Climbing fiber innervation of NG2-expressing glia in the mammalian cerebellum.

The molecular layer of the cerebellar cortex is populated by glial progenitors that express ionotropic glutamate receptors and extend numerous processes among Purkinje cell dendrites. Here, we show that release of glutamate from climbing fiber (CF) axons produces AMPA receptor currents with rapid kinetics in these NG2-immunoreactive glial cells (NG2+ cells) in cerebellar slices. NG2+ cells may receive up to 70 discrete inputs from one CF and, unlike mature Purkinje cells, are often innervated by multiple CFs. Paired Purkinje cell-NG2+ cell recordings show that one CF can innervate both cell types. CF boutons make direct synaptic junctions with NG2+ cell processes, indicating that this rapid neuron-glia signaling occurs at discrete sites rather than through ectopic release at CF-Purkinje cell synapses. This robust activation of Ca2+-permeable AMPA receptors in NG2+ cells expands the influence of the olivocerebellar projection to this abundant class of glial progenitors.