Contributing mechanisms for cysteine excitotoxicity in cultured cerebellar granule cells

Several possible mechanisms for cysteine toxicity on rat cerebellar granule cells were studied and compared with the excitotoxic effect of glutamate. It was shown that the excitotoxic potency of both cysteine and glutamate increased in the presence of elevated concentrations, of bicarbonate or increased pH. Pharmacological studies showed that the cysteine toxicity was specifically coupled to the NMDA receptor, whereas the glutamate toxicity was mediated to a smaller extent also by non-NMDA receptors. Treatment of cerebellar granule cells with cysteine led to an increased extracellular level of glutamate. In addition, cysteine sensitized NMDA receptors by reducing disulfide bonds in the receptor to sulfhydryl groups. A mechanism for cysteine excitotoxicity may therefore be formation of cysteine-sensitized NMDA receptors that are stimulated either by cysteine and/or by endogenous glutamate. This mechanism may also be important for the effects observed during regulated physiological release of cysteine.