Protease‐activated receptor‐4 (PAR4): a role as inhibitor of visceral pain and hypersensitivity

Protease‐activated receptor‐4 (PAR4) belongs to the family of receptors activated by the proteolytic cleavage of their extracellular N‐terminal domain and the subsequent binding of the newly released N‐terminus. While largely expressed in the colon, the role of PAR4 in gut functions has not been defined. We have investigated the effects of PAR4 agonist on colonic sensations and sensory neuron signalling, and its role in visceral pain. We observed that a single administration of the PAR4 agonist peptide (AYPGKF‐NH2), but not the control peptide (YAPGKF‐NH2) into the colon lumen of mice significantly reduced the visceromotor response to colorectal distension at different pressures of distension. Further, intracolonic administration of the PAR4 agonist, but not the control peptide, was able to significantly inhibit PAR2 agonist‐ and transcient receptor potential vanilloid‐4 (TRPV4) agonist‐induced allodynia and hyperalgesia in response to colorectal distension. Protease‐activated receptor‐4 was detected in sensory neurons projecting from the colon, and isolated from the dorsal root ganglia, where it co‐expressed with PAR2 and TRPV4. In total sensory neurons, PAR4 agonist exposure inhibited free intracellular calcium mobilization induced by the pro‐nociceptive agonists of PAR2 and TRPV4. Finally, PAR4‐deficient mice experienced increased pain behaviour in response to intracolonic administration of mustard oil, compared with wild‐type littermates. These results show that PAR4 agonists modulate colonic nociceptive response, inhibit colonic hypersensitivity and primary afferent responses to pro‐nociceptive mediators. Endogenous activation of PAR4 also plays a major role in controlling visceral pain. These results identify PAR4 as a previously unknown modulator of visceral nociception.