Mud sticks after all: Chronically implanted microelectrodes but not electrical stimulation cause c-fos expression along their trajectory

Abstract

The goal of CNS implanted devices is to build a stable brain-machine-interface. The brain tissue response to the foreign body limits the functionality and viability of this brain-machine connection. Notably the astrocytic glial scar formation and inflammation with resulting neuronal cell loss is considered to be responsible for the signal deterioration over time. We chronically implanted a polyimide microelectrode in the dorsolateral striatum of rats. First, we analyzed the c-fos immunoreactivity following high frequency stimulation (HFS) of the dorsolateral striatum and second, using GFAP and ED1 immunocytochemistry, the brain tissue response. Acute as well as chronic HFS showed no significant change of neuronal c-fos expression in the dorsolateral striatum and corresponding cortical areas. We found that the sole chronic implantation of a polyimide microelectrode leads to a reaction of the surrounding neurons, i.e. c-fos expression, along the implantation trajectory. We also observed the formation of a glial scar around the microelectrode with a low number of inflammation cells. Histological and statistical analysis of NeuN positive cells showed no kill zone, which accompanied neuronal cell death around the implantation site.