Activation of PAR‐2 Elicits NO‐Dependent and CGRP‐Independent Dilation of the Dural Artery

(Headache 2010;50:1017‐1030) Objectives.— The goal of this study was to determine the vascular effects of protease‐activated receptor‐2 (PAR‐2) activation in the rat cranial vasculature. Background.— The role of PAR‐2 in pain and inflammatory conditions has been established but the information available on its effects and receptor distribution in the trigeminal vascular axis is limited. We studied the dilatory function and expression of PAR‐2 in the neuro‐vascular circuit, critical in migraine pathogenesis. We also investigated the interaction of PAR‐2 with calcitonin gene‐related peptide (CGRP) and dural mast cells. Methods.— We used an improved model of intravital microscopy on the closed cranial window in rats to study the vascular effects of PAR‐2 activating peptides (PAR‐2 APs; SLIGRL‐NH₂, 2‐Furoyl‐LIGRLO‐NH₂) in the dural vasculature. Measurement of immunoreactive CGRP in skull halves and in trigeminal nucleus caudalis was done by using an enzyme‐linked immunosorbent assay. We also analyzed the presence of PAR‐2 in different migraine relevant tissues by quantitative real‐time PCR and Western blot analysis. Results.— PAR‐2 APs and trypsin induced a dose‐dependent increase in dural artery diameter. The topical application of a nonspecific nitric oxide synthase (NOS) inhibitor, L‐N^G‐Nitroarginine methyl ester, attenuated SLIGRL‐NH₂ responses. Olcegepant, a CGRP receptor antagonist, did not a have significant effect on the SLIGRL‐NH₂ responses, though exogenous CGRP responses were completely blocked. There was no significant release of CGRP from skull halves incubated with SLIGRL‐NH₂ as compared with those incubated with the corresponding negative peptide. Chronic mast cell degranulation did not change the vascular effects of PAR‐2 APs. mRNA and protein expression of PAR‐2 were found throughout trigeminovasuclar axis. Conclusion.— PAR‐2 activation leads to vasodilation of dural arteries and these responses are partially mediated by nitric oxide. As PAR‐2 is present throughout trigeminovasuclar axis, it may have a role in migraine pathogenesis, independent of CGRP and mast cell mediated mechanism.