Optical recording of epileptiform voltage changes in the neocortical slice

In vitro intrinsic optical imaging can be used for source determination in cortical slices

In the last decades intrinsic optical imaging has become a widely used technique for monitoring activity in vivo and in vitro. It is assumed that in brain slices the source of intrinsic optical signals (IOSs) is the change in light scattering caused by cell swelling or shrinkage. The aim of the present study was to find a correlation between electrical activity and parallel optical characteristics, elicited by 4-aminopyridine-containing or Mg(2+) -free medium in rat cortical brain slices. Electrophysiological signals and reflected light alterations were recorded during spontaneous seizure activity. Current source density (CSD) analysis was performed on the electrophysiological records. Direct correlation analysis of IOS to CSD was made, and source distribution provided by IOS and CSD methods was compared by determining Matthews correlation coefficient. The gradual development of seizure-like activity elicited the reduction of light reflectance. The main findings of our experiments are that long-term epileptiform activity resulted in persistent alteration in IOSs of brain slices. The observed IOS pattern remained stable after 1 h incubation in convulsants. The pattern of IOS shows good correlation with the data obtained from the CSD analysis. Persistent IOS changes provide information about the area-specific changes of basic excitability, which can serve as a background for ictal and interictal-like epileptiform activity. We can conclude that changes in IOSs correlate well with electrophysiological recordings under different conditions. Our experiments provide evidence that underlying synchronised neuronal processes produce parallel alterations in IOSs and electrophysiological activity.

Dynamic neurovascular coupling and uncoupling during ictal onset, propagation, and termination revealed by simultaneous in vivo optical imaging of neural activity and local blood volume

Traditional models of ictal propagation involve the concept of an initiation site and a progressive outward march of activation. The process of neurovascular coupling, whereby the brain supplies oxygenated blood to metabolically active neurons presumably results in a similar outward cascade of hyperemia. However, ictal neurovascular coupling has never been assessed in vivo using simultaneous measurements of membrane potential change and hyperemia with wide spatial sampling. In an acute rat ictal model, using simultaneous intrinsic optical signal (IOS) and voltage-sensitive dye (VSD) imaging of cerebral blood volume and membrane potential changes, we demonstrate that seizures consist of multiple dynamic multidirectional waves of membrane potential change with variable onset sites that spread through a widespread network. Local blood volume evolves on a much slower spatiotemporal scale. At seizure onset, the VSD waves extend beyond the IOS signal. During evolution, spatial correlation with hemodynamic signal only exists briefly at the maximal spread of the VSD signal. At termination, the IOS signal extends spatially and temporally beyond the VSD waves. Hence, vascular reactivity evolves in a separate but parallel fashion to membrane potential changes resulting in a mechanism of neurovascular coupling and uncoupling, which is as dynamic as the seizure itself.

In vitro functional imaging in brain slices using fast voltage-sensitive dye imaging combined with whole-cell patch recording

In many brain areas, circuit connectivity is segregated into specific lamina or glomerula. Functional imaging in these anatomically discrete areas is particularly useful in characterizing circuit properties. Voltage-sensitive dye (VSD) imaging directly assays the spatiotemporal dynamics of neuronal activity, including the functional connectivity of the neurons involved. In spatially segregated structures, VSD imaging can define how physiology and connectivity interact, and can identify functional abnormalities in models of neurological and psychiatric disorders. In the following protocol, we describe the in vitro slice preparation, epifluorescence setup and analyses necessary for fast charge-coupled device (CCD)-based VSD imaging combined with simultaneous whole-cell patch recording. The addition of single-cell recordings validates imaging results, and can reveal the relationship between single-cell activity and the VSD-imaged population response; in synchronously activated neurons, this change in whole-cell recorded V(m) can accurately represent population V(m) changes driving the VSD responses. Thus, the combined VSD imaging and whole-cell patch approach provides experimental resolution spanning single-cell electrophysiology to complex local circuit responses.

