Voltage-sensitive dyes for monitoring multineuronal activity in the intact central nervous system

Optical recording of oscillatory activity in the absence of external Ca2+ in the embryonic chick olfactory bulb

We applied 464/1020-site optical recording systems with a voltage-sensitive dye (NK2761) to the embryonic chick olfactory system and detected oscillatory activity in the olfactory bulb (OB) in the absence of synaptic transmission. In embryonic day 8-10 (E8-E10) chick olfactory nerve (N.I)-OB-forebrain preparations, the removal of Ca2+ from the external solution completely blocked the glutamatergic excitatory postsynaptic potential (EPSP) from the N.I to the OB as well as oscillations following the EPSP. However, a novel type of oscillatory activity was detected in the OB with the long-term perfusion of a Ca2+-free solution. The characteristics of oscillatory activity in the Ca2+-free solution differed from those in normal physiological solution. The present results suggest the existence of a neural communication system in the absence of synaptic transmission at the early stage of embryonic development.

Functional development of olfactory nerve-related neural circuits in the embryonic chick forebrain revealed by voltage-sensitive dye imaging

Multiple-site optical recordings with NK2761, a voltage-sensitive absorption dye, were applied to the embryonic chick olfactory system, and the functional development of olfactory nerve (N.I)-related neural circuits was examined in the forebrain. The stimulation of the N. I elicited neural responses in N.I-olfactory bulb (OB)-forebrain preparations at the embryonic 8-12 day (E8-E12) stages. At the E11 stage, we functionally identified two circuits projecting from the OB to the forebrain. The first circuit passed through the ventral side of the forebrain and spread in the dorso-caudal direction, while the second circuit passed through the dorsal side to the first circuit. Pharmacological experiments showed that NMDA receptor function was more significant for the transfer of sensory information in these circuits. The functional development of N.I-related circuits was investigated, and the results obtained revealed that the ventral circuit was generated earlier than the dorsal circuit. Neural responses in the ventral circuit were detected from the E9 stage in normal physiological solution and the E8 stage in Mg²⁺ -free solution, which activated NMDA receptor function. At the E10 stage, neural responses in the dorsal circuit were clearly recognized in addition to ventral responses. We attempted to identify possible candidates for relay nuclei in the forebrain by comparing contour line maps of the optical signal amplitude with previously reported neuroanatomical data. The present results suggest that N.I-related neural circuits from the periphery to the subpallium functionally mature earlier than those to the pallium during ontogenesis.

Genetically Encoded Fluorescent Calcium and Voltage Indicators

Fluorescent probes that indicate biologically important quantities are widely used for many different types of biological experiments across life sciences. During recent years, limitations of small molecule-based indicators have been overcome by the development of genetically encoded indicators. Here we focus on fluorescent calcium and voltage indicators and point to their applications mainly in neurosciences.

Functiogenesis of the embryonic central nervous system revealed by optical recording with a voltage-sensitive dye

Clarification of the functiogenesis of the embryonic central nervous system (CNS) has long been problematic, because conventional electrophysiological techniques have several limitations. First, early embryonic neurons are small and fragile, and the application of microelectrodes is challenging. Second, the simultaneous monitoring of electrical activity from multiple sites is limited, and as a consequence, spatiotemporal response patterns of neural networks cannot be assessed. We have applied multiple-site optical recording with a voltage-sensitive dye to the embryonic CNS and paved a new way to analyze the functiogenesis of the CNS. In this review, we discuss key points of optical recording in the embryonic CNS and introduce recent progress in optical investigations on the embryonic CNS with special emphasis on the development of the chick olfactory system. The studies clearly demonstrate the usefulness of voltage-sensitive dye recording as a powerful tool for elucidating the functional organization of the vertebrate embryonic CNS.

Oscillations in the embryonic chick olfactory bulb: initial expression and development revealed by optical imaging with a voltage-sensitive dye

In a previous study, we applied a multiple-site optical recording technique with a voltage-sensitive dye to the embryonic chick olfactory system and showed that functional synaptic transmission in the olfactory bulb was expressed at embryonic 6-7-day stages. It is known that oscillations, i.e. stereotyped sinusoidal neural activity, appear in the olfactory system of various species. The focus of the present study is to determine whether the oscillation is also generated in the embryonic chick olfactory bulb and, if this is the case, when the oscillation appears and how its profiles change during embryogenesis. At the early stages of development (embryonic 6- to 8-day stages), postsynaptic response-related optical signals evoked by olfactory nerve stimulation exhibited a simple monophasic waveform that lasted for a few seconds. At embryonic 9-day stage, the optical signal became multi-phasic, and the oscillatory event was detected in some preparations. The oscillation was restricted to the distal half of the olfactory bulb. As development proceeded, the incidence and duration of the oscillation gradually increased, and the waveform became complicated. In some cases at embryonic 12-day stage, the oscillation lasted for nearly a minute. The frequency of the oscillation increased slightly with development, but it remained in the range of theta oscillation during the 9- to 12-day stages. We discuss the ontogenetic dynamics of the oscillation and the significance of this activity in the developing olfactory bulb.

Route to genetically targeted optical electrophysiology: development and applications of voltage-sensitive fluorescent proteins

The invention of membrane voltage protein indicators widens the reach of optical voltage imaging in cell physiology, most notably neurophysiology, by enabling membrane voltage recordings from genetically defined cell types in chronic and life-long preparations. While the last years have seen a dramatic improvement in the technical performance of these indicators, concomitant innovations in optogenetics, optical axon tracing, and high-speed digital microscopy are beginning to fulfill the age-old vision of an all-optical analysis of neuronal circuits, reaching beyond the limits of traditional electrode-based recordings. We will present our personal account of the development of protein voltage indicators from the pioneering days to the present state, including their applications in neurophysiology that has inspired our own work for more than a decade.

