The pronociceptive effect of proteinase-activated receptor-4 stimulation in rat knee joints is dependent on mast cell activation

Proteinase-activated receptor-4 (PAR4) is a G-protein-coupled receptor activated by serine proteinases released during tissue repair and inflammation. We have previously shown that PAR4 activation sensitises articular primary afferents leading to joint pain. This study examined whether mast cells contribute to this PAR4-induced sensitisation and consequent heightened pain behaviour. The expression of PAR4 on synovial mast cells was confirmed with immunofluorescent staining of rat knee joint sections. Electrophysiological recordings were made from joint primary afferents in male Wistar rats during both nonnoxious and noxious rotations of the knee. Afferent firing rate was recorded for 15 minutes after close intra-arterial injection of 10−9 to 10−5 mol of the PAR4 activating peptide, AYPGKF-NH2, or the inactive peptide, YAPGKF-NH2 (100-μl bolus). Rats were either naive or pretreated with the mast cell stabilise, cromolyn (20 mg/kg). Mechanical withdrawal thresholds were determined using a dynamic planter aesthesiometer and weight bearing determined using an incapacitance tester. These behavioural measurements were taken before and after intra-articular AYPGKF-NH2, or the inactive peptide, YAPGKF-NH2 (100 μg). Local administration of AYPGKF-NH2 caused a significant increase in joint primary afferent firing rate and pain behaviour compared with the control peptide YAPGKF-NH2. These effects were blocked by pretreatment with cromolyn. These data reveal that PAR4 is expressed on synovial mast cells and the activation of PAR4 has a pronociceptive effect that is dependent on mast cell activation.