Spatio-temporal dynamics of oscillatory network activity in the neonatal mouse cerebral cortex

We used a 60-channel microelectrode array to study in thick (600-1000 microm) somatosensory cortical slices from postnatal day (P)0-P3 mice the spatio-temporal properties of early network oscillations. We recorded local non-propagating as well as large-scale propagating spontaneous oscillatory activity. Both types of activity patterns could never be observed in neocortical slices of conventional thickness (400 microm). Local non-propagating spontaneous oscillations with an average peak frequency of 15.6 Hz, duration of 1.7 s and maximal amplitude of 66.8 microV were highly synchronized in a network of approximately 200 microm in diameter. Spontaneous oscillations of lower frequency (10.4 Hz), longer duration (23.8 s) and larger amplitude (142.9 microV) propagated with 0.11 mm/s in the horizontal direction over at least 1 mm. These propagating oscillations were also synchronized in a columnar manner, but these waves synchronized the activity in a larger neuronal network of 300-400 microm in diameter. Both types of spontaneous network activity could be blocked by the gap junction antagonist carbenoxolone. Electrical stimulation of the subplate (SP) or bath application of the cholinergic agonist carbachol also elicited propagating network oscillations, emphasizing the role of the SP and the cholinergic system in the generation of early cortical network oscillations. Our data demonstrate that a sufficiently large network in thick neocortical slice preparations is capable of generating spontaneous and evoked network oscillations, which are highly synchronized via gap junctions in 200-400-microm-wide columns. These via synchronized oscillations coupled networks may represent a self-organized functional template for the activity-dependent formation of neocortical modules during the earliest stages of development.

Voltage imaging reveals the CA1 region at the CA2 border as a focus for epileptiform discharges and long-term potentiation in hippocampal slices

Voltage-sensitive-dye imaging was used to study the initiation and propagation of epileptiform activity in transverse hippocampal slices. A portion of the slices tested generated epileptiform discharges in response to electrical shocks under normal physiological conditions. The fraction of slices showing epileptiform responses increased from 44 to 86% when bathing [K+] increased from 3.2 to 4 mM. Regardless of stimulation site in the dentate gyrus and hippocampus, discharges generally initiated in the CA3 region. After onset, discharges abruptly appeared in the CA1 region, right at the CA2 border. This spread from the CA3 region to the CA1 region was saltatory, occurring before detectable activity in the intervening CA2 and CA3 regions. Discharges did eventually propagate smoothly through the intervening CA3 region into the CA2 region, but on a slower timescale. The surge in the CA1 region did not spread back into the CA2 region, but spread through the CA1 region toward the subiculum. Tetanic stimulation, theta bursts, and GABA(A) receptor antagonists failed to alter this characteristic pattern, but did reduce the latency of discharge onset. The part of the CA1 region at the CA2 border, where epileptic responses emerged with relatively short latency, also expressed stronger long-term potentiation (LTP) than the rest of the CA1 region. The CA2 region, where discharges had long latencies and low amplitudes, expressed weaker LTP. Thus the CA1 region at the CA2 border has unique properties, which make this part of the hippocampus an important locus for both epileptiform activity and plasticity.

Neurovascular coupling and oximetry during epileptic events

Epilepsy is an abnormal brain state in which a large population of neurons is synchronously active, causing an enormous increase in metabolic demand. Recent investigations using highresolution imaging techniques, such as optical recording of intrinsic signals and voltagesensitive dyes, as well as measurements with oxygen-sensitive electrodes have elucidated the spatiotemporal relationship between neuronal activity, cerebral blood volume, and oximetry in vivo. A focal decrease in tissue oxygenation and a focal increase in deoxygenated hemoglobin occurs following both interictal and ictal events. This "epileptic dip" in oxygenation can persist for the duration of an ictal event, suggesting that cerebral blood flow is inadequate to meet metabolic demand. A rapid focal increase in cerebral blood flow and cerebral blood volume also accompanies epileptic events; however, this increase in perfusion soon (>2 s) spreads to a larger area of the cortex than the excitatory change in membrane potential. Investigations in humans during neurosurgical operations have confirmed the laboratory data derived from animal studies. These data not only have clinical implications for the interpretation of noninvasive imaging studies such as positron emission tomography, single-photon emission tomography, and functional magnetic resonance imaging but also provide a mechanism for the cognitive decline in patients with chronic epilepsy.

