Role of depolarizing GABAergic transmission for cortical network development

Abstract

GABA mediates synaptic inhibition in the adult brain. However, in early development elevated intracellular Cl concentration ([Cl]in) by NKCC1 causes GABAA receptor (GABAAR) activation to be depolarizing in immature cortical neurons. Despite the potential promotion effect of depolarizing GABA for neuronal network development, a coherent picture cannot be drawn due to a lack of appropriate tools to manipulate the [Cl]in in the central nervous system (CNS). An Emx1-dependent conditional NKCC1 knockout mouse line which harbors disrupted NKCC1 function in cortical pyramidal cells and glia cells but leaving interneurons unaffected was used to explore the role of depolarizing GABAergic transmission on cortical network development. Optimized photo-stimulation of the optogenetic Cl pump eNpHR3.0 enables stable photo-currents, hence Cl loading, on longer time-scales. Next, combined with the optogenetic tool and patch-clamp techniques, the present work shows that NKCC1 contributes to GABAergic depolarization, and eNpHR3.0-mediated artificial loading of Cl in Emx1 positive cells contributes to the generation of spontaneous correlated network activity in the hippocampus in vitro. Disruption of NKCC1 function in Emx1 positive cells impairs such activity. However, this impairment is not due to an altered intrinsic excitability. In addition, the synaptic maturation is not affected in both GABAergic and glutamatergic synaptic transmission. The GABAAR-mediated mGPSCs in cells from NKCC1 KOEmx1 animals undergo a normal developmental increase in frequency and acceleration in decay kinetics, while the AMPAR-mediated mEPSCs undergo a developmental increase in frequency and slowdown in decay kinetics. Three-dimensional voxel-based two-photon imaging revealed column-like Ca2+ clusters in visual cortex representing early spontaneous network activity. With the use of in vivo wide-field imaging technique, the present work shows that the deletion of NKCC1 in Emx1 positive cells does not affect the development of spontaneous network activity in the visual cortex, as the Ca2+ cluster frequency and cluster size undergo a developmental increase and they do not significantly differ between WT and NKCC1 KOEmx1 animals. In conclusion, NKCC1-mediated depolarizing action of GABA is not required for major aspects of cortical network development although it contributes to the generation of spontaneous network activity in the hippocampus in vitro.