# Microglia surveillance is directed toward neuron activation during sustained intracortical microstimulation

## TL;DR

This study shows that microglia track and respond to neuron activity during brain stimulation, suggesting they play an active role in modulating brain circuits.

## Contribution

The study reveals stimulus-dependent and neuron-specific microglial surveillance behaviors during early intracortical microstimulation.

## Key findings

- 10 Hz ICMS increased microglial process motility without causing classical activation.
- Microglial processes preferentially engaged neurons with high calcium activity after stimulation.
- Neurons with suppressed activity received more microglial contacts immediately after implantation.

## Abstract

Objective. Intracortical microstimulation (ICMS) is a widely used tool for neuroprostheses, but its long-term efficacy is often limited by foreign body response 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. Approach. We investigated how 10 Hz ICMS influences microglia–neuron interactions during the first three days post-implantation using in vivo two-photon imaging in Cx3cr1-GFP/jRGECO1a mice. Microglial motility, morphology, and process orientation were tracked relative to electrode placement and neuronal calcium activity (measured as change in fluorescence, ΔF/F). Main results. A 1 h session of 10 Hz ICMS did not induce overt classical morphological activation of microglia but robustly increased process motility, with extensions dynamically tracking neurons showing early activation or subsequent functional suppression. By post-implantation Day 2, microglial processes were significantly more likely to engage neurons with high early calcium responses after stimulation onset (mean orientation angle: 74.3° ± 11.8°), but this engagement shifted during prolonged stimulation (117.0° ± 9.2°, p = 0.0017), indicating context-dependent interactions. Contact frequency scaled with neuronal adaptation profiles, and neurons exhibiting depressed activity received the most contacts immediately after implantation (1.2 ± 0.3 contacts, p = 0.046). Significance. These findings reveal stimulus-associated, neuron-dependent surveillance behaviors of microglia during early post-implantation ICMS and suggest that microglia actively participate in short-term modulation of stimulated cortical circuits.