High-resolution multitransistor array recording of electrical field potentials in cultured brain slices.

We report on the recording of electrical activity in cultured hippocampal slices by a multitransistor array (MTA) with 16,384 elements. Time-resolved imaging is achieved with a resolution of 7.8 microm on an area of 1 mm2 at 2 kHz. A read-out of fewer elements allows an enhanced time resolution. Individual transistor signals are caused by local evoked field potentials. They agree with micropipette measurements in amplitude and shape. The spatial continuity of the records provides time-resolved images of evoked field potentials and allows the detection of functional correlations over large distances. As examples, fast propagating waves of presynaptic action potentials are recorded as well as patterns of excitatory postsynaptic potentials across and along cornu ammonis.

Noninvasive neuroelectronic interfacing with synaptically connected snail neurons immobilized on a semiconductor chip.

A hybrid circuit of a semiconductor chip and synaptically connected neurons was implemented and characterized. Individual nerve cells from the snail Lymnaea stagnalis were immobilized on a silicon chip by microscopic picket fences of polyimide. The cells formed a network with electrical synapses after outgrowth in brain conditioned medium. Pairs of neurons were electronically interfaced for noninvasive stimulation and recording. Voltage pulses were applied to a capacitive stimulator on the chip to excite the attached neuron. Signals were transmitted in the neuronal net and elicited an action potential in a second neuron. The postsynaptic excitation modulated the current of a transistor on the chip. The implementation of the silicon-neuron-neuron-silicon circuit constitutes a proof-of-principle experiment for the development of neuroelectronic systems to be used in studies on neuronal signal processing, neurocomputation, and neuroprosthetics.

No correlation of focal contacts and close adhesion by comparing GFP-vinculin and fluorescence interference of Dil.

In regions of focal adhesion, cells adhere to a substrate through the interaction of extracellular matrix proteins and transmembrane integrins which are coupled to the cell skeleton. It is generally assumed that the plasma membrane is brought to close proximity to the substrate there. We used the novel method of fluorescence interference contrast (FLIC) microscopy to measure the distance of the plasma membrane of GD25 fibroblasts on silica coated with fibronectin. We correlated the distance map with the distribution of vinculin tagged with green fluorescent protein. We found that the major part of the membrane was separated by 50 nm from the substrate. With respect to this plateau, we found spots of upward deformation and of close adhesion as well as a general ruffling of the membrane. There was no correlation between the areas of close adhesion and the distribution of vinculin. We conclude that focal adhesion does not imply a close attachment of membrane and substrate.

Fast voltage transients in capacitive silicon-to-cell stimulation detected with a luminescent molecular electronic probe.

The capacitive stimulation of nerve cells from semiconductor chips is a prerequisite for the development of neuroelectronic devices. We report on the primary response of a cell membrane to a voltage step applied to oxidized silicon. It is observed with a luminescent voltage-sensitive dye. We find exponential voltage transients with a time constant of 1-5 micros. We assign the short response to an electrical decoupling by a thin film of electrolyte between oxide and membrane. The high-pass filtering of stimulation is a crucial constraint for the development of silicon-to-neuron interfaces.

Interfacing a silicon chip to pairs of snail neurons connected by electrical synapses.

Future hybrid neuron-semiconductor chips will consist of complex neural networks that are directly interfaced to electronic integrated circuits. They will help us to understand the dynamics of neuronal networks and may lead to novel computational facilities. Here we report on an elementary step towards such neurochips. We designed and fabricated a silicon chip for multiple two-way interfacing, and cultured on it pairs of neurons from the pedal ganglia of the snail Lymnaea stagnalis. These neurons were joined to each other by an electrical synapse, and to the chip by a capacitive stimulator and a recording transistor. We obtained a set of neuroelectronic units with sequential and parallel signal transmission through the neuron-silicon interface and the synapse, with a bidirectionally interfaced neuron-pair and with a signal path from the chip through a synaptically connected neuron pair back to the chip. The prospects for assembling more involved hybrid networks on the basis of these neuroelectronic units are considered.

