Neurochemical Characterization of Excitatory Amino Acid Receptors in Hippocampus
Glutamate and aspartate may be excitatory neurotransmitters within central nervous system (CNS) (6,9). The recent development of specific antagonists has revealed at least three pharmacologically distinct classes of receptors for excitatory amino acids in the vertebrate CNS (19). Cysteine sulfinic acid (CSA) is an intermediate in taurine biosynthesis (Fig. 1), in which the final product, taurine, has inhibitory effects in the CNS. CSA has a strong excitatory effect in the central neurons when applied iontophoretically, as do glutamate and aspartate, and thus it was classified as an excitatory amino acid (7). Among the metabolites appearing in Fig 1, on intraventricular injection, only CSA caused severe behavioral and EEG seizures, which were markedly reduced by a simultaneous injection of taurine (14,15). With respect to the neurochemical aspects of CSA, several studies were reported during the last five years. In brief, CSA is unevenly distributed in rat brain but its content is much lower than that of glutamate (5,8,12). Like many other substances, CSA is accumulated in synaptosomes by a Na+-dependent high affinity uptake system. The Km value is 12 μM (17). Glutamate, aspartate and cysteic acid are competitive inhibitors for the uptake of CSA, indicating that CSA may be transported Into synaptosomes by the high affinity carrier system for acidic amino acids (17).
KeywordsExcitatory Amino Acid Gaba Release Cysteic Acid Excitatory Amino Acid Receptor Vertebrate Centra1 Nervous System
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