Voltage-sensitive dye recording of action potentials and synaptic potentials from sympathetic microcultures

A simultaneous optical and electrical in-vitro neuronal recording system to evaluate microelectrode performance

Objectives

In this paper, we aim to detail the setup of a high spatio-temporal resolution, electrical recording system utilising planar microelectrode arrays with simultaneous optical imaging suitable for evaluating microelectrode performance with a proposed ′performance factor′ metric.

Methods

Techniques that would facilitate low noise electrical recordings were coupled with voltage sensitive dyes and neuronal activity was recorded both electrically via a customised amplification system and optically via a high speed CMOS camera. This technique was applied to characterise microelectrode recording performance of gold and poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) coated electrodes through traditional signal to noise (SNR) calculations as well as the proposed performance factor.

Results

Neuronal activity was simultaneously recorded using both electrical and optical techniques and this activity was confirmed via tetrodotoxin application to inhibit action potential firing. PEDOT/PSS outperformed gold using both measurements, however, the performance factor metric estimated a 3 fold improvement in signal transduction when compared to gold, whereas SNR estimated an 8 fold improvement when compared to gold.

Conclusion

The design and functionality of a system to record from neurons both electrically, through microelectrode arrays, and optically via voltage sensitive dyes was successfully achieved.

Significance

The high spatiotemporal resolution of both electrical and optical methods will allow for an array of applications such as improved detection of subthreshold synaptic events, validation of spike sorting algorithms and a provides a robust evaluation of extracellular microelectrode performance.

Label-free optical detection of action potential in mammalian neurons

We describe an optical technique for label-free detection of the action potential in cultured mammalian neurons. Induced morphological changes due to action potential propagation in neurons are optically interrogated with a phase sensitive interferometric technique. Optical recordings composed of signal pulses mirror the electrical spike train activity of individual neurons in a network. The optical pulses are transient nanoscale oscillatory changes in the optical path length of varying peak magnitude and temporal width. Exogenous application of glutamate to cortical neuronal cultures produced coincident increase in the electrical and optical activity; both were blocked by application of a Na-channel blocker, Tetrodotoxin. The observed transient change in optical path length in a single optical pulse is primarily due to physical fluctuations of the neuronal cell membrane mediated by a yet unknown electromechanical transduction phenomenon. Our analysis suggests a traveling surface wave in the neuronal cell membrane is responsible for the measured optical signal pulses.

Absolute spectroscopic determination of cross-membrane potential

Spectroscopic determination of the cross-membrane electric potential has been used for more than 20 years. This method, which usually employs absorption or fluorescence measurements, allows for a rapid and noninvasive study of the electrical properties of the membranes of cells and liposomes. However, the usual fluorescence techniques preferably allow monitoring changes in the potential on triggerable or excitable membranes, and not the absolute value of the potential. They also do not provide means for measuring the potential on single cells. This paper reviews three methods that solve these issues. Nernstian dyes which partition between intra-and extracompartmental volumes enable a fluorescence microscopic determination of a single cell and even a single organelle. Dual-wavelength ratiometric recording from membrane-staining dyes also provides means for measuring the field on a single cell. Resonance Raman probes provide a spectroscopic method with a natural internal standard for the absolute measurement of membrane potential.

Recall of spatial patterns stored in a hippocampal slice by long-term potentiation

Nervous systems are thought to encode information as patterns of electrical activity distributed sparsely through networks of neurons. These networks then process information by transforming one pattern of electrical activity into another. To store information as a pattern, a neural network must strengthen synapses between designated neurons so that activation of some of these neurons corresponding to some features of an object can spread to activate the larger group representing the complete object. This operation of pattern completion endows a neural network with autoassociative memory. Pattern completion by neural networks has been modeled extensively with computers and invoked in behavioral studies, but experiments have yet to demonstrate pattern completion in an intact neural circuit. In the present study, imaging with voltage-sensitive dye in the CA3 region of a hippocampal slice revealed different spatial patterns of activity elicited by electrical stimulation of different sites. Stimulation of two separate sites individually, or both sites simultaneously, evoked "partial" or "complete" patterns, respectively. A complete pattern was then stored by applying theta burst stimulation to both sites simultaneously to induce long-term potentiation (LTP) of synapses between CA3 pyramidal cells. Subsequent stimulation of only one site then activated an extended pattern. Quantitative comparisons between response maps showed that the post-LTP single-site patterns more closely resembled the complete dual-site pattern. Thus, LTP induction enabled the CA3 region to complete a dual-site pattern upon stimulation of only one site. This experiment demonstrated that LTP induction can store information in the CA3 region of the hippocampus for subsequent retrieval.

