Mammalian brain tissue neurotransmitter

Glutamine synthetase catalyzes the ATP-dependent biosynthesis of glutamine according to equation (1). This enzyme plays a central role in nitrogen metabolism. In mammalian brain tissue, glutamine synthetase converts the neurotransmitter glutamate into glutamine. 

NADPH diaphorase is an enzyme that catalyzes NADPH-dependent reduction of a tetrazoUum salt, such as nitro blue tetrazohum (NBT), into an insoluble colored formazan. In theory, any NADPH-requinng enzyme could exhibit NADPH diaphorase activity. However, when NADPH diaphorase histochemistry was applied to aldehyde-fixed mammalian brain tissue, only a specific population of neurons stained (6). These NADPH-diaphorase-positive neurons did not correspond to any of the known classical neurotransmitter systems and they remained a mystery until the discovery of NO-producing neurons in the brain. 

As opposed to the two other endogenous excitatory amino acid candidates, cystelc acid and cysteine sulphinlc acid (15), glutamate (16) and aspartate (13) are found in abundant quantities in the mammalian CNS. Metabolically, aspartate and glutamate are related and their metabolism is quite complex. Thus, there are undoubtedly several metabolic pools of glutamate and aspartate, in addition to any neurotransmitter pool, making it difficult to study the biochemical aspects of their neurotransmitter action. Nevertheless, it has been shown that both substances are accumulated into brain tissue by a high affinity process, and that both can be released from brain tissue by electrical field stimulation. This behavior is characteristic of neurotransmitters. 

Aminobutanoic acid (y-aminobutyric acid, or GABA) is found in high concentration (0.8 mM) in the brain, but in no significant amounts in any other mammalian tissue. GABA is synthesized in neural tissue by decarboxylation of the a-carboxyl group of glutamic acid and is a neurotransmitter in the central nervous system of invertebrates and possibly in humans as well ... 

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