Submitted on September 23, 2004
Accepted on January 28, 2005
U-46619 but not Serotonin Increases Endocannabinoid Content in the Middle Cerebral Artery: Evidence for Functional Relevance
David J Rademacher1, Sachin Patel1, W.-S. Vanessa Ho1, Amanda M Savoie1, Nancy J Rusch1, Kathryn M Gauthier1, and Cecilia J Hillard1*
1 Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, WI, USA
* To whom correspondence should be addressed. E-mail: chillard{at}mcw.edu.
Cerebral vascular smooth muscle cells express the CB1 cannabinoid receptor and CB1 receptor 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 CB1 receptors, 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, while 5-hydroxytrypamine (5-HT) decreased AEA and 2-AG content. If eCBs play a feedback role in the regulation of MCA tone, then CB1 receptor 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 CB1 receptor antagonists, SR141716 and AM251, decreased the EC50 value for U-46619 to constrict endothelium-denuded MCA without affecting Emax. A low concentration of CB1 receptor agonist Win 55212-2 (30 nM) produced vasodilation of MCAs constricted with low but not saturating concentrations of U-46619. SR141716 had no effect on the 5-HT concentration-contraction relationship. These data suggest that TXA2 receptor activation increases MCA eCB content which, via activation of CB1 receptors, reduces the constriction produced by moderate concentrations of the TXA2 agonist. Although 5-HT-induced vasoconstriction is reduced by exogenous CB1 receptor 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 TXA2 receptors and not 5-HT receptors.
