Excitability and synaptic communication within the oligodendrocyte lineage

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⁺ cells) form synapses and generate action potentials, suggesting that neuralevoked excitation of these progenitors may regulate oligodendrogenesis. However, recent studies also suggest that NG2⁺ 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⁺ 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,wefound that NG2⁺cells in diverse brain regions share a core set of physiological properties, including expression of voltage-gated Na⁺ (NaV) channels and ionotropic glutamate receptors, and formation of synapses with glutamatergic neurons. Although small amplitude Na⁺ spikes could be elicited in some NG2⁺ 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⁺ cells may reflect analysis of cells in later stages of maturation. These results suggest that NG2⁺ cells are uniquely positioned within the oligodendrocyte lineage to monitor the firing patterns of surrounding neurons.