Design and Use of Organic Voltage Sensitive Dyes

The chemistry and the physics of voltage sensitive dyes (VSDs) should be understood and appreciated as a prerequisite for their optimal application to problems in neuroscience cardiology. This chapter provides a basic understanding of the properties of the large variety of available organic VSDs. The mechanisms by which the dyes respond to voltage guides the best set up of the optics for recording or imaging electrophysiological activity. The physical and chemical properties of the dyes can be tuned to optimize delivery to and staining of the cells in different experimental preparations. The aim of this chapter is to arm the experimentalists who use the dyes with enough information and data to be able to intelligently choose the best dye for their specific requirements.

Monitoring membrane potential with second-harmonic generation

This protocol describes the nonlinear optical phenomenon known as second-harmonic generation (SHG) and discusses its special attributes for imaging membrane-potential changes in single cells and multicellular preparations. Undifferentiated N1E-115 mouse neuroblastoma cells are used as a model cellular system for membrane electrophysiology. Styryl and naphthylstyryl dyes, also known as hemicyanines, are a class of electrochromic membrane-staining probes that have been used to monitor membrane potential by fluorescence; they also produce SHG images of cell membranes with SHG intensities that are sensitive to voltage. These experiments allow for the precise characterization of the voltage sensitivity of SHG and identification of the optimal wavelength for the incident laser fundamental light. This protocol presents the steps for the culture, staining, patching, and imaging of cells. The details of the imaging system and the measurements obtained are discussed, as are the prospects of this technology for imaging membrane potential changes in neuronal preparations.

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.

Optical analysis of developmental changes in synaptic potentiation in the neonatal rat corticostriatal projection

We applied voltage-sensitive dye imaging to neonatal rat cortical slice preparations and analyzed developmental changes in synaptic plasticity, long-term potentiation (LTP), in the corticostriatal projection. Coronal slice preparations were dissected from postnatal 1- to 21-day (P1-P21) rats, and the transmembrane voltage-related optical signals evoked by cortical stimulation were recorded using a 464ch optical recording system with the voltage-sensitive absorption dye. In the striatum, the optical signal was composed of a fast spike-like signal followed by a slow signal, which corresponded to an action potential and an excitatory postsynaptic potential (EPSP), respectively. The slow signal could be detected at the P1 stage, suggesting that the EPSP is already expressed in the corticostriatal projection at least at early stages after birth. On the other hand, the slow signal was potentiated with a single shot of tetanic stimulation and the potentiation lasted at least 1 h, which is considered to correspond to long-term potentiation. With ontogenetic examinations, we found that (1) the EPSP could be potentiated with tetanic stimulation from the P9 stage and that (2) after the LTP induction, the potentiation was maintained for a longer time in the postnatal 3W stage than in the 2W stage. These results suggest that characteristics of LTP change dynamically during postnatal development.

The embryonic brain and development of vagal pathways

To regulate the autonomic function, the vagus nerve transfers various sensory information from peripheral organs, and appropriate motor reflexes are produced in the neural circuit. The functional development of the vagal pathway during the early phase of embryonic development has long been unclear. Optical recording with voltage-sensitive dyes has provided a new approach to the analysis of the functional development of the embryonic central nervous system. In this review, we present recent progress in optical studies on the vagal pathway in the embryonic chick and rat brainstems. The topics include how neural excitability is initially expressed in the motor and sensory nuclei [e.g. the dorsal motor nucleus of the vagus nerve (DMNV) and the nucleus of the tractus solitarius (NTS)] and how synapse networks are formed in the primary and higher-ordered sensory nuclei [e.g. the parabrachial nucleus (PBN)]. We also refer to the functional development of the glossopharyngeal nuclei and compare the developmental steps with those of the vagal nuclei.

A novel path for rapid transverse communication of vestibular signals in turtle cerebellum

Voltage-sensitive dye activity within the thin, unfoliated turtle cerebellar cortex (Cb) was recorded in vitro during eighth cranial nerve (nVIII) stimulation. Short latency responses were localized to the middle of the lateral edges of both ipsilateral and contralateral Cb [vestibulocerebellum (vCb)]. Even with a severed contralateral Cb peduncle, stimulation of the nVIII ipsilateral to the intact peduncle evoked contralateral vCb responses with a mean latency of only 0.25 ms after the ipsilateral responses, even though the distance between them was ∼ 5 mm. We investigated whether a rapidly conducting commissure exists between each vCb by stimulating one of them directly. Responses in both vCb spread sagittally, but, surprisingly, there was no sequential activation along a transverse Cb beam between them. In contrast, stimulation medial to either vCb evoked transverse beams that required ∼ 20 ms to cross the Cb. Therefore, the rapid commissural connection between each vCb is not mediated by slowly conducting parallel fibers. Also, the vCb was not strongly activated by climbing fiber stimulation, suggesting that inputs to vCb involve distinct cerebellar circuits. Responses between the two vCb remained following knife cuts through the rostral and caudal Cb along the midline, through both peduncles, and even shallow midline cuts to the middle Cb through its white matter and granule cell layer. Commissural responses were still observed only with a narrow transverse bridge between each vCb or in thick transverse Cb slices. Horseradish peroxidase transport from one vCb labeled transverse axons traveling within the Purkinje cell layer that were larger than parallel fibers and lacked varicosities. In sagittal sections, cross-section profiles of myelinated axons were observed around Purkinje cells midway between the rostral and caudal Cb. This novel pathway for transverse communication between lateral edges of turtle Cb suggests that afferents may directly conduct vestibular information rapidly across the Cb to coordinate vestibulomotor reflex behaviors.

