Neither nociceptin nor its receptor are present in human synovial fluid or tissue.

Our aim was to identify the nociceptin receptor and its endogenous ligand, nociceptin, in human peripheral tissue. Synovial tissue was obtained from 11 patients (ASA I-III, 66-84 yr) undergoing elective total knee replacement. Synovial fluid was obtained from another 10 patients (ASA I-III, 57-81 yr). Fluid was mixed with trifluoroacetic acid and the tissue with isopentone before freezing at -70 degrees C. Nociceptin receptor identification was performed using a [3H]nociceptin binding assay and nociceptin detection by radioimmunoassay. There was no specific [3H]nociceptin binding to knee synovial tissue and radioimmunoassay did not detect nociceptin. Neither the nociceptin receptor nor nociceptin was found in human synovial tissue or fluid.