Ca2+- and K+-dependent communication between central nervous system myelinated axons and oligodendrocytes revealed by voltage-sensitive dyes

Ultrafast four-dimensional imaging of cardiac mechanical wave propagation with sparse optoacoustic sensing

Propagation of electromechanical waves in excitable heart muscles follows complex spatiotemporal patterns holding the key to understanding life-threatening arrhythmias and other cardiac conditions. Accurate volumetric mapping of cardiac wave propagation is currently hampered by fast heart motion, particularly in small model organisms. Here we demonstrate that ultrafast four-dimensional imaging of cardiac mechanical wave propagation in entire beating murine heart can be accomplished by sparse optoacoustic sensing with high contrast, ∼115-µm spatial and submillisecond temporal resolution. We extract accurate dispersion and phase velocity maps of the cardiac waves and reveal vortex-like patterns associated with mechanical phase singularities that occur during arrhythmic events induced via burst ventricular electric stimulation. The newly introduced cardiac mapping approach is a bold step toward deciphering the complex mechanisms underlying cardiac arrhythmias and enabling precise therapeutic interventions.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Voltage-Sensitive Dye Imaging of Neocortical Activity

Neural computations underlying sensory perception, cognition, and motor control are performed by populations of neurons at different anatomical and temporal scales. Few techniques are currently available for exploring the dynamics of local and large range populations. Voltage-sensitive dye imaging (VSDI), based on organic voltage probes, reveals neural population activity in areas ranging from a few tens of micrometers to a couple of centimeters, or two areas up to ~10 cm apart. VSDI provides a submillisecond temporal resolution and a spatial resolution of ~50 µm. The dye signal emphasizes subthreshold synaptic potentials. VSDI has been applied in the mouse, rat, gerbil, ferret, tree shrew, cat, and monkey cortices to explore the lateral spread of retinotopic or somatotopic activation; the dynamic spatiotemporal pattern resulting from sensory activation, including the somatosensory, olfactory, auditory, and visual modalities; and motor preparation and the properties of spontaneously occurring population activity. In this introduction, we focus on VSDI in vivo and review results obtained mostly in the visual system in our laboratory.

Characterization of early cortical population response to thalamocortical input in vitro

The in vitro thalamocortical slice preparation of mouse barrel cortex allows for stimulation of the cortex through its natural afferent thalamocortical pathway. This preparation was used here to investigate the first stage of cortical processing in the large postsynaptic dendritic networks as revealed by voltage sensitive dye imaging (VSDI). We identified the precise location and dimensions of two clearly distinguishable dendritic networks, one in the granular layer (GL) IV and one in the infragranular layer (IGL) V and VI and showed that they have different physiological properties. DiI fluorescent staining further revealed that thalamocortical axons project on to these two networks in the typical barrel like form, not only in the granular but also in the IGL. Finally we investigated the short-term dynamics of both the VSDI signal and the local field potential (LFP) in response to a train of eight-pulses at various frequencies in both these layers. We found evidence of differences in the plasticity between the first two response peaks compared to the remaining six peaks as well as differences in short-term plasticity between the VSDI response and the LFP. Our findings suggest, that at least early cortical processing takes place in two separate dendritic networks that may stand at the beginning of further parallel computation. The detailed characterization of the parameters of these networks may provide tools for further research into the complex dynamics of large dendritic networks and their role in cortical computation.

Optical mapping in the developing zebrafish heart

Understanding the developmental basis of cardiac electrical activity has proven technically challenging, largely as a result of the inaccessible nature of the heart during cardiogenesis in many organisms. The emergence of the zebrafish as a model organism has availed the very earliest stages of heart formation to experimental exploration. The zebrafish also offers a robust platform for genetic and chemical screening. These tools have been exploited in screens for modifiers of cardiac electrophysiologic phenotypes and in screens for novel drugs.

Optical imaging of voltage and calcium in cardiac cells & tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Subthreshold membrane potential oscillations in inferior olive neurons are dynamically regulated by P/Q- and T-type calcium channels: a study in mutant mice

The role of P/Q- and T-type calcium channels in the rhythmic oscillatory behaviour of inferior olive (IO) neurons was investigated in mutant mice. Mice lacking either the CaV2.1 gene of the pore-forming alpha1A subunit for P/Q-type calcium channel, or the CaV3.1 gene of the pore-forming alpha1G subunit for T-type calcium channel were used. In vitro intracellular recording from IO neurons reveals that the amplitude and frequency of sinusoidal subthreshold oscillations (SSTOs) were reduced in the CaV2.1-/- mice. In the CaV3.1-/- mice, IO neurons also showed altered patterns of SSTOs and the probability of SSTO generation was significantly lower (15%, 5 of 34 neurons) than that of wild-type (78%, 31 of 40 neurons) or CaV2.1-/- mice (73%, 22 of 30 neurons). In addition, the low-threshold calcium spike and the sustained endogenous oscillation following rebound potentials were absent in IO neurons from CaV3.1-/- mice. Moreover, the phase-reset dynamics of oscillatory properties of single neurons and neuronal clusters in IO were remarkably altered in both CaV2.1-/- and CaV3.1-/- mice. These results suggest that both alpha1A P/Q- and alpha1G T-type calcium channels are required for the dynamic control of neuronal oscillations in the IO. These findings were supported by results from a mathematical IO neuronal model that incorporated T and P/Q channel kinetics.

Retinohypothalamic tract synapses in the rat suprachiasmatic nucleus demonstrate short-term synaptic plasticity

The master circadian pacemaker located in the suprachiasmatic nucleus (SCN) is entrained by light intensity-dependent signals transmitted via the retinohypothalamic tract (RHT). Short-term plasticity at glutamatergic RHT-SCN synapses was studied using stimulus frequencies that simulated the firing of light sensitive retinal ganglion cells. The evoked excitatory postsynaptic current (eEPSC) was recorded from SCN neurons located in hypothalamic brain slices. The eEPSC amplitude was stable during 0.08 Hz stimulation and exhibited frequency-dependent short-term synaptic depression (SD) during 0.5 to 100 Hz stimulus trains in 95 of 99 (96%) recorded neurons. During SD the steady-state eEPSC amplitude decreased, whereas the cumulative charge transfer increased in a frequency-dependent manner and saturated at 20 Hz. SD was similar during subjective day and night and decreased with increasing temperature. Paired-pulse stimulation (PPS) and voltage-dependent Ca(2+) channel (VDCC) blockers were used to characterize a presynaptic release mechanism. Facilitation was present in 30% and depression in 70% of studied neurons during PPS. Synaptic transmission was reduced by blocking both N- and P/Q-type presynaptic VDCCs, but only the N-type channel blocker significantly relieved SD. Aniracetam inhibited AMPA receptor desensitization but did not alter SD. Thus we concluded that SD is the principal form of short-term plasticity at RHT synapses, which presynaptically and frequency-dependently attenuates light-induced glutamatergic RHT synaptic transmission protecting SCN neurons against excessive excitation.

