Abstract
Microglial cells reciprocally interact with neurons and increasing evidence indicates that these interactions are activity-dependent and influence the maturation, fate and activity of neurons and synapses. However, the mechanisms through which neuronal activity triggers microglial responses are not fully elucidated. Here we used acute hippocampal slices of transgenic mice expressing the genetically encoded Ca2+ indicator GCaMP5G and fluorescent marker tdTomato in microglia to study the impact of neuronal activity on microglial Ca2+ signaling. In standard recording conditions, we observed only rare Ca2+ transients occurring spontaneously in microglia somata. Triggering epileptiform activity by applying a low Mg2+ and 4-AP containing extracellular solution induced somatic Ca2+ transients in microglia from the first neuronal population burst in synchronization with neuronal bursts. Ca2+ transients were blocked by TTX. The probability to observe a Ca2+ transient during epileptiform activity significantly decreased by blocking N-Methyl-D-Aspartate receptors (NMDARs) or by degrading extracellular ATP/ADP, suggesting involvement of NMDAR-dependent ATP release. Pharmacological experiments further showed that microglial Ca2+ transients induced by epileptiform activity were dependent on the activation of purinergic microglial P2Y12 receptors and on Ca2+ release from intracellular stores. Ca2+ signaling in astrocytes was also enhanced by epileptiform activity. However, buffering intracellular Ca2+ is astrocytes did not affect the probability to observe a Ca2+ transients in microglia during neuronal bursts. Finally, initiating epileptiform activity in the presence of a P2Y12 receptor antagonist led to a higher frequency of neuronal bursts during the initial phase. Thus, microglia respond in real time to neuronal epileptiform activity by somatic Ca2+ transients that are induced by P2Y12 receptor activation and could contribute to the negative feedback control of neuronal activity by microglia and to the initiation of microglia reactivity.