Glutamate signalling and secretory phospholipase A2 modulate the release of arachidonic acid from neuronal membranes

The lipid mediators generated by phospholipases A 2 (PLA 2 ), free arachidonic acid (AA), eicosanoids, and platelet-activating factor, modulate neuronal activity; when overproduced, some of them become potent neurotoxins. We have shown, using primary cortical neuron cultures, that glutamate and secretory PLA 2 (sPLA 2 ) from bee venom (bv sPLA 2 ) and Taipan snake venom (OS2) elicit synergy in inducing neuronal cell death. Low concentrations of sPLA 2 are selective ligands of cell-surface sPLA 2 receptors. We investigated which neuronal arachidonoyl phospholipids are targeted by glutamate-activated cytosolic calcium-dependent PLA 2 (cPLA 2 ) and by sPLA 2 . Treatment of 3 H-AA-labeled cortical neurons with mildly toxic concentrations of sPLA 2 (25 ng/ml, 1.78 nM) for 45 min resulted in a two- to threefold higher loss of H-AA from phosphatidylcholine (PC) than from phosphatidylethanolamine (PE) and in minor changes in other phospholipids. A similar profile, although of greater magnitude, was observed 20 hr posttreatment. Glutamate (80 μM) induced much less mobilization of 3 H-AA than did sPLA 2 and resulted in a threefold greater degradation of 3 H-AA PE than of 3 H-AA PC by 20 hr posttreatment. Combining sPLA 2 and glutamate resulted in a greater degradation of PC and PE, and the N-methyl-D-aspartate receptor antagonist MK-801 only blocked glutamate effects. Thus, activation of the arachidonate cascade induced by glutamate and sPLA 2 under experimental conditions that lead to neuronal cell death involves the hydrolysis of different (perhaps partially overlapping) cellular phospholipid pools.