NMDA receptors (N-methyl-D-aspartate receptors - they are named after the also effective selective agonist N-methyl-D-aspartate) are ionotropic glutamate receptors. Ionotropic receptors activate membrane channels where ions can pass through when open. They differ from AMPA receptors in that they are permeable to calcium when activated. Their properties make them particularly important for learning and memory.
The ion channel of the NMDA receptor-channel complex is well permeable to Na+ , K+ and Ca2+ ions (Monaghan et al. 1989). Ca2+ ion influx triggers intracellular processes that appear to be responsible for both neuronal plasticity and NMDA receptor-mediated neurotoxicity (Hardingham and Bading 2003). The ion channel of the NMDA receptor is blocked in a voltage-dependent manner by Mg2+ ions (Nowak et al.1984). This blocking is released when the resting membrane potential of the neuron becomes more positive than -20 to -30 mV (Mayer et al.1984). Another characteristic of the NMDA receptor is the slow time course of the excitatory postsynaptic currents (EPSC; Lester et al. 1990) mediated by it. The NMDA receptor is a protein composed of four different transmembrane subunits (Cull-Candy et al. 2001).