Voltage-clamp Amplifier Research Articles

A test potential booster for fast-scan cyclic voltammetry with an electrophysiological amplifier

Fast-scan cyclic voltammetry (FSCV) is a powerful technique for studying the local dynamics of neurotransmitters and neuromodulators. FSCV is attractive to researchers employing electrophysiological techniques because it can be performed using an electrophysiological voltage-clamp amplifier. However, the narrow test potential range of electrophysiological amplifiers (typically, ±1V) limits testable species of analytes. Here we devised a booster that extends the test potential range. Using the booster, we could detect the oxidation current of adenosine peaking near a test potential of +1.5V. The booster should promote combined electrophysiological and electrochemical studies of synaptic release and neural secretion.

A Four Microelectrode Method to Study Intracellular Ion Concentration and Transport in Skeletal Muscle Fibers

Steady state myoplasmic ion concentration and slow ion transport across the sarcolemma can be quantitatively studied by means of ion sensing electrodes (ISE). ISE's readout represents the sum of the electrochemical potential of the ion of interest and the resting membrane potential (Vm). Vm is measured either by using an independent standard microelectrode or a double-barreled capillary. These approaches have known limitations. Ion transport and membrane potential are intricately connected, the resting potential may be unstable or be far from the desired value, ion transport can be electrically silent, and membrane currents may be carried by more than one ion. Thus, an electrophysiological method able to simultaneously measure the concentration of more than one ion, and to measure and control Vm and membrane current (Im) is desirable for physiological and pathophysiological studies. For skeletal fibers, a triple-barreled electrode has been used in the past allowing for current injection to modify Vm under current clamp conditions. Here we describe a novel four-microelectrode system composed of a two-microelectrode voltage-clamp amplifier (TEV-200A, Dagan) and a two channel high impedance electrometer (FD223a, WPI). This approach allows for potentiometric measurements of the concentration and transport of up to two ions in short murine skeletal muscle fibers subjected to either current- or voltage-clamp conditions. Ion translocation by primary and secondary active transport mechanisms, ion exchangers or passive ion diffusion can be studied. The system performance is exemplified by using Na, K and H selective electrodes made with commercially available ionophores and fibers enzymatically dissociated from the flexor digitorum brevis muscles.

Determination and compensation of series resistances during whole-cell patch-clamp recordings using an active bridge circuit and the phase-sensitive technique

We present a technique which combines two methods in order to measure the series resistance (R S) during whole-cell patch-clamp recordings from excitable and non-excitable cells. R S is determined in the amplifier's current-clamp mode by means of an active bridge circuit. The correct neutralization of the electrode capacitance and the adjustment of the bridge circuit is achieved by the so-called phase-sensitive method: Short sine wave currents with frequencies between 3 and 7kHz are injected into the cells. Complete capacitance neutralization is indicated by the disappearance of the phase lag between current and voltage, and correct bridge balance is indicated by a minimized voltage response to the sine wave current. The R S value determined in the current-clamp mode then provides the basis for R S compensation in the voltage-clamp recording mode. The accuracy of the procedure has been confirmed on single-compartment cell models where the error amounted to 2-3%. Similar errors were observed during dual patch clamp recordings from single neocortical layer 5 pyramidal cells where one electrode was connected to the bridge amplifier and the other one to a time-sharing, single-electrode current- and voltage-clamp amplifier with negligible R S. The technique presented here allows R S compensation for up to 80-90%, even in cells with low input resistances (e.g., astrocytes). In addition, the present study underlines the importance of correct R S compensation by showing that significant series resistances directly affect the determination of membrane conductances as well as the kinetic properties of spontaneous synaptic currents with small amplitudes.

Smartphone Operated Signal Transduction by Ion Nanogating (STING) Amplifier for Nanopore Sensors: Design and Analytical Application.

In this report, we demonstrated a handheld wireless voltage-clamp amplifier for current measurement of nanopore sensors. This amplifier interfaces a sensing probe and connects wirelessly with a computer or smartphone for the required stimulus input, data processing and storage. To test the proposed Signal Transduction by Ion Nanogating (STING) wireless amplifier, in the current study the system was tested with a nano-pH sensor to measure pH of standard buffer solutions and the performance was compared against the commercial voltage-clamp amplifier. To our best knowledge, STING amplifier is the first miniaturized wireless voltage-clamp platform operated with a customized smart-phone application (app).

