The mammalian CNS contains an abundant, widely distributed population of glial cells that serve as oligodendrocyte progenitors. It has been reported that these NG2-immunoreactive cells (NG2 ϩ cells) form synapses and generate action potentials, suggesting that neuralevoked excitation of these progenitors may regulate oligodendrogenesis. However, recent studies also suggest that NG2 ϩ cells are comprised of functionally distinct groups that differ in their ability to respond to neuronal activity, undergo differentiation, and experience injury following ischemia. To better define the physiological properties of NG2 ϩ cells, we used transgenic mice that allowed an unbiased sampling of this population and unambiguous identification of cells in discrete states of differentiation. Using acute brain slices prepared from developing and mature mice, we found that NG2 ϩ cells in diverse brain regions share a core set of physiological properties, including expression of voltage-gated Na ϩ (NaV) channels and ionotropic glutamate receptors, and formation of synapses with glutamatergic neurons. Although small amplitude Na ϩ spikes could be elicited in some NG2 ϩ cells during the first postnatal week, they were not capable of generating action potentials. Transition of these progenitors to the premyelinating stage was accompanied by the rapid removal of synaptic input, as well as downregulation of AMPA and NMDA receptors and NaV channels. Thus, prior reports of physiological heterogeneity among NG2 ϩ cells may reflect analysis of cells in later stages of maturation. These results suggest that NG2 ϩ cells are uniquely positioned within the oligodendrocyte lineage to monitor the firing patterns of surrounding neurons.

SUMMARY Microglia play critical roles in tissue homeostasis and can also modulate neuronal function and synaptic connectivity. In contrast to astrocytes and oligodendrocytes, which arise from multiple progenitor pools, microglia arise from yolk sac progenitors and are widely considered to be equivalent throughout the CNS. However, little is known about basic properties of deep brain microglia, such as those within the basal ganglia (BG). Here, we show that microglial anatomical features, lysosome content, membrane properties, and transcriptomes differ significantly across BG nuclei. Region-specific phenotypes of BG microglia emerged during the second postnatal week and were re-established following genetic or pharmacological microglial ablation and repopulation in the adult, indicating that local cues play an ongoing role in shaping microglial diversity. These findings demonstrate that microglia in the healthy brain exhibit a spectrum of distinct functional states and provide a critical foundation for defining microglial contributions to BG circuit function.

Structural plasticity of dendritic spines and synapses is a fundamental mechanism governing neuronal circuits and may form an enduring basis for information storage in the brain. We find that the p65 subunit of the NF-κB transcription factor, which is required for learning and memory, controls excitatory synapse and dendritic spine formation and morphology in murine hippocampal neurons. Endogenous NF-κB activity is elevated by excitatory transmission during periods of rapid spine and synapse development. During in-vitro synaptogenesis, NF-κB enhances dendritic spine and excitatory synapse density and loss of endogenous p65 decreases spine density and spine head volume. Cell-autonomous function of NF-κB within the postsynaptic neuron is sufficient to regulate the formation of both pre- and post-synaptic elements. During synapse development in-vivo, loss of NF-κB similarly reduces spine density and also diminishes the amplitude of synaptic responses. In contrast, after developmental synaptogenesis has plateaued, endogenous NF-κB activity is low and p65-deficiency no longer attenuates basal spine density. Instead, NF-κB in mature neurons is activated by stimuli that induce demand for new synapses, including estrogen and short-term bicuculline, and is essential for upregulating spine density in response to these stimuli. p65 is enriched in dendritic spines making local protein-protein interactions possible; however, the effects of NF-κB on spine density require transcription and the NF-κB-dependent regulation of PSD-95, a critical postsynaptic component. Collectively, our data define a distinct role for NF-κB in imparting transcriptional regulation required for the induction of changes to, but not maintenance of, excitatory synapse and spine density.

Oligodendrocyte precursor cells (OPCs) express NMDA receptors (NMDARs) and form synapses with glutamatergic neurons throughout the central nervous system (CNS). Although glutamate influences the proliferation and maturation of these progenitors in vitro, the role of NMDAR signaling in oligodendrogenesis and myelination in vivo is not known. Here, we investigated the consequences of genetically deleting the obligatory NMDAR subunit NR1 from OPCs and their oligodendrocyte progeny in the CNS of developing and mature mice. NMDAR-deficient OPCs proliferated normally, achieved appropriate densities in gray and white matter, and differentiated to form major white matter tracts without delay. OPCs also retained their characteristic physiological and morphological properties in the absence of NMDAR signaling, and were able to form synapses with glutamatergic axons. However, expression of calcium permeable AMPA receptors was enhanced in NMDAR-deficient OPCs. These results suggest that NMDAR signaling is not used to control OPC development, but to regulate AMPAR-dependent signaling with surrounding axons, pointing to additional functions for these ubiquitous glial cells.