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FIGURE 1–7. The glutamatergic system.This figure depicts the various regulatory processes involved in glutamatergic neurotransmission. The biosynthetic pathway for glutamate involves synthesis from glucose and the transamination of -ketoglutarate; however, a small proportion of glutamate is formed more directly from glutamine by glutamine synthetase. The latter is actually synthesized in glia and, via an active process (requiring ATP), is transported to neurons, where in the mitochondria glutaminase is able to convert this precursor to glutamate. Furthermore, in astrocytes glutamine can undergo oxidation to yield -ketoglutarate, which can also be transported to neurons and participate in glutamate synthesis. Glutamate is either metabolized or sequestered and stored in secretory vesicles by vesicle glutamate transporters (VGluTs). Glutamate can then be released by a calcium-dependent excitotoxic process. Once released from the presynaptic terminal, glutamate is able to bind to numerous excitatory amino acid (EAA) receptors, including both ionotropic (e.g., NMDA [N-methyl-d-aspartate]) and metabotropic (mGluR) receptors. Presynaptic regulation of glutamate release occurs through metabotropic glutamate receptors (mGluR2 and mGluR3), which subserve the function of autoreceptors; however, these receptors are also located on the postsynaptic element. Glutamate has its action terminated in the synapse by reuptake mechanisms utilizing distinct glutamate transporters (labeled VGT in figure) that exist on not only presynaptic nerve terminals but also astrocytes; indeed, current data suggest that astrocytic glutamate uptake may be more important for clearing excess glutamate, raising the possibility that astrocytic loss (as has been documented in mood disorders) may contribute to deleterious glutamate signaling, but more so by astrocytes. It is now known that a number of important intracellular proteins are able to alter the function of glutamate receptors (see diagram). Also, growth factors such as glial-derived neurotrophic factor (GDNF) and S100 secreted from glia have been demonstrated to exert a tremendous influence on glutamatergic neurons and synapse formation. Of note, serotonin1A (5-HT1A) receptors have been documented to be regulated by antidepressant agents; this receptor is also able to modulate the release of S100. AKAP = A kinase anchoring protein; CaMKII = Ca2+/calmodulin–dependent protein kinase II; ERK = extracellular response kinase; GKAP = guanylate kinase–associated protein; Glu = glutamate; Gly = glycine; GTg = glutamate transporter glial; GTn = glutamate transporter neuronal; Hsp70 = heat shock protein 70; MEK = mitogen-activated protein kinase/ERK; mGluR = metabotropic glutamate receptor; MyoV = myosin V; NMDAR = NMDA receptor; nNOS = neuronal nitric oxide synthase; PKA = phosphokinase A; PKC = phosphokinase C; PP-1, PP-2A, PP-2B = protein phosphatases; RSK = ribosomal S6 kinase; SHP2 = src homology 2 domain–containing tyrosine phosphatase.Source. Adapted from Cooper JR, Bloom FE, Roth RH: The Biochemical Basis of Neuropharmacology, 7th Edition. New York, Oxford University Press, 2001. Copyright 1970, 1974, 1978, 1982, 1986, 1991, 1996, 2001 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. Modified from Nicholls 1994.


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