Fig. 4
The potential synovial interoception circuit in OA synovitis. The PGE2 level is increased in ST of OA. The elevated PGE2 may activate EP receptors on sensory nerves, thus transmitting the interoceptive signals to the brain (need verification). The brain, after integrating and interpreting interoceptive signals, downregulates sympathetic nerve activity. The SNS exhibits anti-inflammatory effects during the chronic inflammation phase of the synovium, while suppressed sympathetic activity promotes synovial inflammation and the release of pro-inflammatory cytokines such as TNF-α and IL-1β. Further verifying the existence of synovial interoception circuit is warranted, as well as a more comprehensive exploration of the cerebral nucleis involved in synovial interoception. PGE2 prostaglandin E2, ST synovial tissue, EP prostaglandin E, SNS sympathetic nervous system, TNF-α tumor necrosis factor α, IL-1β interleukin 1β, NE norepinephrine
