Urocortin is a newly identified member of the corticotropin-releasing factor (CRF) neuropeptide family, and is known to be involved in the modulation of the inflammatory process. We examined the expression of urocortin, CRF and their receptors (CRF receptor; CRF-R) in the synovial tissue of patients with rheumatoid arthritis (RA) in order to study the possible biological roles of urocortin. Synovial tissues/fluids were obtained from 38 patients with RA, nine patients with osteoarthritis and four with trauma. We studied the concentration of urocortin in the synovial fluid using RIA, and the expression of urocortin in synovial tissue using immunohistochemistry, mRNA in situ hybridization and reverse transcriptase-PCR (RT-PCR). In addition, we examined the immunolocalization of CRF and the expression of CRF-R1, -R2-alpha and -R2-beta mRNAs utilizing RT-PCR in these synovial tissues. Urocortin concentrations in synovial fluid were higher in RA patients (79.8+/-154 pg/ml) than in control patients (12.3+/-4.8 pg/ml; P< or =0.05). Urocortin immunoreactivity and mRNA signals were both detected in synovial cells, lymphocytes, fibroblasts and macrophages. The number of urocortin-positive cells in the synovium was significantly higher in RA (73.1+/-32.1 cells per high-power field) than in control (18.4+/-10.4 cells per high-power field) patients. In addition, both urocortin immunoreactivity and mRNA signals in the synovium reached maximum levels in the active stage of RA inflammation. Moreover, the number of immunoreactive urocortin-positive cells was significantly correlated with the urocortin concentration in synovial fluid (r=0.705; P<0.001) and with histologically defined local inflammatory activity (r=0.641; P<0.001). The distribution and number of immunoreactive CRF-positive cells in synovial tissue were similar to those of urocortin-positive cells (r=0.701; P<0.001). Urocortin, CRF-R1 and CRF-R2-alpha mRNAs detected by RT-PCR were expressed in in the synovium of 10/10, 10/10 and 2/10 RA patients respectively, but CRF-R2-beta was not expressed. Urocortin was actively synthesized in the synovium of RA patients. The present study suggests that urocortin may play an important role as an autocrine and/or paracrine regulator of synovial inflammation in RA.