Agonists of proteinase-activated receptor 1 induce plasma extravasation bya neurogenic mechanism

1 Thrombin, generated in the circulation during injury, cleaves proteinase- activated receptor I (PAR1) to stimulate plasma extravasation and granulocy te infiltration. However, the mechanism of thrombin-induced inflammation in intact tissues is unknown. We hypothesized that thrombin cleaves PAR1 on s ensory nerves to release substance P (SP), which interacts with the neuroki nin I receptor (NK1R) on endothelial cells to cause plasma extravasation. 2 PAR1 was detected in small diameter neurons known to contain SP in rat do rsal root ganglia by immunohistochemistry and in situ hybridization. 3 Thrombin and the PAR1 agonist TFLLR-NH2 (TF-NH2) increased [Ca2+](i), > 5 0% of cultured neurons (EC(50)s 24 mu ml(-1) and 1.9 muM, respectively), as sessed using Fura-2 AM. The PAR1 agonist completely desensitized responses to thrombin, indicating that thrombin stimulates neurons through PAR1. 4 Injection of TF-NH2 into the rat paw stimulated a marked and sustained oe dema. An NK1R antagonist and ablation of sensory nerves with capsaicin inhi bited oedema by 44% at 1 h and completely by 5 h. 5 In wild-type but not PAR1(-/-) mice, TF-NH2 stimulated Evans blue extrava sation in the bladder, oesophagus, stomach, intestine and pancreas by 2-8 f old. Extravasation in the bladder, oesophagus and stomach was abolished by an NK1R antagonist. 6 Thus, thrombin cleaves PARI on primary spinal afferent neurons to release SP, which activates the NKIR on endothelial cells to stimulate gap formati on, extravasation of plasma proteins, and oedema. In intact tissues, neurog enic mechanisms are predominantly responsible for PAR1-induced oedema.