Comparative study of the release of glutamate and GABA, newly synthesized from glutamine, in various regions of the central nervous system

The in vitro release of ³H]glutamate and ³H]γ-aminobutyric acid, both newly synthesized from ³H]glutamine, were studied in various regions of the rat and pigeon brain. The amount of transmitter released from slices during stimulation by excess K⁺, as well as the ratio of the amounts of glutamate and γ-aminobutyrate released, varied considerably from region to region. Very high levels of release of glutamate were observed in rat striatum and in rat and pigeon hippocampus whereas only moderate release of glutamate was found in rat and pigeon cerebellum, rat cochlear nucleus and rat substantia nigra. The highest levels of release of γ-aminobutyrate were observed in the rat substantia nigra, hippocampus and striatum. Particularly low levels of release of glutamate was observed in the pigeon optic tectum. The release of γ-aminobutyrate and glutamate in all structures was largely calciumdependent.These results suggest that the release of glutamate and γ-aminobutyrate newly synthesized from glutamine reflects the neurotransmitter function of the two amino acids within various CNS regions. They further suggest that the glutamate synthesizing and releasing pool is different from the γ-aminobutyrate synthesizing and releasing pool. The similarity between the release of glutamate in rat and pigeon cerebellum and hippocampus suggest some structural homology in both species.