Combined in vivo recording of neural signals and iontophoretic injection of pathway tracers using a hollow silicon microelectrode

•Spatially controlled neuronal labeling is performed with a hollow silicon microelectrode.•Combination of iontophoretic injection with electrophysiology using a single micromachined device is demonstrated in acute animal experiment.•Mapping the projectome of locally labeled neuronal population is demonstrated as possible application of microiontophoresis function of a silicon neural probe. This paper presents the results of in vivo local release of a neuronal tracer, biotinylated dextran amine (BDA) in the rat somatosensory cortex using monolithically integrated microfluidic channel of a silicon neural microelectrode. The tracer injection is controlled by iontophoresis using Pt electrodes in the vicinity of the outlet of the microfluidic channel. Using 3–5μA, 5–7s on/off cycle and 15–20min total injection time the localized injection resulted in clear anterograde and retrograde BDA labeling both within the cortex and in subcortical structures. Anterograde and retrograde labeling revealed the fine details of neuronal processes including dendritic spines and axon terminal-like endings. Injection sites appeared clear lacking any strong diffuse background labeling. Electrophysiological recording performed with the same microdevice immediately after the iontophoresis indicated normal cortical functioning. The results prove that the combination of in vivo multichannel neural recording and controlled tracer injection using a single implanted microdevice is feasible, and therefore it can be a powerful tool for studying the connectome of the brain.