In Vivo Release of Endogenous Amino Acids from the Rat Striatum: Further Evidence for a Role of Glutamate and Aspartate in Corticostriatal Neurotransmission

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Abstract
By means of the push-pull cannula method, the outflow of endogenous amino acids was studied in the striatum of halothane-anesthetized rats. Addition of K+ ions (30 mM for 4 min) to the superfusion fluid increased the release of aspartate (+ 116%), glutamate (+217%), taurine (+109%), and .gamma.-aminobutyric acid (GABA) (+429%) whereas a prolonged decrease in the outflow of glutamine (-28%) and a delayed reduction in the efflux of tyrosine (-25%) were observed. In the absence of Ca2+, the K+-induced release of aspartate, glutamate, and GABA was blocked whereas the K+-induced release of taurine was still present. Under these conditions, the decrease in glutamine efflux was reduced and that of tyrosine was abolished. Local application of tetrodotoxin (5 .mu.M) decreased only the outflow of glutamate (-25%). One week following lesion of the ipsilateral sensorimotor cortex the spontaneous outflow of glutamine and of tyrosine was enhanced. Despite the lack of change in their spontaneous outflow, the K+-evoked release of aspartate and glutamate was less pronounced in lesioned than in control animals, whereas the K+-evoked changes in GABA and glutamine efflux were not modified. Our data indicate that the push-pull cannula method is a reliable approach for the study of the in vivo release of endogenous amino acids. In addition, they provide further evidence for a role for glutamate and aspartate as neurotransmitters of corticostriatal neurons.

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