Smoothing of, and parameter estimation from, noisy biophysical recordings

Biophysically detailed models of single cells are difficult to fit to real data. Recent advances in imaging techniques allow simultaneous access to various intracellular variables, and these data can be used to significantly facilitate the modelling task. These data, however, are noisy, and current approaches to building biophysically detailed models are not designed to deal with this. We extend previous techniques to take the noisy nature of the measurements into account. Sequential Monte Carlo ("particle filtering") methods, in combination with a detailed biophysical description of a cell, are used for principled, model-based smoothing of noisy recording data. We also provide an alternative formulation of smoothing where the neural nonlinearities are estimated in a non-parametric manner. Biophysically important parameters of detailed models (such as channel densities, intercompartmental conductances, input resistances, and observation noise) are inferred automatically from noisy data via expectation-maximization. Overall, we find that model-based smoothing is a powerful, robust technique for smoothing of noisy biophysical data and for inference of biophysical parameters in the face of recording noise.

Caged neuron MEA: a system for long-term investigation of cultured neural network connectivity

Traditional techniques for investigating cultured neural networks, such as the patch clamp and multi-electrode array, are limited by: (1) the number of identified cells which can be simultaneously electrically contacted, (2) the length of time for which cells can be studied, and (3) the lack of one-to-one neuron-to-electrode specificity. Here, we present a new device - the caged neuron multi-electrode array - which overcomes these limitations. This micro-machined device consists of an array of neurocages which mechanically trap a neuron near an extracellular electrode. While the cell body is trapped, the axon and dendrites can freely grow into the surrounding area to form a network. The electrode is bi-directional, capable of both stimulating and recording action potentials. This system is non-invasive, so that all constituent neurons of a network can be studied over its lifetime with stable one-to-one neuron-to-electrode correspondence. Proof-of-concept experiments are described to illustrate that functional networks form in a neurochip system of 16 cages in a 4 x 4 array, and that suprathreshold connectivity can be fully mapped over several weeks. The neurochip opens a new domain in neurobiology for studying small cultured neural networks.

Caged neuron MEA: a system for long-term investigation of cultured neural network connectivity

Traditional techniques for investigating cultured neural networks, such as the patch clamp and multi-electrode array, are limited by: (1) the number of identified cells which can be simultaneously electrically contacted, (2) the length of time for which cells can be studied, and (3) the lack of one-to-one neuron-to-electrode specificity. Here, we present a new device - the caged neuron multi-electrode array - which overcomes these limitations. This micro-machined device consists of an array of neurocages which mechanically trap a neuron near an extracellular electrode. While the cell body is trapped, the axon and dendrites can freely grow into the surrounding area to form a network. The electrode is bi-directional, capable of both stimulating and recording action potentials. This system is non-invasive, so that all constituent neurons of a network can be studied over its lifetime with stable one-to-one neuron-to-electrode correspondence. Proof-of-concept experiments are described to illustrate that functional networks form in a neurochip system of 16 cages in a 4 x 4 array, and that suprathreshold connectivity can be fully mapped over several weeks. The neurochip opens a new domain in neurobiology for studying small cultured neural networks.

Rhythmic and dysrhythmic thalamocortical dynamics: GABA systems and the edge effect

Brain function is fundamentally related in the most general sense to the richness of thalamocortical interconnectivity, and in particular to the rhythmic oscillatory properties of thalamocortical loops. Such rhythmicity is involved in the genesis of cognition, in the sleep-wake cycle, and in several neurological and psychiatric disorders. The role of GABA-mediated transmission in regulating these functional states is addressed here. At the cortical level, inhibition determines the spread of cortical activation by sculpting the precise activity patterns that underlie the details of cognition and motor control. At the thalamic level, GABA-mediated inhibition modulates and resets distribution of the ongoing thalamocortical rhythmic oscillations that bind multisensory inputs into a single cognitive experience and regulate arousal levels.

Voltage-sensitive dye imaging of population neuronal activity in cortical tissue

Voltage-sensitive dyes (VSDs) and optical imaging are useful for studying spatiotemporal patterns of population neuronal activity in cortical tissue. Using a photodiode array and absorption dyes we were able to detect neuronal activity in single trials before it could be detected by local field potential (LFP) recordings. Simultaneous electrical and optical recordings from the same tissue also showed that VSD and LFP signals have different waveforms during different activities, suggesting that they are sensitive to different aspects of the synchronization across the population. Noise, dye bleaching, phototoxicity and optical filter selection are important to the quality of the VSD signal and are discussed in this report. With optimized signal-to-noise ratio (S/N) and total recording time, we can optically monitor approximately 500 locations in an area of 1 mm(2) of cortical tissue with a sensitivity comparable to that of LFP electrodes. The total recording time and S/N of fluorescence and absorption dyes are also compared. At S/N of 8-10, absorption dye NK3630 allows a total recording time of 15-30 min, which can be divided into hundreds of 4-8 s recording trials over several hours, long enough for many kinds of experiments. In conclusion, the VSD method provides a reliable way for examining neuronal activity and pharmacological properties of synapses in brain slices.

