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NG2 cells, oligodendrocyte progenitors, receive a major synaptic input from interneurons in the developing neocortex. It is presumed that these precursors integrate cortical networks where they act as sensors of neuronal activity. We show that NG2 cells of the developing somatosensory cortex form a transient and structured synaptic network with interneurons that follows its own rules of connectivity. Fast-spiking interneurons, highly connected to NG2 cells, target proximal subcellular domains containing GABAA receptors with γ2 subunits. Conversely, non-fast-spiking interneurons, poorly connected with these progenitors, target distal sites lacking this subunit. In the network, interneuron-NG2 cell connectivity maps exhibit a local spatial arrangement reflecting innervation only by the nearest interneurons. This microcircuit architecture shows a connectivity peak at PN10, coinciding with a switch to massive oligodendrocyte differentiation. Hence, GABAergic innervation of NG2 cells is temporally and spatially regulated from the subcellular to the network level in coordination with the onset of oligodendrogenesis.

NG2 cells, a main pool of glial progenitors, express γ-aminobutyric acid A (GABA(A)) receptors (GABA(A)Rs), the functional and molecular properties of which are largely unknown. We recently reported that transmission between GABAergic interneurons and NG2 cells drastically changes during development of the somatosensory cortex, switching from synaptic to extrasynaptic communication. Since synaptic and extrasynaptic GABA(A)Rs of neurons differ in their subunit composition, we hypothesize that GABA(A)Rs of NG2 cells undergo molecular changes during cortical development accompanying the switch of transmission modes. Single-cell RT-PCR and the effects of zolpidem and α5IA on evoked GABAergic currents reveal the predominance of functional α1- and α5-containing GABA(A)Rs at interneuron-NG2 cell synapses in the second postnatal week, while the α5 expression declines later in development when responses are exclusively extrasynaptic. Importantly, pharmacological and molecular analyses demonstrate that γ2, a subunit contributing to the clustering of GABA(A)Rs at postsynaptic sites in neurons, is down-regulated in NG2 cells in a cell type-specific manner in concomitance with the decline of synaptic activity and the switch of transmission mode. In keeping with the synaptic nature of γ2 in neurons, the down-regulation of this subunit is an important molecular hallmark of the change of transmission modes between interneurons and NG2 cells during development., (© The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

Oligodendrocyte precursor cells (OPCs) are the major source of myelinating oligodendrocytes during development. These progenitors are highly abundant at birth and persist in the adult where they are distributed throughout the brain. The large abundance of OPCs after completion of myelination challenges their unique role as progenitors in the healthy adult brain. Here we show that adult OPCs of the barrel cortex sense fine extracellular K(+) increases generated by neuronal activity, a property commonly assigned to differentiated astrocytes rather than to progenitors. Biophysical, pharmacological, and single-cell RT-PCR analyses demonstrate that this ability of OPCs establishes itself progressively through the postnatal upregulation of Kir4.1 K(+) channels. In animals with advanced cortical myelination, extracellular stimulation of layer V axons induces slow K(+) currents in OPCs, which amplitude correlates with presynaptic action potential rate. Moreover, using paired recordings, we demonstrate that the discharge of a single neuron can be detected by nearby adult OPCs, indicating that these cells are strategically located to detect local changes in extracellular K(+) concentration during physiological neuronal activity. These results identify a novel unitary neuron-OPC connection, which transmission does not rely on neurotransmitter release and appears late in development. Beyond their abundance in the mature brain, the postnatal emergence of a physiological response of OPCs to neuronal network activity supports the view that in the adult these cells are not progenitors only.