Recombinant maxi-K channels on transistor, a prototype of iono-electronic interfacing.

We report on the direct electrical interfacing of a recombinant ion channel to a field-effect transistor on a silicon chip. The ion current through activated maxi-K(Ca) channels in human embryonic kidney (HEK293) cells gives rise to an extracellular voltage between cell and chip that controls the electronic source-drain current. A comparison with patch-clamp recording shows that the channels at the cell/chip interface are fully functional and that they are significantly accumulated there. The direct coupling of potassium channels to a semiconductor on the level of an individual cell is the prototype for an iono-electronic interface of ligand-gated or G protein-coupled ion channels and the development of screening biosensors with many transfected cells on a chip with a large array of transistors.

Electrical synapses by guided growth of cultured neurons from the snail Lymnaea stagnalis.

The ability to assemble neuronal networks with designed topology would allow uniquely defined experiments on neurocomputing. We describe a fundamental step, the controlled formation of synapses by guided outgrowth, in vitro for the first time combining simple neuritic geometry with predefined connectivity. We used neurons from the A-clusters in the pedal ganglia of the snail Lymnaea stagnalis. They were cultured on a substrate with linear patterns made by adsorption of brain-derived conditioning factors and photolithography. We induced and observed the frontal collision of two growth cones on narrow lanes. Following such encounters. individual electrical synapses formed that were sometimes strong enough for prolonged presynaptic stimulation to reach the threshold of postsynaptic firing.

Transistor probes local potassium conductances in the adhesion region of cultured rat hippocampal neurons.

Adhesion interactions of neurons in a tissue may affect the ion conductance of the plasma membrane, inducing selective localization and modulation of channels. We studied the adhesion region of cultured neurons from rat hippocampus as a defined model where such effects could be observed electrophysiologically, taking advantage of extracellular recording by a transistor integrated in the substrate. We observed the K(+) current through the region of soma adhesion under voltage-clamp and compared it with the current through the whole cell. We found that the specific A-type conductance was depleted, even completely, in the region of adhesion, whereas the specific K-type conductance was enhanced up to a factor of 12. The electrophysiological approach opens a new way to investigate targeting of ion channels in the cell membrane as a function of adhesion processes.

Extracellular recording with transistors and the distribution of ionic conductances in a cell membrane.

Intracellular voltage transients of cultured cells are recorded by transistors and other planar electrodes as local extracellular voltages. The theoretical relationship between extra- and intracellular voltage is investigated with a two-compartment circuit using the approximation of a fast, weak and small cell-silicon junction. It is shown that extracellular recording relies on the difference of specific ionic conductances in the attached and free regions of the cell membrane. The result rationalizes various observations with neuron transistors. It guides the optimization of extracellular recording and the development of cell-based chemical sensors.

Ca2+ activation of hSlo K+ channel is suppressed by N-terminal GFP tag.

The human slow poke (hSlo) K+ channel was tagged with GFP (green fluorescent protein) at the N-terminus of its alpha-subunit. The fusion protein was expressed transiently in HEK293 cells; it formed functional voltage-gated channels as shown by whole cell patch-clamp measurements. However, the tag lowered the voltage dependence of gating and it suppressed the typical left-shift of gating by intracellular binding of Ca2+. The location of the GFP-tagged N-terminus was confirmed to be on the extracellular side by application of a monoclonal antibody to nonpermeabilized cells. Structural interpretations of the effects are discussed.

Neuron-silicon junction with voltage-gated ionic currents.

We recorded the signals of firing Retzius neurons from Hirudo medicinalis by field-effect transistors. The axon stump of dissociated cells was attached to an open gate coated with concanavalin A. We observed a new type of neuron-transistor coupling: the extracellular voltage transients beneath the neuron were dominated by a negative peak during the rising phase of the action potential with a weaker positive transient in the falling phase. The biphasic response was opposite to the signal of capacitive coupling. We simulated the junction on the basis of the Hodgkin-Huxley equations. We found that the negative transient corresponded to an inward flow of sodium and the positive response to an outward flow of potassium. The field-effect transistors are able to probe the local flow of ionic currents in a membrane which is hidden in the region of cell adhesion. They may become a novel tool in neuroscience.

