Any receptor protein that binds the neurotransmitter glutamate as a ligand. Glutamate receptors fall into two main types: ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels, responsible for fast excitatory transmission; and metabotropic glutamate receptors (mGluRs), which work via G proteins and cause slower longer-lasting effects in postsynaptic cells. Ionotropic glutamate receptors are further subdivided into three classes, each named according to their sensitivity to specific agonists: NMDA receptors (named after N-methyl-d-aspartate), AMPA receptors (named after α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), and kainate receptors (see kainate). Unusually, NMDA receptors must bind not only glutamate but also glycine, and the membrane must be depolarized, before the ion channel will open. Glutamate receptors are thought to mediate changes in the strength of synapses that form the basis of learning and memory. See synaptic plasticity.