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 CA] 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 mu M DHPG produced an inward current of -48 +/- 10pA (I-mGluR), which was blocked by application of group I mGluR antagonists. In contrast, bath application of 100 VM glutamate in the presence of a ionotropic glutamate receptor antagonist and TTX did not activate I-mGluR in CA1 cells patch-clamped at a depth of similar to 30 mu m. Similarly, sole inhibition of glutamate transporters by the broad-spectrum glutamate transporter antagonist TBOA did not induce I-mGluR under the same conditions. Only if glutamate was co-applied with TBOA an I-mGluR 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 decree that a CA I neuron at a depth of 30 ILm is exposed to low extracellular glutamate levels that are not sufficient to induce a detectable activation of group I mGluRs (<2 mu M). (C) 2007 Elsevier Ireland Ltd. All rights reserved.