Recordings from human myenteric neurons using voltage-sensitive dyes

Voltage-sensitive dye (VSD) imaging became a powerful tool to detect neural activity in the enteric nervous system, including its routine use in submucous neurons in freshly dissected human tissue. However, VSD imaging of human myenteric neurons remained a challenge because of limited visibility of the ganglia and dye accessibility. We describe a protocol to apply VSD for recordings of human myenteric neurons in freshly dissected tissue and myenteric neurons in primary cultures. VSD imaging of guinea-pig myenteric neurons was used for reference. Electrical stimulation of interganglionic fiber tracts and exogenous application of nicotine or elevated KCl solution was used to evoke action potentials. Bath application of the VSDs Annine-6Plus, Di-4-ANEPPS, Di-8-ANEPPQ, Di-4-ANEPPDHQ or Di-8-ANEPPS revealed no neural signals in human tissue although most of these VSD worked in guinea-pig tissue. Unlike methylene blue and FM1-43, 4-Di-2-ASP did not influence spike discharge and was used in human tissue to visualize myenteric ganglia as a prerequisite for targeted intraganglionic VSD application. Of all VSDs, only intraganglionic injection of Di-8-ANEPPS by a volume controlled injector revealed neuronal signals in human tissue. Signal-to-noise ratio increased by addition of dipicrylamine to Di-8-ANEPPS (0.98±0.16 vs. 2.4±0.62). Establishing VSD imaging in primary cultures of human myenteric neurons led to a further improvement of signal-to-noise ratio. This allowed us to routinely record spike discharge after nicotine application. The described protocol enabled reliable VSD recordings from human myenteric neurons but may also be relevant for the use of other fluorescent dyes in human tissues.

Imaging activity of neuronal populations with new long-wavelength voltage-sensitive dyes

We have assessed the utility of five new long-wavelength fluorescent voltage-sensitive dyes (VSD) for imaging the activity of populations of neurons in mouse brain slices. Although all the five were capable of detecting activity resulting from activation of the Schaffer collateral-CA1 pyramidal cell synapse, they differed significantly in their properties, most notably in the signal-to-noise ratio of the changes in dye fluorescence associated with neuronal activity. Two of these dyes, Di-2-ANBDQPQ and Di-1-APEFEQPQ, should prove particularly useful for imaging activity in brain tissue and for combining VSD imaging with the control of neuronal activity via light-activated proteins such as channelrhodopsin-2 and halorhodopsin.

Origin of the earliest correlated neuronal activity in the chick embryo revealed by optical imaging with voltage-sensitive dyes

Spontaneous correlated neuronal activity during early development spreads like a wave by recruiting a large number of neurons, and is considered to play a fundamental role in neural development. One important and as yet unresolved question is where the activity originates, especially at the earliest stage of wave expression. In other words, which part of the brain differentiates first as a source of the correlated activity, and how does it change as development proceeds? We assessed this issue by examining the spatiotemporal patterns of the depolarization wave, the optically identified primordial correlated activity, using the optical imaging technique with voltage-sensitive dyes. We surveyed the region responsible for the induction of the evoked and spontaneous depolarization waves in chick embryos, and traced its developmental changes. The results showed that the wave initially originated in a restricted area near the obex and was generated by multiple regions at later stages. We suggest that the upper cervical cord/lower medulla near the obex is the kernel that differentiates first as the source of the correlated activity, and that regional and temporal differences in neuronal excitability might underlie the developmental profile of wave generation in early chick embryos.

Topography and response timing of intact cerebellum stained with absorbance voltage-sensitive dye

Physiological activity of the turtle cerebellar cortex (Cb), maintained in vitro, was recorded during microstimulation of inferior olive (IO). Previous single-electrode responses to such stimulation showed similar latencies across a limited region of Cb, yet those recordings lacked spatial and temporal resolution and the recording depth was variable. The topography and timing of those responses were reexamined using photodiode optical recordings. Because turtle Cb is thin and unfoliated, its entire surface can be stained by a voltage-sensitive dye and transilluminated to measure changes in its local absorbance. Microstimulation of the IO evoked widespread depolarization from the rostral to the caudal edge of the contralateral Cb. The time course of responses measured at a single photodiode matched that of single-microelectrode responses in the corresponding Cb locus. The largest and most readily evoked response was a sagittal band centered about 0.7 mm from the midline. Focal white-matter (WM) microstimulation on the ventricular surface also activated sagittal bands, whereas stimulation of adjacent granule cells evoked a radial patch of activation. In contrast, molecular-layer (ML) microstimulation evoked transverse beams of activation, centered on the rostrocaudal stimulus position, which traveled bidirectionally across the midline to the lateral edges of the Cb. A timing analysis demonstrated that both IO and WM microstimulation evoked responses with a nearly simultaneous onset along a sagittal band, whereas ML microstimulation evoked a slowly propagating wave traveling about 25 cm/s. The response similarity to IO and WM microstimulation suggests that the responses to WM microstimulation are dominated by activation of its climbing fibers. The Cb's role in the generation of precise motor control may result from these temporal and topographic differences in orthogonally oriented pathways. Optical recordings of the turtle's thin flat Cb can provide insights into that role.

