Acute spatiotemporal changes in neuronal density surrounding microelectrode arrays implanted in rat motor cortex

Neuronal density and proximity to recording electrodes is perhaps the best indicator of recording longevity from chronically implanted microelectrodes. However, little is known about this important parameter in the hours following implantation. In this study, 1 hour, 24 hours, and 2 weeks post implantation with Si "Michigan" arrays, rats were sacrificed and cortical sections were obtained and immunostained with the neuron-specific marker NeuN. Images were quantified and compared between time points within a zone of interest (0-100 mum from the insertion sites) and examined as a function of depth below the cortical surface. Differences in neuronal density between these three time points were dependent on cortical depth. These data strongly suggest that neuronal density in the cortex respond differently to injury based on their location within the cortical column. The need to account for depth information in evaluating tissue reaction to microelectrode implantation is highlighted.