Endogenous opioid system inadequate to ease chronic pain

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Endogenous opioid system inadequate to ease chronic pain



Oct 13, 2005



Gandey

Calgary, AB - While researchers had hoped that tapping into the

body's own supply of painkilling peptides would provide a safe and

natural treatment for arthritis pain, a new animal study has shown

that this approach is ineffective in curbing chronic pain [1]. " We

had initially aspired to control pain using naturally occurring

compounds in the body. We anticipated that the endogenous opioid

system would be effective for pain management, " senior author Dr

McDougall (University of Calgary, Alberta) told rheumawire. " We

were surprised to find that while the morphinelike compound found in

knee joints was able to reduce electrical activity and joint pain in

normal and acutely inflamed joints, this was not the case for chronic

arthritis. We did not expect this. " 

The morphinelike compound McDougall refers to is known as endomorphin

1, and his team studied whether peripheral administration of this

endogenous µ-opioid peptide could reduce knee joint pain. Looking at

male rats with induced acute and chronic arthritis, they evaluated

the effectiveness of endomorphin 1 in comparison with treatment with

morphine. The work appears in the October 2005 issue of Arthritis and

Rheumatism.

According to a news release about the study, previous research on µ-

opioid therapy for arthritis has primarily focused on changes

occurring in the hours immediately following tissue inflammation.

This study is reportedly the first to examine the impact on chronic

inflammation.

Endomorphin 1 had no observable effect in chronic arthritis

In the study, rats were randomly assigned to the different treatment

groups, ranging from acute (48-hour) inflammation to chronic (one-

week and three-week) inflammation and normal controls. Under

anesthesia, the rats were injected endomorphin 1 into arthritic knee

joints. Led by Dr Zongming Li (University of Calgary, Alberta), the

researchers assessed therapeutic effectiveness by measures of joint

edema formation and sensory nerve activity associated with pain.



Future study will need to examine why the body's own pain relievers

are not working anymore.





They found that in rats with acute arthritis, endomorphin 1 worked to

significantly reduce the hypersensitivity of joint nerves by as much

as 75%. But in rats with chronic arthritis, endomorphin 1 had no

observable benefit. " This shouldn't be seen as a negative result, "

McDougall told rheumawire. " Instead, it opens the door to new

questions surrounding why the body's analgesic system isn't working

in chronic inflammatory diseases like arthritis. " He added, " Future

study will need to examine why the body's own pain relievers are not

working anymore. "

Li and colleagues conclude, " It is clear that a better understanding

of the signaling mechanisms that control µ-opioid-receptor expression

during arthritis progression and the identification of systemic

factors that can alter this process could have major implications in

our understanding of opioid-induced control of chronic joint pain. "

http://www.jointandbone.org/viewArticle.do?primaryKey=576645