ROLE OF P-450 ARACHIDONIC-ACID EPOXYGENASE IN THE RESPONSE OF CEREBRAL BLOOD-FLOW TO GLUTAMATE IN RATS

Background and Purpose Glutamate, a major excitatory neurotransmitter in the brain, has been implicated in the hyperemic response to increas es in the activity of neurons, but the mechanism of glutamate-induced dilation of cerebral blood vessels is unknown. Glutamate has been show n to enhance the release of arachidonic acid (AA) in brain tissue and cultured astrocytes. We have previously shown that astrocytes metaboli ze AA to vasodilator products, epoxyeicosatrienoic acids (EETs), and e xpress a P-450 AA epoxygenase, P-450 2C11. We tested the hypothesis th at glutamate-induced dilation of cerebral arterioles is mediated in pa rt by changes in the formation and release of EETs by perivascular ast rocytes. Methods Primary astrocyte cultures were prepared from 3-day-o ld rat pups. The cells were labeled with [C-14]AA, and the effect of g lutamate on the formation of EETs from [C-14]AA by cultured astrocytes was studied. The expression of P-450 2C11 protein in the microsomal f ractions of cultured astrocytes was assessed by Western blot. In vivo cerebral blood flow measurements were made in adult rats by laser-Dopp ler flowmetry after administration of glutamate into the subdural spac e of the rat before and after treatment with miconazole. Results Gluta mate treatment (100 mu mol/L for 30 minutes) induced a threefold incre ase in the formation of EETs from [C-14]AA by cultured astrocytes, and the increase was inhibited by miconazole (20 mu mol/L), an inhibitor of P-450 AA epoxygenase. Treatment with glutamate (100 mu mol/L) for 1 2 hours increased the expression of P-450 2C11 protein in the microsom al fraction of cultured astrocytes. The response of laser-Doppler cere bral blood flow to administration of glutamate (500 mu mol/L) into the subdural space of the rat was significantly attenuated after treatmen t with miconazole (20 mu mol/L for 30 minutes). Conclusions These find ings suggest a role for a P-450 AA epoxygenase in astrocytes in the co upling between the metabolic activity of neurons and regional blood fl ow in the brain.