Protease-activated receptors (PARs) are a family of G-protein-coupled receptors activated by a tethered ligand amino acid sequence within the amino terminal that is revealed by site-specific proteolysis. In the vascular endothelium, activation of PAR-2 by treatment with trypsin or by using the amino acid ligand sequence (SLIGRL) produces endothelium-dependent relaxation of isolated noncerebral vascular segments. In this study, we first tested whether PAR-2 activation produces cerebral vasodilatation in vivo and then examined whether PAR-2-mediated vasodilatation is dependent on the production of nitric oxide.Concentration-dependent vasodilator effects of the PAR-2 agonist peptide SLIGRL and trypsin were examined on the basilar artery using a cranial window in anesthetized rats. In addition, the vasodilator effects of SLIGRL, acetylcholine (ACh), and sodium nitroprusside (SNP) were examined in the absence and presence of N(G)-nitro-L-arginine (L-NNA), an inhibitor of nitric oxide synthase, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-l-one (ODQ), an inhibitor of soluble guanylate cyclase.Baseline diameter of the basilar artery averaged 239+/-4 microm. Under control conditions, SLIGRL (10(-6) to 10(-4) mol/L) and trypsin (0.01 to 10 U/mL) produced concentration-dependent vasodilator responses. In time-control experiments, SLIGRL (3 x 10(-6) and 10(-5) mol/L), ACh (10(-6) and 10(-5) mol/L), and SNP (10(-8) and 10(-7) mol/L) elicited reproducible dilatation of the basilar artery. In another group of rats, L-NNA (10(-4) mol/L) markedly inhibited dilator responses to both SLIGRL (13+/-3% versus 1+/-1% and 39+/-7% versus 11+/-2%; both P<0.05) and ACh (8+/-1% versus 0+/-0% and 13+/-2% versus 3+/-1%; both P<0.05). By contrast, responses to SNP were significantly augmented after treatment with L-NNA (P<0.05 versus control), indicating that inhibitory effects of L-NNA were specific for responses mediated by endogenous nitric oxide. Furthermore, in another group ODQ (10(-5) mol/L) inhibited responses to SLIGRL to a degree similar to that seen with L-NNA, consistent with a mechanism of PAR-2-mediated vasodilatation that involves activation of guanylate cyclase by nitric oxide.To the best of our knowledge, this study is the first to examine whether PAR-2-mediated vasodilatation is functional in cerebral arteries and is also the first to directly assess the effects of PAR-2 activation on vascular tone in vivo. The results suggest that activation of PAR-2 is an effective and powerful vasodilator mechanism in cerebral arteries in vivo. Cerebral vasodilator responses to PAR-2 activation are mediated by nitric oxide and are likely to be endothelium dependent.