TBOA-sensitive uptake limits glutamate penetration into brain slices to a few micrometers

Removal of neurotransmitter from the extracellular space is crucial for normal functioning of the central nervous system. In this study, we have used high-affinity metabotropic glutamate receptors (mGluRs) expressed by hippocampal CA1 pyramidal cells to test how far bath-applied glutamate penetrates into slice tissue before being removed by uptake mechanisms. Activation of group I mGluRs by 100 μM DHPG produced an inward current of −48 ± 10 pA (ImGluR), which was blocked by application of group I mGluR antagonists. In contrast, bath application of 100 μM glutamate in the presence of a ionotropic glutamate receptor antagonist and TTX did not activate ImGluR in CA1 cells patch-clamped at a depth of ∼30 μm. Similarly, sole inhibition of glutamate transporters by the broad-spectrum glutamate transporter antagonist TBOA did not induce ImGluR under the same conditions. Only if glutamate was co-applied with TBOA an ImGluR of −39 ± 8 pA was recorded which was also blocked by group I antagonists. The data suggest that TBOA-sensitive uptake mechanisms are able to maintain a steep concentration gradient of glutamate to such a degree that a CA1 neuron at a depth of 30 μm is exposed to low extracellular glutamate levels that are not sufficient to induce a detectable activation of group I mGluRs (<2 μM).