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Characterization of Separated Cell Types from the Developing Rat Cerebellum: Transport of Glutamate and Aspartate by Preparations Enriched in Purkinje Cells, Granule Neurones, and Astrocytes

Authors


Address correspondence and reprint requests to R. D. Gordon, Muscular Dystrophy Research Laboratories, Institute of Neurology, Queen Square, London WCIN 3BG, U.K.

Abstract:

Preparations of structurally preserved cerebellar perikarya (cells) were found to express high-affinity transport systems for glutamate but not for certain putative transmitter substances (including monoamines, glycine and taurine) and non-transmitter amino acids. The characteristics of the high-affinity glutamate transport system were similar to those of other preparations of brain tissue: [3H]glutamate uptake by the cells was Na+-dependent and was inhibited competetively by other acidic amino acids. The rank order of apparent affinities of the carrier for acidic amino acids was L-aspartate > L-glutamate > D-aspartate ≫ D-glutamate (the affinity for D-glutamate being over two orders of magnitude lower than for the other three amino acids). Comparison of high-affinity [3H]glutamate uptake in preparations enriched in different cell types showed that although the affinities are similar (2-4 fiM), the rate is outstandingly high in astrocytes (Vmax 18 nmol/min per mg protein). Significantly, uptake into the putatively glutamatergic granule cells was very low. These observations were supported by autoradiographic findings which showed that the predominant sites of [3H]glutamate uptake in cerebellar cultures enriched in interneurones are the astrocytes. Furthermore, the Vmax in cultures enriched in astrocytes was as high as that in separated astrocytes. Thus, it seems that the principal cell type involved in acidic amino acid uptake in the cerebellum is the astrocyte, and this must be taken into consideration when high-affinity uptake is used as a marker for glutamatergic transmitter systems. Furthermore, the selective cellular distribution of glutamate transport sites, together with the uneven distribution of enzymes related to glutamate metabolism observed previously, indicates that a metabolic interaction takes place between the different cell types, supporting the current hypothesis on metabolic compartmentation in the brain.

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