LOCAL ANALGESIC EFFECT OF ENDOGENOUS OPIOID-PEPTIDES

Opioids produce analgesia by interacting with local opioid receptors i n peripheral inflamed tissue. This study investigated whether endogeno us ligands of these receptors are present in synovia and whether such opioid peptides can inhibit pain by activation of intra-articular opio id receptors. Samples of synovium from 8 patients undergoing arthrosco pic knee surgery were examined by immunohistochemistry for the presenc e of beta-endorphin, met-encephalin, and dynorphin. All tissue samples showed synovitis. Inflammatory cells stained strongly for beta-endorp hin and met-encephalin but not for dynorphin. To find out whether bloc kade of intra-articular opioid receptors affected pain, we randomly as signed 22 patients undergoing arthroscopic knee surgery to receive nal oxone (0.04 mg) intra-articularly (n = 10) or intravenously (n = 12); each patient received a placebo injection into the other site. Postope rative pain was assessed by visual analogue scale, a numerical rating scale, the McGill pain questionnaire, and supplementary analgesic cons umption during the next 24 h. All pain scores were higher in the intra -articular naloxone group than in the intravenous naloxone group. The differences were significant (p < 0.05) during the first 4 h. Suppleme ntary analgesic consumption was significantly higher in the intra-arti cular group (52.5 14.0! vs 15.6 8.0! mg diclofenac, p < 0.05). Opioi d peptides are present in inflamed synovial tissue and can inhibit pai n after knee surgery through an action specific to intra-articular opi oid receptors, These findings expand the gate control theory of pain a nd suggest new approaches such as the development of peripherally acti ng opioid analgesics without central side-effects.