CYTOTOXIC ACTIONS AND EFFECTS ON INTRACELLULAR CA2+ AND CGMP CONCENTRATIONS OF SULFUR-CONTAINING EXCITATORY AMINO-ACIDS IN CULTURED CEREBRAL CORTICAL-NEURONS

Effects of the sulphur-containing acidic amino acids (SAAs) cysteic acid (CA), homocysteic acid (HCA), cysteine sulphinic acid (CSA), homocysteine sulphinic acid (HCSA), and S-sulphocysteine (SC) on intracellular concentrations of Ca2+ ([Ca2+]i) and cGMP ([cGMP]i) as well as their cytotoxic actions were investigated in cultured cerebral cortical neurons. The glutamate receptor subtype selective antagonists APV (D-(-)-2-amino-5-phosphonopentanoate) acting on N-methyl-D-aspartate (NMDA) receptors and DNQX (6,7-dinitroquinoxaline-2,3-dione) acting on non-NMDA receptors were employed to obtain information about the involvement of glutamate receptor subtypes in these actions of the SAAs. It was found that all SAAs exerted a cytotoxic action on the neurons. The ED50 values for CSA, CA, HCSA, and HCA were around 30 to 50 muM and that for SC was about 150 muM. The glutamate transport blocker L-aspartate-beta-hydroxamate increased the efficacy of CSA and CA but had no effect on the cytotoxic actions of the remaining SAAs. In case of CA, HCA, and SC the cytotoxicity could be prevented by APV alone and for HCSA, DNQX could block the toxic action. DNQX reduced the toxicity of HCA somewhat but the presence of APV was required for complete protection. CSA toxicity could only be blocked by the combination of APV and DNQX. All SAAs induced an increase in [cGMP]i and [Ca2+li and with regard to [Ca2+]i SC was the most potent and CA the least potent SAA. The effect of all SAAs on [cGMP]i could be blocked by APV alone whereas DNQX had no effect except in the case of HCSA where the response was blocked completely and HCA where the response was inhibited by 75%. The SAA-induced increase in [Ca2+]i could in all cases be significantly reduced by 0.6 MM Mg2+ and in the presence of Mg2+, APV dose dependently blocked the remaining SAA induced increase in [Ca2+li completely. Under these conditions DNQX was also found to block the SAA-induced increase in [Ca2+]i dose dependently. In the absence of Mg2+, DNQX (25 muM) inhibited the response of the SAAs only by 65-75%. Under these conditions all SAA responses except that to SC could be fully antagonized by 300 muM APV. The SC-induced increase in [Ca2+]i was inhibited by 60% by APV. The results show that no simple correlation exists between SAA-induced cytotoxicity and their ability to increase intracellular levels of Ca2+ and cGMP. However, when both NMDA and non-NMDA receptors were antagonized no toxicity or changes in calcium or cGMP were observed.