Voltage-sensitive dye imaging: Technique review and models

In this review, we present the voltage-sensitive dye imaging (VSDI) method. The possibility offered for in vivo (and in vitro) brain imaging is unprecedented in terms of spatial and temporal resolution. However, the unresolved multi-component origin of the optical signal encourages us to perform a detailed analysis of the method limitation and the existing models. We propose a biophysical model at a mesoscopic scale in order to understand and interpret this signal.

Non-linear population firing rates and voltage sensitive dye signals in visual areas 17 and 18 to short duration stimuli

Visual stimuli of short duration seem to persist longer after the stimulus offset than stimuli of longer duration. This visual persistence must have a physiological explanation. In ferrets exposed to stimuli of different durations we measured the relative changes in the membrane potentials with a voltage sensitive dye and the action potentials of populations of neurons in the upper layers of areas 17 and 18. For durations less than 100 ms, the timing and amplitude of the firing and membrane potentials showed several non-linear effects. The ON response became truncated, the OFF response progressively reduced, and the timing of the OFF responses progressively delayed the shorter the stimulus duration. The offset of the stimulus elicited a sudden and strong negativity in the time derivative of the dye signal. All these non-linearities could be explained by the stimulus offset inducing a sudden inhibition in layers II-III as indicated by the strongly negative time derivative of the dye signal. Despite the non-linear behavior of the layer II-III neurons the sum of the action potentials, integrated from the peak of the ON response to the peak of the OFF response, was almost linearly related to the stimulus duration.

White matter in learning, cognition and psychiatric disorders

White matter is the brain region underlying the gray matter cortex, composed of neuronal fibers coated with electrical insulation called myelin. Previously of interest in demyelinating diseases such as multiple sclerosis, myelin is attracting new interest as an unexpected contributor to a wide range of psychiatric disorders, including depression and schizophrenia. This is stimulating research into myelin involvement in normal cognitive function, learning and IQ. Myelination continues for decades in the human brain; it is modifiable by experience, and it affects information processing by regulating the velocity and synchrony of impulse conduction between distant cortical regions. Cell-culture studies have identified molecular mechanisms regulating myelination by electrical activity, and myelin also limits the critical period for learning through inhibitory proteins that suppress axon sprouting and synaptogenesis.

Cortical dynamics subserving visual apparent motion

Motion can be perceived when static images are successively presented with a spatial shift. This type of motion is an illusion and is termed apparent motion (AM). Here we show, with a voltage sensitive dye applied to the visual cortex of the ferret, that presentation of a sequence of stationary, short duration, stimuli which are perceived to produce AM are, initially, mapped in areas 17 and 18 as separate stationary representations. But time locked to the offset of the 1st stimulus, a sequence of signals are elicited. First, an activation traverses cortical areas 19 and 21 in the direction of AM. Simultaneously, a motion dependent feedback signal from these areas activates neurons between areas 19/21 and areas 17/18. Finally, an activation is recorded, traveling always from the representation of the 1st to the representation of the next or succeeding stimuli. This activation elicits spikes from neurons situated between these stimulus representations in areas 17/18. This sequence forms a physiological mechanism of motion computation which could bind populations of neurons in the visual areas to interpret motion out of stationary stimuli.

Electron tomographic analysis of cytoskeletal cross-bridges in the paranodal region of the node of Ranvier in peripheral nerves

The node of Ranvier is a site for ionic conductances along myelinated nerves and governs the saltatory transmission of action potentials. Defects in the cross-bridging and spacing of the cytoskeleton are a prominent pathological feature in diseases of the peripheral nerve. Electron tomography was used to examine cytoskeletal-cytoskeletal, membrane-cytoskeletal, and heterologous cell connections in the paranodal region of the node of Ranvier in peripheral nerves. Focal attachment of cytoskeletal filaments to each other and to the axolemma and paranodal membranes of the Schwann cell via narrow cross-bridges was visualized in both neuronal and glial cytoplasm. A subset of intermediate filaments associates with the cytoplasmic surfaces of supramolecular complexes of transmembrane structures that are presumed to include known and unknown junctional proteins. Mitochondria were linked to both microtubules and neurofilaments in the axoplasm and to neighboring smooth endoplasmic reticulum by narrow cross-bridges. Tubular cisternae in the glial cytoplasm were also linked to the paranodal glial cytoplasmic loop juxtanodal membrane by short cross-bridges. In the extracellular matrix between axon and Schwann cell, junctional bridges formed long cylinders linking the two membranes. Interactions between cytoskeleton, membranes, and extracellular matrix associations in the paranodal region are likely critical not only for scaffolding, but also for intracellular and extracellular communication.

Somatomotor and oculomotor inferior olivary neurons have distinct electrophysiological phenotypes

The electrophysiological properties of rat inferior olive (IO) neurons in the dorsal cap of Kooy (DCK) and the adjacent ventrolateral outgrowth (VLO) were compared with those of IO neurons in the principal olive (PO). Whereas DCK/VLO neurons are involved in eye movement control via their climbing fiber projection to the cerebellar flocculus, PO neurons control limb and digit movements via their climbing fiber projection to the lateral cerebellar hemisphere. In vitro patch recordings from DCK/VLO neurons revealed that low threshold calcium currents, Ih currents, and subthreshold oscillations are lacking in this subset of IO neurons. The recordings of activity in DCK neurons obtained by using voltage-sensitive dye imaging showed that activity is not limited to a single neuron, but rather that clusters of DCK neurons can be active in unison. These electrophysiological results show that the DCK/VLO neurons have unique properties that set them apart from the neurons in the PO nucleus. This finding indicates that motor control, from the perspective of the olivocerebellar system, is fundamentally different for the oculomotor and the somatomotor systems.

