Coincident glutamatergic depolarizations enhance GABA(A) receptor-dependent Cl(-) influx in mature and suppress Cl(-) efflux in immature neurons

The impact of GABAergic transmission on neuronal excitability depends on the Cl(-)-gradient across membranes. However, the Cl(-)-fluxes through GABA(A) receptors alter the intracellular Cl(-) concentration ([Cl(-)](i)) and in turn attenuate GABAergic responses, a process termed ionic plasticity. Recently it has been shown that coincident glutamatergic inputs significantly affect ionic plasticity. Yet how the [Cl(-)](i) changes depend on the properties of glutamatergic inputs and their spatiotemporal relation to GABAergic stimuli is unknown. To investigate this issue, we used compartmental biophysical models of Cl(-) dynamics simulating either a simple ball-and-stick topology or a reconstructed CA3 neuron. These computational experiments demonstrated that glutamatergic co-stimulation enhances GABA receptor-mediated Cl(-) influx at low and attenuates or reverses the Cl(-) efflux at high initial [Cl(-)](i). The size of glutamatergic influence on GABAergic Cl(-)-fluxes depends on the conductance, decay kinetics, and localization of glutamatergic inputs. Surprisingly, the glutamatergic shift in GABAergic Cl(-)-fluxes is invariant to latencies between GABAergic and glutamatergic inputs over a substantial interval. In agreement with experimental data, simulations in a reconstructed CA3 pyramidal neuron with physiological patterns of correlated activity revealed that coincident glutamatergic synaptic inputs contribute significantly to the activity-dependent [Cl(-)](i) changes. Whereas the influence of spatial correlation between distributed glutamatergic and GABAergic inputs was negligible, their temporal correlation played a significant role. In summary, our results demonstrate that glutamatergic co-stimulation had a substantial impact on ionic plasticity of GABAergic responses, enhancing the attenuation of GABAergic inhibition in the mature nervous systems, but suppressing GABAergic [Cl(-)](i) changes in the immature brain. Therefore, glutamatergic shift in GABAergic Cl(-)-fluxes should be considered as a relevant factor of short-term plasticity.