Studies on [3H]Glutamate Binding in Nervous Tissues. What are the Pitfalls?
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Some free acidic amino acids enriched in the brain, such as L-glutamic acid (Glu) and L-aspartic acid (Asp), are believed to play a role as excitatory amino acid neurotransmitters in the mammalian central nervous system (Curtis and Johnston, 1974; Fonnum, 1984). For instance, Glu is synthesized in presynaptic nerve terminals from which it is released during the neuronal excitation in a Ca2+-dependent fashion. A high-affinity and Na+-dependent uptake system is responsible for the termination of neurotransmission mediated by these acidic amino acids. Recently it has been demonstrated that Glu is released from an exocytotic pool in cerebral cortical synaptosomes on depolarization of the plasma membranes (Nicholls and Sihra, 1986; Sanchez-Prieto et al., 1987). These findings are consistent with the successful isolation of synaptic vesicles with a Na+-independent and ATP-dependent accumulation system for the acidic amino acid, which is distinctly different from the abovementioned Na+-dependent uptake system (Naito and Ueda, 1983, 1985). An immunohistochemical study has revealed the localization of vesicular structures accumulating Glu in nerve terminals (Storm-Mathisen et al., 1983). In addition, human fibroblasts are shown to contain an acidic amino acid exchange system between cystine and Glu across plasma membranes (Bannai, 1986).
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