NMDA-receptor activation increases cytoplasmic calcium concentration in cultured spinal cord neurones

Excitatory amino acids act via receptor subtypes in the mammalian central nervous system (CNS)^1-3. The receptor selectively activated by N-methyl-D-aspartic acid (NMDA) has been best characterized using voltage-clamp and single-channel recording; the results suggest that NMDA receptors gate channels that are permeable to Na⁺, K⁺ and other monovalent cations^4-7. Various experiments suggest that Ca ²⁺ flux is also associated with the activation of excitatory amino-acid receptors on vertebrate neurones^8-11. Whether Ca ²⁺ enters through voltage-dependent Ca²⁺ channels or through excitatory amino-acid-activated channels of one or more subtype is unclear. Mg²⁺ can be used to distinguish NMDA-receptor-activated channels from voltage-dependent Ca²⁺ channels, because at micromolar concentrations Mg²⁺ has little effect on voltage-dependent Ca ²⁺ channels¹² while it enters and blocks NMDA receptor channels^4,5,7,13,14. Marked differences in the potency of other divalent cations acting as Ca²⁺ channel blockers compared with their action as NMDA antagonists also distinguish the NMDA channel from voltage-sensitive Ca²⁺ channels^5,7. However, we now directly demonstrate that excitatory amino acids acting at NMDA receptors on spinal cord neurones increase the intracellular Ca²⁺ activity, measured using the indicator dye arsenazo III, and that this is the result of Ca²⁺ influx through NMDA receptor channels. Kainic acid (KA), which acts at another subtype of excitatory amino-acid receptor, was much less effective in triggering increases in intracellular free Ca²⁺.