Tonic release of glutamate into the extracellular space of the hippocampus and striatum has been attributed to a cystine-glutamate exchanger which is blockable by the glutamate analogue CPG, and adaptation of this release in the striatum has been suggested to underlie relapse in the use of cocaine (Jabaudon et al. 1999; Baker et al. 2002, 2003). We monitored tonic glutamate release in area CA1 of hippocampal slices (from P12 rats killed humanely by cervical dislocation) by using whole-cell patch-clamping to measure the glutamate receptor mediated current evoked in pyramidal cells when Na-dependent glutamate uptake was blocked with the non-transported glutamate analogue TBOA (200 µM). The superfusion solution contained TTX (0.1-1µM) to block action potentials.

TBOA evoked a slowly developing inward current at -30 mV which reached ~125 pA after 1.5 mins, and was abolished by the NMDA receptor blocker D-AP5 (50 µM). The TBOA-evoked current was not significantly affected by CPG (50µM), even though cystine (300 µM) increased the TBOA-evoked current 2.8 ± 1.9 fold (mean ± S.E.M., P = 0.04 by 2 tailed t test, n = 5,) and sometimes evoked an inward current even in the absence of TBOA (as seen previously in cerebellum and cortex: Warr et al. 1999), demonstrating that cystine-glutamate exchange is present in area CA1. The TBOA-evoked current was reduced by 49 ± 11 % (P = 0.0004, n = 5) by DIDS (1 mM). DIDS has been claimed to block NMDA receptor mediated currents (Tauskela et al. 2003), but this was found to be an artefact (produced by the co-application of NMDA and DIDS) reflecting the superposition of a DIDS-evoked outward current on an NMDA-evoked inward current.

These data suggest that although cystine-glutamate exchange is present in the hippocampus it does not generate the tonic release of glutamate, and that the tonic release is mediated by a DIDS-sensitive mechanism.

This work was supported by the Wellcome Trust, the European Union and a Wolfson-Royal Society Award.