Imaging nervous system activity with voltage-sensitive dyes

Optical recording with a voltage-sensitive dye is advantageous where membrane potential must be recorded in many sites at once. This unit describes methods for making voltage-sensitive dye measurements on different preparations to study (1) how a neuron integrates its synaptic input into its action potential output by measuring membrane potential everywhere synaptic input occurs and where spikes are initiated; (2) how a nervous system generates a behavior in Aplysia abdominal ganglion; and (3) responses to sensory stimuli and generation of motor output in the vertebrate brain by simultaneous measurement of population signals from many areas. The approach is three-pronged: (1) find the dye with the largest signal-to-noise ratio; (2) reduce extraneous sources of noise; and (3) maximize the number of photons measured to reduce the relative shot noise. A discussion of optical recording methods including the choice of dyes, light sources, optics, cameras, and minimizing noise is also provided.

"Nanosized voltmeter" enables cellular-wide electric field mapping

Previously, all biological measurements of intracellular electric fields (E fields), using voltage dyes or patch/voltage clamps, were confined to cellular membranes, which account for <0.1% of the total cellular volume. These membrane-dependent techniques also frequently require lengthy calibration steps for each cell or cell type measured. A new 30-nm "photonic voltmeter", 1000-fold smaller than existing voltmeters, enables, to our knowledge, the first complete three-dimensional E field profiling throughout the entire volume of living cells. These nanodevices are calibrated externally and then applied for E field determinations inside any live cell or cellular compartment, with no further calibration steps. The results indicate that the E fields from the mitochondrial membranes penetrate much deeper into the cytosol than previously estimated, indicating that, electrically, the cytoplasm cannot be described as a simple homogeneous solution, as often approximated, but should rather be thought of as a complex, heterogeneous hydrogel, with distinct microdomains.

Optical mapping of spatiotemporal emergence of functional synaptic connections in the embryonic chick olfactory pathway

In order to understand the functional maturation of the CNS, it is essential to first describe the functional maturation of sensory processing. We have approached this topic by following the ontogenetic patterning of neural circuit formation related to cranial and spinal sensory input using voltage-sensitive dye imaging. In previous studies, we have described the functional maturation of synapses in brainstem/midbrain neural circuits. Here, we elucidate the functional maturation of forebrain circuits by investigating neural networks related to the olfactory nerve (N. I) of chicken embryo. In the isolated N. I-olfactory bulb-forebrain preparation, application of electrical stimulation to N. I elicited excitatory postsynaptic potential (EPSP)-related slow optical signals in the olfactory bulb. The slow signal was mainly mediated by glutamate, and was easily fatigued with repetitive stimuli because of the immaturity of synapses in the embryonic CNS. Ontogenetically, the slow signal was detected from the 6-day embryonic stage, suggesting that functional synaptic connections between N. I and olfactory bulb emerge around this stage. In addition, from the 8-day embryonic stage, another response area was discriminated within the forebrain, which corresponded to the higher-ordered nucleus of the olfactory pathway. In comparison with our previous studies concerning the functional development of other cranial nerve-related sensory nuclei in the embryonic brainstem and midbrain, these results suggest that the olfactory pathway is functionally generated in the early stages of development when neural networks related to other visceral and somatic sensory inputs are also in the process of developing.

Optical recording of vagal pathway formation in the embryonic brainstem

Multiple-site optical recording with a fast voltage-sensitive dye, absorption dye NK2761, was used to study the developmental organization of functional synaptic networks in the vagal pathway. Glutamatergic excitatory postsynaptic potentials (EPSPs) evoked by vagus nerve stimulation was first detected from the nucleus of the tractus solitarius (NTS) at embryonic day 7 (E7) in chick embryos and E15 in rat embryos, when morphological differentiation of pre- and postsynaptic neurons is incomplete. When extracellular Mg2+ was removed, small EPSPs were elicited at E6 in chick embryos and E14 in rat embryos. These results suggest that synaptic function mediated by N-methyl-D-aspartate (NMDA) receptors is latently generated 1 day before the expression of glutamatergic EPSP. Functional synapses related to the glossophyaryngeal nerve appear to be generated at the same time as the vagus nerve, but their spatial distribution was different from that of the vagus nerve. We further investigated the development of second synaptic pathways from the NTS to higher centers, and found that neuronal circuits from the NTS are already generated when the primary afferents form functional synapses with NTS neurons.

Using recurrence quantification analysis determinism for noise removal in cardiac optical mapping

Selecting signal processing parameters in optical imaging by utilizing the change in Determinism, a measure introduced in Recurrence Quantification Analysis, provides a novel method using the change in residual noise Determinism for improving noise quantification and removal across signals exhibiting disparate underlying tissue pathologies. The method illustrates an improved process for selecting filtering parameters and how using measured signal-to-noise ratio alone can lead to improper parameter selection.

Synthesis, spectra, delivery and potentiometric responses of new styryl dyes with extended spectral ranges

Styryl dyes have been among the most widely used probes for mapping membrane potential changes in excitable cells. However, their utility has been somewhat limited because their excitation wavelengths have been restricted to the 450-550 nm range. Longer wavelength probes can minimize interference from endogenous chromophores and, because of decreased light scattering, improve recording from deep within tissue. In this paper we report on our efforts to develop new potentiometric styryl dyes that have excitation wavelengths ranging above 700 nm and emission spectra out to 900 nm. We have prepared and characterized dyes based on 47 variants of the styryl chromophores. Voltage-dependent spectral changes have been recorded for these dyes in a model lipid bilayer and from lobster nerves. The voltage sensitivities of the fluorescence of many of these new potentiometric indicators are as good as those of the widely used ANEP series of probes. In addition, because some of the dyes are often poorly water soluble, we have developed cyclodextrin complexes of the dyes to serve as efficient delivery vehicles. These dyes promise to enable new experimental paradigms for in vivo imaging of membrane potential.