Development of a model for microphysiological simulations: small nodes of ranvier from peripheral nerves of mice reconstructed by electron tomography

The node of Ranvier is a complex structure found along myelinated nerves of vertebrate animals. Specific membrane, cytoskeletal, junctional, extracellular matrix proteins and organelles interact to maintain and regulate associated ion movements between spaces in the nodal complex, potentially influencing response variation during repetitive activations or metabolic stress. Understanding and building high resolution three dimensional (3D) structures of the node of Ranvier, including localization of specific macromolecules, is crucial to a better understanding of the relationship between its structure and function and the macromolecular basis for impaired conduction in disease. Using serial section electron tomographic methods, we have constructed accurate 3D models of the nodal complex from mouse spinal roots with resolution better than 7.5 nm. These reconstructed volumes contain 75-80% of the thickness of the nodal region. We also directly imaged the glial axonal junctions that serve to anchor the terminal loops of the myelin lamellae to the axolemma. We created a model of an intact node of Ranvier by truncating the volume at its midpoint in Z, duplicating the remaining volume and then merging the new half volume with mirror symmetry about the Z-axis. We added to this model the distribution and number of Na+ channels on this reconstruction using tools associated with the MCell simulation program environment. The model created provides accurate structural descriptions of the membrane compartments, external spaces, and formed structures enabling more realistic simulations of the role of the node in modulation of impulse propagation than have been conducted on myelinated nerve previously.

Role of gap junctions in synchronized neuronal oscillations in the inferior olive

Inferior olivary (IO) neurons are electrically coupled through gap junctions and generate synchronous subthreshold oscillations of their membrane potential at a frequency of 1-10 Hz. Whereas the ionic mechanisms of these oscillatory responses are well understood, their origin and ensemble properties remain controversial. Here, the role of gap junctions in generating and synchronizing IO oscillations was examined by combining intracellular recordings with high-speed voltage-sensitive dye imaging in rat brain stem slices. Single cell responses and ensemble synchronized responses of IO neurons were compared in control conditions and in the presence of 18beta-glycyrrhetinic acid (18beta-GA), a pharmacological gap junction blocker. Under our experimental conditions, 18beta-GA had no adverse effects on intrinsic electroresponsive properties of IO neurons, other than the block of gap junction-dependent dye coupling and the resulting change in cells' passive properties. Application of 18beta-GA did not abolish single cell oscillations. Pharmacologically uncoupled IO neurons continued to oscillate with a frequency and amplitude that were similar to those recorded in control conditions. However, these oscillations were no longer synchronized across a population of IO neurons. Our optical recordings did not detect any clusters of synchronous oscillatory activity in the presence of the blocker. These results indicate that gap junctions are not necessary for generating subthreshold oscillations, rather, they are required for clustering of coherent oscillatory activity in the IO. The findings support the view that oscillatory properties of single IO neurons endow the system with important reset dynamics, while gap junctions are mainly required for synchronized neuronal ensemble activity.

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.

Optical imaging of the heart

Optical techniques have revolutionized the investigation of cardiac cellular physiology and advanced our understanding of basic mechanisms of electrical activity, calcium homeostasis, and metabolism. Although optical methods are widely accepted and have been at the forefront of scientific discoveries, they have been primarily applied at cellular and subcellular levels and considerably less to whole heart organ physiology. Numerous technical difficulties had to be overcome to dynamically map physiological processes in intact hearts by optical methods. Problems of contraction artifacts, cellular heterogeneities, spatial and temporal resolution, limitations of surface images, depth-of-field, and need for large fields of view (ranging from 2x2 mm2 to 3x3 cm2) have all led to the development of new devices and optical probes to monitor physiological parameters in intact hearts. This review aims to provide a critical overview of current approaches, their contributions to the field of cardiac electrophysiology, and future directions of various optical imaging modalities as applied to cardiac physiology at organ and tissue levels.

VSDI: a new era in functional imaging of cortical dynamics

During the last few decades, neuroscientists have benefited from the emergence of many powerful functional imaging techniques that cover broad spatial and temporal scales. We can now image single molecules controlling cell differentiation, growth and death; single cells and their neurites processing electrical inputs and sending outputs; neuronal circuits performing neural computations in vitro; and the intact brain. At present, imaging based on voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution for imaging neocortical functions in the living brain, and has paved the way for a new era in the functional imaging of cortical dynamics. It has facilitated the exploration of fundamental mechanisms that underlie neocortical development, function and plasticity at the fundamental level of the cortical column.

Neuronal assemblies: single cortical neurons are obedient members of a huge orchestra

Spontaneous cortical activity of single neurons is often either dismissed as noise, or is regarded as carrying no functional significance and hence is ignored. Our findings suggest that such concepts should be revised. We explored the coherent population activity of neuronal assemblies in primary sensory area in the absence of a sensory input. Recent advances in real-time optical imaging based on voltage-sensitive dyes (VSDI) have facilitated exploration of population activity and its intimate relationship to the activity of individual cortical neurons. It has been shown by in vivo intracellular recordings that the dye signal measures the sum of the membrane potential changes in all the neuronal elements in the imaged area, emphasizing subthreshold synaptic potentials and dendritic action potentials in neuronal arborizations originating from neurons in all cortical layers whose dendrites reach the superficial cortical layers. Thus, the VSDI has allowed us to image the rather illusive activity in neuronal dendrites that cannot be readily explored by single unit recordings. Surprisingly, we found that the amplitude of this type of ongoing subthreshold activity is of the same order of magnitude as evoked activity. We also found that this ongoing activity exhibited high synchronization over many millimeters of cortex. We then investigated the influence of ongoing activity on the evoked response, and showed that the two interact strongly. Furthermore, we found that cortical states that were previously associated only with evoked activity can actually be observed also in the absence of stimulation, for example, the cortical representation of a given orientation may appear without any visual input. This demonstration suggests that ongoing activity may also play a major role in other cortical function by providing a neuronal substrate for the dependence of sensory information processing on context, behavior, memory and other aspects of cognitive function.

Role of calcineurin in calcium-mediated hypoxic injury to white matter

BACKGROUND CONTEXT

Calcium influx into cells is responsible for initiating the "final pathway" to cell death in neuronal tissue after traumatic or hypoxic injury. The specific pathways in this cascade are myriad and the importance each one plays is controversial. It is clear, though, that blocking individual pathways confers protection to these tissues.

PURPOSE

In the present study we examined the role of Cyclosporin A (CsA), FK-506 and rapamycin in modulating the effects of Ca(2+) influx through their interactions with immunophilins and specifically the end result of calcineurin modulation.

METHODS

Dorsal columns were isolated from the spinal cord of adult rats and injured by exposure to hypoxic conditions for 60 minutes. The samples were monitored electrophysiologically in an in vitro recording chamber (maintained at 37 C degrees ) during injury, and the compound action potential (CAP) was monitored with glass microelectrodes. The dorsal column was exposed to hypoxic Ringers solution alone or with the different immunosuppressants and compared with baseline readings. Functional recovery of the dorsal column was then assessed by recovery of the CAP.