Epileptiform discharges with in-vivo-like features in slices of rat piriform cortex with longitudinal association fibers

Brain slices serve as useful models for the investigation of epilepsy. However, the preparation of brain slices disrupts circuitry and severs axons, thus complicating efforts to relate epileptiform activity in vitro to seizure activity in vivo. This issue is relevant to studies in transverse slices of the piriform cortex (PC), the preparation of which disrupts extensive rostrocaudal fiber systems. In these slices, epileptiform discharges propagate slowly and in a wavelike manner, whereas such discharges in vivo propagate more rapidly and jump abruptly between layers. The objective of the present study was to identify fiber systems responsible for these differences. PC slices were prepared by cutting along three different nearly orthogonal planes (transverse, parasagittal, and longitudinal), and epileptiform discharges were imaged with a voltage-sensitive fluorescent dye. Interictal-like epileptiform activity was enabled by either a kindling-like induction process or disinhibition with bicuculline. The pattern of discharge onset was very similar in slices cut in different planes. As described previously in transverse PC slices, discharges were initiated in the endopiriform nucleus (En) and adjoining regions in a two-stage process, starting with low-amplitude "plateau activity" at one site and leading to an accelerating depolarization and discharge onset at another nearby site. The similar pattern of onset in slices of various orientations indicates that the local circuitry and neuronal properties in and around the En, rather than long-range fibers, assume dominant roles in the initiation of epileptiform activity. Subtle variations in the onset site indicate that interneurons can fine tune the site of discharge onset. In contrast to the mode of onset, discharge propagation showed striking variations. In longitudinal slices, where rostrocaudal association fibers are best preserved, discharge propagation resembled in vivo seizure activity in the following respects: propagation was as rapid as in vivo and about two to three times faster than in other slices; discharges jumped abruptly between the En and PC; and discharges had large amplitudes in superficial layers of the PC. Cuts in longitudinal slices that partially separated the PC from the En eliminated these unique features. These results help clarify why epileptiform activity differs between in vitro and in vivo experiments and suggest that rostrocaudal pyramidal cell association fibers play a major role in the propagation of discharges in the intact brain. The longitudinal PC slice, which best preserves these fibers, is ideally suited for the study their role.

Dendritic spatial flicker of local membrane potential due to channel noise and probabilistic firing of hippocampal neurons in culture

Whole-cell recordings and imaging of dissociated hippocampal neurons stained with voltage sensitive dye provide a new microscopic picture of neuronal excitation. This is the first attempt to combine imaging of active channel clusters on the geometry of live neurons and a theoretical approach. During single somatic action potentials and the back-invasion into the neurites, local mean potentials are generated at sites of active channel clusters which are unevenly distributed in the neuronal membrane. Similar mean membrane potentials are observed in the neurites and at the soma. Identical action potentials produce different spatial patterns of mean membrane potentials from trial to trial. This spatial variability is explained by the stochastic behavior of the channels in the clusters. When hippocampal neurons are excited by synaptic inputs, their evoked responses are probabilistic and generate variable spatial patterns of mean membrane potential trial after trial. Our stochastic model reproduces this random behavior by assuming that the voltage fluctuations generated by channel noise are added to the synaptic potentials reaching the soma. We demonstrate that the probability of action potential initiation depends on the strength of the synaptic input, the diameter of the dendrites and the relative positions of the channel clusters, of the synapse and of the soma.

Imaging stochastic spatial variability of active channel clusters during excitation of single neurons

Topographical maps of membrane voltages were obtained during action potentials by imaging, at 1 microm resolution, live dissociated neurons stained with the voltage sensitive dye RH237. We demonstrate with a theoretical approach that the spatial patterns in the images result from the distribution of net positive charges condensed in the inner sites of the membrane where clusters of open ionic channels are located. We observed that, in our biological images, this spatial distribution of open channels varies randomly from trial to trial while the action potentials recorded by the microelectrode display similar amplitudes and time-courses. The random differences in size and intensity of the spatial patterns in the images are best evidenced when the time of observation coincides with the duration of single action potentials. This spatial variability is explained by the fact that only part of the channel population generates an action potential and that different channels open in turn in different trials due to their stochastic operation. Such spatial flicker modifies the direction of lateral current along the neuronal membrane and may have important consequences on the intrinsic processing capabilities of the neuron.