Recent developments in oximetry and perfusion-based mapping techniques and their role in the surgical treatment of neocortical epilepsy

Detailed understanding of neurovascular coupling during epilepsy is critical for the interpretation of various perfusion-based imaging techniques, such as positron emission tomography, single-photon-emission computed tomography, and functional magnetic resonance imaging, which are used to guide surgical therapy. We used high-resolution intrinsic signal- and voltage-sensitive dye imaging, as well as oxygen-sensitive electrodes, to map the precise spatiotemporal relationship between excitatory and inhibitory neuronal activity, cerebral blood volume, and oximetry during epilepsy. We observed a rapid focal decrease in tissue oxygenation and an increase in deoxygenated hemoglobin in association with both interictal and ictal events. This "epileptic dip" in oxygenation lasts several seconds following both interictal and ictal events, implying that for a period, cerebral blood flow is inadequate to meet metabolic demand. We also observed a rapid focal increase in cerebral blood volume that soon spread to adjacent nonepileptic gyri. Likewise, a diffuse decrease in deoxygenated hemoglobin, related to the blood oxygen level-dependent signal recorded with functional magnetic resonance imaging, spread to adjacent gyri and was poorly localized.

Horizontal spread of activity in neocortical inhibitory networks

In the presence of 4-aminopyridine (4-AP) and excitatory amino acid receptor blockers, GABAergic networks in the neocortex give rise to large spontaneous GABA-mediated depolarizations. We used voltage-sensitive dye techniques to explore the network properties of depolarizing GABA responses. Voltage-sensitive dye signals demonstrated that the superficial layers support the propagation of depolarizing GABA responses, with only minimal signals detected in deeper cortical layers. GABA responses propagated at a speed of 2.7 +/- 0.2 mm/s, a rate intermediate to fast synaptic transmission and spreading depression. Changes in the extracellular potassium concentration altered the propagation speed of the depolarizing GABA response. Taken together, these data support a role for both direct synaptic action of GABA at GABA(A) receptors and nonsynaptic mechanisms in the generation and propagation of depolarizing GABA responses.

Dopamine Enhances Spatiotemporal Spread of Activity in Rat Prefrontal Cortex

Dopaminergic modulation of prefrontal cortex (PFC) is important for neuronal integration in this brain region known to be involved in cognition and working memory. Because of the complexity and heterogeneity of the effect of dopamine on synaptic transmission across layers of the neocortex, dopamine's net effect on local circuits in PFC is difficult to predict. We have combined whole cell patch-clamp recording and voltage-sensitive dye imaging to examine the effect of dopamine on the excitability of local excitatory circuits in rat PFC in vitro. Whole cell voltage-clamp recording from visually identified layer II/III pyramidal neurons in rat brain slices revealed that, in the presence of bicuculline (10 μM), bath-applied dopamine (30–60 μM) increased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by weak intracortical stimulus. The effect was mimicked by the selective D1 receptor agonist SKF 81297 (1 μM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 μM) significantly lowered the threshold stimulus required for generating epileptiform discharges to 83% of control. In the imaging experiments, bath application of dopamine or SKF 81297 enhanced the spatiotemporal spread of activity in response to weak stimulation and previously subthreshold stimulation resulted in epileptiform activity that spread across the whole cortex. These effects could be blocked by the selective D1 receptor antagonist SCH 23390 (10 μM) but not by the D2 receptor antagonist eticlopride (5 μM). These results indicate that dopamine, by a D1 receptor–mediated mechanism, enhances spatiotemporal spread of synaptic activity and lowers the threshold for epileptiform activity in local excitatory circuits within PFC.