Microchip amplifier for in vitro, in vivo, and automated whole cell patch-clamp recording.

Patch clamping is a gold-standard electrophysiology technique that has the temporal resolution and signal-to-noise ratio capable of reporting single ion channel currents, as well as electrical activity of excitable single cells. Despite its usefulness and decades of development, the amplifiers required for patch clamping are expensive and bulky. This has limited the scalability and throughput of patch clamping for single-ion channel and single-cell analyses. In this work, we have developed a custom patch-clamp amplifier microchip that can be fabricated using standard commercial silicon processes capable of performing both voltage- and current-clamp measurements. A key innovation is the use of nonlinear feedback elements in the voltage-clamp amplifier circuit to convert measured currents into logarithmically encoded voltages, thereby eliminating the need for large high-valued resistors, a factor that has limited previous attempts at integration. Benchtop characterization of the chip shows low levels of current noise [1.1 pA root mean square (rms) over 5 kHz] during voltage-clamp measurements and low levels of voltage noise (8.2 μV rms over 10 kHz) during current-clamp measurements. We demonstrate the ability of the chip to perform both current- and voltage-clamp measurement in vitro in HEK293FT cells and cultured neurons. We also demonstrate its ability to perform in vivo recordings as part of a robotic patch-clamping system. The performance of the patch-clamp amplifier microchip compares favorably with much larger commercial instrumentation, enabling benchtop commoditization, miniaturization, and scalable patch-clamp instrumentation.

A Miniaturized Single Channel Amplifier for Various Different Electrophysiology Setup

Different electrophysiology setups allow single ion channel recordings using Lipid Bilayer Membrane (BLM) for protein insertion. Ion channels are transmembrane proteins involved in many biological process of cell life. They are used as target for drug discovery and single-molecule sensors. Synthetic nanopores can also be used as nano-coulter counters and very promising transducers for DNA sequencing. Complex protocols are under investigation to assembly ion channels on reconstituted lipid membranes. They use different setups, ranging from suspended BLM, microfluidic and lab-on-a-chip devices, planar patch clamp, tip-dip method. These studies, combined to new emerging nanopore applications, require high sensitive and high resolution instrumentation, able to detect small currents (in the pA range) over large bandwith (up to 100kHz), providing flexible voltage stimuli generation. We present a compact and complete system for single ion channels and synthetic nanopores experiment, useful for scientists and students for academic purposes, composed by:- a miniaturized low noise single channel voltage-clamp amplifier and digitizer integrated into a single ASIC, with direct connection to a computer USB port;- a software interface for real time data display and analysis featuring flexible voltage stimuli control;- customised electrode interfaces for various different electrophysiology platforms and microfluidic devices.As a proof of concept, we demonstrate the system performances using different test proteins (KcsA, Gramicidin, Alpha-hemolysin) embedded into reconstituted lipid membranes. The acquired data are presented.

Amphetamine Activates an Amine-gated Chloride Channel to Generate Behavioral Effects in Caenorhabditis elegans

Amphetamine is a highly addictive psychostimulant, which is thought to generate its effects by promoting release of dopamine through reverse activation of dopamine transporters. However, some amphetamine-mediated behaviors persist in dopamine transporter knock-out animals, suggesting the existence of alternative amphetamine targets. Here we demonstrate the identification of a novel amphetamine target by showing that in Caenorhabditis elegans, a large fraction of the behavioral effects of amphetamine is mediated through activation of the amine-gated chloride channel, LGC-55. These findings bring to light alternative pathways engaged by amphetamine, and urge rethinking of the molecular mechanisms underlying the effects of this highly-addictive psychostimulant.

MATLAB implementation of a dynamic clamp with bandwidth of >125 kHz capable of generating I Na at 37 °C

