Characterisation of extracellular amino acids in striatum of freely moving rats by in vivo microdialysis

Summary

To investigate the characteristics of extracellular amino acids released from the striatum, we performedin vivo microdialysis in non-anaesthetised, freely moving rats. Amino acids were determined after precolumn derivatisation witho-phthalaldehyde by high-performance liquid chromatography and fluorescence detection. The omission of Ca²⁺ in the perfusion medium partially decreased the basal concentration of aspartate and glutamate. This shows that a small fraction of basal concentration of aspartate and glutamate is of neuronal origin. The effect of high K⁺ and veratrine stimulation was evaluated in the presence or absence of Ca²⁺ or tetrodotoxin (1 μM). High K⁺ and veratrine caused a remarkable increase in the aspartate and glutamate efflux. The omission of Ca²⁺ only partially decreased K⁺-stimulated aspartate and glutamate efflux. Tetrodotoxin completely antagonised veratrine-stimulated aspartate and glutamate efflux. Although glycine and taurine releases were stimulated by high K⁺ and veratrine, their release was not always antagonised with Ca²⁺ omission or tetrodotoxin inclusion. Thus, the neuronal origin of stimulated release of glycine and taurine is unclear. Although tetrodotoxin sensitivity and Ca²⁺-dependency are regarded as a basic criterion for classical neurotransmitters in microdialysis experiments, they should not be adapted to the physiological characteristics of the release of amino acids.