Neuronal avalanches are diverse and precise activity patterns that are stable for many hours in cortical slice cultures

A major goal of neuroscience is to elucidate mechanisms of cortical information processing and storage. Previous work from our laboratory (Beggs and Plenz, 2003) revealed that propagation of local field potentials (LFPs) in cortical circuits could be described by the same equations that govern avalanches. Whereas modeling studies suggested that these "neuronal avalanches" were optimal for information transmission, it was not clear what role they could play in information storage. Work from numerous other laboratories has shown that cortical structures can generate reproducible spatiotemporal patterns of activity that could be used as a substrate for memory. Here, we show that although neuronal avalanches lasted only a few milliseconds, their spatiotemporal patterns were also stable and significantly repeatable even many hours later. To investigate these issues, we cultured coronal slices of rat cortex for 4 weeks on 60-channel microelectrode arrays and recorded spontaneous extracellular LFPs continuously for 10 hr. Using correlation-based clustering and a global contrast function, we found that each cortical culture spontaneously produced 4736 +/- 2769 (mean +/- SD) neuronal avalanches per hour that clustered into 30 +/- 14 statistically significant families of spatiotemporal patterns. In 10 hr of recording, over 98% of the mutual information shared by these avalanche patterns were retained. Additionally, jittering analysis revealed that the correlations between avalanches were temporally precise to within +/-4 msec. The long-term stability, diversity, and temporal precision of these avalanches indicate that they fulfill many of the requirements expected of a substrate for memory and suggest that they play a central role in both information transmission and storage within cortical networks.

Stimulus-induced patterns of bioelectric activity in human neocortical tissue recorded by a voltage sensitive dye

Stimulus-induced pattern of bioelectric activity in human neocortical tissue was investigated by use of the voltage sensitive dye RH795 and a fast optical recording system. During control conditions stimulation of layer I evoked activity predominantly in supragranular layers showing a spatial extent of up to 3000 microm along layer III. Stimulation in white matter evoked distinct activity in infragranular layers with a spatial extent of up to 3000 microm measured along layer V. The mean amplitude of optical signals close to the stimulated sites in layer I and white matter determined 25 ms following the stimulus, decreased by 50% at a lateral distance of approximately 900 microm and 1200 microm, respectively. Velocity of spread along the vertical stimulation axis reached 0.24 m/s in the supragranular layers (layers I to III) and then decreased to 0.09 m/s following layer I activation; stimulation of white matter induced a velocity of spread in layer V of 0.38 m/s, which slowed down to 0.12 m/s when passing the lower border of lamina IV. The horizontal velocities of spread determined from the stimulation site to a lateral distance of 500 microm reached 0.26-0.28 m/s and 0.28-0.35 m/s for layer I and white matter stimulation, respectively. At larger distances velocity of spread decreased. Increased excitability (Mg(2+)-free solution) had no significant effect on the spatio-temporal distribution of evoked activity as compared with control conditions. There were also no obvious differences between the results obtained in slices, which generated spontaneously sharp waves and those which were not spontaneously active. About 30% of the slices (n=7) displayed a greatly different response pattern, which seemed not to be related in a simple way to the stimulation as was the case in the majority of the investigated slices. The activity pattern of those slices appeared atypical in regard to their deviations of the vertical and horizontal extent of activity, to their reduced spatial extent of activity during increased excitability, to their layer-related distribution of activity, and to the appearance of afterdischarges.Concluding, in 30% of the human temporal lobe slices atypical activity pattern occurred which obviously reflect intrinsic epileptiform properties of the resected tissue. The majority of slices showed stereotyped activity pattern without evidence for increased excitability.

Optical analysis of acute spontaneous epileptiform discharges in the in vivo rat cerebral cortex

We examined the spatiotemporal patterns of spontaneous epileptiform activity observed in the in vivo rat cerebral cortex using an optical recording technique of detecting transmembrane voltage changes. The surface of the cerebral cortex was exposed under anesthesia and stained with a fluorescent voltage-sensitive dye, RH414. Acute spontaneous epileptiform discharges were induced by application of a GABA(A) receptor antagonist, bicuculline. Changes in the intensity of fluorescence were recorded from the cerebral cortex using a 464-channel optic fiber photodiode system. We succeeded in recording spontaneous epileptiform discharges, and constructed their initiation-site maps. We found that the initiation site was neither unique nor randomly located, but exhibited a multimodal distribution pattern. The incidence of epileptiform discharges was different between the initiation sites, and some sites showed dominance in the induction of spontaneous epileptiform discharges.