RESULTS

The mean CAP decreased to about 20% of baseline control levels during hypoxia and returned 53.8+/-7.6% of baseline (p<.05) after reoxygenation. CsA, an immunosuppressant known to inhibit calcineurin, promoted a significantly greater recovery of CAP amplitude to 76.8+/-5.2% and 72.1+/-13.2% of control (p<.05) after hypoxic injury and reoxygenation of dorsal column white matter when applied at concentrations of 1 microM and 10 microM, respectively. FK-506, which also inhibits calcineurin, was applied at a concentration of 0.1 microM, and promoted CAP amplitude recovery to 82.6+/-5.0% of control after hypoxic injury and reoxygenation of dorsal column white matter. The addition of rapamycin (1 microM), which binds to the same immunophilin as FK-506, to the FK-506 (0.1 microM) solution during hypoxic injury showed recovery of CAP amplitudes to only 56.9+/-6.7% of control. Electron microscopy revealed remarkable protection of axons and prevention of organelle disruption in segments treated with CsA and FK-506 during hypoxia when compared with hypoxic controls.

CONCLUSION

In conclusion, both CsA and FK-506 confer in vitro protection to dorsal columns during hypoxic injury at physiological temperatures, and rapamycin blocks the protective effect of FK-506. Thus, calcineurin may play an important role in the physiology of neuronal injury.

Multiple-site optical recording for characterization of functional synaptic organization of the optic tectum of rainbow trout

To map the functional synaptic organization over a wide area in the optic tectum, we directly monitored two-dimensional propagation of postsynaptic depolarization evoked by firing of retinotectal afferents in optic tectum slices prepared from rainbow trout (Oncorhynchus mykiss), using a voltage-sensitive dye and a photodiode array system. The postsynaptic responses to afferent stimulation first propagated in the stratum opticum and stratum fibrosum et griseum superficiale in an anterograde fashion in the afferents and then expanded vertically into the deep layers. This vertical propagation appeared to occur along a bundle-like structure that corresponded well with a cluster of neurons whose somata are located in the stratum periventriculare. Pharmacological studies showed that these postsynaptic responses were mediated by ionotropic glutamate receptors. On the other hand, the optical signals appeared to consist of at least two components (a transient signal and a slow signal). The second transient signal summated with the first slow signal by paired stimulation, suggesting that the transient and slow signals originated from different cell types. Taken together, these results showed that the functional synaptic organization of the teleost optic tectum comprises of two depolarization-signal propagating paths along a horizontal layer structure and a vertical bundle-like structure and that these synaptic responses occur via glutamatergic transmission.

Patch-clamp recordings from white matter glia in thin longitudinal slices of adult rat spinal cord

We developed a technique of whole cell patch-clamp recordings from white matter oligodendrocytes and astrocytes in 200-250 microm-thick horizontal slices of adult (>2 months, 240-260 g) rat thoracic spinal cord. The viability of the white matter, sectioned in Na(+)-free, low Ca(2+) media, and the function of axons were preserved for >8 h, as demonstrated by the propagation of TTX-sensitive compound action potentials (CAPs) and the sensitivity of their refractory period to K(+) channel blocker 4-aminopyridine (1 microM). Glial cells were visually identified within the slices with a 40 x water immersion objective using infra-red differential interference contrast (IR-DIC) video microscopy, and the details of their morphology were further elucidated after filling the cells with Lucifer Yellow or Alexa 350 fluorescent dyes during whole-cell recording. Using voltage steps and ramps, we revealed pronounced non-linearity of I-V relationships in both oligodendrocytes and astrocytes. Both types of cells expressed TEA-sensitive outward delayed rectifier-type currents activated at positive voltages but showed little, if any, signs of inward rectification at voltages up to -140 mV. At -70 mV holding voltage, bath-applied kainic acid (100 microM) activated inward currents in both types of cells. This novel horizontal slice preparation of adult rat thoracic cord will facilitate the examination of mature glial cell physiology, glial-axonal signaling and the pathophysiology of spinal cord trauma and ischemia.

Electrotonically mediated oscillatory patterns in neuronal ensembles: an in vitro voltage-dependent dye-imaging study in the inferior olive

Spatiotemporal profiles of ensemble subthreshold neuronal oscillation were studied in brainstem slices using high-speed voltage-sensitive dye imaging. After local electrical stimuli, the overall voltage profile demonstrated coherent oscillatory waves that spread over the inferior olive (IO). These oscillations were also observed in concurrently obtained intracellular recordings from IO neurons. Over the first few seconds after the stimuli, the optically recorded oscillations clustered into coherent groups comprising hundreds of neurons. Statistical analysis of the spatial profiles of these clusters revealed size fluctuation around stable core regions that were surrounded by a rim the diameter of which varied in time during the oscillation period. The neuronal ensemble oscillations were calcium derived and had an average frequency range of 1-7 Hz. This rhythmic response demonstrated a different spatiotemporal distribution in the presence of picrotoxin, which induced the merging of neuronal clusters into larger areas of coherent activity. The possibility that such clustering is a consequence of intrinsic oscillations in ensembles of coupled neurons was tested using mathematical modeling.

Role of RyRs and IP3 receptors after traumatic injury to spinal cord white matter

Calcium influx and elevation of intracellular free calcium (Ca2+i), with subsequent activation of degenerative enzymes is hypothesized to cause cell injury and death after trauma. We examined the effects of traumatic compressive injury on (Ca2+)i dynamics in spinal cord white matter. We conducted electrophysiological studies with ryanodine and inositol (1,4,5)-triphosphate (IP3) receptor agonists and antagonists in an in vitro model of spinal cord injury (SCI). A 25-30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (37 degrees C) and injured with a modified clip (2-g closing force) for 15 sec. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes. The CAP decreased to 55.2+/-6.8% of control (p < 0.05) after spinal cord injury (SCI). Chelation of Ca2+i with BAPTA-AM (a high-affinity calcium chelator) promoted significantly greater recovery of CAP amplitude (83.2+/-4.2% of control; p < 0.05) after injury. Infusion of caffeine (1 and 10 mM) exacerbated CAP amplitude decline (45.1+/-5.9% of control; p < 0.05; 44.6+/-3.1% of control; p < 0.05) postinjury. Blockade of Ca2+i release through ryanodine-sensitive receptors (RyRs) with dantrolene (10 microM) and ryanodine (50 microM), conferred significant (p < 0.05) improvement in CAP amplitude after injury. On the other hand, blockade of Ca2+i with inositol (1,4,5)-triphosphate receptor (IP3Rs) blocker 2APB (10 microM) also conferred significant improvement in CAP amplitude after injury (82.9+/-7.9%; p < 0.05). In conclusion, the injurious effects of Ca2+i in traumatic central nervous system (CNS) white matter injury appear to be mediated both by RyRs and through IP3Rs calcium-induced calcium release receptors (CICRs).