Macroscopic optical mapping of excitation in cardiac cell networks with ultra-high spatiotemporal resolution

Optical mapping of cardiac excitation using voltage- and calcium-sensitive dyes has allowed a unique view into excitation wave dynamics, and facilitated scientific discovery in the cardiovascular field. At the same time, the structural complexity of the native heart has prompted the design of simplified experimental models of cardiac tissue using cultured cell networks. Such reduced experimental models form a natural bridge between single cells and tissue/organ level experimental systems to validate and advance theoretical concepts of cardiac propagation and arrhythmias. Macroscopic mapping (over >1cm(2) areas) of transmembrane potentials and intracellular calcium in these cultured cardiomyocyte networks is a relatively new development and lags behind whole heart imaging due to technical challenges. In this paper, we review the state-of-the-art technology in the field, examine specific aspects of such measurements and outline a rational system design approach. Particular attention is given to recent developments of sensitive detectors allowing mapping with ultra-high spatiotemporal resolution (>5 megapixels/s). Their interfacing with computer platforms to match the high data throughput, unique for this new generation of detectors, is discussed here. This critical review is intended to guide basic science researchers in assembling optical mapping systems for optimized macroscopic imaging with high resolution in a cultured cell setting. The tools and analysis are not limited to cardiac preparations, but are applicable for dynamic fluorescence imaging in networks of any excitable media.

Spatiotemporal patterns of dorsal root-evoked network activity in the neonatal rat spinal cord: optical and intracellular recordings

Spatiotemporal patterns of dorsal root-evoked potentials were studied in transverse slices of the rat spinal cord by monitoring optical signals from a voltage-sensitive dye with multiple-photodiode optic camera. Typically, dorsal root stimulation generated two basic waveforms of voltage images: dual-component images consisting of fast, spike-like signal followed by a slow signal in the dorsal horn, and small, slow signals in the ventral horn. To qualitatively relate the optical signals to membrane potentials, whole cell recordings were combined with measurements of light absorption in the area around the soma. The slow optical signals correlated closely with subthreshold postsynaptic potentials in all regions of the cord. The spike-like component was not associated with postsynaptic action potentials, suggesting that the fast signal was generated by presynaptic action potentials. Firing in a single neuron could not be detected optically, implying that local voltage images originated from synchronously activated neuronal ensembles. Blocking glutamatergic synaptic transmission inhibited excitatory postsynaptic potentials (EPSPs) and significantly reduced the slow optical signals, indicating that they were mediated by glutamatergic synapses. Suppressing glycine-mediated inhibition increased the amplitude of both optical signals and EPSPs, while blocking GABA(A) receptor-mediated synapses, increased the amplitude and time course of EPSPs and prolonged the duration of voltage images in larger areas of the slice. The close correlation between evoked EPSPs and their respective local voltage images shows the advantage of the high temporal resolution optical system in measuring both the spatiotemporal dynamics of segmental network excitation and integrated potentials of neuronal ensembles at identified sites.

Electroporation loading of calcium-sensitive dyes into the CNS

Calcium imaging of neural network function has been limited by the extent of tissue labeled or the time taken for labeling. We now describe the use of electroporation-an established technique for transfecting cells with genes-to load neurons with calcium-sensitive dyes in the isolated spinal cord of the neonatal mouse in vitro. The dyes were injected subdurally, intravascularly, or into the central canal. This technique results in rapid and extensive labeling of neurons and their processes at all depths of the spinal cord, over a rostrocaudal extent determined by the position and size of the electrodes. Our results suggest that vascular distribution of the dye is involved in all three types of injections. Electroporation disrupts local reflex and network function only transiently (approximately 1 h), after which time they recover. We describe applications of the method to image activity of neuronal populations and individual neurons during antidromic, reflex, and locomotor-like behaviors. We show that these different motor behaviors are characterized by distinct patterns of activation among the labeled populations of cells.

Optical Mapping Reveals Developmental Dynamics of Mg2+-/APV-Sensitive Components of Glossopharyngeal Glutamatergic EPSPs in the Embryonic Chick NTS

To examine whether there are any differences in functional organization between the glossopharyngeal nerve (N. IX)– and vagus nerve (N. X)–projecting areas in the nucleus of the tractus solitarius (NTS), we performed optical recording of neural responses evoked by N. IX stimulation in 5- to 9-day-old embryonic chick brain stem preparations and compared the results with those in our previous studies concerning the N. X-related NTS. First, we investigated dl-2-amino-5-phosphonovaleric acid (APV)/Mg2+sensitivity of the glutamatergic excitatory postsynaptic potentials (EPSPs) in the N. IX-related NTS. In 7- to 9-day-old preparations, we found regional differences in the degree of both the APV-induced reduction and Mg2+-free–induced enhancement of the EPSPs. We constructed developmental maps of spatial patterns of the APV- and Mg2+-sensitive components and showed that functional expression of the N-methyl-d-aspartate (NMDA) receptor dynamically changed during development. Second, we studied initial expression of synaptic functions in the N. IX-related NTS. In 6-day-old preparations, although action potentials alone were usually detected in normal Ringer solution, small EPSPs were elicited in a Mg2+-free solution. This result suggests that the NMDA receptor–mediated synaptic function is latently generated in the N. IX-related NTS at the 6-day-old embryonic stage and that external Mg2+regulates the onset of synaptic functions. Developmental patterns of APV/Mg2+sensitivity and the stage of initial expression of the glossopharyngeal EPSP were similar to those of the N. X, suggesting that the developmental sequence of the synaptic function in the NTS is the same for the N. IX- and N. X-related NTS.

