Onset Of Spreading Depression Research Articles

Time-resolved quantification of the dynamic extracellular space in the brain: study of cortical spreading depression.

Extracellular diffusion in the brain is customarily characterized by two parameters, the extracellular space (ECS) volume fraction α and the diffusion tortuosity λ. How these two parameters are temporarily modified and correlated in a physiological/pathological event remains unclear to date. Using tetramethylammonium (TMA+) as an ECS ion tracer in a newly updated iontophoretic sinusoidal method, we studied in this work the dynamic α(t) and λ(t) in rat somatosensory cortex during spreading depression (SD). Temporal variations of α(t) and λ(t), as evoked by SD, were obtained through analyses of the extracellular TMA+ diffusion waveform resulting from a sinusoidally modulated point source. Most of the time, cortical SD induced coordinated α(t) decreases and λ(t) increases. In rare occasions, SD induced sole decreases of α(t) with no changes in λ(t). The independent modulation of α(t) and λ(t) was neither associated with cortical anatomy nor with the specific shape of the SD field potential wave. Changes of α(t) and λ(t) often took place acutely at the onset of SD, followed by a more transient modulation. Compared with the prior iontophoretic methods of TMA+, the sinusoidal method provides time-resolved quantification of α(t) and λ(t) in relative terms but also raises a higher property requirement on the TMA+-selective microelectrode. The sinusoidal method could become a valuable tool in the studies of the dynamic ECS response in various brain events. NEW & NOTEWORTHY An iontophoretic sinusoidal method was applied to study the dynamic changes of two extracellular space parameters, the extracellular volume fraction α(t) and tortuosity λ(t), in the brain during cortical spreading depression. Both parameters showed coordinated (most often) and independent (rarely) modulations in spreading depression. The sinusoidal method is equally applicable to other acute pathological events and a valuable tool to study the functional role of extracellular space in brain events.

Cell swelling increases the severity of spreading depression in Locusta migratoria.

Progressive accumulation of extracellular potassium ions can trigger propagating waves of spreading depression (SD), which are associated with dramatic increases in extracellular potassium levels ([K(+)]o) and arrest in neural activity. In the central nervous system the restricted nature of the extracellular compartment creates an environment that is vulnerable to disturbances in ionic homeostasis. Here we investigate how changes in the size of the extracellular space induced by alterations in extracellular osmolarity affect locust SD. We found that hypotonic exposure increased susceptibility to experimentally induced SD evidenced by a decrease in the latency to onset and period between individual events. Hypertonic exposure was observed to delay the onset of SD or prevent the occurrence altogether. Additionally, the magnitude of extracellular K(+) concentration ([K(+)]o) disturbance during individual SD events was significantly greater and they were observed to propagate more quickly under hypotonic conditions compared with hypertonic conditions. Our results are consistent with a conclusion that hypotonic exposure reduced the size of the extracellular compartment by causing cell swelling and thus facilitated the accumulation of K(+) ions. Lastly, we found that pharmacologically reducing the accumulation of extracellular K(+) using the K(+) channel blocker tetraethylammonium slowed the rate of SD propagation while increasing [K(+)]o through inhibition of the Na-K-2Cl cotransporter increased propagation rates. Overall our findings indicate that treatments or conditions that act to reduce the accumulation of extracellular K(+) help to protect against the development of SD and attenuate the spread of ionic disturbance adding to the evidence that diffusion of K(+) is a leading event during locust SD.

Contribution of astrocyte glycogen stores to progression of spreading depression and related events in hippocampal slices

Spreading depression (SD) is a wave of coordinated cellular depolarization that propagates slowly throughout brain tissue. SD has been associated with migraine aura, and related events have been implicated in the enlargement of some brain injuries. Selective disruption of astrocyte oxidative metabolism has previously been shown to increase the propagation rate of SD in vivo, but it is currently unknown whether astrocyte glycogen stores make significant contributions to the onset or propagation of SD. We examined SD in acutely-prepared murine hippocampal slices, using either localized microinjections of KCl or oxygen and glucose deprivation (OGD) as stimuli. A combination of glycogenolysis inhibitors 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) and 1-deoxynojirimycin (DNJ) increased the propagation rates of both high K(+)-SD and OGD-SD. Consistent with these observations, exposure to l-methionine-dl-sulfoximine (MSO) increased slice glycogen levels and decreased OGD-SD propagation rates. Effects of glycogen depletion were matched by selective inhibition of astrocyte tricarboxylic acid (TCA) cycle activity by fluoroacetate (FA). Prolonged exposure to reduced extracellular glucose (2 mM) has been suggested to deplete slice glycogen stores, but significant modification SD of propagation rate was not observed with this treatment. Furthermore, decreases in OGD-SD latency with this preexposure paradigm appeared to be due to depletion of glucose, rather than glycogen availability. These results suggest that astrocyte glycogen stores contribute to delaying the advancing wavefront of SD, including during the severe metabolic challenge of OGD. Approaches to enhance astrocyte glycogen reserves could be beneficial for delaying or preventing SD in some pathologic conditions.