Temporal binding via cortical coincidence detection of specific and nonspecific thalamocortical inputs: a voltage-dependent dye-imaging study in mouse brain slices

Voltage-sensitive dye imaging of mouse thalamocortical slices demonstrated that electrical stimulation of the centrolateral intralaminar thalamic nucleus (CL) resulted in the specific activation of thalamic reticular nucleus, striatum/putamen, and cortical layers 5, 6, and 1. By contrast, ventrobasal (VB) thalamic stimulation, while activating the reticular and basal ganglia nuclei, also activated directly layers 4 and deep 5 of the cortex. Conjoined stimulation of the VB and CL nuclei resulted in supralinear summation of the two inputs at cortical output layer 5, demonstrating coincidence detection along the apical dendrites. This supralinear summation was also noticed at gamma band stimulus frequency ( approximately 40 Hz). Direct stimulation of cortical layer 1, after a radial section of the cortex that spared only that layer, was shown to sum supralinearly with the cortical activation triggered by VB stimulation, providing a second demonstration for coincidence detection. Coincidence detection by coactivation of the specific (VB) and nonspecific (CL) thalamic nuclei has been proposed as the basis for the temporal conjunction that supports cognitive binding in the brain.

Neuron-independent Ca(2+) signaling in glial cells of snail's brain

To directly monitor the glial activity in the CNS of the pond snail, Lymnaea stagnalis, we optically measured the electrical responses in the cerebral ganglion and median lip nerve to electrical stimulation of the distal end of the median lip nerve. Using a voltage-sensitive dye, RH155, we detected a composite depolarizing response in the cerebral ganglion, which consisted of a fast transient depolarizing response corresponding to a compound action potential and a slow depolarizing response. The slow depolarizing response was observed more clearly in an isolated median lip nerve and also detected by extracellular recording. In the median lip nerve preparation, the slow depolarizing response was suppressed by an L-type Ca(2+) channel blocker, nifedipine, and was resistant to tetrodotoxin and Na(+)-free conditions. Together with the fact that a delay from the compound action potential to the slow depolarizing response was not constant, these results suggested that the slow depolarizing response was not a postsynaptic response. Because the signals of the action potentials appeared on the saturated slow depolarizing responses during repetitive stimulation, the slow depolarizing response was suggested to originate from glial cells. The contribution of the L-type Ca(2+) current to the slow depolarizing response was confirmed by optical recording in the presence of Ba(2+) and also supported by intracellular Ca(2+) measurement. Our results suggested that electrical stimulation directly triggers glial Ca(2+) entry through L-type Ca(2+) channels, providing evidence for the generation of glial depolarization independent of neuronal activity in invertebrates.

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.

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Multiple-site optical recording of mouse brainstem evoked by vestibulocochlear nerve stimulation

We used optical imaging to investigate the mouse cochlear and vestibular nucleus in brainstem slices using a voltage-sensitive dye, RH 155. As a result, the spatiotemporal patterns of excitatory propagation were shown. These optical signals consisted of two components consisting of a spike-like fast signal and a long-lasting slow signal. All responses were abolished by tetrodotoxin. The slow signals were eliminated under a Ca(2+)-free solution. In addition, synaptic fatigue was also observed. The present study indicated the feasibility of optical recording for visually revealing the synaptic transmission in both the vestibular and cochlear nucleus.

N-type calcium channels and their regulation by GABAB receptors in axons of neonatal rat optic nerve

Axons of neonatal rat optic nerves exhibit fast calcium transients in response to brief action potential stimulation. In response to one to four closely spaced action potentials, evoked calcium transients showed a fast-rising phase followed by a decay with a time constant of approximately 2-3 sec. By selective staining of axons or glial cells with calcium dyes, it was shown that the evoked calcium transient originated from axons. The calcium transient was caused by influx because it was eliminated when bath calcium was removed. Pharmacological profile studies with calcium channel subtype-specific peptides suggested that 58% of the evoked calcium influx was accounted for by N-type calcium channels, whereas L- and P/Q-type calcium channels had little, if any, contribution. The identity of the residual calcium influx remains unclear. GABA application caused a dramatic reduction of the amplitude of the action potential and the associated calcium influx. When GABAA receptors were blocked by bicuculline, the inhibitory effect of GABA on the action potential was eliminated, whereas that on the calcium influx was not, indicating involvement of GABAB receptors. Indeed, the calcium influx was inhibited by the GABAB receptor agonist baclofen. This baclofen effect was occluded by a previous block of N-type calcium channels and was unaffected by the broad-spectrum K+ channel blocker 4-AP. We conclude that neonatal rat optic nerve axons express N-type calcium channels, which are subjected to regulation by G-protein-coupled GABAB receptors. We suggest that receptor-mediated inhibition of axonal calcium channels plays a protective role in neonatal anoxic and/or ischemic injury.

N-type calcium channels and their regulation by GABAB receptors in axons of neonatal rat optic nerve

Axons of neonatal rat optic nerves exhibit fast calcium transients in response to brief action potential stimulation. In response to one to four closely spaced action potentials, evoked calcium transients showed a fast-rising phase followed by a decay with a time constant of approximately 2-3 sec. By selective staining of axons or glial cells with calcium dyes, it was shown that the evoked calcium transient originated from axons. The calcium transient was caused by influx because it was eliminated when bath calcium was removed. Pharmacological profile studies with calcium channel subtype-specific peptides suggested that 58% of the evoked calcium influx was accounted for by N-type calcium channels, whereas L- and P/Q-type calcium channels had little, if any, contribution. The identity of the residual calcium influx remains unclear. GABA application caused a dramatic reduction of the amplitude of the action potential and the associated calcium influx. When GABAA receptors were blocked by bicuculline, the inhibitory effect of GABA on the action potential was eliminated, whereas that on the calcium influx was not, indicating involvement of GABAB receptors. Indeed, the calcium influx was inhibited by the GABAB receptor agonist baclofen. This baclofen effect was occluded by a previous block of N-type calcium channels and was unaffected by the broad-spectrum K+ channel blocker 4-AP. We conclude that neonatal rat optic nerve axons express N-type calcium channels, which are subjected to regulation by G-protein-coupled GABAB receptors. We suggest that receptor-mediated inhibition of axonal calcium channels plays a protective role in neonatal anoxic and/or ischemic injury.

