Rapid Determination of Glutamate Using HPLC Technology

Abstract

Glutamate is an excitatory transmitter released from neurons. It communicates information rapidly by activating receptors in other neurons (1). Physiologic concentrations of glutamate are 1 μM extracellularly and several millimolar intracellularly. Higher extracellular levels lead to cytotoxicity in the neurons (2). Glutamate exerts its excitatory action via ligand-gated ion channels to enhance Na⁺/Ca⁺⁺conductance. There are two types of glutamate receptors: ionotropic and metabotropic. The ionotropic receptors are comprised of three families with intrinsic cation permeable channels (Na⁺and K⁺): N-methyl-D-aspartate (NMDA), 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and kainate. The latter two families are known as the nonNMDA receptors (3,4). The metabotropic receptors, on the other hand, are G protein-coupled subunits, which release secondary messengers in the cytoplasm or influence ion channels through the release of G-protein subunits within the membrane (4). Vesicles in presynaptic terminals release glutamate via a calcium-dependent mechanism. Glutamate released from pre-synaptic structures is not enzymatically degraded. Instead, it is taken up by transporters, which directly regulate extracellular glutamate concentrations and limit excitotoxicity (5,6).