Chronic arthritis down‐regulates peripheral μ‐opioid receptor expression with concomitant loss of endomorphin 1 antinociception
Bibliographic record
Abstract
OBJECTIVE: To determine whether peripheral administration of the endogenous mu-opioid peptide endomorphin 1 could reduce knee joint pain, using animal models of acute and chronic arthritis. METHODS: Extracellular electrophysiologic recordings were made of rat knee joint primary afferent nerve activity in response to noxious hyperrotation of the joint. Neuronal activity was assessed before and following local injection of endomorphin 1. Comparisons were made between normal knees and knees with adjuvant-induced monarthritis, tested at 48 hours and 1 week posttreatment. Expression of mu-opioid receptors in the dorsal root ganglia ipsilateral to the chronically inflamed joints was determined by real-time polymerase chain reaction (PCR) and immunohistochemical analysis. RESULTS: In normal knees, endomorphin 1 caused up to a 75% reduction in joint afferent nerve activity, which was blocked by the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-amide. The inhibitory effect of endomorphin 1 was sustained in acutely inflamed knees. Conversely, in chronically inflamed joints, endomorphin 1 had no observable effect on the primary afferent nerve firing rate elicited by a noxious mechanical stimulus and, as such, was significantly different from the rate in normal joints. Immunohistochemical and real-time PCR analysis of the L3-L5 dorsal root ganglia ipsilateral to the chronic arthritis lesion revealed a reduction in mu-opioid receptor protein and gene expression compared with that in normal control animals. CONCLUSION: Taken together, these results provide the first electrophysiologic evidence that selective activation of peripheral mu-opioid receptors reduces normal knee joint mechanosensitivity to a noxious stimulus. Furthermore, the analgesic effect of endomorphin 1 is lost during chronic inflammation due to down-regulation of mu-opioid receptor expression in afferent nerve cell bodies. These findings begin to explain the ambiguous efficacy of peripherally administered mu-opioid drugs in controlling chronic inflammatory joint pain.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".