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.

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.

Imaging epileptiform discharges in slices of piriform cortex with voltage-sensitive fluorescent dyes

Voltage imaging techniques were used to investigate epileptiform discharges in brain slices containing piriform cortex (PC). These experiments pinpointed the site of discharge onset in the endopiriform nucleus (En). Under some conditions, discharge onset also occurred simultaneously in adjoining neocortex. With slightly suprathreshold electrical stimulation, discharge generation was a two-stage process in which onset was preceded by a sustained spatially localized depolarization denoted as plateau activity. Plateau activity was seen away from the onset site, in a border region between En and layer III of PC. A similar two-stage sequence was seen for slices taken from a variety of planes, using two different interictal models as well as an ictal model. Plateau activity was found to be necessary for the generation of both kinds of discharge. Synaptic transmission at the site of onset was found to be required for the generation of interictal-like discharges, but ictal-like discharges were different in that they could still be generated when synaptic transmission at this site was impaired. These studies identify specialized regions with potentially important roles in epileptogenesis and help to elucidate the neuronal circuitry that can produce epileptiform activity.

Simultaneous optical recording of membrane potential and intracellular calcium from brain slices

Optical recording techniques provide a constantly evolving and increasingly powerful set of tools for investigations of cellular physiology. These techniques rely on the use of optical indicators, molecules that change their optical properties depending on the cellular parameter of interest. In this paper we discuss some of the general considerations involved in recording optical signals from multiple indicators. Specifically, we describe a technique for simultaneously recording transients of membrane potential and intracellular calcium concentration, two parameters that have a very complex interrelationship in neuronal functioning. This technique relies on the use of two fluorescent indicators (the voltage-sensitive dye RH-414 and the calcium-sensitive dye Calcium Orange) that have overlapping excitation spectra but separable emission spectra. This fact, in combination with the use of fast, spatially resolving photodetectors (10 x 10-element photodiode matrices), allows for truly simultaneous recording of these transients from brain slices with high spatial ( approximately 200 x 200 microm with a 10x microscope objective) and temporal ( approximately 500 micros) resolution. Furthermore, the quality of the signals obtained is sufficient to allow for recording of spontaneous synchronized activity such as epileptiform activity induced by the potassium channel blocker 4-aminopyridine. The nature of the signals obtained by these indicators recorded from guinea pig hippocampal slices and some applications of this technique are discussed.

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.

Spatio-temporal distribution of epileptiform activity in slices from human neocortex: recordings with voltage-sensitive dyes

The spatio-temporal distribution of epileptiform activity was investigated in slices from human temporal neocortex resected during epilepsy surgery. Activity was recorded by use of a voltage-sensitive dye and an optical recording system. Epileptiform activity was induced with 10 microM bicuculline and electrical stimulation of layer I. In 10 slices from six patients investigated, epileptiform activity spread across most of the slice. Largest amplitudes were located in layer II/III. Epileptiform activity was characterized by long-lasting potentials with slow rising phases and a low velocity of spread in the horizontal direction (0.044 m/s). This spatio-temporal pattern of epileptiform activity in human slices was similar to that found previously in neocortical slices from guinea pigs with bicuculline. In four of nine human slices investigated under control bath conditions (in non-epileptogenic medium), the spatio-temporal activity patterns were similar to those of guinea pigs in non-epileptogenic medium. In the remaining five human slices, however, the spread in the horizontal direction was significantly larger (4188 microm) in non-epileptogenic medium than that found in slices from guinea pigs (2171 microm). Activity in human slices showing such 'wide spread' in control bath conditions occasionally had characteristic features of epileptiform activity. Further work will have to clarify whether these epileptiform features reflect intrinsic epileptiform properties in human tissue slices.

