Methods: Densities of nerve fibres and BDNF⁺ cells were detected by quantitative immunohistochemistry in fresh synovial tissue from 52 patients with RA, 59 with OA, and 26 controls (Co). BDNF was also detected by in situ hybridisation.

Results: Sympathetic nerve fibre density was similar in Co and OA but markedly reduced in RA (p = 0.002), whereas density of substance P positive (SP⁺) sensory nerve fibres was lower in OA than in Co and RA (p = 0.002). The ratio of sympathetic/SP⁺ sensory nerve fibre density was highest in OA and Co, followed by RA. The correlation between density of sympathetic nerve fibres and SP⁺ sensory nerve fibres in OA (R = 0.425, p = 0.001) was strongly positive, had a positive trend in Co (R = 0.243, NS), but was negative in RA (R = –0.292, p = 0.040). In RA and OA tissue the density of BDNF⁺ cells was high in sublining areas but markedly lower in Co (p = 0.001). BDNF⁺ cell density correlated positively with the ratio of sympathetic/SP⁺ sensory nerve fibre density in Co (R = 0.433, p = 0.045) and in OA (R = 0.613, p = 0.015), but not in RA (R = 0.101, NS). Immunohistochemical double staining demonstrated that some macrophages and fibroblasts were positive for BDNF.