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|Title:||NMDA RECEPTOR AGONISTS DILATE ISOLATED MOUSE MIDDLE CEREBRAL ARTERIES BY AN ENOS-MEDIATED MECHANISM|
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|Authors/Affiliations:||1 Jillian LeMaistre*; 2 Hope Anderson; 1 Christopher Anderson; |
1 Department of Pharmacology and Therapeutics- University of Manitoba, Winnipeg, MB, Canada; 2 Faculty of Pharmacy- University of Manitoba, Winnipeg, MB, Canada
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|Content:||Objective: Cerebral blood flow regulation involves a relationship between neurons, astrocytes and blood vessels, comprising the neurovascular unit. Astrocytes extensively encircle arterioles and function by releasing numerous gliotransmitters via calcium-dependent pathways in response to neuronal stimulation. One such gliotransmitter is D-serine, a NMDA receptor co-agonist, which may participate in cerebral blood flow regulation, since NMDA receptors have been shown to influence cerebrovascular tone. NMDA receptors are expressed by endothelial cells in culture, but the specific function of these receptors remains unclear. We hypothesize that D-serine from astrocytes initiates vasoactive signals mediated by endothelial NMDA receptors. The objective of the current study was to determine whether NMDA receptors are involved in glutamate and D-serine-induced vasomotor responses in isolated cerebral arteries.|
Materials and methods: Middle cerebral arteries (MCAs) from 15 week old male CD1 mice were isolated, mounted between glass cannulae, pressurized to 30 mmHg and preconstricted with norepinephrine (1 ÁM). Dilatory responses to NMDA receptor agonists, antagonists and other tests compounds were measured by video microscopy.
Results: Neither glutamate nor D-serine alone produced significant vasodilation up to concentrations of 10 mM. However, glutamate and NMDA produced a concentration-dependent dilation of MCAs (31 ▒ 5.1% at 100 ÁM glutamate) in the presence of D-serine (100 ÁM). Vasodilatory effects of glutamate and D-serine together (both 100 ÁM) were significantly reduced by both the competitive NMDA receptor antagonists, 2-amino-5-phosphonopentanoic acid (AP5, 100 nM, 10 ÁM) and the selective glycine site antagonist, 5,7-dichloro-kynurenic acid (DCKA, 300 nM, 30 ÁM), while a blocker of AMPA/kainate receptors (6-cyano-7-nitroquinoxaline-2,3-dione, CNQX, 10 ÁM) did not affect glutamate/D-serine induced vasodilation. Denuding the endothelium abolished vasodilation induced by glutamate/D-serine, as did pre-treatment of MCAs with the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 10 ÁM). Also, a neuronal nitric oxide synthase inhibitor (1-(2-(trifluoromethylphenyl))imidazole, TRIM, 50 ÁM) failed to prevent relaxation, but an endothelium nitric oxide synthase antagonist (Diphenylene iodonium, DPI, 1 ÁM) blocked vasodilation.
CONCLUSION: These results provide functional evidence that NMDA receptors are expressed by cerebral arterial endothelium, and that the gliotransmitter D-serine is capable of producing NMDA receptor-mediated cerebral vasodilation in combination with glutamate. Furthermore, our results indicate that mouse cerebrovascular NMDA receptors may mediate vasodilation by stimulating endothelial nitric oxide production. Supported by the Ajinomoto Amino Acids Research Program and the Canadian Institutes of Health Research.
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