An Optogenetic Approach for Investigation of Excitatory and Inhibitory Network GABA Actions in Mice Expressing Channelrhodopsin-2 in GABAergic Neurons

Abstract

To investigate excitatory and inhibitory GABA actions in cortical neuronal networks, we present a novel optogenetic approach using a mouse knock-in line with conditional expression of channelrhodopsin-2 (ChR2) in GABAergic interneurons. During whole-cell recordings from hippocampal and neocortical slices from postnatal day (P) 2-P15 mice, photostimulation caused depolarization and excitation of interneurons and evoked barrages of postsynaptic GABAergic currents. Excitatory/inhibitory GABA actions on pyramidal cells were assessed by monitoring the alteration in the frequency of EPSCs during photostimulation of interneurons. We found that in slices from P2-P8 mice, photostimulation evoked an increase in EPSC frequency, whereas in P9 -P15 mice the response switched to a reduction in EPSC frequency, indicating a developmental excitatory-to-inhibitory switch in GABA actions on glutamatergic neurons. Using a similar approach in urethane-anesthetized animals in vivo, we found that photostimulation of interneurons reduces EPSC frequency at ages P3-P9. Thus, expression of ChR2 in GABAergic interneurons of mice enables selective photostimulation of interneurons during the early postnatal period, and these mice display a developmental excitatory-to-inhibitory switch in GABA action in cortical slices in vitro, but so far show mainly inhibitory GABA actions on spontaneous EPSCs in the immature hippocampus and neocortex in vivo.

Developmental Emergence of Sparse Coding: A Dynamic Systems Approach

“…Therefore, our modeling results (Fig. 4b) are in agreement with recent in vivo findings that while GABA acts as a mainly depolarizing transmitter during the first postnatal week, it exerts an inhibitory effect on the underlying network activity in the immature cortex and hippocampus 20,31 .…”

“…This simulated manipulation led to an increase in the cluster activity size only when GABAergic transmission was inhibitory at the network level. Therefore, our results support the recent in vivo findings 31,33 . Importantly, this finding implies that even when GABA depolarizes immature neurons, GABA still can attenuate the instability effect on network activity, thereby restricting cluster activity size.…”

“…However, the effect of GABA at the network level is still an open question. While most of the previous in vitro studies reported that GABA is excitatory (or both excitatory and inhibitory) at the network level 32 , recent in vivo studies harnessing new experimental techniques found that GABA inhibits intact neonatal networks in neocortex and hippocampus 20,29,31 . In particular, GABAergic transmission was shown to limit the spatial extent of cluster activity 20 .…”

“…These studies were focused on a single postnatal stage and this stage was mostly restricted to the first postnatal week, like P5. Moreover, these authors probably did not consider some important biophysical considerations which have only recently been revealed by in vivo studies: the abolishment of cluster activity following the removal of glutamatergic synapses 20,29 , the existence of a profound inhibitory effect of GABAergic transmission at the network level 20,29 and its non-excitatory (but depolarizing) effect at the neuron level 20,31 during the first postnatal week, or that the loss of large cluster activity occurs around eye-opening 2,3 and not around P7 41 . In contrast, here, (i) we used a unified computational model which takes into account most of these recently reported biophysical considerations (see Supplementary Method and Table 1), (ii) our work shows results based on, and mainly in accordance to the recent in vivo studies, where these studies revealed some critical features different from those reported for in vitro data, (iii) we studied the whole developmental period, comprising the time course of sparsification, and not only a single postnatal stage, (iv) we not only address the possible underlying mechanism of the postnatal cluster activity generation, but also the developmental changes in the operating regimes and information processing capabilities, as well as the possible refinements in network properties governing the sparsification.…”

Functions of GABAergic transmission in the immature brain

“…What are the consequences for the generation of early network activity? In order to address the question, Valeeva and colleagues recently employed an optogenetic strategy by selectively expressing channelrhodopsin-2 in GABAergic interneurons and recording glutamatergic PSCs (EPSCs) (Valeeva et al, 2016). The latter serve as a measure of activity of presynaptic glutamatergic neurons.…”