U-46619 but not serotonin increases endocannabinoid content in middle cerebral artery: evidence for functional relevance

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Abstract
Cerebral vascular smooth muscle cells express the CB1cannabinoid receptor, and CB1receptor agonists produce vasodilation of cerebral arteries. The purpose of this study was to determine whether vasoconstriction of rat middle cerebral artery (MCA) results in the local formation of endocannabinoids (eCBs), which, via activation of CB1receptors, oppose the vasoconstriction in a feedback manner. The thromboxane A2(TXA2) mimetic U-46619 significantly increased N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG) content of isolated MCA, whereas 5-hydroxytrypamine (5-HT) decreased AEA and 2-AG content. If eCBs play a feedback role in the regulation of MCA tone, then CB1receptor antagonists should enhance the constriction of MCA produced by U-46619 but not 5-HT. U-46619 caused concentration-dependent constrictions of endothelium-denuded MCA. Two CB1receptor antagonists SR-141716 and AM-251 decreased the EC50value for U-46619 to constrict endothelium-denuded MCA without affecting the maximal effect. A low concentration of CB1receptor agonist Win-55212-2 (30 nM) produced vasodilation of MCAs constricted with low but not saturating concentrations of U-46619. SR-141716 had no effect on the 5-HT concentration-contraction relationship. These data suggest that TXA2receptor activation increases MCA eCB content, which, via activation of CB1receptors, reduces the constriction produced by moderate concentrations of the TXA2agonist. Although 5-HT-induced vasoconstriction is reduced by exogenous CB1receptor agonist, activation of 5-HT receptors does not increase eCB content. These results suggest that MCA production of eCBs is not regulated by constriction per se but likely via a signaling pathway that is specific for TXA2receptors and not 5-HT receptors.