Optical recording of trisynaptic pathway in rat hippocampal slices with a voltage-sensitive dye

Changes in membrane potentials were recorded from rat hippocampal slices with a voltage-sensitive dye using a real-time optical recording system, which had high spatial resolution of 128 x 128 points with a high time resolution of 0.6 ms. Serial excitatory propagation was recorded in the dentate gyrus. CA3 and CA1 after stimulation of the perforant pathway, and the optical signals were clearly divided into two components in the dentate gyrus adjacent to the stimulus site. The slow component was suppressed in Ca(2+)-free solution, but the fast component in the molecular layer was not affected. However, the application of 1 microM tetrodotoxin fully abolished both components. These results suggest that the fast and slow components mainly reflect Na(+)-dependent action potentials and excitatory postsynaptic potentials, respectively. The excitatory response duration in the stratum radiatum of CA3 was significantly longer than that in other hippocampal areas. The long-lasting excitation in CA3 is probably related to the CA3 associational projections, because direct stimulation of CA3 pyramidal cell layer also produced similar results. The long-lasting dendritic excitation is probably important to integrate synaptic transmission and may be related to epileptogenesis. When long-term potentiation was induced by a tetanic stimulation (100 Hz for 1 s), the onset latency in the stratum radiatum of CA1 was reduced to as much as 65%, suggesting an increase of excitatory propagation. The analysis of the spatial-temporal optical signals contributes to understanding information processes in the hippocampus, related to learning and memory including long-term potentiation.

Partial suppression of GABAA-mediated inhibition induces spatially restricted epileptiform activity in guinea pig neocortical slices

The spatio-temporal distribution of evoked activity in guinea pig neocortical slices was investigated during partial suppression of gamma-aminobutyric acid (GABA)A-mediated synaptic inhibition with different concentrations of bicuculline. Activity patterns were recorded by use of a voltage-sensitive dye and a fast optical recording technique. At non-epileptogenic concentrations of bicuculline (0.6-2.5 microM), evoked potentials were of longer duration and larger amplitude, but the spatial extent of spread in the horizontal direction was unaffected. At threshold epileptogenic concentrations of bicuculline (1.25-5 microM), spatially restricted epileptiform activity developed at a distance from the stimulation site which was clearly separated from potentials with non-epileptic characteristics close to the stimulation site. It is concluded that, under moderate disinhibition, stimulus-evoked activity has a suppressive effect on spread and development of epileptiform activity, probably through synchronous activation of still-functioning inhibitory circuits.

Simultaneous optical recording of evoked and spontaneous transients of membrane potential and intracellular calcium concentration with high spatio-temporal resolution

We have developed a system for simultaneous optical recording of transients of membrane potential and intracellular calcium concentration from mammalian brain slice preparations with high spatio-temporal resolution. Simultaneous recording was achieved by using two dedicated photodetectors together with two fluorescent indicators. Specifically, the calcium-sensitive dye Calcium Orange and the voltage-sensitive dye RH-414 were selected because they have overlapping excitation spectra, but separable emission spectra. Transverse guinea pig hippocampal slices were double-loaded by bath application of the membrane-permeant form of Calcium Orange and RH-414. Transients of intracellular calcium concentration and membrane potential associated with evoked neural activity in hippocampal areas CA1 and CA3 were recorded. Furthermore, we have recorded calcium and voltage transients associated with spontaneous epileptiform activity induced by bath application of an epileptogenic drug, 4-aminopyridine. The use of photodiode matrices (10 x 10 elements each) as detectors gives the high spatial (200 x 200 microns/element with a 10 x objective) and temporal resolution (570 microseconds/frame). The recording system also includes a CCD camera for obtaining images of the preparation and overlaying the image with the optically detected signals. A software package has been developed for setting up the experimental protocol(s) and for collecting, processing, displaying, and analyzing the data in an user-friendly, windows-based environment.

