Age-related and function-dependent regional alterations of free L- and D-aspartate in postembryonic chick brain.

D-aspartate (D-Asp) modulates adult neural plasticity and embryonic brain development by promoting cell proliferation, survival and differentiation. Here, developmental changes of the excitatory amino acids (EAAs) L-Glu, L-Asp and D-Asp were determined during the first postembryonic days, a time window for early learning, in selected brain regions of domestic chickens after chiral separation and capillary electrophoresis. Extracellular concentration (ECC) of EAAs was measured in microdialysis samples from freely moving chicks. ECC of D-Asp (but not L-EAAs) decreased during the first week of age, with no considerable regional or learning-related variation. ECC of L-Asp and L-Glu (but not of D-Asp) were elevated in the mSt/Ac in response to a rewarding stimulus, suggesting importance of Asp-Glu co-release in synaptic plasticity of basal ganglia. Potassium-evoked release of D-Asp, with a protracted transient, was also demonstrated. D-Asp constitutes greater percentage of total aspartate in the extracellular space than in whole tissue extracts, thus the bulk of D-Asp detected in tissue appears in the extracellular space. Conversely, only a fraction of tissue L-EAAs can be detected in extracellular space. The lack of changes in tissue D-Asp following avoidance learning indicates a tonic, rather than phasic, mechanism in the neuromodulatory action of this amino acid.

Chiral analysis of amino acid neurotransmitters and neuromodulators in mouse brain by CE-LIF.

Chiral CE method has been developed for quantitative determination of d-amino acid modulators of NMDA glutamate receptor; d-serine and d-aspartate along with l-glutamate and l-aspartate in biological samples. These ligands are suggested to be involved in regulation of NMDA receptor related brain functions, such as neurogenesis, neuronal plasticity, and memory formation. For sensitive determination of the amino acids LIF detection was chosen, and a fluorogenic reagent, 7-fluoro-4-nitro-2,1,3-benzoxadiazole was used for derivatization. An amino-modified ß-CD, 6-monodeoxy-6-mono(3-hydroxy)propylamino-ß-CD (HPA-ß-CD) was applied as chiral selector. Determinations were accomplished in a polyacrylamide coated capillary and reverse polarity was used for the analysis of the negatively charged analytes. The method was optimized and validated; 6 mM HPA-ß-CD in 50 mM HEPES buffer, pH 7 was appropriate to achieve baseline separation of the analytes. The limit of quantification with acceptable accuracy is 0.05 µM for both d-amino acids. The method was used for the determination of d-aspartate and d-serine content in various brain regions of adult mice.

Chiral separation and determination of excitatory amino acids in brain samples by CE-LIF using dual cyclodextrin system.

Chiral capillary electrophoresis method has been developed to separate aspartate and glutamate enantiomers to investigate the putative neuromodulator function of D-Asp in the central nervous system. To achieve appropriate detection sensitivity fluorescent derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole and laser-induced fluorescence detection was applied. Although, simultaneous baseline separation of the two enantiomer pairs could be achieved by using 3 mM 6-monodeoxy-6-mono(3-hydroxy)propylamino-ß-cyclodextrin (HPA-ß-CD), further improvement of the chemical selectivity was required because of the high excess of L-enantiomers in real samples to be analyzed. The system selectivity was fine-tuned by combination of 8 mM heptakis(2,6-di-O-methyl)-ß-cyclodextrin and 5 mM HPA-ß-CD in order to increase the resolution between aspartate and glutamate enantiomers. The method was validated for biological application. The limits of detection for D-Asp and D-Glu were 17 and 9 nM, respectively, while the limit of quantification for both analytes was 50 nM. This is the lowest quantification limit reported so far for NBD-tagged D-Asp and D-Glu obtained by validated capillary electrophoresis laser-induced fluorescence method. The applicability of the method was demonstrated by analyzing brain samples of 1-day-old chickens. In all the studied brain areas, the D-enantiomer contributed 1-2 % of the total aspartate content, corresponding to 17-45 nmol/g wet tissue.