We have previously demonstrated that antithrombin (AT) reduces ischemia/reperfusion (I/R)-induced liver injury by inhibiting leukocyte activation through the promotion of hepatic production of prostacyclin (PGI2) . However, AT does not directly promote the endothelial production of PGI2 in cultured endothelial cells . Capsaicin-sensitive sensory neurons (CSSN) are nociceptive neurons that release calcitonin gene-related peptide (CGRP) upon activation. Since CGRP increases the endothelial production of PGI2, it is possible that AT may increase the hepatic PGI2 production in rats subjected to I/R through the activation of CSSN. Recent studies have demonstrated that CGRP increases the endothelial production of nitric oxide (NO). Since NO has been shown to activate endothelial cyclooxygenase-1 (COX-1) activity, we further examined whether AT-induced increase in hepatic level of PGI2 can be mediated by nitric oxide synthase (NOS) activation. Male Wistar rats were subjected to 60-min ischemia and subsequent reperfusion. Both tissue levels of CGRP and the expression of immunohistochemical CGRP in the liver were significantly increased in rats subjected to I/R 1 hour after reperfusion. AT (250 U/kg, i.v.) significantly enhanced the I/R-induced increase in both hepatic levels of CGRP and the expression of immunohisto-chemical CGRP. AT-induced increase in hepatic level of CGRP and CGRP expression were completely inhibited by capsazepine (CPZ), a vanilloid receptor-1 antagonist. Furthermore, AT-induced increase in hepatic level of 6-keto-PGF1a, a stable metabolite of PGI2, were significantly inhibited by CPZ, CGRP (8-37), a CGRP receptor antagonist, and L-nitro-arginine-methyl-ester (L-NAME), a non-selective inhibitor of NOS. AT reduced the I/R-induced liver injury by inhibiting the I/R-induced increase in hepatic tumor necrosis factor (TNF)-a. Pretreatment of rats with CPZ, CGRP (8-37), and L-NAME completely abrogated such preventive effects of AT. Administration of rat a-CGRP produced effects similar to those of AT. These results strongly suggest that AT might reduce the I/R- induced liver injury by increasing the hepatic level of PGI2 through the activation of CSSN. Thus, AT might sensitize hepatic CSSN in rats subjected to hepatic I/R, leading to the increase in the hepatic tissue level of PGI2. In this process, CGRP-induced activation of endothelial NOS and COX-1 could be critically involved.