Immunocytochemical identification of GABA in astrocytes located in white matter after inhibition of GABA-transaminase with gamma-acetylenic GABA.

Several lines of evidence suggest that astrocytes contribute to the uptake and degradation of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Recent immunohistochemical studies have shown that GABA-like immunoreactivity can be demonstrated in astrocytes in the grey matter of the rat's brainstem. The present study investigates whether GABA is also present in astrocytes located in the white matter. Adult rats were given gamma-acetylenic GABA (GAG), which inhibits the GABA-degratory enzyme GABA-alpha-ketoglutaric acid aminotransferase, and tissue sections from the cerebral cortex and brainstem were processed for GABA immunohistochemistry using an antiserum to GABA. Light microscopic examination of the sections showed numerous small GABA-immunoreactive cells in fibre tracts as well as in nuclear regions. Electron microscopic examination of the immunoreactive cells showed that they were fibrous astrocytes. The results provide evidence that the large increase in GABA in fibre tracts found in biochemical studies of rats injected with GAG is due to an increase in astrocytic GABA and suggest that fibrous astrocytes regulate GABA levels in the extracellular space of white matter.