Neuronal glutamate transporters regulate synaptic transmission in single synapses on CA1 hippocampal neurons

Glutamate is the major excitatory transmitter in CNS although it causes severe brain damage by pathologic excitotoxicity. Efficient neurotransmission is controlled by powerful protection and support afforded by specific high-affinity glutamate transporters in neurons and glia, clearing synaptic glutamate. While the role of glial cells in glutamate uptake is well defined, the role of neuronal transporters remains poorly understood. The evaluation of impact of neuronal transporters on spontaneous and evoked EPSC in hippocampal CA1 neurons within a model 'single bouton preparation' by pre- and postsynaptic uptake was addressed. In whole-cell patch clamp experiments the influence of blocking, pre- or both pre- and postsynaptic glutamate transporters (GluT) on spontaneous and evoked postsynaptic currents (sEPSC and eEPSC), was examined by manipulating the content of intracellular solution. Suppressing GluT by non-transportable inhibitor TBOA (10 microM) led to remarkable alteration of glutamate uptake process and was reflected in measurable changes of general properties of synaptic currents. Elimination of intracellular K(+) concentration required for glutamate transporter operation by using Cs(+)-based internal solution (postsynaptic GluTs are non-functional apriori), causes the deficient of presynaptic glutamate transporters. Applied in such conditions glutamate transporter inhibitor TBOA (10 microM) affected the occurrence of synaptic event and thus unregulated the transmitter release. eEPSCs were generally suppressed both in amplitude (to 48.73+/-7.03% vs. control) and in success rate (R(suc)) by TBOA (from 91.1+/-7.5% in control to 79.57+/-13.2%). In contrast, with K(+)-based solution in patch pipette (pre- and postsynaptic GluT are intact), amplitude of eEPSC was substantially potentiated by pre-treatment with TBOA (152.1+/-11%), whereas (R(suc)) was reduced to 79.8+/-8.3% in average. The identical reduction of event success rate as well as increased pair-pulse ratios (PPF ratio) for eEPSC in both cases indicates the effect of TBOA on presynaptic uptake. sEPSCs simultaneously recorded from neurons, showed the same pattern of regulation but with less potency, indicating the similar processes in most of excitatory synapses. In conclusion, presynaptic transporters are suggested to act mainly as negative feedback signal on presynaptic release and/or referred to vesicle refilling processes.