We describe the construction of a dynamic clamp with a bandwidth of >125 kHz that utilizes a high-performance, yet low-cost, standard home/office PC interfaced with a high-speed (16 bit) data acquisition module. High bandwidth is achieved by exploiting recently available software advances (code-generation technology and optimized real-time kernel). Dynamic-clamp programs are constructed using Simulink, a visual programming language. Blocks for computation of membrane currents are written in the high-level MATLAB language; no programming in C is required. The instrument can be used in single- or dual-cell configurations, with the capability to modify programs while experiments are in progress. We describe an algorithm for computing the fast transient Na(+) current (I Na) in real time and test its accuracy and stability using rate constants appropriate for 37 °C. We then construct a program capable of supplying three currents to a cell preparation: I Na, the hyperpolarizing-activated inward pacemaker current (I f) and an inward-rectifier K(+) current (I K1). The program corrects for the IR drop due to electrode current flow and also records all voltages and currents. We tested this program on dual patch-clamped HEK293 cells where the dynamic clamp controls a current-clamp amplifier and a voltage-clamp amplifier controls membrane potential, and current-clamped HEK293 cells where the dynamic clamp produces spontaneous pacing behavior exhibiting Na(+) spikes in otherwise passive cells.

Fragrant Dioxane Derivatives Identify β1-Subunit-containing GABAA Receptors

Nineteen GABA(A) receptor (GABA(A)R) subunits are known in mammals with only a restricted number of functionally identified native combinations. The physiological role of beta1-subunit-containing GABA(A)Rs is unknown. Here we report the discovery of a new structural class of GABA(A)R positive modulators with unique beta1-subunit selectivity: fragrant dioxane derivatives (FDD). At heterologously expressed alpha1betaxgamma2L (x-for 1,2,3) GABA(A)R FDD were 6 times more potent at beta1- versus beta2- and beta3-containing receptors. Serine at position 265 was essential for the high sensitivity of the beta1-subunit to FDD and the beta1N286W mutation nearly abolished modulation; vice versa the mutation beta3N265S shifted FDD sensitivity toward the beta1-type. In posterior hypothalamic neurons controlling wakefulness GABA-mediated whole-cell responses and GABAergic synaptic currents were highly sensitive to FDD, in contrast to beta1-negative cerebellar Purkinje neurons. Immunostaining for the beta1-subunit and the potency of FDD to modulate GABA responses in cultured hypothalamic neurons was drastically diminished by beta1-siRNA treatment. In conclusion, with the help of FDDs we reveal a functional expression of beta1-containing GABA(A)Rs in the hypothalamus, offering a new tool for studies on the functional diversity of native GABA(A)Rs.

Constitutive Activity of the Human TRPML2 Channel Induces Cell Degeneration

The mucolipin (TRPML) ion channel proteins represent a distinct subfamily of channel proteins within the transient receptor potential (TRP) superfamily of cation channels. Mucolipin 1, 2, and 3 (TRPML1, -2, and -3, respectively) are channel proteins that share high sequence homology with each other and homology in the transmembrane domain with other TRPs. Mutations in the TRPML1 protein are implicated in mucolipidosis type IV, whereas mutations in TRPML3 are found in the varitint-waddler mouse. The properties of the wild type TRPML2 channel are not well known. Here we show functional expression of the wild type human TRPML2 channel (h-TRPML2). The channel is functional at the plasma membrane and characterized by a significant inward rectification similar to other constitutively active TRPML mutant isoforms. The h-TRPML2 channel displays nonselective cation permeability, which is Ca(2+)-permeable and inhibited by low extracytosolic pH but not Ca(2+) regulated. In addition, constitutively active h-TRPML2 leads to cell death by causing Ca(2+) overload. Furthermore, we demonstrate by functional mutation analysis that h-TRPML2 shares similar characteristics and structural similarities with other TRPML channels that regulate the channel in a similar manner. Hence, in addition to overall structure, all three TRPML channels also share common modes of regulation.

Ionflux: A Microfluidic Approach to Ensemble Recording and Block of Whole-Cell Current from Voltage-Gated Ion Channels

Automated patch clamp (APC) addresses a need for high throughput screening of chemical entities altering ion channel function. Systems that can produce pharmacologically relevant data rapidly and consistently find considerable utility in the pharmaceutical industry, but also increasingly in academic laboratories. Here we present data obtained with a novel APC platform utilizing a well-plate microfluidic design. Unlike existing devices, the IonFlux system incorporates no internal robotic liquid handling, and features continuous recording from cell ensembles during rapid solution switches with a bench-top footprint resembling a conventional plate reader.True whole cell voltage clamp was applied to linear arrays of up-to thirty cells in parallel, utilizing fully-featured 16 or 64 channel voltage-clamp amplifiers under computer control. Laminar flow of solutions in a microfluidic network delivered cells in suspension to the recording sites and enabled fast exchange of bathing solutions via an electro-pneumatic interface, on either 96 or 384 well-plate formats. Electrophysiological characterization was achieved for KV 2.1 and hERG potassium channels, and examples of NaV sodium channels. Our results show the voltage-dependence of these currents and their block by pharmacological agents. The recordings also demonstrate the potential for microfluidics-enabled, exceptionally fast superfusion and washout of candidate drugs. Incorporation of multiple experiments per well-plate enables many investigations to be performed in parallel, especially if multiple compounds are applied to each individual cell ensemble recording array. Data on recording success rates, throughput and assay reproducibility show that high throughput experiments can be performed with enhanced reliability.