Optical detection of neural function in the chick visual pathway in the early stages of embryogenesis

We investigated the developmental pattern of functional synaptogenesis in the chick visual pathway using a multiple-site optical recording method. Responses to optic nerve stimulation were recorded from the diencephalon and mesencephalon of the chick embryo. The first excitatory postsynaptic responses to optic nerve stimulation appeared in the contralateral diencephalon at Hamburger-Hamilton stage 27, which corresponds to an incubation day 5.5 (E5.5). At more developed stages, the optical signals evoked by optic nerve stimulation spread to several different regions, including the tectum and extra-tectal visual nuclei. We constructed maps of neural activity in the diencephalon and mesencephalon at different stages to investigate the spatio-temporal patterns of functional development in the chick visual system. The maps revealed that distinct postsynaptic response areas in the extra-tectal regions showed different onsets of activity, suggesting that the corresponding visual nuclei exhibit different time courses of functional synaptogenesis. We also identified the onset and location of the first functional synaptic connection in the optic tectum, which had been a point of controversy in earlier studies. In the tectal region, the action potential and the excitatory postsynaptic potential first appeared at E8, although these signals were recognized in the tecto/tegmental region at E7. The response area expanded with retinotectal fibre elongation, and reached the area centralis at E9. These results show that the onset of synaptic function in the tectum occurs 2-3 days earlier than was previously reported.

Optical mapping of the functional organization of the rat trigeminal nucleus: initial expression and spatiotemporal dynamics of sensory information transfer during embryogenesis

We examined the functional organization of the rat trigeminal nuclear complex and its developmental dynamics using a multiple-site optical recording technique. Brainstem preparations were dissected from embryonic day 12 (E12)-E16 rat embryos, and stimulation was applied individually to the three branches of the trigeminal nerve (V1-V3). The action potential activity of presynaptic fibers was detected from E13, and the glutamate-mediated postsynaptic response was significantly observed from E15 on. At E14, the evoked signals usually consisted of only the action potential-related fast component. However, when extracellular Mg2+ was removed, a significant dl-2-amino-5-phosphonovaleric acid-sensitive slow component appeared. These results suggest that postsynaptic function mediated by NMDA receptors is latently generated as early as E14. The response area of the three branches of the trigeminal nerve showed some functional somatotopic organization, with the ophthalmic (V1) nerve area medially located and the mandibular (V3) nerve area laterally located. The center of the trigeminal nuclear complex in which the activity of neurons and synaptic function was greatest shifted caudally with development, suggesting that the functional architecture of the trigeminal nuclear complex is not fixed but changes dynamically during embryogenesis. By electron microscopy, we could not observe clear correlations between functional data and morphological information; when we surveyed E16 preparations, we could not identify typical synaptic structures between the 1,1'-dioctyldecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-labeled trigeminal nerve terminals and the neurons in the trigeminal nuclear complex. This implies that postsynaptic function in the trigeminal nuclear complex is generated before the appearance of the morphological structure of conventional synapses.

Depolarization waves in the embryonic CNS triggered by multiple sensory inputs and spontaneous activity: optical imaging with a voltage-sensitive dye

Previously, we discovered a novel type of depolarization wave in the embryonic chick brain by using a multiple-site optical recording technique with a fast voltage-sensitive dye. This depolarization wave traveled widely over almost all the region of the CNS. This profile has raised the possibility that the depolarization wave plays some global roles in development of the CNS, rather than contributing to a specific neuronal circuit formation. To obtain more information concerning this issue, in the present study, we examined whether the depolarization wave was triggered by various types of peripheral nerve inputs. Stimulation applied to the vagus, glossopharyngeal, cochlear and trigeminal nerves evoked widely spreading depolarization waves with similar spatiotemporal distribution patterns. The developmental sequence of wave expression was parallel to the development of the excitatory postsynaptic potentials in each sensory nucleus. The depolarization wave was accompanied by a Ca(2+)-wave, suggesting that not only electrical synchrony, but also large-scale Ca(2+)-transients may affect developmental processes in the embryonic brain. Furthermore, we found that the depolarization wave also occurred spontaneously. The waveform and distribution patterns of the spontaneous optical signals were similar to those of the cranial nerve-evoked depolarization wave. These results demonstrated that the depolarization wave in the embryonic chick brain is triggered by multiple sources of external and endogenous activity. This profile supports the idea that this depolarization wave may not serve as a simple regulator of specific neuronal circuit formation, but might play more global roles in CNS development.

Optical analysis of depolarization waves in the embryonic brain: a dual network of gap junctions and chemical synapses

Correlated neuronal activity plays a fundamental role in the development of the CNS. Using a multiple-site optical recording technique with a voltage-sensitive dye, we previously described a novel type of depolarization wave that was evoked by cranial or spinal nerve stimulation and spread widely over the whole brain region in the chick embryo. We have now investigated developmental expression and neuronal network mechanisms of this depolarization wave by applying direct stimulation to the brain stem or upper cervical cord of E5-E11 embryos, which elicited wave activity similar to that evoked by nerve stimulation. Spatial distribution patterns of the depolarization wave changed dynamically with development, and this change appeared to be related to the regional differences in neuronal differentiation. The depolarization wave was completely eliminated by application of either gap junction blockers or an N-methyl-D-aspartate (NMDA)-receptor antagonist, indicating that functions of both gap junctions and NMDA receptors are indispensable for wave propagation. A possible interpretation of the results is that dual networks of gap junctions and chemical synaptic coupling mediate large-scale depolarization waves in the developing chick CNS.

