Endomorphin 1 Ineffective For Chronic Arthritis Pain

Chronic arthritis reduces μ-opioid receptor expression

FRIDAY, Sept. 30 (HealthDay News) -- Endomorphin 1, a natural morphine-like compound found in joints, is not effective in treating knee joint pain associated with chronic arthritis, according to a new study published in the October issue of Arthritis & Rheumatism.

Using rats with induced acute or chronic arthritis, Zongming Li, M.D., Ph.D., of the University of Calgary in Alberta, Canada, and colleagues tested the effectiveness of endomorphin 1 in combating knee joint pain.

Endomorphin 1 was injected into the knees of rats with normal knees and those with induced monarthritis. Tests for expression of μ-opioid receptor were conducted at 48 hours and one week. In rats with normal knees, the injection of endomorphin 1 caused a reduction in nerve activity of up to 75%. The same effect was also observed in rats with knees inflamed with acute arthritis. However, in rats with chronic joint inflammation, there was no observable effect on nerve activity, and PCR analysis revealed reduced expression of μ-opioid receptor protein.

"The analgesic effect of endomorphin 1 is lost during chronic inflammation due to down-regulation of μ-opioid receptor expression in afferent nerve cell bodies. These findings begin to explain the ambiguous efficacy of peripherally administered μ-opioid drugs in controlling inflammatory joint pain," the authors conclude.

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