Population Patch Clamp Improves Data Consistency and Success Rates in the Measurement of Ionic Currents

Present whole-cell patch-clamp methodology has only moderate consistency and throughput, rendering impractical functional measurements on large numbers of ion channel ligands or on large numbers of unknown or mutant channel genes. In the population patch clamp (PPC) described herein, a single voltage-clamp amplifier sums the whole-cell currents from multiple cells at once, each sealed to a separate aperture in a planar substrate well. The resulting ensemble currents are more consistent from well to well, and the success rate for each recording attempt is >95%. The PPC was implemented by modifying the PatchPlate substrate and amplifiers in the IonWorks patch-clamp instrument. The increased data consistency and likelihood of a successful recording in each well, combined with 384-well measurements in parallel, allow the direct electrophysiological recording of thousands of ensemble ionic currents per day. Therapeutic groups in drug discovery programs require this order of throughput to screen directed compound libraries against ion channel targets. The potential for studying the function of large numbers of ion channel mutants may be realized with the technique. The procedure incorporates subtraction methods that correct for expected distortions and also reliably produces data that agree with previous patch-clamp studies.

Direct electrical detection of DNA synthesis

Rapid, sequence-specific DNA detection is essential for applications in medical diagnostics and genetic screening. Electrical biosensors that use immobilized nucleic acids are especially promising in these applications because of their potential for miniaturization and automation. Current DNA detection methods based on sequencing by synthesis rely on optical readouts; however, a direct electrical detection method for this technique is not available. We report here an approach for direct electrical detection of enzymatically catalyzed DNA synthesis by induced surface charge perturbation. We discovered that incorporation of a complementary deoxynucleotide (dNTP) into a self-primed single-stranded DNA attached to the surface of a gold electrode evokes an electrode surface charge perturbation. This event can be detected as a transient current by a voltage-clamp amplifier. Based on current understanding of polarizable interfaces, we propose that the electrode detects proton removal from the 3'-hydroxyl group of the DNA molecule during phosphodiester bond formation.

Dopamine receptor activation can reduce voltage-gated Na+ current by modulating both entry into and recovery from inactivation.

We tested whether dopamine receptor activation modulates the voltage-gated Na+ current of goldfish retinal ganglion cells, using a fast voltage-clamp amplifier, perforated-patch whole cell mode, and a physiological extracellular Na+ concentration. As found in other cells, activators of D1-type dopamine receptors and of protein kinase A reduced the amplitude of current activated by depolarizations from resting potential without altering the current kinetics or activation range. However, D1-type dopamine receptor activation also accelerated the rate of entry into inactivation during subthreshold depolarizations and slowed the rate of recovery from inactivation after single, brief depolarizations. Our results provide the first evidence in any preparation that D1-type receptor activation can produce both of these latter effects.

Multiple Structural Elements Contribute to the Slow Kinetics of the Cav3.3 T-type Channel

Molecular cloning and expression studies established the existence of three T-type Ca(2+) channel (Ca(v)3) alpha(1) subunits: Ca(v)3.1 (alpha(1G)), Ca(v)3.2 (alpha(1H)), and Ca(v)3.3 (alpha(1I)). Although all three channels are low voltage-activated, they display considerable differences in their kinetics, with Ca(v)3.1 and Ca(v)3.2 channels activating and inactivating much faster than Ca(v)3.3 channels. The goal of the present study was to determine the structural elements that confer the distinctively slow kinetics of Ca(v)3.3 channels. To address this question, a series of chimeric channels between Ca(v)3.1 and Ca(v)3.3 channels were constructed and expressed in Xenopus oocytes. Kinetic analysis showed that the slow activation and inactivation kinetics of the Ca(v)3.3 channel were not completely abolished by substitution with any one portion of the Ca(v)3.1 channel. Likewise, the Ca(v)3.1 channel failed to acquire the slow kinetics by simply adopting one portion of the Ca(v)3.3 channel. These findings suggest that multiple structural elements contribute to the slow kinetics of Ca(v)3.3 channels.