Characteristics of plateau activity during the latent period prior to epileptiform discharges in slices from rat piriform cortex

The deep piriform region has an unusually high seizure susceptibility. Voltage imaging previously located the sites of epileptiform discharge onset in slices of rat piriform cortex and revealed the spatiotemporal pattern of development of two types of electrical activity during the latent period prior to discharge onset. A ramplike depolarization (onset activity) appears at the site of discharge onset. Onset activity is preceded by a sustained low-amplitude depolarization (plateau activity) at another site, which shows little if any overlap with the site of onset. Because synaptic blockade at either of these two sites blocks discharges, it was proposed that both forms of latent period activity are necessary for the generation of epileptiform discharges and that the onset and plateau sites work together in the amplification of electrical activity. The capacity for amplification was examined here by studying subthreshold responses in slices of piriform cortex using two different in vitro models of epilepsy. Under some conditions electrically evoked responses showed a nonlinear dependence on stimulus current, suggesting amplification by strong polysynaptic excitatory responses. The sites of plateau and onset activity were mapped for different in vitro models of epilepsy and different sites of stimulation. These experiments showed that the site of plateau activity expanded into deep layers of neighboring neocortex in parallel with expansions of the onset site into neocortex. These results provide further evidence that interactions between the sites of onset and plateau activity play an important role in the initiation of epileptiform discharges. The site of plateau activity showed little variation with different stimulation sites in the piriform cortex, but when stimulation was applied in the endopiriform nucleus (in the sites of onset of plateau activity), plateau activity had a lower amplitude and became distributed over a much wider area. These results indicate that in the initiation of epileptiform discharges, the location of the circuit that generates plateau activity is not rigidly defined but can exhibit flexibility.

Sustained plateau activity precedes and can generate ictal-like discharges in low-Cl(-) medium in slices from rat piriform cortex

Interictal and ictal discharges represent two different forms of abnormal brain activity associated with epilepsy. Ictal discharges closely parallel seizure activity, but depending on the form of epilepsy, interictal discharges may or may not be correlated with the frequency, severity, and location of seizures. Recent voltage-imaging studies in slices of piriform cortex indicated that interictal-like discharges are generated in a two-stage process. The first stage consists of a sustained, low-amplitude depolarization (plateau activity) lasting the entire latent period prior to discharge onset. Plateau activity takes place at a site distinct from the site of discharge onset and serves to sustain and amplify activity initiated by an electrical stimulus. In the second stage a rapidly accelerating depolarization begins at the onset site and then spreads over a wide region. Here, we asked whether ictal-like discharges can be generated in a similar two-stage process. As with interictal-like activity, the first sign of an impending ictal-like discharge is a sustained depolarization with a plateau-like time course. The rapidly accelerating depolarization that signals the start of the actual discharge develops later at a separate onset site. As found previously with interictal-like discharges, local application of kynurenic acid to the plateau site blocked ictal-like discharges throughout the entire slice. However, in marked contrast to interictal-like activity, blockade of synaptic transmission at the onset site failed to block the ictal-like discharge. This indicates that interictal- and ictal-like discharges share a common pathway in the earliest stage of their generation and that their mechanisms subsequently diverge.

Sustained plateau activity precedes and can generate ictal-like discharges in low-Cl(-) medium in slices from rat piriform cortex

Interictal and ictal discharges represent two different forms of abnormal brain activity associated with epilepsy. Ictal discharges closely parallel seizure activity, but depending on the form of epilepsy, interictal discharges may or may not be correlated with the frequency, severity, and location of seizures. Recent voltage-imaging studies in slices of piriform cortex indicated that interictal-like discharges are generated in a two-stage process. The first stage consists of a sustained, low-amplitude depolarization (plateau activity) lasting the entire latent period prior to discharge onset. Plateau activity takes place at a site distinct from the site of discharge onset and serves to sustain and amplify activity initiated by an electrical stimulus. In the second stage a rapidly accelerating depolarization begins at the onset site and then spreads over a wide region. Here, we asked whether ictal-like discharges can be generated in a similar two-stage process. As with interictal-like activity, the first sign of an impending ictal-like discharge is a sustained depolarization with a plateau-like time course. The rapidly accelerating depolarization that signals the start of the actual discharge develops later at a separate onset site. As found previously with interictal-like discharges, local application of kynurenic acid to the plateau site blocked ictal-like discharges throughout the entire slice. However, in marked contrast to interictal-like activity, blockade of synaptic transmission at the onset site failed to block the ictal-like discharge. This indicates that interictal- and ictal-like discharges share a common pathway in the earliest stage of their generation and that their mechanisms subsequently diverge.

