Prosthetic devices that are controlled by intracortical
electrodes recording one's 'thoughts' are a reality today, and no longer merely in the realm of science fiction. However, widespread clinical use of
implanted electrodes is hampered by a lack of reliability in chronic recordings, independent of the type of
electrodes used. One major hypothesis has been that astroglial
scar electrically impedes the
electrodes. However, there is a temporal discrepancy between stabilization of
scar's electrical properties and recording failure with recording failure lagging by 1 month. In this study, we test a possible explanation for this discrepancy: the hypothesis that chronic
inflammation, due to the persistent presence of the
electrode, causes a local neurodegenerative state in the immediate vicinity of the
electrode. Through modulation of chronic
inflammation via
stab wound,
electrode geometry and age-matched control, we found that after 16 weeks, animals with an increased level of chronic
inflammation were associated with increased neuronal and dendritic, but not axonal, loss. We observed increased neuronal and dendritic loss 16 weeks after implantation compared to 8 weeks after implantation, suggesting that the local neurodegenerative state is progressive. After 16 weeks, we observed axonal pathology in the form of hyperphosphorylation of the
protein tau in the immediate vicinity of the
microelectrodes (as observed in
Alzheimer's disease and other
tauopathies). The results of this study suggest that a local, late onset
neurodegenerative disease-like state surrounds the chronic
electrodes and is a potential cause for chronic recording failure. These results also inform strategies to enhance our capability to attain reliable long-term recordings from
implantable electrodes in the CNS.