Expression of glutamate receptors on cultured cerebral endothelial cells

Activation of glutamate receptors has been shown to mediate a large number of neuronal processes such as long-term potentiation and ischemic damage. In addition to neurons and glia, glutamate receptors may occur on cerebral endothelial cells (CECs). The aim of the present study was to determine which glutamate receptors are expressed in CECs and to demonstrate the functional presence of such channels. By using reverse transcriptase-polymerase chain reaction, we showed that primary cultures of rat CECs express N-methyl-D-aspartate (NMDA) receptors (NR1 subunit, which is necessary for the formation of functional NMDA receptors, and NR2A-C subunits), 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl-propionate (AMPA) receptors (GLUR1-4 subunits), and metabotropic receptors (mGLUR). Exposure of the cultures to 2 mM glutamate, a well-established mediator of ischemic damage, for 30 min increased significantly the phosphorylation of calcium/calmodulin-dependent protein kinase II even after 10- and 60-min recovery times. This effect could be prevented by the NMDA blocker MK-801. The presence of multiple glutamate receptor types may confer a finely tuned responsiveness of the cerebral endothelium to glutamate in physiological and pathological conditions.

Effect of subtype-specific Ca(2+)-antagonists and Ca(2+)-free media on the field stimulation-evoked release of ATP and [3H]acetylcholine from rat habenula slices

The involvement of different subtypes of voltage-sensitive (Ca2+ channels in the initiation of field stimulation-induced endogenous adenosine triphosphate (ATP) and [3H]acetylcholine ([3H]ACh) release was investigated in the superfused rat habenula slices. ATP, measured by the luciferin-luciferase assay, and [3H]ACH were released simultaneously from the tissue in response to low frequency electrical stimulation (2 Hz, 2.5 msec, 360 shocks). The N-type Ca(2+)-channel blocker omega-conotoxin GVIA (omega-CgTX, 0.01-1 microM) reduced the stimulation-evoked release of ATP and [3H]ACh in a dose-dependent manner. Similarly, the P-type Ca2+ channel antagonist omega-agatoxin IVA (omega-Aga IVA) (0.05 microM) and the inorganic Ca(2+)-channel blocker Ca2+ (0.2 mM) inhibited the outflow of both transmitters, while Ni2+ (0.1 mM) was without significant effect. A high correlation was observed between the percent inhibition of ATP release and percent inhibition of ACh release caused by the different Ca2+ antagonists. Long-term perfusion (i.e., 90 min) with Ca(2+)-free solution inhibited the evoked-release of ATP and [3H]ACh. In contrast, perfusion of slices with the same media for a shorter time (i.e., 20 min) did not reduce the release of [3H]ACh and ATP but even increased the evoked-release of ATP about fourfold. The breakdown of extracellular ATP was not blocked under low [Ca2+]0 condition, measured by the creatine phosphokinase assay and HPLC-UV technique. Application of extra- or intracellular Ca2+ chelators, and dipyridamole (2 microM), the nucleoside transporter inhibitor, did not reduce the excess release of ATP after short-term perfusion with Ca(2+)-free media. Tetrodotoxin (TTX, 1 microM), while inhibiting the majority of ATP release under normal conditions, was also unable to reduce release under low [Ca2+]0 conditions. In summary, we showed that both N- and P-type Ca2+ channels are involved in the initiation of electrical stimulation-evoked release of ATP and [3H]ACh in the rat habenula under normal extracellular calcium concentration. Under low [CA2+]0 conditions an additional release of ATP occurs, which is not associated with action potential propagation.