Sustained and accelerating activity at two discrete sites generate epileptiform discharges in slices of piriform cortex

When near-threshold electrical stimulation is used to evoke epileptiform discharges in brain slices, a latent period of up to 150 msec elapses before the discharge begins. During this period most neurons are silent, and abnormal electrical activity is difficult to detect with microelectrodes. A fundamental question about epileptiform activity concerns how synchronous discharges arise abruptly in a relatively quiescent slice. This issue was addressed here by using voltage imaging techniques to study epileptiform discharges in rat piriform cortex slices. These experiments revealed two distinct forms of electrical activity during the latent period. (1) A steeply increasing depolarization, referred to here as onset activity, has been described previously and occurs at the site of discharge onset. (2) A sustained depolarization that precedes onset activity, referred to here as plateau activity, has not been described previously. Plateau and onset activity occurred in different subregions of the endopiriform nucleus (a region of high seizure susceptibility). When cobalt or kynurenic acid was applied focally to inhibit electrical activity at the site of plateau activity, discharges were blocked. However, application of these agents to other nearby sites (except the site of onset) failed to block discharges. Plateau activity represents a novel form of electrical activity that precedes and is necessary for epileptiform discharges. Discharges thus are generated in a sequential process by two spatially distinct neuronal circuits. The first circuit amplifies and sustains activity initiated by the stimulus, and the second generates the actual discharge in response to an excitatory drive from the first.

Sustained and accelerating activity at two discrete sites generate epileptiform discharges in slices of piriform cortex

When near-threshold electrical stimulation is used to evoke epileptiform discharges in brain slices, a latent period of up to 150 msec elapses before the discharge begins. During this period most neurons are silent, and abnormal electrical activity is difficult to detect with microelectrodes. A fundamental question about epileptiform activity concerns how synchronous discharges arise abruptly in a relatively quiescent slice. This issue was addressed here by using voltage imaging techniques to study epileptiform discharges in rat piriform cortex slices. These experiments revealed two distinct forms of electrical activity during the latent period. (1) A steeply increasing depolarization, referred to here as onset activity, has been described previously and occurs at the site of discharge onset. (2) A sustained depolarization that precedes onset activity, referred to here as plateau activity, has not been described previously. Plateau and onset activity occurred in different subregions of the endopiriform nucleus (a region of high seizure susceptibility). When cobalt or kynurenic acid was applied focally to inhibit electrical activity at the site of plateau activity, discharges were blocked. However, application of these agents to other nearby sites (except the site of onset) failed to block discharges. Plateau activity represents a novel form of electrical activity that precedes and is necessary for epileptiform discharges. Discharges thus are generated in a sequential process by two spatially distinct neuronal circuits. The first circuit amplifies and sustains activity initiated by the stimulus, and the second generates the actual discharge in response to an excitatory drive from the first.

The neurochip: a new multielectrode device for stimulating and recording from cultured neurons

The neurochip is a silicon micromachined device upon which cultured mammalian neurons can be continuously and individually monitored and stimulated. The neurochip is based upon a 4 x 4 array of metal electrodes, each of which has a caged well structure designed to hold a single mature cell body while permitting normal outgrowth of neural processes. We demonstrate that this device is capable of maintaining cell survival, and that the electrodes can both record and stimulate electrical activity in individual cells with no crosstalk between channels.

Voltage imaging of epileptiform activity in slices from rat piriform cortex: onset and propagation

The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.

High-speed, random-access fluorescence microscopy: I. High-resolution optical recording with voltage-sensitive dyes and ion indicators

The design and implementation of a high-speed, random-access, laser-scanning fluorescence microscope configured to record fast physiological signals from small neuronal structures with high spatiotemporal resolution is presented. The laser-scanning capability of this nonimaging microscope is provided by two orthogonal acousto-optic deflectors under computer control. Each scanning point can be randomly accessed and has a positioning time of 3-5 microseconds. Sampling time is also computer-controlled and can be varied to maximize the signal-to-noise ratio. Acquisition rates up to 200k samples/s at 16-bit digitizing resolution are possible. The spatial resolution of this instrument is determined by the minimal spot size at the level of the preparation (i.e., 2-7 microns). Scanning points are selected interactively from a reference image collected with differential interference contrast optics and a video camera. Frame rates up to 5 kHz are easily attainable. Intrinsic variations in laser light intensity and scanning spot brightness are overcome by an on-line signal-processing scheme. Representative records obtained with this instrument by using voltage-sensitive dyes and calcium indicators demonstrate the ability to make fast, high-fidelity measurements of membrane potential and intracellular calcium at high spatial resolution (2 microns) without any temporal averaging.

