  Graphene has the potential to make a very significant impact on society, with
important applications in the biomedical field. The possibility to engineer
graphene-based medical devices at the neuronal interface is of particular
interest, making it imperative to determine the biocompatibility of graphene
materials with neuronal cells. Here we conducted a comprehensive analysis of
the effects of chronic and acute exposure of rat primary cortical neurons to
few-layers pristine graphene (GR) and monolayer graphene oxide (GO) flakes. By
combining a range of cell biology, microscopy, electrophysiology and omics
approaches we characterized the graphene neuron interaction from the first
steps of membrane contact and internalization to the long-term effects on cell
viability, synaptic transmission and cell metabolism. GR/GO flakes are found in
contact with the neuronal membrane, free in the cytoplasm and internalized
through the endolysosomal pathway, with no significant impact on neuron
viability. However, GO exposure selectively caused the inhibition of excitatory
transmission, paralleled by a reduction in the number of excitatory synaptic
contacts, and a concomitant enhancement of the inhibitory activity. This was
accompanied by induction of autophagy, altered Ca2+ dynamics and by a
downregulation of some of the main players in the regulation of Ca2+
homeostasis in both excitatory and inhibitory neurons. Our results show that,
although graphene exposure does not impact on neuron viability, it does
nevertheless have important effects on neuronal transmission and network
functionality, thus warranting caution when planning to employ this material
for neuro-biological applications.
