In neuroscience,<unk> refers to the anion of glutamic acid in its role as a neurotransmitter: a chemical that nerve cells use to send signals to other cells. It is by a wide margin the most abundant excitatory neurotransmitter in the vertebrate nervous system. It is used by every major excitatory function in the vertebrate brain, accounting in total for well over 90% of the synaptic connections in the human brain. It also serves as the primary neurotransmitter for some localized brain regions, such as cerebellum granule cells. Biochemical receptors for<unk> fall into three major classes, known as AMPA receptors, NMDA receptors, and metabotropic<unk> receptors. A fourth class, known as kainate receptors, are similar in many respects to AMPA receptors, but much less abundant. Many synapses use multiple types of<unk> receptors. AMPA receptors are ionotropic receptors specialized for fast excitation: in many synapses they produce excitatory electrical responses in their targets a fraction of a millisecond after being stimulated. NMDA receptors are also ionotropic, but they differ from AMPA receptors in being permeable, when activated, to calcium. Their properties make them particularly important for learning and memory. Metabotropic receptors act through second messenger systems to create slow, sustained effects on their targets. Because of its role in synaptic plasticity,<unk> is involved in cognitive functions such as learning and memory in the brain. The form of plasticity known as long-term potentiation takes place at<unk> gic synapses in the hippocampus, neocortex, and other parts of the brain.<unk> works not only as a point-to-point transmitter, but also through spill-over synaptic crosstalk between synapses in which summation of<unk> released from a neighboring synapse creates extrasynaptic signaling/volume transmission. In addition,<unk> plays important roles in the regulation of growth cones and synaptogenesis during brain development.
Glutamate 
Acetylcholine
Serotonin
Gamma-Hydroxybutyric acid
Phenserine
Glutamine
N-Acetylserotonin
Dopamine