Pain signalling in rodent joints by serine proteases

Purpose: Serine proteases modulate pain by either activating or disarming protease-activated receptors (PARs). The aim of the present study was to assess the effect of two serine proteases (cathepsin G and neutrophil elastase) on knee joint mechanosensitivity and pain in rodents. Methods: Electrophysiological recordings from knee joint primary afferents were carried out in deeply anaesthetised male Wistar rats in response to normal and hyper-rotation of the knee. Animals were treated with either normal or denatured cathepsin G (1ng - 10μg; close intra-arterial injection) and nerve recordings repeated. In separate experiments, the effect of intra-articular injection of human neutrophil elastase (1-50μg rat; 1-5μg mouse) on pain behaviour in awake and unrestrained Wistar rats and c57bl/6 mice was determined by measurement of hindlimb incapacitance (weight bearing) and von Frey hair tactile allodynia. Results: Peripheral administration of cathepsin G caused a dose-dependent reduction in joint mechanosensitivity compared to boiled enzyme, which had no discernible effect (P<0.05). The desensitizing effect of cathepsin G occurred with both normal and hyper-rotation of the knee, with the top dose of the enzyme reducing afferent activity by around 75%. Intra-articular injection of neutrophil elastase produced a significant (P<0.05) pain behavioural response in both rats and mice lasting several hours. Conclusions: Cathepsin G reduced joint mechanosensitivity, probably by disarming PAR4 (which is pro-nociceptive), or activating PAR1, which is known to be anti-nociceptive in joints. Local injection of neutrophil elastase produced a robust pain response in rats and mice, possibly by activating PAR2. Future studies will determine the distinct PAR pathways involved in protease-induced pain regulation in arthritic joints.