Chronic Arthritis Impairs Pain Control Mechanisms

October 21, 2005

NEW YORK (Reuters Health) Oct 13 - Murine studies indicate that

chronic arthritis downregulates the expression of receptors that play

a key role in pain control, Canadian researchers report in the

October issue of Arthritis & Rheumatism.

" What may be effective in relieving pain during the acute phase of

the disease, " Dr. J. McDougall from the University of Calgary,

Alberta, Canada told Reuters Health, " does not necessarily equate

during chronic arthritis and we may have to revise our treatment

strategies accordingly. "

Dr. McDougall and colleagues investigated the effects of peripheral

administration of the endogenous mu-opioid peptide endomorphin 1 on

knee joint pain in rat models of acute and chronic arthritis.

Injection of endomorphin 1 reduced knee joint afferent nerve activity

in response to noxious rotation of the knee by about 75% in normal

knees and by just over 50% in knees with adjuvant-induced arthritis

of 48 hours duration, the authors report.

The analgesic effect of endomorphin 1 was blocked by the specific mu-

opioid receptor antagonist CTOP, the researchers note. In contrast,

injection of endomorphin 1 a week after induction of monoarthritis

had no observable effect on joint afferent nerve activity following

hyperrotation of the knee.

Mu-opioid receptor was clearly demonstrated by immunostaining in

dorsal root ganglion cells from normal rats and from rats with acute

inflammation, the investigators report, but no such staining was seen

in dorsal root ganglion cells from rats with monoarthritis of 1 week

and 3 weeks duration.

Similarly, the results indicate, expression of mu-opioid receptor

mRNA did not differ between normal rats and those with acute

inflammation, but mRNA expression was markedly and significantly

reduced in animals examined 3 weeks after administration of adjuvant.

" The next logical step, " Dr. McDougal concluded, " is to find out why

these receptors are disappearing during chronic arthritis and

determine what interventions we can implement to try to prevent this

from happening. "

Arthritis Rheum 2005;52:3210-3219.

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