The phosphoinositide signaling cycle in myelin requires cooperative interaction with the axon

Previous studies on the origin of myelin phosphoinositides involved in signaling mechanisms indicated axon to myelin transfer of phosphatidylinositol followed by myelin-localized incorporation of axon-derived phosphate groups into phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate. This is in agreement with other studies showing the presence of phosphorylating activity in myelin that converts phosphatidylinositol into the mono-and diphospho derivatives. It was also found that the second messenger, inositol 1,4,5-trisphosphate, is hydrolyzed to inositol 1,4-bisphosphate by a myelin-localized enzyme. The present study was undertaken to determine the locus of the remaining reactions leading to formation of free inositol and completion of the cycle by resynthesis of phosphatidylinositol. The latter reaction was found to occur preferentially in isolated axons, and to a limited extent if at all in myelin. On the other hand, hydrolytic reactions which sequentially convert inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, inositol 1-phosphate, and free inositol were found to occur more prominently in myelin. Thus, restoration of phosphoinositides following signal-induced breakdown of PIP2 in myelin is seen as requiring metabolic interplay between myelin and axon.

Optical recording of the rat piriform cortex activity

The piriform cortex (PCx) is a phylogenetically old brain structure which presents characteristics of a content-addressable memory. Taking into account its particular anatomo-functional organization, we hypothesized that this cortex could behave rather as an assembly of different functional units than as a functionally homogeneous structure. This hypothesis was tested by using both anatomical and functional approaches. Immunohistological and tracing experiments demonstrated that both the connections of the PCx with the higher nervous centres, and its monoaminergic and cholinergic modulatory afferents exhibited a heterogeneous distribution. Then, optical monitoring of its neuronal activity with a voltage-sensitive dye pointed out that the PCx is a functionally heterogeneous structure. Electrical stimulations of the olfactory bulb showed that the inhibitory processes which control the cortical responsiveness were not identical in all the PCx area. Two different functional areas at least could be distinguished: in the ventromedial PCx, the afferent activity is privileged since the level of inhibition of disynaptic activation remained large during repetitive stimuli. Contrarily, in the posterior PCx, the disynaptic activity remained unchanged in response to successive stimulations and the responses of neighbouring sites were statistically more synchronized than in its anterior part. Moreover, a late depolarization wave was significantly larger in the posterior PCx. These data are in good agreement with the results provided by computational models of the PCx. In the future, theoretical and experimental investigations of this cortex will be useful for understanding olfactory information processing and as a model of brain functioning at the neocortical level as well.

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

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

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Current source density profiles of optical recording maps: a new approach to the analysis of spatio-temporal neural activity patterns

Spatio-temporal spreading of activity in the CA1 region of the rat hippocampal slice was studied by two experimental approaches. At identical locations in the tissue we measured both the extracellular field potential distribution with microelectrode recordings and the intracellular potential distribution by optical recording, using voltage-sensitive fluorescent dyes. Current source density analysis (CSD) was applied to the extracellular field potential distributions (eCSD) to enhance the spatial resolution. In order to obtain an analogous improvement for the optical recordings, we developed a new CSD transformation, which calculates the locations of the transmembrane current generators from the intracellular potential distributions (iCSD). Compared to the underlying fluorescence maps, the new iCSD profiles exhibit a considerable improvement in spatial resolution. Results can be directly interpreted in terms of physiological membrane processes, such as postsynaptic potentials and action potentials. The iCSD profiles show a surprisingly good correspondence with the classical eCSD profiles both qualitatively and quantitatively, the only difference being that cell body activity is reduced in amplitude. Thus, this new optical CSD analysis paves the way for a quantitative interpretation, rather than the hitherto predominantly qualitative interpretation of spatio-temporal activity profiles from optical recording measurements.

Neurally evoked calcium transients in terminal Schwann cells at the neuromuscular junction

We examined the effects of motor-nerve stimulation on the intracellular Ca2+ levels of Schwann cells, the glial cells at the frog neuromuscular junction. Schwann cells, which were loaded with the fluorescent Ca2+ indicator fluo-3 and examined by confocal microscopy, showed a transient increase in free Ca2+ within a few seconds of the onset of tetanic stimulation of the motor nerve. The Ca2+ response was specific to the synapse in that it was found in the terminal Schwann cells at the junction but not in the myelinating Schwann cells along the axon. The Ca2+ transients occurred in the presence of d-tubocurare, indicating that they were not mediated by nicotinic acetylcholine receptors and recurred when the stimulus was repeated. The Ca2+ response persisted after degeneration of the postsynaptic muscle fiber, demonstrating that the terminal Schwann cell was stimulated directly by presynaptic activity. The finding that terminal Schwann cells at the neuromuscular junction respond to presynaptic activity suggests that glial-cell function is modulated by synaptic transmission.

Axon-myelin transfer of phospholipids and phospholipid precursors. Labeling of myelin phosphoinositides through axonal transport

Previous studies have provided evidence for axon-to-myelin transfer of intact lipids and lipid precursors for reutilization by myelin enzymes. Several of the lipid constituents of myelin showed significant contralateral/ipsilateral ratios of incorporated radioactivity, indicative of axonal origin, whereas proteins and certain other lipids did not participate in this transfer-reutilization process. The present study will examine the labeling of myelin phosphoinositides by this pathway. Both 32PO4 and [3H]inositol were injected monocularly into 7-9-wk-old rabbits and myelin was isolated 7 or 21 days later from pooled optic tracts and superior colliculi. In total lipids 32P counts of the isolated myelin samples showed significant contralateral/ipsilateral ratios as well as increasing magnitude of contralateral-ipsilateral differences during the time interval. Thin-layer chromatographic isolation of the myelin phosphoinositides revealed significant 32P-labeling of these species, with PIP and PIP2 showing time-related increases. This resembled the labeling pattern of the major phospholipids from rabbit optic system myelin in a previous study and suggested incorporation of axon-derived phosphate by myelin-associated enzymes. The 32P label in PI, on the other hand, remained constant between 7 and 21 days, suggesting transfer of intact lipid. This was supported by the labeling pattern with [3H]inositol, which also showed no increase over time for PI. These results suggest axon-myelin transfer of intact PI followed by myelin-localized incorporation of axon-derived phosphate groups into PIP and PIP2. The general topic of axon-myelin transfer of phospholipids and phospholipid precursors is reviewed.