NG2 cells, oligodendrocyte precursors, play a critical role in myelination during postnatal brain maturation, but a pool of these precursors is maintained in the adult and recruited to lesions in demyelinating diseases. NG2 cells in immature animals have recently been shown to receive synaptic inputs from neurons, and these have been assumed to persist in the adult. Here, we investigated the GABAergic synaptic activity of NG2 cells in acute slices of the barrel cortex of NG2-DsRed transgenic mice during the first postnatal month, which corresponds to the period of active myelination in the neocortex. Our data demonstrated that the frequency of spontaneous and miniature GABAergic synaptic activity of cortical NG2 cells dramatically decreases after the second postnatal week, indicating a decrease in the number of synaptic inputs onto NG2 cells during development. However, NG2 cells still receive GABAergic inputs from interneurons in the adult cortex. These inputs do not rely on the presence of functional synapses but involve a form of GABA spillover. This GABA volume transmission allows interneurons to induce phasic responses in target NG2 cells through the activation of extrasynaptic GABA(A) receptors. Hence, after development is complete, volume transmission allows NG2 cells to integrate neuronal activity patterns at frequencies occurring during in vivo sensory stimulation.

Background: Advanced light microscopy offers sensitive and non-invasive means to image neural activity and to control signaling with photolysable molecules and, recently, light-gated channels. These approaches require precise and yet flexible light excitation patterns. For synchronous stimulation of subsets of cells, they also require large excitation areas with millisecond and micrometric resolution. We have recently developed a new method for such optical control using a phase holographic modulation of optical wave-fronts, which minimizes power loss, enables rapid switching between excitation patterns, and allows a true 3D sculpting of the excitation volumes. In previous studies we have used holographic photololysis to control glutamate uncaging on single neuronal cells. Here, we extend the use of holographic photolysis for the excitation of multiple neurons and of glial cells., Methods/principal Findings: The system combines a liquid crystal device for holographic patterned photostimulation, high-resolution optical imaging, the HiLo microscopy, to define the stimulated regions and a conventional Ca(2+) imaging system to detect neural activity. By means of electrophysiological recordings and calcium imaging in acute hippocampal slices, we show that the use of excitation patterns precisely tailored to the shape of multiple neuronal somata represents a very efficient way for the simultaneous excitation of a group of neurons. In addition, we demonstrate that fast shaped illumination patterns also induce reliable responses in single glial cells., Conclusions/significance: We show that the main advantage of holographic illumination is that it allows for an efficient excitation of multiple cells with a spatiotemporal resolution unachievable with other existing approaches. Although this paper focuses on the photoactivation of caged molecules, our approach will surely prove very efficient for other probes, such as light-gated channels, genetically encoded photoactivatable proteins, photoactivatable fluorescent proteins, and voltage-sensitive dyes.

In the postnatal central nervous system, glial cells expressing the chondroitin sulfate proteoglycan NG2 (NG2-cells) constitute a cell population exhibiting several properties of oligodendrocyte precursors such as the ability to proliferate. One particular feature of NG2-cells is that they express several glutamatergic and GABAergic ionotropic receptors activated by synaptic neurotransmitter release. Here, we used patch-clamp recordings, immunostaining, calcium imaging, and intracellular labeling to test for the presence of ionotropic nicotinic acetylcholine receptors (nAChRs) in NG2-cells identified in acute hippocampal slices of mice. We demonstrated that these cells express functional nAChRs during the second postnatal week, i.e., the period in which they become the most abundant proliferative cell type of CA1 stratum radiatum. Pharmacological experiments showed that NG2-cells express alpha 7-containing nAChRs. In particular, the powerful positive allosteric modulator of these receptors PNU-120596 induced a 20-fold increase of agonist-induced currents and revealed rises in intracellular calcium concentration upon agonist applications. In addition, nanomolar concentrations of nicotine, which did not induce any response in these cells, largely desensitized nAChR-mediated currents. These data indicate that the functional expression of Ca(2+)-permeable alpha 7-containing nAChRs in hippocampal slices is not restricted to neurons and that the receptors of NG2-cells can be desensitized by low concentrations of nicotine., ((c) 2009 Wiley-Liss, Inc.)