The effects of excitatory amino acids on intracellular calcium in single mouse striatal neurons in vitro

Using microspectrofluorimetry and the calcium-sensitive dye fura-2, we examined the effect of excitatory amino acids on [Ca²⁺](i) in single striatal neurons in vitro. N-methyl-D-aspartic acid (NMDA) produced rapid increases in [Ca²⁺](i). These were blocked by DL-2-amino-5-phosphonovaleric acid (AP5), by Mg²⁺, by phencyclidine, and by MK801. The block produced by Mg²⁺ and MK801 could be relieved by depolarizing cells with veratridine. When external Ca²⁺ was removed, NMDA no longer increased [Ca²⁺](i). Furthermore, the effects of NMDA were not blocked by concentrations of La³⁺ that blocked depolarization induced rises in [Ca²⁺](i). Substitution of Na⁺(o) by Li⁺ did not block the effects of NMDA. Concentrations of L-glutamate ≥ 10^-6 M also increased [Ca²⁺](i). The effects of moderate concentrations of glutamate were blocked by AP5 but not by La³⁺ or by substitution of Na⁺ by Li⁺. The effects of glutamate were blocked by removal of external Ca²⁺ but were not blocked by concentrations of Mg²⁺ or MK801 that completely blocked the effects of NMDA. The glutamate analogs kainic acid (KA) and quisqualic acid also increased [Ca²⁺](i). The effects of KA were blocked by removal of external Ca²⁺ but not by La³⁺, Mg²⁺, MK801, or replacement of Na⁺ by Li⁺. Although AP5 was able to block the effects of KA partially, very high concentrations were required. These results may be explained by considering the properties of glutamate-receptor-linked ionophores. Excitatory amino acid induced increases in [Ca²⁺](i) are consistent with the possibility that Ca²⁺ mediates excitatory amino acid induced neuronal degeneration.