Role of desensitization of AMPA receptors on the neuronal viability and onthe [Ca (2+)](i) changes in cultured rat hippocampal neurons

We investigated the role of desensitization of alpha-amino-3-hydroxy-5-meth yl-isoxazole-4-propionate (AMPA) receptors on the neurotoxicity and on the [Ca2+](i) changes induced by kainate or by AMPA in cultured rat hippocampal neurons. The neuronal viability was evaluated either by the 3-(4,5-dimethy lthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, or by analysis of cell morphology. Short-term exposure of the neurons to kainate or AMPA ( 30 min) was not toxic, but the exposure for 24 h to the excitotoxic drugs c aused a concentration-dependent neurotoxic effect which was prevented by LY 303070, a noncompetitive AMPA receptor antagonist. In the presence of cycl othiazide (CTZ), kainate or AMPA was toxic (30 min exposure), or the toxic effect was significantly enhanced (24 h exposure), but in this case LY 3030 70 did not completely protect the cells against kainate-induced toxicity. T he alterations in the [Ca2+](i) caused by kainate or AMPA showed a great ce ll-to-cell variability. LY 303070 completely or partially inhibited the res ponses stimulated by kainate. CTZ differentially affected the responses evo ked by kainate or AMPA. In the majority of hippocampal neurons, CTZ did not potentiate, or only slightly potentiated, the kainate-stimulated responses but in 11% of neurons there was a great potentiation. In AMPA-stimulated n eurons, the responses were slightly or greatly potentiated in the majority of neurons, but not in all of them. The results show that AMPA and kainate may be toxic, depending on the time of exposure and on the blockade of the desensitization of the AMPA receptors. Overall, our results clearly show th at there exist different populations of hippocampal neurons with different sensitivities to kainate, AMPA, CTZ and LY 303070. Moreover, the effects of CTZ on both [Ca2+](i) alterations and neurotoxicity are not fully correlat ed.