Rapid microglial phenotype changes modulate neuronal networks and sharp wave-ripple activity in acute slice preparations

Microglia, the main immune cells of the central nervous system (CNS) have long been known for their remarkable sensitivity to tissue disturbance or injury, but its implications to the interpretation of results from ex vivo models of the CNS have remained largely unclear to date. To this end, we have followed the course of microglial phenotype changes and contribution to neuronal network organisation and functioning in acute brain slices prepared from mice, widely used to study the physiology of the brain from nanoscale events to complex circuits. We found that upon acute slice preparation, microglial cell bodies dislocate and migrate towards the surface of slices, alongside with rapidly progressing morphological changes and altered interactions with neurons. This is accompanied by gradual depolarization and downregulation of P2Y12 receptors, which are instrumental for microglia-neuron communication. Quantitative post-embedding immunofluorescent labelling reveals time-dependent increase in the number of excitatory and inhibitory synapses upon slice preparation in the cerebral cortex, which are markedly influenced by microglia. In line with this, the absence of microglia diminishes the incidence, amplitude and frequency of sharp wave-ripple activity in hippocampal slices. Collectively, our data suggest that microglia are not only inherent modulators of complex neuronal networks, but their specific actions on network reorganisation and functioning must be taken into account when learning lessons from ex vivo models of the CNS. ### Competing Interest Statement The authors have declared no competing interest.