Three-dimensional fine structure of cytoskeletal-membrane interactions at nodes of Ranvier

Cytoskeleton-membrane-extracellular matrix interactions at the node of Ranvier were examined in both central and peripheral axons by combining three different methods for tissue preparation with three different electron microscopic techniques for imaging supramolecular structure. Conventional and three-dimensional high voltage electron microscopy of thin and semithick sections of tissues stained en bloc with ferric chloride revealed the presence of transcellular structures across the nodal gap traversing the paranodal glial-axonal junction. These structures penetrate both axonal and glial membranes and are further traced to the cortical axoplasm. This observation was verified by an examination of similar regions in rapidly-frozen freeze-substituted fresh axons. The filamentous nature of these structures, their focal attachment to the external true surface of the nodal and paranodal axolemma and their association with membrane particles were visualized in deep etch rotary-shadow replicas. At the node, both extracellular gap-crossing filaments and membrane-cytoskeletal linkers in the nodal axoplasm are joined to one of the prominent membrane particles of the nodal axolemma. At the paranodal axo-glial junction, the anchoring site of these membrane-cytoskeleton linkers are found on the linear arrays of 16 nm particles. Thus, cytoplasmic filaments and extracellular filaments or bridge structures are involved in the membrane-cytoskeletal interaction at the node and paranode. Some of these membrane particles are known to play a role in ionic conductances known to occur at this site. An additional role in cell adhesion or maintenance of the membrane specialization of this functionally important site of axolemma is now indicated.

Optical determination of impulse conduction velocity during development of embryonic chick cervical vagus nerve bundles

1. Employing an optical method for multiple-site simultaneous recording of electrical activity, we have determined the conduction velocity in cervical vagus nerve bundles isolated from 5- to 21-day-old chick embryos, and investigated its developmental changes. 2. The preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761), and action potential- (impulse-) related optical signals were elicited by brief stimuli applied to the end of the vagus nerve bundle with a suction electrode. Optical signals were recorded simultaneously from many contiguous regions using a 12 x 12-element photodiode array. 3. The optical signals spread with small delay from the site of stimulation. From the relationship between the delay and distance from the current-applying electrode, conduction velocities were estimated in each tested preparation: the conduction velocity was very small and increased monotonically from about 0.1 m s-1 at 5 days embryonic age to about 0.4 m s-1 by hatching. The increase in the conduction velocity was closely related to a developmental increase in the diameter of the vagus nerve bundle. 4. In addition, we have examined the spread of electrotonic potentials. The space constant was very small (200-450 microns) and increased as development proceeded. 5. Compound optical action signals having two distinct components were also recorded. They often appeared to be concentrated in the preparations from 8- to 12-day-old embryos. The conduction velocity of the second component was slower than that of the first. We suggest that appearance of the second component reflects degeneration of a subset of axons resulting from 'neural cell death' during the development of the vagus nerve.

Ultrastructural identification of HRP-injected oligodendrocytes in the intact rat optic nerve

Glial cells in the rat optic nerve were visualized by intracellular injections of horseradish peroxidase (HRP). A novel class of cell was encountered that was presumed to be an oligodendrocyte on the basis of arguments related to its light microscopic appearance after intracellular staining (Butt and Ransom, Glia 1989;2:470-475). These cells had 10-20 parallel processes 200-300 microns long that were oriented exclusively along the long axis of the optic nerve; the parallel processes were connected to the cell body by thin branches 15-30 microns long. To determine if these HRP-filled cells were oligodendrocytes, they were examined ultrastructurally; all cells examined in this way were unequivocally found to be myelin-producing oligodendrocytes. The oligodendrocytes contained intracellular organelles that were characteristic of this cell type, including abundant Golgi profiles and microtubules. In addition, HRP was found to fill the inner and outer tongue processes of myelin sheaths and the paranodal loops at nodes of Ranvier, proving that the entire cytoplasmic border surrounding the myelin sheath rapidly communicates by intracellular diffusion with the cell body. This electron microscopic study demonstrates that oligodendrocytes in the rat optic nerve can be positively identified by their distinctive light microscopic appearance after intracellular dye injection, and provides light microscopic criteria for establishing the number, distribution, and dimensions of the myelin segments provided by individual oligodendrocytes.

Optical recording of epileptiform voltage changes in the neocortical slice

Voltage sensitive probes were used to monitor the development, distribution, and spread of epileptiform potentials with a photodiode array in neocortical slices of guinea pigs. Epileptiform activity was induced by bath application of bicuculline-methiodide or 3,4-diaminopyridine and electrical stimulation of white matter or cortical layer I. Stimulation evoked a primary or early potential which was followed by a delayed epileptiform potential with a larger spatial extent. Shape, duration and amplitude of the delayed epileptiform potential varied strongly among slices and across the recording area and could reach largest amplitudes at a distance from the stimulation point. At a specific recording site, however, with repeated stimulation, potentials were generated in a stereotyped way. Intracellularly recorded delayed epileptiform potentials corresponded very closely at least to the early part of the optical response. Epileptiform activity appeared in layer III as soon as the primary potential reached sufficient amplitude there. Apart from this relationship, the distribution and spread of maximal amplitudes of delayed epileptiform potentials were segregated from those of early potentials. Early potentials reached maximal amplitudes close to the stimulation site. In contrast, the largest amplitudes of delayed epileptiform potentials were always found in layer III. A second maximum occasionally occurred in layer V. The horizontal amplitude distribution of epileptiform potentials was asymmetric, i.e. amplitudes increased to one side and decreased to the other. Early potential maxima spread from deeper to upper layers when initiated by white matter stimulation and from upper to deeper layers when initiated by layer I stimulation. In contrast, delayed epileptiform potentials always spread from layer III to lower layers and to the sides. Velocity of spread of early potentials and delayed epileptiform potentials differed systematically along the vertical and horizontal axis. The distribution of maximal amplitudes, shape, and pattern, of spread of epileptiform potentials was the same whether white matter or layer I was stimulated. The independence of delayed epileptiform potential characteristics from the point of stimulation and from early potential characteristics suggests that epileptiform activity is determined by intrinsic properties of the cortex and not by afferent activation.

Optical measurement of conduction in single demyelinated axons

Demyelination was initiated in Xenopus sciatic nerves by an intraneural injection of lysolecithin over a 2-3-mm region. During the next week macrophages and Schwann cells removed all remaining damaged myelin by phagocytosis. Proliferating Schwann cells then began to remyelinate the axons, with the first few lamellae appearing 13 d after surgery. Action potentials were recorded optically through the use of a potential-sensitive dye. Signals could be detected both at normal nodes of Ranvier and within demyelinated segments. Before remyelination, conduction through the lesion occurred in only a small fraction of the fibers. However, in these particular cases we could demonstrate continuous (nonsaltatory) conduction at very low velocities over long (greater than one internode) lengths of demyelinated axons. We have previously found through loose patch clamp experiments that the internodal axolemma contains voltage-dependent Na+ channels at a density approximately 4% of that at the nodes. These channels alone, however, are insufficient for successful conduction past the transition point between myelinated and demyelinated regions. Small improvements in the passive cable properties of the axon, adequate for propagation at this site, can be realized through the close apposition of macrophages and Schwann cells. As the initial lamellae of myelin appear, the probability of success at the transition zone increases rapidly, though the conduction velocity through the demyelinated segment is not appreciably changed. A detailed computational model is used to test the relative roles of the internodal Na+ channels and the new extracellular layer. The results suggest a possible mechanism that may contribute to the spontaneous recovery of function often seen in demyelinating disease.

