Microglia surveillance is directed toward neuron activation during sustained intracortical microstimulation

Intracortical microstimulation (ICMS) is a widely used tool for neuroprostheses, but its long-term efficacy is often limited by biofouling and neuroinflammatory responses at the electrode-tissue interface. Microglia orchestrate neuroinflammation and regulate synaptic plasticity, and low-frequency stimulation has been shown to promote anti-inflammatory microglial phenotypes. We therefore examined, over the first three days post-implantation, how 10-Hz ICMS influences microglial-neuronal interactions in vivo using two-photon imaging in Cx3cr1-GFP/jRGECO1a mice.

A one-hour session of 10-Hz ICMS did not induce overt morphological activation of microglia but increased their process motility, directing extensions toward both the electrode and neurons exhibiting elevated calcium activity. By post-implantation Day 2, microglial extensions were significantly biased toward neurons whose ΔF/F surpassed a 3 standard deviation threshold after stimulation onset (74.26° ± 11.83°) but shifted away from those same neurons after 40 min of continuous stimulation (116.99° ± 9.19°) (p = 0.001732), suggesting a dynamic, homeostatic response to sustained neuronal calcium elevations. Although multi-day electrode insertion accelerated microglial motility and polarization toward the device, 10-Hz ICMS alone did not alter microglial branching or soma shape. Microglial contact frequency scaled with neuronal adaptation profiles: depressed neurons received the most contacts immediately post-implant (1.15 ± 0.3 contacts; p = .0460). These findings reveal stimulus-associated, neuron-dependent surveillance behaviors of microglia during early post-implantation ICMS and implicate them as active participants in short-term modulation of cortical circuits.