In vivo two-photon microscopy reveals immediate microglial reaction to implantation of microelectrode through extension of processes

Objective. Penetrating cortical neural probe technologies allow investigators to record electrical signals in the brain. Implantation of probes results in acute tissue damage, and microglia density increases around implanted devices over weeks. However, the mechanisms underlying this encapsulation are not well understood in the acute temporal domain. The objective here was to evaluate dynamic microglial response to implanted probes using two-photon microscopy. Approach. Using two-photon in vivo microscopy, cortical microglia similar to 200 mu m below the surface of the visual cortex were imaged every minute in mice with green fluorescent protein-expressing microglia. Main results. Following probe insertion, nearby microglia immediately extended processes toward the probe at (1.6 +/- 1.3) mu m min(-1) during the first 30-45 min, but showed negligible cell body movement for the first 6 h. Six hours following probe insertion, microglia at distances <130.0 mu m (p = 0.5) from the probe surface exhibit morphological characteristics of transitional stage (T-stage) activation, similar to the microglial response observed with laser-induced blood-brain barrier damage. T-stage morphology and microglia directionality indexes were developed to characterize microglial response to implanted probes. Evidence suggesting vascular reorganization after probe insertion and distant vessel damage was also observed hours after probe insertion. Significance. A precise temporal understanding of the cellular response to microelectrode implantation will facilitate the search for molecular cues initiating and attenuating the reactive tissue response.