A nonlinear electrostatic potential change in the T-system of skeletal muscle detected under passive recording conditions using potentiometric dyes

Voltage-sensing dyes were used to examine the electrical behavior of the T-system under passive recording conditions similar to those commonly used to detect charge movement. These conditions are designed to eliminate all ionic currents and render the T-system potential linear with respect to the command potential applied at the surface membrane. However, we found an unexpected nonlinearity in the relationship between the dye signal from the T-system and the applied clamp potential. An additional voltage- and time-dependent optical signal appears over the same depolarizing range of potentials where change movement and mechanical activation occur. This nonlinearity is not associated with unblocked ionic currents and cannot be attributed to lack of voltage clamp control of the T-system, which appears to be good under these conditions. We propose that a local electrostatic potential change occurs in the T-system upon depolarization. An electrostatic potential would not be expected to extend beyond molecular distances of the membrane and therefore would be sensed by a charged dye in the membrane but not by the voltage clamp, which responds solely to the potential of the bulk solution. Results obtained with different dyes suggest that the location of the phenomena giving rise to the extra absorbance change is either intramembrane or at the inner surface of the T-system membrane.

Effects of barium and bicarbonate on glial cells of Necturus optic nerve. Studies with microelectrodes and voltage-sensitive dyes

We have studied the effects of Ba++, a known K+ channel blocker, on the electrophysiological properties of the glial cells of Necturus optic nerve. The addition of Ba++ reversibly depolarized glial cells by 25-50 mV; the half maximal deplorization was obtained with a Ba++ concentration of approximately 0.3 mM. In the presence of Ba++, the sensitivity of the membrane to changes in K+ was reduced and there was evidence of competition between K+ and Ba++ for the K+ channel. These effects, which were accompanied by a large increase in the input resistance of the glial cells, indicate that Ba++ blocks the K+ conductance in glial cells of Necturus optic nerve. With the K+ conductance reduced, we were able to investigate the presence of other membrane conductances. We found that in the presence of Ba++, the addition of HCO3- caused a Na+-dependent hyperpolarization that was sensitive to the disulfonic stilbene SITS (4-acetamido-4'-isothiocyanostilbene-2, 2'-disulfonic acid). Removal of Na+ resulted in a HCO3- -dependent, SITS-sensitive depolarization. These results are consistent with the presence in the glial membrane of an electrogenic Na+/HCO3- cotransporter in which Na+, HCO3-, and net negative charge are transported in the same direction. In Cl- -free solutions, the Ba++-induced depolarization increased, suggesting a small permeability to Cl-. Using voltage-sensitive dyes and a photodiode array for multiple site optical recording, the distribution of potential changes in response to square pulses of intracellularly injected current were recorded before and after the addition of increased and the decay of amplitude as a function of distance decreased. Such results indicate that Ba++ increases the membrane resistance more than the resistance of the intercellular junctions.

Optical recording of electrical activity from axons and glia of frog optic nerve: potentiometric dye responses and morphometrics

Voltage-sensitive dyes were used to study the changes in membrane potential in axons and glial cells of the frog optic nerve following electrical stimulation. The lack of a signal in the unstained nerve and the multiphasic action spectra after staining indicated that the optical responses were from the extrinsic dyes. Changes in dye absorption and fluorescence had rapid and slow phases. The rapid phases resulted from action potentials in myelinated and unmyelinated axons. The kinetics of the slow phase of the optical response were similar to the depolarization recorded from the glial cells with intracellular electrodes. The ratio of the amplitudes of the fast and slow phases was characteristic for each type of dye. Pharmacological analysis of the action potential of the unmyelinated axons revealed tetrodotoxin-sensitive sodium channels and 4-aminopyridine-sensitive potassium channels. Repeated exposure of the stained preparation to light led to photodynamic damage as shown by a block of recovery of the glial depolarization. An electron microscopic morphometric study of the nerve was carried out in an effort to understand the contribution of the various anatomical elements to the compound optical response. The ratio of unmyelinated axon membrane to glial membrane was much greater than was the ratio of the fast and slow components of the signal, suggesting that the dyes either had a higher affinity for glial membrane or did not penetrate the nerve uniformly.

Optical recording of electrical activity from parallel fibres and other cell types in skate cerebellar slices in vitro

1. A reliable and simple fish brain slice preparation was obtained from the cerebellum of the skate, and its properties were described. 2. A potentiometric oxonol dye, RH-482, and multiple site optical recording of transmembrane voltage (MSORTV) were used to reveal the electrophysiological properties of the parallel fibre action potential and to measure its conduction (0.13 m/s). The parallel fibre action potential was blocked in the presence of tetrodotoxin (TTX) and prolonged by tetraethylammonium (TEA), suggesting that the upstroke depends upon sodium entry and the repolarization upon potassium efflux. An after-hyperpolarization results from a calcium-dependent potassium conductance. 3. A second potentiometric dye, RH-155, differing only slightly from RH-482, exhibited a high affinity for glial cell membrane, and could be used to monitor changes in extracellular potassium concentration by detecting changes in glial membrane potential. 4. Calcium channel blockers such as cadmium ions blocked the optical signal that reflected the extracellular accumulation of potassium. 5. Interventions that modified the extracellular volume, and thereby affected the accumulation of potassium, produced large changes in the optical signal that monitored glial depolarization. Hypertonic and hypotonic bathing solutions resulted in decreases and increases, respectively, in the magnitude of the extrinsic absorption change that tracked potassium accumulation. 6. Blocking sodium-potassium pump activity by means of ouabain prolonged the time course of the optical signal that was related to potassium accumulation in the extracellular space. 7. Extracellular potassium accumulation was revealed to be critically dependent upon intracellular calcium ions.

Activity-dependent calcium transients in central nervous system myelinated axons revealed by the calcium indicator Fura-2

Optical measurements from rat optic nerve, loaded with the new Ca2+ indicator Fura-2, provide the first evidence for the presence of activity-dependent fast intracellular [Ca2+] transients in mammalian central nervous system (CNS) myelinated axons. The results suggest that voltage-dependent Ca2+ channels are present in some of the myelinated axons. Optical measurements from axons stained with anterogradely transported voltage-sensitive dye suggest the presence of Ca2+-dependent potassium conductances in these axons. This report also demonstrates that Fura-2 can readily detect changes in [Ca2+] inside cells as a result of electrical activity, and establishes its suitability for measurements of intracellular Ca2+ transients in the millisecond time domain.