Voltage-clamp-controlled current-clamp recordings from neurons: an electrophysiological technique enabling the detection of fast potential changes at preset holding potentials.

Investigations of the properties of fast, transient potential changes (e.g. receptor potentials or synaptic potentials) in excitable cells by means of current-clamp recording techniques require the exact adjustment and control of membrane potentials. Usually, the desired membrane potential values are set by current injection via the recording electrode and are controlled manually by regulating the current strength necessary to maintain a constant potential. However, this technique is associated with a number of disadvantages. A single-electrode current- and voltage-clamp amplifier was therefore modified to compensate for slow membrane potential changes without affecting faster voltage responses. Basically, low-pass filters with selectable time constants were incorporated into the voltage-clamp feedback circuit to control the amplifier's response speed. In addition, the amplifier's electronic circuits were altered to enable current pulse injection into the cells. Thus, while recording at preset and controlled membrane potentials, it was possible to monitor the cell's input resistance or current/voltage relationship. This new recording technique has been designated "voltage-clamp-controlled current clamp" (VCcCC) and its performance was tested by intracellular recordings from neocortical and neostriatal neurons in vitro using either conventional microelectrodes or patch-clamp electrodes.

Switched single-electrode voltage-clamp amplifiers allow precise measurement of gap junction conductance.

Measurement of gap junction conductance (gj) with patch-clamp amplifiers can, due to series resistance problems, be subject to considerable errors when large currents are measured. Formulas developed to correct for these errors unfortunately depend on exact estimates of series resistance, which are not always easy to obtain. Discontinuous single-electrode voltage-clamp amplifiers (DSEVCs) were shown to overcome series resistance problems in single whole cell recording. With the use of two synchronized DSEVCs, the simulated gj in a model circuit can be measured with a maximum error of <5% in all recording situations investigated (series resistance, 5-47 MOmega; membrane resistance, 20-1,000 MOmega; gj, 1-100 nS). At a very low gj of 100 pS, the error sometimes exceeded 5% (maximum of 15%), but the error was always <5% when membrane resistance was >100 MOmega. The precision of the measurements is independent of series resistance, membrane resistance, and gj. Consequently, it is possible to calculate gj directly from Ohm's law, i.e., without using correction formulas. Our results suggest that DSEVCs should be used to measure gj if large currents must be recorded, i.e., if cells are well coupled or if membrane resistance is low.

Study of the inhibitor of the crayfish neuromuscular junction by presynaptic voltage control.

The inhibitor of the crayfish opener muscle was investigated by a presynaptic voltage control method. Two microelectrodes were inserted into the inhibitor and the amplitude and duration of presynaptic depolarization were controlled by a voltage-clamp amplifier. The inhibitory postsynaptic potential (IPSP) was measured from a muscle fiber located near the presynaptic voltage electrode. Nonlinear summation of IPSP amplitudes was corrected after chloride equilibrium potential was measured. With the use of 5-ms presynaptic pulses, the depolarization-release coupling (D-R) curve constructed from IPSP peak amplitudes (IPSPcor) had a threshold of about -35 mV and reached its maximal level at -5 to -10 mV. Depolarization beyond the maximum led to a suppression of neurotransmitter release. When transmitter release during a presynaptic pulse was completely suppressed, IPSPs activated by tail current could be identified with an average synaptic delay of 2.5 ms. Transmitter secretion triggered by a calcium current activated during the 5-ms pulses (IPSPon) was also measured on the rising phase of an IPSP, at 2.5 ms after the end of the 5-ms pulses. D-R coupling plots measured from IPSPon exhibited a more pronounced suppression than that obtained from IPSPcor. The effect of presynaptic pulse duration on the level of transmitter release was analyzed. Transmitter release increased with increasing duration and was nearly saturated by 20-ms pulses depolarized to 0 mV. The following conditions were identified as necessary to obtain a consistent D-R curve with a clear suppression: 1) small animals, 3.8 cm head to tail, 2) 15 degrees C, 3) 40 mM tetraethylammonium and 1 mM 4-aminopyridine, 4) an extracellular calcium concentration of < or = 10 mM. In addition, a consistent correlation was found among the branching pattern of the inhibitor, the placement of the presynaptic electrode, and the characteristics of the D-R curves. An ideal presynaptic electrode configuration involved placing the voltage electrode in a secondary branch, approximately 100 microns from the main branch point, and placing the current electrode at the branch point. Postsynaptically, optimal recordings were obtained from muscle fibers innervated by a single branch of the inhibitor that originated from a point near the presynaptic voltage electrode. A cable-release model was constructed to evaluate the relationship between the shape of the D-R coupling curves and the space constants of the presynaptic terminals. A comparison between the model and the D-R coupling curves suggested that the space constant of an inhibitor branch on a muscle fiber is > or = 8 times longer than its actual length. Therefore the upper limit estimate of the space constant of a typical preparation is approximately 3 mm. Results reported here outline morphological and physiological conditions needed to achieve optimal control of the presynaptic branch of the crayfish inhibitor. The cable-release model quantitatively defines the extent of presynaptic voltage control.

