of the drugs.
The study conducted using a mouse model showed that mice bred without TRPA1 genes demonstrate no pain when the drugs are administered and used, Ahern said.
"Most general anesthetics activate the mustard oil receptor, and animals that don't have the receptor don't have irritation," he said.
The nerve-mediated inflammation was greater when pungent (chemical irritants) versus non-pungent inhaled general anesthetics were used.
Ahern said that both findings suggest that sensory nerve stimulation throughout the body just before and during surgery adds to the pain that is felt after the patient is awake.
"This is a provocative finding in terms of the clinical setting, because it was not really recognized that use of these drugs results in release of lots of chemicals that recruit immune cells to the nerves, which causes more pain or inflammation.
"The choice of anesthetic appears to be an important determinant of post-operative pain. We hope these findings are ultimately helpful in providing more comfort to patients," he added.
The study appears in June 23rd issue of the Proceedings of the National Academy of Sciences (PNAS).