Cutting-edge technology. III. Imaging and the gastrointestinal tract: mapping the human enteric nervous system

Monitoring membrane potentials by multisite optical recording techniques using voltage-sensitive dyes is ideal for direct analysis of network signaling. We applied this technology to monitor fast and slow excitability changes in the enteric nervous system and in hundreds of neurons simultaneously at cellular and subcellular resolution. This imaging technique presents a powerful tool to study activity patterns in enteric pathways and to assess differential activation of nerves in the gut to a number of stimuli that modulate neuronal activity directly or through synaptic mechanisms. The optical mapping made it possible to record from tissues such as human enteric nerves, which were, until now, inaccessible by other techniques.

Optical imaging of spreading depolarization waves triggered by spinal nerve stimulation in the chick embryo: possible mechanisms for large-scale coactivation of the central nervous system

Using a multiple-site optical recording technique with a voltage-sensitive dye, we found that widely spreading depolarization waves were evoked by dorsal root stimulation in embryonic chick spinal cords. Spatiotemporal maps of the depolarization waves showed that the signals were mainly distributed in the ventral half of the slice, with the highest activity in the ventrolateral area. The propagation velocity of the waves was estimated to be in the order of mm/s. Depolarization waves were evoked in the ventral root-cut preparation, but not in the dorsal root-cut preparation, suggesting that the wave was triggered by synaptic inputs from the primary afferents, and that activation of the motoneurons was not essential for wave generation. In intact spinal cord-brain preparations, the depolarization wave propagated rostrally and caudally for a distance of several spinal segments in normal Ringer's solution. In a Mg(2+)-free solution, the amplitude and extent of the signals were markedly enhanced, and the depolarization wave triggered in the cervical spinal cord propagated to the brainstem and the cerebellum. The depolarization wave demonstrated here had many similarities with the vagus nerve-evoked depolarization wave reported previously. The results suggest that functional cell-to-cell communication systems mediated by the depolarization wave are widely generated in the embryonic central nervous system, and could play a role in large-scale coactivation of the neurons in the spinal cord and brain.

Multiple-site optical recording reveals embryonic organization of synaptic networks in the chick spinal cord

We examined embryonic expression of postsynaptic potentials in stages 26-31 (E5 to E7) chick spinal cord slices. Slow optical signals related to the postsynaptic potentials which were evoked by electrical stimulation of afferent fibers were identified in the dorsal grey matter and the ventral motoneuronal area. In cervical spinal cord (C13) preparations, the dorsal slow signal appeared from stage 28 (E6), whilst the ventral slow signal was recognized from stage 29. At stages 26 and 27 (E5), no slow signal was observed in either the dorsal or ventral regions. On the other hand, in lumbosacral spinal cord (LS5) preparations, the dorsal, as well as ventral, slow signals appeared from stage 29; at stage 28 no slow signal was detected in the dorsal or ventral regions. These results suggest that there are differences in the ontogenetic expression of synaptic functions between the dorsal and ventral regions, and between the cervical and lumbosacral spinal cords. In embryos older than stage 29, removal of Mg2+ from the bathing solution markedly enhanced the amplitude and incidence of the ventral slow signal. In addition, in C13 preparations at stage 28, removal of Mg2+ elicited small slow signals in the ventral region in which no synaptic response was evoked in normal Ringer's solution. The slow signals induced in the Mg2+-free solution were blocked by 2-amino-5-phosphonovaleric acid (APV), showing that they are attributable to N-methyl- D-aspartate (NMDA) receptors. These results suggest that functional synaptic connections via polysynaptic pathways are already generated on motoneurons, but are suppressed by a Mg2+ block on the NMDA receptors at developmental stages when synaptic transmission from the primary afferents to the dorsal interneurons is initially expressed in the dorsal region.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Independent component analysis at the neural cocktail party

'Independent component analysis' is a technique of data transformation that finds independent sources of activity in recorded mixtures of sources. It can be used to recover fluctuations of membrane potential from individual neurons in multiple-detector optical recordings. There are some examples in which more than 100 neurons can be separated simultaneously. Independent component analysis automatically separates overlapping action potentials, recovers action potentials of different sizes from the same neuron, removes artifacts and finds the position of each neuron on the detector array. One limitation is that the number of sources--neurons and artifacts--must be equal to or less than the number of simultaneous recordings. Independent component analysis also has many other applications in neuroscience including, removal of artifacts from EEG data, identification of spatially independent brain regions in fMRI recordings and determination of population codes in multi-unit recordings.

Odors elicit three different oscillations in the turtle olfactory bulb

We measured the spatiotemporal aspects of the odor-induced population response in the turtle olfactory bulb using a voltage-sensitive dye, RH414, and a 464-element photodiode array. In contrast with previous studies of population activity using local field potential recordings, we distinguished four signals in the response. The one called DC covered almost the entire area of the olfactory bulb; in addition, three oscillations, named rostral, middle, and caudal according to their locations, occurred over broad regions of the bulb. In a typical odor-induced response, the DC signal appeared almost immediately after the start of the stimulus, followed by the middle oscillation, the rostral oscillation, and last, the caudal oscillation. The initial frequencies of the three oscillations were 14.1, 13.0, and 6.6 Hz, respectively. When the rostral and caudal oscillations occurred together, their frequencies differed by a factor of 1.99 +/- 0.01. The following evidence suggests that the four signals are functionally independent: (1) in different animals some signals could be easily detected whereas others were undetectable; (2) the four signals had different latencies and frequencies; (3) the signals occurred in different locations and propagated in different directions; (4) the signals responded differently to changes in odor concentration; (5) the signals had different shapes; and (6) the rostral and caudal signals added in a simple, linear manner in regions where the location of the two signals overlapped. However, the finding that the frequency of the rostral oscillation is precisely two times that of the caudal oscillation suggests a significant relationship between the two. The location of the caudal oscillation in the bulb changed from cycle to cycle, implying that different groups of neurons are active in different cycles. This result is consistent with the earlier findings in the olfactory system of the locust (). Our results suggest an additional complexity of parallel processing of olfactory input by multiple functional population domains.

