Jw. Phillis et Mh. Oregan, MECHANISMS OF GLUTAMATE AND ASPARTATE RELEASE IN THE ISCHEMIC RAT CEREBRAL-CORTEX, Brain research, 730(1-2), 1996, pp. 150-164
Elevated levels of glutamate and aspartate have been implicated in the
pathogenesis of neural injury and death induced by ischemia. The mech
anism(s) whereby they escape into the extracellular environment have b
een a subject of controversy. This study evaluated the contribution of
phospholipases and protein kinases to ischemia-evoked glutamate and a
spartate release from the ischemic/reperfused rat cerebral cortex. Cha
nges in the extracellular levels of these amino acids during four-vess
el occlusion elicited global cerebral ischemia were examined using a c
ortical cup technique. Ischemia-evoked amino acid release was compared
in control vs. drug treated animals, in which selective inhibitors of
phospholipases and protein kinases were applied topically onto the ce
rebral cortex. The phospholipase inhibitors tested included 4-bromophe
nacyl bromide, a non-selective inhibitor; 7,7-dimethyleicosadienoic (D
EDA), an inhibitor of secretory type phospholipase A(2) (PLA(2)); AACO
CF(3), an inhibitor of the Ca2+-dependent cytoplasmic form of PLA(2),
HELSS, which inhibits a Ca2+-independent cytoplasmic PLA(2), and U7312
2, a selective inhibitor of phospholipase C (PLC). All five phospholip
ase inhibitors significantly attenuated glutamate and aspartate releas
e into the extracellular milieu, indicating the possibility that sever
al forms of the enzyme are likely to be involved. The protein kinase C
(PKC) inhibitor, chelerythrine chloride, also reduced excitatory amin
o acid efflux, whereas the PKC activator phorbol 12-myristate 13-aceta
te (PMA) enhanced their release. The non-selective kinase inhibitor, s
taurosporine: and H-89, which selectively inhibits protein kinase A, d
id not reduce ischemia-evoked amino acid efflux. These results suggest
that ischemia-evoked release of the excitatory transmitters amino aci
ds is a result, in part, of the activation of phospholipases A, and C,
with PKC involvement in the transduction process. Destabilization and
deterioration of the plasma membrane, as a consequence of phospholipi
d hydrolysis, may allow these transmitter amino acids to diffuse down
their concentration gradients into the extracellular fluid.