Optical Current Source Density Analysis in Hippocampal Organotypic Culture Shows That Spreading Depression Occurs with Uniquely Reversing Currents

Spreading depression (SD) involves current flow through principal neurons, but the pattern of current flow over the expanse of susceptible tissues or individual principal neurons remains undefined. Accordingly, tissue and single cell maps made from digital imaging of voltage-sensitive dye changes in hippocampal organotypic cultures undergoing SD were processed via optical current source density analysis to reveal the currents associated with pyramidal neurons. Two distinctive current flow patterns were seen. The first was a trilaminar pattern (420 microm2) that developed with the onset of SD in CA3 pyramidal neurons, in which SD most often began. This initial pattern comprised a somatic current sink with current sources to either side in the dendrites that lasted for seconds extending into the first aspect of the classical "inverted saddle" interstitial direct current waveform of SD. Next, the somatic sink backpropagated at a speed of millimeters per minute into the proximal dendrites, resulting in a reversal of the initial current flow pattern to its second orientation, namely dendritic sinks associated with a somatic source. The latter persisted for the remainder of SD in CA3 and was the only pattern seen in CA1, in which SD was rarely initiated. This backpropagating SD current flow resembles that of activity-dependent synaptic activation. Retrograde and associative signaling via principal neuron current flow is a key means to affect tissue function, including synaptic activation and, by extension, perhaps SD. Such current-related postsynaptic signaling might not only help explain SD but also neuroprotection and migraine, two phenomena increasingly recognized as being related to SD.

P/Q Ca2+ channel blockade stops spreading depression and related pyramidal neuronal Ca2+ rise in hippocampal organ culture.

Ca2+ channels and pyramidal cell Ca2+ are involved in hippocampal spreading depression (SD), but their roles remain elusive. Accordingly, we characterized Ca2+ changes during SD in CA3 pyramidal neurons and determined whether Ca2+ channel antagonists could prevent SD. SD was induced in hippocampal organotypic cultures (HOTCs), in which experimental conditions can be rigorously controlled. SD was triggered by transient exposure to sodium acetate (NaAc)-based Ringer's coupled to an electrical pulse in the dentate gyrus and its occurrence confirmed with interstitial DC recordings. Pyramidal cell Ca2+ was measured with fura-2 filled cells and was quantified at the soma, proximal and more distal apical dendrites. Regional Ca2+ changes began simultaneously with the triggering pulse of SD and reached three distinct peaks before returning to baseline concomitant with the interstitial DC potential of SD. The first peak occurred within 5 s of the triggering pulse, was smallest, and heralded the onset of SD. The second Ca2+ change was the greatest and reached a peak 6 s later, during the early phase of SD. The third was intermediate in size and occurred 18 s later, as SD reached its maximum interstitial DC change. SD was prevented by nonselective Ca2+ blockade (Ni2+ and Cd2+) but not by either L-Ca2+ channel (nifedipine) or N-Ca2+ channel inhibition (omega-conotoxin GVIA). Importantly, SD was blocked by P/Q Ca2+ channel antagonism (omega-agatoxin-IVA), which also prompted a significant reduction in pyramidal cell Ca2+ change and hyperexcitability. These results show that the spatiotemporal pattern of pyramidal cell Ca2+ change with SD is multiphasic; they provide further evidence that these changes begin before electrophysiologic evidence of SD. Furthermore, they show that P/Q Ca2+ channel antagonism can prevent SD in HOTCs and it appears to do so by preventing the NaAc-induced increased pyramidal cell excitability from NaAc exposure, which may involve altered GABAergic transmission.

Conditions for the Triggering of Spreading Depression Studied With Computer Simulations