Oscillatory dynamics and information processing in olfactory systems

Oscillatory dynamics is a universal design feature of olfactory information-processing systems. Recent results in honeybees and terrestrial slugs suggest that oscillations underlie temporal patterns of olfactory interneuron responses critical for odor discrimination. Additional general design features in olfactory information-processing systems include (1) the use of central processing areas receiving direct olfactory input for odor memory storage and (2) modulation of circuit dynamics and olfactory memory function by nitric oxide. Recent results in the procerebral lobe of the terrestrial slug Limax maximus, an olfactory analyzer with oscillatory dynamics and propagating activity waves, suggest that Lucifer Yellow can be used to reveal a band-shaped group of procerebral neurons involved in the storage of an odor memory. A model has been constructed to relate wave propagation and odor memory bands in the procerebral lobe of L. maximus and to relate these findings to glomerular odor representations in arthropods and vertebrates.

Fast optical measurement of membrane potential changes at multiple sites on an individual nerve cell

In the past 15 years, there has been renewed interest in the detailed spatial analyses of signalling in individual neurons. The behaviour of many nerve cells is difficult to understand on the basis of microelectrode measurements from the soma. Regional electrical properties of neurons have been studied using sharp microelectrode and patch-electrode recordings from neuronal processes, high-resolution multisite optical recordings of Ca2+ concentration changes and by using models to predict the distribution of membrane potential in the entire neuronal arborization. Additional, direct evidence about electrical signalling in neuronal processes of individual cells in situ can now be obtained by recording of membrane potential changes using voltage-sensitive dyes. A number of recent studies have shown that active regional electrical properties of individual neurons are extraordinarily complex, dynamic and, in the general case, impossible to predict by present models. This places a great significance on measuring capabilities in experiments studying the detailed functional organization of individual neurons. The main difficulty in obtaining a more accurate description was that experimental techniques for studying regional electrical properties of neurons were not available. With this motivation, we worked on the development of multisite voltage-sensitive dye recording as a potentially powerful approach. The results described here demonstrate that the sensitivity of voltage-sensitive dye recording from branches of individual neurons was brought to a level at which it can be used routinely in physiologically relevant experiments. The crucial figure-of-merit in this approach, the signal-to-noise ratio from neuronal processes in intact ganglia, has been improved by a factor of roughly 150 over previously available signals. The improvement in the sensitivity allowed, for the first time, direct investigation of several important aspects of the functional organization of an individual neuron: (1) the direction and the velocity of action potential propagation in different neuronal processes in the neuropile was determined; and (2) the interaction of two independent action potentials (spike collision) was monitored directly in a neurite in the neuropile; (3) it was demonstrated that several action potentials are initiated in the same neuron at different sites (multiple spike trigger zones) by a single stimulus; (4) the exact location and the size of one of the remote spike trigger zones was determined; (5) the spread of passive subthreshold signals was followed in the neurites in the neuropile. This kind of information was not previously available. Preliminary experiments on vertebrate neurons indicate partial success in the effort to use intracellularly applied voltage-sensitive dyes to record from neurons in a mammalian brain slice preparation. The results suggest that, with further improvements, it may be possible to follow optically synaptic integration and spike conduction in the dendrites of vertebrate nerve cells. The main impact of these results is a demonstration of a new way of analysing how individual neurons are functionally organized. Limitations and prospects for the further refinement of the technique are discussed mostly in terms of the signal-to-noise ratio; both improvements in the apparatus and design of more sensitive dyes are addressed.

Monitoring activity in neuronal populations with single-cell resolution in a behaving vertebrate

Vertebrate behaviours are produced by activity in populations of neurons, but the techniques typically used to study activity allow only one or very few nerve cells to be monitored at a time. This limitation has prompted the development of methods of imaging activity in the nervous system. The overall goal of these methods is to image neural activity non-invasively in populations of neurons, ideally with high spatial and temporal resolution. We have moved closer to this goal by using confocal calcium imaging to monitor neural activity in the transparent larvae of zebrafish. Neurons were labelled either by backfilling from injections of the calcium indicator (Calcium Green dextran) into muscle or spinal cord of larvae or by injections into blastomeres early in development. The labelled neurons were bright enough at resting calcium levels to allow the identification of individual neurons in the live, intact fish, based upon their dendritic and axonal morphology. The neurons from the live animal could also be reconstructed in three dimensions for morphometric study. Neurons increased their fluorescence during activity produced by direct electrical stimulation and during escape behaviours elicited by an abrupt touch to the head or tail of the fish. The rise in calcium associated with a single action potential could be detected as an increase in fluorescence of at least 7-10%, but neurons typically showed much larger increases during behaviour. Calcium signals in the dendrites, soma and nucleus could be resolved, especially when using the line-scanning mode, which provides 2-ms temporal resolution. The imaging was used to study activity in populations of motoneurons and hindbrain neurons during the escape behaviour fish use to avoid predators. We found a massive activation of the motoneuron pool and a differential activation of populations of hindbrain neurons during escapes. The latter finding confirms predictions that the activity pattern of hindbrain neurons may help to determine the directionality of the escape. This approach should prove useful for studying the activity of populations of neurons throughout the nervous system in both normal and mutant lines of fish.