Optical responses evoked by white matter stimulation in rat visual cortical slices and their relation to neural activities

To characterize optical responses (ORs) evoked by white matter (WM) stimulation in slices of rat visual cortex (VC) stained with voltage sensitive dyes, time course of ORs in each layer was investigated by recording ORs with a linearly aligned photodiode array, and the spatial patterns of the ORs at specified time after stimulation were investigated by a CCD camera in combination with stroboscopic illumination. The ORs recorded by the photodiode array were an increase in absorption at 700 nm and a decrease in the wavelength below 650 nm, suggesting that the ORs were dye related. The ORs were compared with field potentials (FPs) to clarify that neural events were represented by the ORs, and in support of this view, we found that the first order spatial differentials of ORs and that of FPs were in good agreement. We further compared ORs with intracellular responses, and found that the ORs mainly represent postsynaptic potentials (PSPs) of VC neurons except for the deeper part of layer VI, where a component representing action potentials in fibers stimulated directly was observed. The time-lapse imaging of ORs showed that excitation first propagated vertically up to layer I and subsequently in the horizontal direction along layers II-III and V-VI as in previous investigations. Spatio-temporal patterns of ORs under blockade of synaptic transmission were also investigated to reveal activity of fibers evoked by WM stimulation which produced such patterns of propagation.

Multichannel optical recording of neuronal network activity and synaptic potentiation in dissociated cultures from rat hippocampus

The activity of neuronal networks formed by dissociated rat hippocampal neurons was observed with a 128-channel optical recording apparatus using an absorptive voltage-sensitive dye, RH482. Two-dimensional patterns of neural electrical events along somata and neurites in the networks were visualized as the responses to pulse stimuli applied to the somata of the presynaptic neurons by patch-clamp electrodes. Synaptic delay was analyzed from propagation delay of the responses along the neurites. Synaptic potentiation was also observed in postsynaptic responses that were amplified by a factor of 1.24 after tetanization. In contrast, presynaptic components were unaffected by the procedure. In the light of the present results, multichannel optical recording promises to promote our understanding of neuronal interactions at cellular level.

Fluorescence imaging of local membrane electric fields during the excitation of single neurons in culture

The spatial distribution of depolarized patches of membrane during the excitation of single neurons in culture has been recorded with a high spatial resolution (1 micron2/pixel) imaging system based on a liquid-nitrogen-cooled astronomical camera mounted on an inverted microscope. Images were captured from rat nodose neurons stained with the voltage-sensitive dye RH237. Conventional intracellular microelectrode recordings were made in synchrony with the images. During an action potential the fluorescence changes occurred in localized, unevenly distributed membrane areas, which formed clusters of depolarized sites of different sizes and intensities. When fast conductances were blocked by the addition of tetrodotoxin, a reduction in the number and the intensities of the depolarized sites was observed. The blockade by tetrodotoxin of voltage-clamped neurons also reduced the number of depolarized sites, although the same depolarizing voltage step was applied. Similarly, when a voltage-clamped neuron was depolarized by a constant-amplitude voltage step, the number of depolarized sites varied according to the degree of activation of the voltage-sensitive channels, which was modified by changing the holding potential. These results suggest that the spatial patterns of depolarization observed during excitation are related to the operations of ionic channels in the membrane.

Use of voltage-sensitive dyes and optical recordings in the central nervous system

Understanding the spatio-temporal features of the information processing occurring in any complex neural structure requires the monitoring and analysis of the activity in populations of neurons. Electrophysiological and other mapping techniques have provided important insights into the function of neural circuits and neural populations in many systems. However, there remain limitations with these approaches. Therefore, complementary techniques which permit the monitoring of the spatio-temporal activity in neuronal populations are of continued interest. One promising approach to monitor the electrical activity in populations of neurons or on multiple sites of a single neuron is with voltage-sensitive dyes coupled with optical recording techniques. This review concentrates on the use of voltage-sensitive dyes and optical imaging as tools to study the activity in neuronal populations in the central nervous system. Focusing on 'fast' voltage-sensitive dyes first, several technical issues and developments in optical imaging will be reviewed. These will include more recent developments in voltage-sensitive dyes as well as newer developments in optical recording technology. Second, studies using voltage-sensitive dyes to investigate information processing questions in the central nervous system and in the invertebrate nervous system will be reviewed. Some emphasis will be placed on the cerebellum, but the major goal is to survey how voltage-sensitive dyes and optical recordings have been utilized in the central nervous system. The review will include optical studies on the visual, auditory, olfactory, somatosensory, auditory, hippocampal and brainstem systems, as well as single cell studies addressing information processing questions. Discussion of the intrinsic optical signals is also included. The review attempts to show how voltage-sensitive dyes and optical recordings can be used to obtain high spatial and temporal resolution monitoring of neuronal activity.

