Sustained depolarizing shift of the GABA reversal potential by glutamate receptor activation in hippocampal neurons

The inhibitory action of GABA is a consequence of a relatively hyperpolarized Cl- reversal potential (E-Cl), which results from the activity of K+-Cl- cotransporter (KCC2). In this study we investigated the effects of glutamate and glutamatergic synaptic activity on E-Cl. In dissociated culture of mature hippocampal neurons, the application of glutamate caused positive E-Cl shifts with two distinct temporal components. Following a large transient depolarizing state, the sustained depolarizing state (E-Cl-sustained) lasted more than 30 min. The E-Cl-sustained disappeared in the absence of external Ca2+ during glutamate application and was blocked by both AP5 and MK801, but not by nifedipine. The E-Cl-sustained was also induced by NMDA. The E-Cl-sustained was blocked by furosemide, a blocker of both KCC2 and NKCC1, but not bumetanide, a blocker of NKCC1. On the other hand, in immature neurons having less expression of KCC2, NMDA failed to induce the sustained depolarizing E-Cl shift. In organotypic slice cultured neurons, repetitive activation of glutamatergic afferents also generated a sustained depolarizing E-Cl shift. These results suggest that Ca2+ influx through NMDA receptors causes the down-regulation of KCC2 and gives rise to long lasting positive E-Cl shifts, which might contribute to hyperexcitability, LTP, and epileptiform discharges. (C) 2008 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.