In spite of five decades of study, the biophysics of spreading depression (SD) is incompletely understood. Earlier we have modeled seizures and SD, and we have shown that currents through ion channels normally present in neuron membranes can generate SD-like depolarization. In the present study, we define the conditions for triggering SD and the parameters that influence its course in a model of a hippocampal pyramidal cell with more complete representation of ions and channels than the previous version. "Leak" conductances for Na(+), K(+), and Cl(-) and an ion pump were present in the membrane of the entire cell; fast inactivating voltage dependent conductances for sodium and potassium in the soma; "persistent" conductances in soma and apical dendrite, and K(+)- and voltage-dependent N-methyl-D-aspartate (NMDA)-controlled conductance in the apical dendrite. The neuron was surrounded by restricted interstitial space and by a "glia-endothelium" system of extracellular ion regulation bounded by a membrane having leak conductances and an ion pump. Ion fluxes and concentration changes were continuously computed as well as osmotic cell volume changes. As long as reuptake into the neuron and "buffering" by glia kept pace with K(+) released from the neuron, stimulating current applied to the soma evoked repetitive firing that stopped when stimulation ceased. When glial uptake was reduced, K(+) released from neurons could accumulate in the interstitium and keep the neuron depolarized so that strong depolarizing pulses injected into the soma were followed either by afterdischarge or SD. SD-like depolarization was ignited when depolarization spreading into the apical dendrite, activated persistent Na(+) current and NMDA-controlled current. With membrane parameters constant, varying the injected stimulating current influenced SD onset but neither the depolarization nor the increase in extracellular K(+). Glial "leak" conductance influenced SD duration and SD ignition point. Varying maximal conductances (representing channel density) also influenced SD onset time but not the amplitude of the depolarization. Hypoxia was simulated by turning off the Na-K exchange pump, and this resulted in SD-like depolarization. The results confirm that, once ignited, SD runs an all-or-none trajectory, the level of depolarization is governed by feedback involving ion shifts and glutamate acting on ion channels and not by the number of channels open, and SD is ignited if the net persistent membrane current in the apical dendrites turns inward.

Na(+) dependence and the role of glutamate receptors and Na(+) channels in ion fluxes during hypoxia of rat hippocampal slices.

Spreading depression (SD) as well as hypoxia-induced SD-like depolarization in forebrain gray matter are characterized by near complete depolarization of neurons. The biophysical mechanism of the depolarization is not known. Earlier we reported that simultaneous pharmacological blockade of all known major Na(+) and Ca(2+) channels prevents hypoxic SD. We now recorded extracellular voltage, Na(+), and K(+) concentrations and the intracellular potential of individual CA1 pyramidal neurons during hypoxia of rat hippocampal tissue slices after substituting Na(+) in the bath by an impermeant cation, or in the presence of channel blocking drugs applied individually and in combination. Reducing extracellular Na(+) concentration [Na(+)](o) to 90 mM postponed the hypoxia-induced extracellular DC-potential deflection (DeltaV(o)) and reduced its amplitude, and it also postponed the SD-like depolarization of neurons. After lowering [Na(+)](o) to 25 mM, SD-like DeltaV(o) became very small, indicating that an influx of Na(+) is required for SD; influx of Ca(2+) ions alone is not sufficient. We then asked whether the SD-related Na(+) current flows through glutamate-controlled and/or through voltage-gated Na(+) channels. Administration of either the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) postponed the hypoxic DeltaV(o) and depressed its amplitude but the effect of the combined administration of these two drugs was not greater than that of either alone. During the early phase of hypoxia, before SD onset, [K(+)](o) increased faster and reached a much higher level in the presence of glutamate antagonists than in their absence. The [K(+)](o) level reached at the height of hypoxic SD was, however, not affected. When TTX was added to DNQX and CPP, SD was prevented in half the trials. When SD did occur, it was greatly delayed, yet eventually neurons depolarized to the same extent as in normal solution. The SD-related sudden drop in [Na(+)](o) was depressed by only 19% in the presence of the three drugs, indicating that Na(+) can flow into cells through pathways other than ionotropic glutamate receptors and TTX-sensitive Na(+) channels. We conclude that, when they are functional, glutamate-receptor-mediated and voltage-gated Na(+) currents are the major generators of the self-regenerative rapid depolarization, but in their absence other pathways can sometimes take their place. The final level of SD-like depolarization is determined by positive feedback and not by the number of channels available. A schematic flow chart of the events generating hypoxic SD is discussed.

Light scattering in brain slices measured with a photon counting fiber optic system

Measurements of intrinsic optical signals (IOSs) from neural tissue, commonly with a reflection-type or transmittance-type set-up, have been used increasingly to study physiological events. Even for the same event, however, such as spreading depression (SD) or osmotic challenge, signals of opposite polarities (increase or decrease) have been obtained by different investigators using similar set-ups under similar conditions. The origin of the inconsistencies is still unknown. It is suggested here that the inconsistencies may be caused by artifacts associated with tissue surface scattering. The main goal of this paper was to present a photon counting fiber optic (PCFO) system designed to exclude surface artifacts and predominantly measure the light scattering (LS) within the tissue. Experiments on rat neocortical slices under osmotic challenges demonstrated the consistency of the PCFO data: hypertonic challenge always increased LS signal while hypotonic challenge decreased it, as long as the challenge did not induce SD. Under strong osmotic challenge (−100 mOsm), the signal suddenly reversed the polarity at the onset of SD induced by the challenge and continued to increase until the challenge was removed. When SD was blocked by high [Mg 2+] o, the LS signal remained decreased during the −100 mOsm challenge. A spectroscopic study with the PCFO system showed that the spectrum of tissue scattering was almost a flat function in most of the visible range (650–470 nm) with a maximum at 500 nm and a rapid drop at 450 nm.

