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Abstract
Extracellular (EC) concentrations of amino acids were determined in the rat dentate
gyrus by means of non-linear regression analysis of 'in vivo' brain dialysis data,
considering a simple model of diffusion through a dialysis membrane. The apparent
diffusion constants (K) of several amino acids were also calculated in the 'in vivo'
situation. While putative amino acid neurotransmitters (glutamate, aspartate and gamma-aminobutyric
acid (GABA) were present in the EC fluid at the low micromolar range (0.8-2.9 microM),
glutamine was by far the most prominent (193.4 microM). The values of intra/extracellular
concentration ratios formed 3 groups: high (greater than 2000) for putative neurotransmitters;
low (less than 100) for serine, glutamine, arginine and alpha-alanine; and intermediate
(about 400) for taurine. The 'in vivo' calculated K values proved useful for estimation
of both basal and changing EC concentrations of amino acids in relatively brief perfusions.
These data were evaluated in terms of the functional significance of absolute EC concentrations
and tissue-EC fluid ratios. Present findings indicate the simultaneous existence of
both an inhibitory and an excitatory tonus as well as the utility of high intra/extracellular
concentration ratios in determination of the possible neurotransmitter role of specific
amino acids.