Microglia are an integral part of the brain´s cellular network and have emerged to be crucial regulators of neuronal homeostasis and development as well of brain wiring. Recent evidence indicates that microglial processes extend towards neuronal axons and dendrites, monitoring and regulating neuronal activities and affecting the final formation of neuronal circuits. Some physical interactions of microglia with synapses, as the developmental pruning of afferent inputs, are dependent on neural activity. In this context, one option would be that microglia directly detect neurotransmitter spillover from synapses during neurotransmission. This communication pathway is already established for astrocytes which sense neurotransmitter release via a large repertory of neurotransmitter receptors and transporters that enable them to detect and control the activity of the surrounding neuronal network. Microglia are equipped with a plethora of classical neurotransmitter receptors, as for purines or GABA. The majority of them are G protein-coupled receptors which activation elicits cytosolic Ca2+ elevations. The functional expression of neurotransmitter receptors, as GABAB, was previously demonstrated by live-cell Ca2+ imaging, on sub-populations of freshly isolated microglia or neonatal and adult primary cultured.
In this study, two novel microglia Ca2+ indicator mouse models were used to detect microglial Ca2+ level changes in response to neuronal activity in situ. In these mice, the expression of a genetically encoded calcium indicator (GCaMP6m) is driven by endogenous Csf1r gene expression and therefore independent of Cre recombinase activity or viruses, enabling the opportunity to monitor Ca2+ level changes in microglia from neonatal and adult hippocampal brain slices. In the present thesis, I demonstrate that electrical stimulation of the Schaffer collateral pathway results in microglial Ca2+ responses in early postnatal, but not adult hippocampus. Preceding the microglial responses, a similar wave-like propagation of Ca2+ responses was present also in astrocytes, and both were dependent on neuronal activity as evidenced by their sensitivity to tetrodotoxin. Blocking the astrocytic glutamate uptake or the GABA transport, as well as antagonizing GABAB receptors, abolished the stimulation-induced microglial responses. These data therefore suggest that the neuronal activity-induced glutamate uptake and release of GABA by astrocytes triggers the activation of GABAB receptors in microglia. This novel neuron, astrocyte and microglia communication pathway is confined to postnatal brain and might then critically modulate microglial activity in developing neuronal networks.
