Intracortical microstimulation induces rapid microglia process convergence

Intracortical microstimulation (ICMS) has the potential to restore vision and hearing by stimulating relevant cortical regions in both animals and humans, offering significant clinical promise for sensory restoration. While the neuronal response to ICMS has been extensively studied at the cellular level through electrophysiology and two-photon (2P) imaging, the response of non-neuronal cells, particularly microglia, as well as the effects on blood-brain barrier (BBB) integrity remain poorly understood. In this study, we applied ICMS under 2P imaging in dual-reporter mice, with green fluorescent protein labeling microglia and a red fluorescent Ca²⁺ indicator labeling neurons. We also monitored vascular dye leakage to assess BBB integrity. Using clinically relevant waveforms, we tested a range of current amplitudes. Surprisingly, we found that microglia responded within 15 min of stimulation by converging their processes (MPC) on areas of high neural activity and the prevalence of MPC increased at higher current amplitudes. Additionally, vascular dye penetration into brain tissue was higher in stimulated animals and increased with current amplitude. This study reveals a rapid microglia and BBB response to ICMS that has not been reported previously, underscoring the need for further research to fully characterize the biological response to ICMS and establish improved safety standards.