Renin-angiotensin system and cell communication in the failing heart.

The influence of heart failure on the process of cell communication was investigated in cell pairs isolated from the ventricle of cardiomyopathic hamsters (11 months old) and the results compared with age-matched normal hamsters. The gap junctional conductance (gj) was measured with two voltage-clamp amplifiers. The results showed two major populations of cell pairs with respect to gj values: one with very low values (0.8 to 2.5 nS) and the other with higher values (7 to 35 nS). In normal hamsters, the most frequent gj values were in the range of 40 to 100 nS. Angiotensin II (Ang 11, 1 microg/mL) caused cell uncoupling in myopathic myocytes with low gj but reduced gj by 53 +/- 6.6 percent (+/- SE) in cell pairs with higher gj values (7 to 35 nS). The effect of Ang II on gj of myopathic cell pairs was suppressed by losartan (10(-7) mol/L). In cardiomyopathic cell pairs with low gj (0.8 to 2.5 nS), enalapril (1 microg/mL) caused an appreciable increase in gj (219 +/- 20.3 percent), whereas in cell pairs with higher gj (7 to 35 nS), the gj increment was smaller (80 +/- 10.8 percent) but still larger than that seen in controls (33 +/- 5.4 percent). Intracellular dialysis of Ang I (10(-8) mol/L) abolished cell communication in myopathic cell pairs with low gj (0.8 to 2.5 nS) and reduced gj by 66 +/- 1.7 percent in the other pairs (7 to 35 nS). The effect of Ang I on gj was greatly reduced by enalaprilat (10(-9) mol/L) added to the cytosol. Dialysis of Ang II (10(-8) mol/L) into the myopathic cell reduced gj by 48 +/- 4.2 percent, an effect abolished by losartan (10(-8) mol/L). The results indicate that the decline in gj seen in the ventricle of cardiomyopathic hamsters is in part due to activation of the cardiac renin-angiotensin system.

An iconographic program for computer-controlled whole-cell voltage clamp experiments

A Macintosh-based system is described for performing instrument control, data acquisition and storage operations in single-electrode whole-cell voltage clamp experiments. The system consists of a commercially available voltage clamp amplifier, multifunction input/output (I/O) board, graphical programming language (LabVIEW 2) and custom built 'virtual instrument' (VI). The I/O board is capable of fast (up to 110 ksample/s) multichannel analog-to-digital (A/D) conversion with 12-bit resolution. It can control the gain settings of the clamp unit through digital I/O lines and generate the P/N leak subtraction protocol to eliminate the linear portion of capacitive currents using analog output voltages and gating pulses. Complete voltage clamp protocols can be implemented using the on-screen front panel controls of the VI. It enables the user to visualize the acquired data, to graph sets of current-voltage (I-V) relations or to fit single-exponential functions to one current trace. To evaluate the adequacy of whole-cell recording, the total membrane capacitance (Cm), the series resistance (Rs) and the time constant (tau c) of the decay of the capacitive current are calculated using the single-exponential function fit to the data. The system is particularly well suited to the study of large quantities of transmembrane I-V relationships. Source code for the crucial elements of the VI as well as sample recordings from a cultured spinal cord neuron, illustrating system operation, are presented.