NMDA receptors

The N-methyl-D-aspartate receptor (also known as the NMDA receptor or NMDAR), is a glutamate receptor and ion channel protein found in nerve cells. It is activated when glutamate and glycine (or D-serine) bind to it, and when activated it allows positively charged ions to flow through the cell membrane. The NMDA receptor is very important for controlling synaptic plasticity and memory function.

The NMDAR is a specific type of ionotropic glutamate receptor. The NMDA receptor is named this because the agonist molecule N-methyl-D-aspartate (NMDA) binds selectively to it, and not to other glutamate receptors. Activation of NMDA receptors results in the opening of an ion channel that is nonselective to cations, with a reversal potential near 0 mV. While the opening and closing of the ion channel is primarily gated by ligand binding, the current flow through the ion channel is voltage dependent. Extracellular magnesium (Mg²⁺) and zinc (Zn²⁺) ions can bind to specific sites on the receptor, blocking the passage of other cations through the open ion channel. Depolarization of the cell dislodges and repels the Mg²⁺ and Zn²⁺ ions from the pore, thus allowing a voltage-dependent flow of sodium (Na⁺) and small amounts of calcium (Ca²⁺) ions into the cell and potassium (K⁺) out of the cell.

Ca²⁺ flux through NMDARs is thought to be critical in synaptic plasticity, a cellular mechanism for learning and memory. The opening and closing (gating) of the NMDA receptor is complex. While it is primarily a ligand-gated channel, it does display weaker voltage-dependence modulation of the ligand-dependent gating. The ligand gating requires co-activation by two ligands: glutamate and either D-serine or glycine. The voltage-dependence of current through the channel is mainly due to binding of Mg²⁺ or Zn²⁺ ions to the protein as described above.

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