Probing projections between brain areas and their modulation by synaptic potentiation requires dense arrays of contacts for noninvasive electrical stimulation and recording. Semiconductor technology is able to provide planar arrays with high spatial resolution to be used with planar neuronal structures such as organotypic brain slices. To address basic methodical issues we developed a silicon chip with simple arrays of insulated capacitors and field-effect transistors for stimulation of neuronal activity and recording of evoked field potentials. Brain slices from rat hippocampus were cultured on that substrate. We achieved local stimulation of the CA3 region by applying defined voltage pulses to the chip capacitors. Recording of resulting local field potentials in the CA1 region was accomplished with transistors. The relation of stimulation and recording was rationalized by a sheet conductor model. By combining a row of capacitors with a row of transistors we determined a simple stimulus-response matrix from CA3 to CA1. Possible contributions of inhomogeneities of synaptic projection, of tissue structure and of neuroelectronic interfacing were considered. The study provides the basis for a development of semiconductor chips with high spatial resolution that are required for longterm studies of topographic mapping.
Silicon chip and cultured hippocampus slice. (A) Micrograph of silicon chip with array of capacitors (diameter 100 µm) and array of field effect transistors (gate 78 µm x 10 µm). Scale bar 100 µm. The reflection colors mark the different areas of the chip (blue: undoped chip, red: doped areas, green: thin oxide). (B) Nissl staining of typical hippocampus slice on silicon chip after 10 days in culture. Scale bar 100 µm. The positions of capacitors and transistors are marked. (C) Micrograph of silicon chip with unstained hippocampus slice used for the representative measurements. The approximate position of CA1, CA3 and gyrus dentatus is indicated by thin lines.
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