Glial Cell Function: Uptake of Transmitter Substances

Rabbit-brain fractions enriched in neuronal cell bodies and in glial cells accumulated norepinephrine, serotonin, dopamine, and gamma-aminobutyric acid, substances believed to serve as neurotransmitters in the central nervous system. Both neurons and glia were able to concentrate the monoamine transmitters about 4-fold from a medium containing 0.1-1 muM concentrations. However, the glial-cell fraction concentrated aminobutyrate over a 100-fold from the medium, in contrast to the neuronal fraction, which concentrated this amino acid only 4-fold. The uptake of aminobutyrate by glial cells was 30-50% of that of synaptosome preparations. Its uptake in all fractions was temperature sensitive, sensitive to metabolic inhibitors, and exhibited K(m) values of 0.72 muM for the neuronal fraction, 0.42 muM for the synaptosomal fraction, and 0.27 muM for the glial-cell fraction. These results are interpreted as evidence that the glial cell is involved in limiting the extracellular build-up of substances that might trigger synaptic transmission by removing any transmitters that may diffuse out of the synaptic cleft during the transmission of impulses. The possible function of the enormous ability of glia and synaptosomes to accumulate aminobutyrate is discussed in light of the actions and distribution of this substance in the central nervous system.