Modulation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/quisqualate receptors by phospholipase A2: a necessary step in long-term potentiation?

The effects of kainate (KA)-induced epileptic seizures on the binding properties of hippocampal glutamate receptors, on the modulation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/quisqualate receptor by phospholipase A2 (PLA2), and on the formation of long-term potentiation (LTP) were studied in hippocampal membranes and hippocampal slices. Systemic administration of KA (10 mg/kg; 15 hr survival) produced specific changes in the binding properties of the AMPA/quisqualate receptors and its regulation. Whereas the binding of various ligands to the N-methyl-D-aspartate receptors was not modified by KA treatment, there was a significant decrease in the maximal number of binding sites for [3H]AMPA. In addition, the increase in [3H]AMPA binding elicited by PLA2 treatment of hippocampal, but not cerebellar, membranes was markedly decreased after KA injection. LTP was also substantially reduced in area CA1 of hippocampal slices from KA-treated animals. The loss of LTP was not due to changes in postsynaptic responses elicited by the bursts that trigger the potentiation effect, thus suggesting that KA treatment disrupts processes that follow N-methyl-D-aspartate receptor activation. Systemic administration of KA was associated with calpain activation as the amount of spectrin breakdown products was increased severalfold in hippocampus but not in cerebellum. Pretreatment of telencephalic membranes with calpain greatly reduced the PLA2-induced increase in [3H]AMPA binding. The results provide evidence in favor of an essential role of PLA2 in the development of LTP and suggest that the order of activation of different calcium-dependent processes is critical for producing the final changes underlying LTP.