Cable properties of dendrites in hippocampal neurons of the rat mapped by a voltage-sensitive dye.

Dendrites of pyramidal neurons from embryonic rat hippocampus are investigated in culture using a voltage-sensitive fluorescent dye. The electrical response to somatic stimulation is observed as a time-resolved map with a resolution of 0.9 microm at a time constant of 0.4 ms without signal averaging. The data are interpreted in terms of a tapering cable with Hodgkin-Huxley parametrization. The spread of short hyperpolarizing transients is damped by capacitive shunting. The invasion of an action potential is boosted by voltage-gated conductances of a low density. No irregularity is observed at a bifurcation. The passive cable parameters of internal resistance and membrane resistance at resting voltage are Ri = 300 omega cm and Rm = 40 (k)omega cm2 respectively, at a maximum sodium conductance of approximately 4.4 mS/cm2. The electrotonic length constant and the dynamic length constant at 1 kHz are 580 and 90 microm respectively. These results are compatible with electrophysiological data of dendrites in slices of adult hippocampus and with optical data of narrow processes of leech neurons in culture. The functional implications of boosting an action potential by voltage-gated channels of low density are considered.

Defined neuronal arborizations by guided outgrowth of leech neurons in culture.

Identified neurons of Hirudo medicinalis were cultivated on a protein extract of the extracellular matrix (ECM) of the leech. Microscopic patterns of active ECM protein were prepared by UV photolithography using aluminium masks. The shape of the patterns was visualized by a colour pattern formed in a dye-polymer substrate. The neurons were explanted on the root of branched ECM patterns. The patterns guided the outgrowth of neurites along linear lanes and they induced a bifurcation of the neurites under certain conditions. Neurons with a reproducible, regular shape of arborization were obtained within 1-2 days.

Voltage-sensitive fluorescence of amphiphilic hemicyanine dyes in a black lipid membrane of glycerol monooleate.

Amphiphilic fluorescent hemicyanine dyes were adsorbed to a hemispherical bimolecular membrane of glycerol monooleate. Their excitation spectra of fluorescence were as in water, their emission spectra were as in hydrocarbon. An AC-voltage was applied across the membrane and the relative changes of the spectra of excitation and of emission were recorded. For all dyes we observed a blue-shift of excitation with positive voltage on the opposite side of staining. The effect is compared with the blue-shift expected for electrochromism. For most dyes we observed a red-shift of emission and a drop of the fluorescence intensity. These effects are compared with the red-shift and the drop of quantum yield expected for a voltage-induced solvatochromism caused by a minute displacement of the dyes in the anisotropic environment at the membrane/water interface.

Cable properties of a straight neurite of a leech neuron probed by a voltage-sensitive dye.

We measured a time-resolved map of electrical activity in a thin straight neurite (1.5 microns thick, 500 microns long) at a resolution of 8 microns and 0.4 ms. The neurite was obtained by guided outgrowth of an identified neuron of the leech on lanes of extracellular matrix protein. The electrical signals were detected by a fluorescent voltage-sensitive dye. We observed the voltage that was caused by an action potential elicited at the soma and by a Gaussian hyperpolarization induced at the soma, respectively. We compared the data with numerical solutions of the cable equation using the Hodgkin-Huxley parametrization. We could attribute the experimental results of depolarization and of hyperpolarization to the propagation of an action potential along an "active" cable and to the spread along a "passive" cable, respectively, if we assigned rather high specific resistances to the cytoplasm (RI = 250 omega.cm) and to the membrane (RM = 22 k omega.cm2). This assignment explained the slow velocity of 150 microns/ms of a pulse by active propagation and the limited range of 200 microns of a pulse by passive spread.