Excitotoxic cell death dependent on inhibitory receptor activation

Although excitotoxic cell death is usually considered a Ca(2+)-dependent process, in certain neuronal systems there is strong evidence that excitotoxic cell death is independent of Ca2+ and is instead remarkably dependent on extracellular Cl-. We have shown (in isolated chick embryo retina) that at least some of the lethal Cl- entry is through GABA and glycine receptors. Here we show that when all the GABA and glycine receptors are blocked by using an appropriate cocktail of inhibitors, agonist-induced excitotoxic cell death can be completely prevented. To determine if ligand-gated Cl- channels contribute to excitotoxic cell death in other neurons, we examined KA-induced cell death in cultured rat cerebellar granule cells. GABA receptor blockade with either a competitive or noncompetitive antagonist provides complete neuroprotection. KA stimulates Cl- uptake by the granule cells, and this is blocked by the GABA antagonists. Granule cell cultures take up [3H]GABA and release it in response to KA treatment. A subpopulation of neurons in the cultures is shown to have GAD and high concentrations of GABA, and this presumably is the source of the GABA that leads to receptor activation and lethal Cl- entry. Finally, we show that retinal cell death due to 1 h of simulated ischemia (combined oxygen and glucose deprivation) is completely prevented by blocking the inhibitory receptors. These results indicate that, paradoxically, excitotoxic cell death is completely dependent on activation of inhibitory receptors, in at least some neuronal systems, and this pathological process may contribute to disease.