Epileptiform activity in the guinea-pig neocortical slice spreads preferentially along supragranular layers--recordings with voltage-sensitive dyes

The spread of epileptiform activity was monitored in guinea-pig neocortical slices by the use of a voltage-sensitive dye (RH795) and a fast optical recording technique. Epileptiform activity induced by bicuculline methiodide (10-20 microM) and single-pulse stimulation spread from the stimulation site in layer I or in the white matter across most of the slice. Different lesions were made in the slice in order to specify the neuronal connections used for spread in the horizontal direction. In the slice, intracortical connections are necessary for the spread of epileptiform activity, as shown by vertical cuts through all cortical layers but sparing the white matter. Horizontal connections were interrupted by cuts parallel to the axis of pyramidal neurons through either supragranular or infragranular layers. Vertical connections were interrupted by cuts perpendicular to the axis of pyramidal neurons separating supragranular and infragranular layers. Spread of epileptiform activity in the horizontal direction was not hindered by horizontal cuts. Vertical cuts through infragranular layers also did not hinder the spread of epileptiform activity. In contrast, vertical cuts through supragranular layers either abolished completely (nine slices) or delayed significantly (ten slices) the spread of epileptiform activity. The mean delay at the supragranular lesion was 44 ms in layer III and 30 ms in layer V; at the infragranular lesion the mean delay was 2 ms in layer III and 6 ms in layer V. Also, with horizontal cuts, in three out of five slices the velocity of spread was significantly lower in infragranular as compared to supragranular layers. It is concluded that both supra- and infragranular layers if isolated possess the ability to initiate and propagate epileptiform activity independently. However, in the intact slice the influence of the supragranular networks on initiation and propagation of epileptiform activity appears to dominate.

Optical responses recorded after local stimulation in rat neostriatal slice preparations: effects of GABA and glutamate antagonists, and dopamine agonists

Effects of GABA and glutamate antagonists as well as dopamine agonists and antagonists on the optical responses of neostriatal (Str) slices to local electrical stimulation were examined using a voltage-sensitive dye and a high-speed image sensor. A single local stimulation applied to the Str slices evoked optical responses lasting for 40-80 ms and propagating in every direction up to about 1.5 mm. Bath application of bicuculline methiodide increased the intensity and duration of optical responses, while their spatial response patterns were unchanged. Bath application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) greatly reduced the late part of responses occurring about 4 ms after stimulation, but the early part of responses was unaffected by CNQX. The early part of the response was eliminated by application of tetrodotoxin. Bath application of N-methyl-D-aspartate antagonists, 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and 2-amino-5-phosphonovaleric acid resulted in only small changes in the optical responses. Bath application of D1 agonist 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5,-tetrahydro-1H-3-benz aze pine hydrobromide consistently increased the intensity but decreased the speed of propagation and duration of the optical response. Bath application of D2 agonist quinpirole had no effect on the optical response. D1 antagonist SCH 23390 and D2 antagonist sulpiride also failed to change optical responses. These results indicate that the early part of the response is due to direct activation of the neuronal elements by electrical stimulation, while the late part of the response is due mainly to glutamatergic ex-citatory postsynaptic potentials (EPSPs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors. This study also suggests that dopamine may modulate AMPA/kainate responses through D1 receptors.

Changes in intrinsic optical signal of rat neocortical slices following afferent stimulation

Changes in intrinsic optical signal of rat neocortical slices following afferent stimulation were recorded using darkfield infrared-videomicroscopy. Response amplitude was linearly related to stimulation intensity. The intensity of the optical signal reached its maximum 3 s after onset of stimulation and redecayed with a mean time constant of 23 +/- 7.1 s. The optical signal had a columnar shape. The size of the column was independent from stimulation intensity with stimuli of medium amplitudes. The extent of the optical signal corresponded to the extent of the electrical activation. Changes in intrinsic optical properties may be a useful tool for the study of spread of excitation in neuronal tissue in vitro.

