and 6 GlaxoSmithKline, Screening and Compound Profiling, Harlow, UK Background and purpose: Atypical cannabinoids are thought to cause vasodilatation through an as-yet unidentified 'CBx' receptor. Recent reports suggest GPR55 is an atypical cannabinoid receptor, making it a candidate for the vasodilator 'CBx' receptor. The purpose of the present study was to test the hypothesis that human recombinant GPR55 is activated by atypical cannabinoids and mediates vasodilator responses to these agents. Experimental approach: Human recombinant GPR55 was expressed in HEK293T cells and specific GTPgS activity was monitored as an index of receptor activation. In GPR55-deficient and wild-type littermate control mice, in vivo blood pressure measurement and isolated resistance artery myography were used to determine GPR55 dependence of atypical cannabinoidinduced haemodynamic and vasodilator responses. Key results: Atypical cannabinoids O-1602 and abnormal cannabidiol both stimulated GPR55-dependent GTPgS activity (EC 50 approximately 2 nM), whereas the CB 1 and CB 2 -selective agonist WIN 55,212-2 showed no effect in GPR55-expressing HEK293T cell membranes. Baseline mean arterial pressure and heart rate were not different between WT and GPR55 KO mice. The blood pressure-lowering response to abnormal cannabidiol was not different between WT and KO mice (WT 2072%, KO 2675% change from baseline), nor was the vasodilator response to abnormal cannabidiol in isolated mesenteric arteries (IC 50 approximately 3 mM for WT and KO). The abnormal cannabidiol vasodilator response was antagonized equivalently by O-1918 in both strains.Conclusions: These results demonstrate that while GPR55 is activated by atypical cannabinoids, it does not appear to mediate the vasodilator effects of these agents. (2007) British Journal of Pharmacology

Tetrahydrobiopterin (BH 4) acts as an important co-factor for endothelial nitric oxide synthase (eNOS). Glucocorticolds have been shown to inhibit expression of the rate-limiting enzyme for tetrahydrobiopterin synthesis, GTP cyclohydrolase, in other cell types. We hypothesized that endothelium-dependent vasodilator responses would be blunted in rats made hypertensive with dexamethasone. Further, we hypothesized that treatment of rat vascular segments with dexamethasone would result in attenuation of endothelial function accompanied by decreased GTP cyclohydrolase expression. We report that endotheliumdependent relaxation responses to the calcium ionophore A23187 are reduced in aortic rings from dexamethasonehypertensive rats compared with sham values. Dexamethasone incubation abolishes contraction to N...-nltro-t-argtnlne (L-NNA, 10-5 M) in endothelium-intact aortic rings, and inhibits expression of GTP cyclohydrolase. We conclude that inhibition of BH 4 synthesis by glucocorticoid regulation of GTP cyclohydrolase expression may contribute to reduced endothellumdependent vasodilation characteristic of glucocortlcoid-induced hypertension.

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