Voltage sensitivity of the fluorescent probe RH421 in a model membrane system

The voltage sensitivity of the fluorescent styrylpyridinium dye RH421 has been investigated in dimyristoylphosphatidylcholine vesicles by inducing an intramembrane electric field through the binding of the hydrophobic ion tetraphenylborate (TPB). To assess the probability of electrochromic and solvatochromic mechanisms for the dye response, the ground-state dipole moment of the dye in chloroform solution was determined from dielectric constant measurements to be 12 (+/- 1) Debye, and the change in dipole moment upon excitation was calculated from measurements of the Stokes shift in solvents of varying polarity to be 25 (+/- 11) Debye. As well as causing absorbance and fluorescence changes of membrane-bound dye, the TPB-induced electrical field was found to reduce significantly the pKa of the dye. The pH at which experiments are carried out is, thus, an important factor in determining the amplitude of the voltage-induced absorbance and fluorescence changes. The observed absorbance changes induced by the field are inconsistent with a pure electrochromic mechanism. A reorientation/solvatochromic mechanism, whereby the electrical field reorients the dye molecules so that they experience a change in polarity of their lipid environment is likely to make a significant contribution to both the spectral changes and to the field effect on the acid-base properties of the dye.

Simultaneous multisite recordings and stimulation of single isolated leech neurons using planar extracellular electrode arrays

Planar extracellular electrode arrays provide a non-toxic, non-invasive method of making long-term, multisite recordings with moderately high spatial frequency (recording sites per unit area). This paper reports advances in the use of this approach to record from and stimulate single identified leech neurons in vitro. A modified enzyme treatment allowed identified neurons to be extracted with very long processes. Multisite extracellular recordings from the processes of such isolated neurons revealed both the velocity and direction of action potential propagation. Propagation in two cell types examined was from the broken stump towards the cell body (antidromic). This was true for spontaneous action potentials, action potentials produced by injecting current into the cell body and extracellular stimulation of the extracted process via a planar extracellular electrode. These results extend previous findings which have shown that the tip of the broken stump of extracted neurons has a high density of voltage-activated sodium channels. Moreover they demonstrate the applicability of extracellular electrode arrays for recording the electrical excitability of single cells.

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.

Static and dynamic studies of the potential-sensitive membrane probe RH421 in dimyristoylphosphatidylcholine vesicles

The dynamics of the potential-sensitive styryl dye RH421 in dimyristoylphosphatidylcholine vesicles have been investigated above and below the main phase transition temperature using iodine-laser temperature-jump relaxation spectrophotometry and time-resolved fluorescence lifetime measurements. Equilibrium fluorescence titrations have shown that the affinity of the dye for the membrane is much higher in the liquid-crystalline state than in the gel state. The interaction can be described by either a partition or a binding model and a theory is presented providing a relation between these two approaches. In the liquid-crystalline state bound dye exhibits steady-state fluorescence relaxation processes in the submicrosecond and millisecond time range following a temperature jump. Time-resolved fluorescence measurements show a variation in the fluorescence lifetime across the emission spectrum, suggesting an excited-state process occurring on the subnanosecond time scale. These processes are most likely related to dye and/or lipid reorientation following the temperature jump or excitation pulse. Temperature-dependent changes in the fluorescence excitation spectrum of bound dye suggest that the dye exists in at least two different sites within the membrane.

Visualization of synaptic structure and function with confocal microscopy: calcium fluctuations and oscillations

This article summarizes the basic principles of confocal microscopy and how they can be employed to visualize synaptic structure and function. Optical 'sectioning' of living cells allows the examination of a large number of biological processes at different subcellular localities. Different fluorescent markers enable the study of processes in the extracellular, intracellular and membrane domains of the nerve cell. The excellent spatial resolution of confocal microscopy permits to study the changes in intracellular calcium concentration in single synaptic boutons, without a substantial interference from supporting cells. Intracellular calcium concentration shows coordinated fluctuations in space and periodic oscillations. Periodic oscillations can serve as time keeping devices in nerve terminals. Oscillations were previously observed also in the process of transmitter release. We speculate therefore that these calcium oscillations may be of significance, if the quantal transmitter release is governed by a sequence of calcium dependent steps, which have a different affinity for calcium.