Spread of epileptiform potentials in the neocortical slice: recordings with voltage-sensitive dyes

The spread of epileptiform potentials in guinea pig neocortical slices was investigated by use of voltage sensitive dyes and a fast optical recording technique. Epileptiform activity was induced in a perfusion medium containing 10-20 microM bicuculline-methiodide and by single pulse stimulation of layer I or the white matter. The location of minimal and maximal amplitudes, the shape of the potentials at specific sites and the velocity of spread were independent from the specific stimulation site. The expression of epileptiform activity appeared to depend on specific, possibly geometrical, properties of the tissue.

The contribution of intracortical connections to horizontal spread of activity in the neocortex as revealed by voltage sensitive dyes and a fast optical recording method

Coronal slices from guinea-pig visual neocortex were stained with voltage-sensitive fluorescence dyes RH414 or RH795. Activity was evoked by electrical stimulation of either the white matter or layer I. Emitted light intensity changes representing summated changes of membrane potential were recorded by a 10 x 10 photodiode array with a temporal resolution of 0.4 ms and a spatial resolution of 94 microns. The distribution and spread of activity in the horizontal direction was analysed. Following stimulation of the white matter or layer I, two regions of activity were differentiated in the medio-lateral direction: a central region (approximately 1 mm wide) of high-amplitude activity close to the stimulation electrode and, distant from the stimulation electrode, peripheral regions of low-amplitude activity. Central and peripheral regions differed in their rates of decline, their relative extent with stimulation of different sites and within different layers. The total extent of non-synaptic evoked activity did not exceed that of the central region of high-amplitude activity. Along the extent of non-synaptic activity, onset latencies of potentials were almost constant. Thus, activity of high amplitude in the central region was likely mediated by simultaneous activation of distributed afferent fibres. In contrast, no non-synaptic activity was found in peripheral regions. Therefore it is suggested that this low-amplitude activity was mediated without direct afferent activation but via long-distance intracortical horizontal pathways. These pathways are known to terminate in layer III, and accordingly latencies of responses in the periphery were shortest in upper cortical layers, whereas in the central region, latencies increased from lower to upper cortical layers.

Slices from the rat olfactory bulb maintained in vitro. Morphological aspects

Transverse, 400-microns-thick slices of 8-day-old rat olfactory bulb were incubated in Krebs-Henseleit medium with and without oxygenation. Following incubation, slices were fixed in aldehyde-osmium and embedded in resin for light and electron microscopy. After 2 h of incubation oxygenated preparations showed a structural preservation comparable to that of the freshly fixed olfactory bulb. Under hypoxic conditions mitral cells located on the medial side of the bulb were the most sensitive to the interruption of gassing, while ventricular cells and glomeruli were remarkably resistant as judged by morphological standards. The effects of short-term (up to 30 min) interruptions of gassing proved to be reversible. Our findings suggest that the incubated olfactory bulb slice may be a useful preparation for functional morphological studies.

Spatio-temporal Distribution of Epileptiform Potentials in the Hippocampal Slice: Recordings with Voltage-sensitive Dyes

Voltage-sensitive dyes and fast optical recording techniques were used to monitor the spatio-temporal activity pattern of epileptiform potentials in hippocampal slices from guinea pigs. Epileptiform potentials were induced by adding 4-aminopyridine to the bath solution and applying single pulse stimulation either to the stratum pyramidale of area CA3 or to the stratum radiatum of area CA1. Optical activity as well as intra- or extracellular electrical activity were recorded from area CA1. There was a good correlation between optical and intracellular records from the same site. The spatio-temporal activity pattern of control and epileptiform potentials elicited by stimulation of CA1 was similar for the initial part of the potential. Then, epileptiform changes became apparent throughout the vertical extent of pyramidal neurons. Qualitative changes occurred in the stratum moleculare, reflecting activity of apical dendrites, such changes occurred even more strongly in the stratum oriens, reflecting activity of basal dendrites. The activity in the stratum oriens occurred relatively late, so that it cannot account for the initiation of epileptic discharges. It might, however, play an important role in the synchronization and spread of epileptiform potentials. The investigation of the horizontal distribution of potentials throughout the entire area CA1 indicates that different mechanisms are involved in the spread of epileptiform activity elicited by stimulation of CA1 and stimulation of CA3.