PAR4 plays an endogenous antinociceptive role in inflammatory but not in stress-induced colorectal hyperalgesia in mice

Activation of proteinase-activated receptor-4 (PAR4) from the colonic lumen by its agonist (PAR4-AP) or cathepsin-G (Cat-G) has antinociceptive effect to colorectal distension (CRD) in mice in basal conditions. Aims: To clarify if PAR4 activation has antihyperalgesic effect in 2 visceral hypersensitivity models, water avoidance stress (WAS) and TNBS colitis, and to test if endogenous PAR4 activation is present in these conditions. Methods: In one group of C57/BL6 mice, animals underwent WAS. A second group was treated with TNBS (20mg/kg intracolonically/ic/). Controls were either SHAM-stressed or received saline ic. After 4 days of WAS sessions or 72h TNBS infusion mice were infused ic with 100µg PAR4-AP or its vehicle. Other groups of TNBS or WAS treated animals received a PAR4 antagonist (pepducin, 0.75mg/kg) or its vehicle intraperitoneally (ip). Visceral hypersensitivity was assessed by recording abdominal contractions provoked by CRD. Results: WAS significantly increased the abdominal EMG response by 760, 123, 68 and 22%, at the distension volumes from 0.02 to 0.08 mL. PAR4-AP completely restored basal sensitivity at all volumes (p<0.05). Pepducin did not change the hyperalgesia provoked by stress. In TNBS colitis, EMG activity was elevated by 1529, 98, 90 and 25% compared to controls. PAR4-AP reduced the increase of visceral sensitivity to the baseline (p<0.05). Pepducin significantly increased the hypersensitivity at the volumes of 0.04, 0.06 and 0.08 mL compared to TNBS alone (p<0.05). Cat-G was elevated in feces after TNBS compared to controls (17.0±2.6 vs. 8.8±1.9 U/mg, p<0.05), but not after WAS (6.9±2.5 U/mg). Conclusions: PAR4 activation displays analgesic properties in both inflammatory and non-inflammatory colorectal hyperalgesia. Endogenous PAR4 activation is present and plays an antinociceptive role in inflammatory, but not in stress induced colorectal hyperalgesia. These data also suggest that Cat-G released by neutrophils in TNBS colitis may be responsible for such endogenous activation. (Grant: TÁMOP-4.2.2–08)