Spectroscopic investigations of the potential-sensitive membrane probe RH421

The absorbance spectra, fluorescence emission and excitation spectra, and fluorescence anisotropy of the potential-sensitive styryl dye RH421 have been investigated in aqueous solution and bound to the lipid membrane. The potential-sensitive response of the dye has been studied using a preparation of membrane fragments containing a high density of Na+, K(+)-ATPase molecules. In aqueous solution the dye is sensitive both to changes in pH and ionic strength. Evidence has been found that the dye readily aggregates in aqueous solution. Aggregation is enhanced by an increase in ionic strength. The aggregates formed display a low fluorescence intensity. At high pH values (above approx. 8) changes in the dye's fluorescence spectra are observed, which may be due to a reaction of the dye with hydroxide ions. When bound to the membrane the dye also exhibits concentration-dependent fluorescence changes. The potential-sensitive response of the dye in Na(+),K(+)-ATPase membrane fragments after addition of MgATP in the presence of Na+ ions cannot be explained by a purely electrochromic mechanism. The results are consistent with either a potential-dependent equilibrium between membrane-bound dye monomers and membrane-bound dimers, similar to that previously proposed for the dye merocyanine 540, or with a field-induced structural change of the membrane.

A naphthyl analog of the aminostyryl pyridinium class of potentiometric membrane dyes shows consistent sensitivity in a variety of tissue, cell, and model membrane preparations

The fast potentiometric indicator di-4-ANEPPS is examined in four different preparations: lipid vesicles, red blood cells, squid giant axon, and guinea pig heart. The dye gives consistent potentiometric responses in each of these systems, although some of the detailed behavior varies. In lipid vesicles, the dye displays an increase in fluorescence combined with a red shift of the excitation spectrum upon hyperpolarization. Similar behavior is found in red cells where a dual wavelength radiometric measurement is also demonstrated. The signal-to-noise ratio of the potentiometric fluorescence response is among the best ever recorded on the voltage-clamped squid axon. The dye is shown to be a faithful and persistent monitor of cardiac action potentials with no appreciable loss of signal or deterioration of cardiac activity for periods as long as 2 hr with intermittent illumination every 10 min. These results, together with previously published applications of the dye to a spherical lipid bilayer model and to cells in culture, demonstrate the versatility of di-4-ANEPPS as a fast indicator of membrane potential.

Noninvasive detection of changes in membrane potential in cultured neurons by light scattering

We report a procedure to detect electrical activity in cultured neurons by changes in their intrinsic optical properties. Using dark-field microscopy to detect scattered light, we observe an optical signal that is linearly proportional to the change in the membrane potential. Action potentials can be recorded without signal averaging. We use the dark-field method to show that there are substantial time delays between activity in the soma and in fine distal processes of identified Aplysia neurons. The biophysical basis for the change in optical properties of the neuron was deduced from measurements of the angular distribution of scattered laser light. An analysis of the data indicates that the radial component of the index of refraction of the membrane increases and the tangential components decrease concomitant with an increase in membrane potential. This is suggestive of a rapid reorientation of dipoles in the membrane during an action potential.

An apparatus for recording synaptic potentials from neuronal cultures using voltage-sensitive fluorescent dyes

Voltage-sensitive dyes offer the promise of noninvasive multicell recording of electrical activity, and should therefore be useful for studying the synaptic interactions of small networks of cultured neurons. We have designed and built a system for recording from microcultures of 1-15 neurons from the rat superior cervical ganglion (SCG), using voltage-sensitive fluorescent dyes of the styryl class. The apparatus comprises a standard inverted epifluorescence microscope; a mercury arc lamp with an optical feedback regulator; a 256-pixel fiber-optic camera with individual photodiode detectors and very low-noise amplifiers; and a personal computer-based data acquisition system. Its dark noise and illumination fluctuations are low enough that at typical fluorescence levels for these cells, it is limited by shot noise (the inherent physical limit of detection). Recording from SCG neurons, the signal-to-noise ratio is high enough to see large subthreshold synaptic potentials without signal averaging. This apparatus should be useful for studying long-term synaptic plasticity in cultures of vertebrate neurons, and several of its features should apply to optical recording from other preparations.