EVIDENCE THAT FUNCTIONAL GLUTAMATE RECEPTORS ARE NOT EXPRESSED ON RATOR HUMAN CEREBROMICROVASCULAR ENDOTHELIAL-CELLS

Excitatory amino acids can modify the tone of cerebral vessels and per meability of the blood-brain barrier (BBB by acting directly on endoth elial cells of cerebral vessels or indirectly by activating receptors expressed on other brain cells. in this study we examined whether rat or human cerebromicrovascular endothelial cells (CEC) express ionotrop ic and metabotropic glutamate receptors. Glutamate and the glutamate r eceptor agonists N-methyl-d-aspartate (NMDA), pha-amino-3-hydroxy-5-me thyl-isoxazole-4-propionic acid (AMPA), and kainate failed to increase [Ca2+](i) in either rat or human microvascular and capillary CEC but elicited robust responses in primary rat cortical neurons, as measured by fura-2 fluorescence. The absence of NMDA and AMPA receptors in rat and human CEC was further confirmed by the lack of immunocytochemical staining of cells by antibodies specific for the AMPA receptor subuni ts GluR1, GluR2/3, and GluR4 and the NMDA receptor subunits NR1, NR2A, and NR2B. We failed to detect mRNA expression of the AMPA receptor su bunits GluR1 to GluR4 or the NMDA receptor subunits NR1(1XX), NR1(0XX) , and NR2A to NR2C in both freshly isolated rat and human microvessels and cultured CEC using reverse transcriptase polymerase chain reactio n (RT-PCR). Cultured rat CEC expressed mRNA for KA1 or KA2- and GluR5 subunits. Primary rat cortical neurons were found to express GluR1 to GluR3 and NR1, NR2A, and NR2B by both immunocytochemistry and RT-PCR a nd KA1, KA7, GluR5, GluR6, and GluR7 by RT-PCR. Moreover, the metabotr opic glutamate receptor agonist 1-amino-cyclopentyl-1S, 3R-dicorboxyla te (1S,3R-trans-ACPD), while eliciting both inositol trisphosphate and [Ca2+](i) increases and inhibiting forskolin-stimulated cyclic AMP in cortical neurons, was unable to induce either of these responses in r at or human CEC. These results strongly suggest that both rat and huma n CEC do not express functional glutamate receptors. Therefore, excita tory amino acid-induced changes in the cerebral microvascular tone and BBB permeability must be affected indirectly, most likely by mediator s released from the adjacent glutamate-responsive cells.