Na+ and K+ Concentrations, Extra- and Intracellular Voltages, and the Effect of TTX in Hypoxic Rat Hippocampal Slices

Severe hypoxia causes rapid depolarization of CA1 neurons and glial cells that resembles spreading depression (SD). In brain slices in vitro, the SD-like depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na(+) currents by tetrodotoxin (TTX). To investigate the role of Na(+) flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measured membrane changes in single CA1 pyramidal neurons simultaneously with extracellular DC potential (V(o)) and either extracellular [K(+)] or [Na(+)]; alternatively, we simultaneously recorded [Na(+)](o), [K(+)](o), and V(o). Confirming previous reports, early during hypoxia, before SD onset, [K(+)](o) began to rise, whereas [Na(+)](o) still remained normal and V(o) showed a slight, gradual, negative shift; neurons first hyperpolarized and then began to gradually depolarize. The SD-like abrupt negative DeltaV(o) corresponded to a near complete depolarization of pyramidal neurons and an 89% decrease in input resistance. [K(+)](o) increased by 47 mM and [Na(+)](o) dropped by 91 mM. Changes in intracellular Na(+) and K(+) concentrations, estimated on the basis of the measured extracellular ion levels and the relative volume fractions of the neuronal, glial, and extracellular compartment, were much more moderate. Because [Na(+)](o) dropped more than [K(+)](o) increased, simple exchange of Na(+) for K(+) cannot account for these ionic changes. The apparent imbalance of charge could be made up by Cl(-) influx into neurons paralleling Na(+) flux and release of Mg(2+) from cells. The hypoxia-induced changes in interneurons resembled those observed in pyramidal neurons. Astrocytes responded with an initial slow depolarization as [K(+)](o) rose. It was followed by a rapid but incomplete depolarization as soon as SD occurred, which could be accounted for by the reduced ratio, [K(+)](i)/[K(+)](o). TTX (1 microM) markedly postponed SD, but the SD-related changes in [K(+)](o) and [Na(+)](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like depolarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from control. We conclude that TTX-sensitive channels mediate only a fraction of the Na(+) influx, and that some of the K(+) is released in exchange for Na(+). Even though TTX-sensitive Na(+) currents are not essential for the self-regenerative membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons, thereby synchronizing the SD-like event.

Glutamate Release through Volume-Activated Channels during Spreading Depression

Volume-sensitive organic anion channels (VSOACs) in astrocytes are activated by cell swelling and are permeable to organic anions, such as glutamate and taurine. We have examined the release of glutamate through VSOACs during the propagation of spreading depression (SD). SD was induced by bath application of ouabain in hippocampal brain slices and was monitored by imaging intrinsic optical signals, a technique that provides a measure of cellular swelling. The onset of SD was associated with increased light transmittance, confirming previous studies that cellular swelling occurs during SD. NMDA receptor antagonists, either noncompetitive (MK-801, 10-50 microM) or competitive (CGS-17355, 100 microM), reduced the rate of propagation of SD, indicating that glutamate release contributes to SD onset. SD still occurred in zero Ca(2+)-EGTA (0-Ca(2+)-EGTA) solution, a manipulation that depresses synaptic transmission. HPLC measurements indicated that, even in this solution, there was significant glutamate release. Two lines of experiments indicated that glutamate was released through VSOACs during SD. First, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a blocker of VSOACs, depressed the rate of propagation of SD in a manner similar to NMDA antagonists. Second, NPPB inhibited the release of glutamate during SD in 0-Ca(2+)-EGTA external solution. These results indicate that cellular swelling during SD causes the activation of VSOACs and the release of glutamate by permeation through this channel. Cellular swelling is a result of neuronal activity and is observed during excitotoxicity. Therefore, glutamate release from VSOAC activation could occur under conditions of cell swelling and contribute to excitotoxic damage.

Endogenous pH shifts facilitate spreading depression by effect on NMDA receptors.

Rapid extracellular alkalinizations accompany normal neuronal activity and have been implicated in the modulation of N-methyl-D-aspartate (NMDA) receptors. Particularly large alkaline transients also occur at the onset of spreading depression (SD). To test whether these endogenous pH shifts can modulate SD, the alkaline shift was amplified using benzolamide, a poorly permeant inhibitor of interstitial carbonic anhydrase. SD was evoked by microinjection of 1.2 M KCl into the CA1 stratum radiatum of rat hippocampal slices and recorded by a proximal double-barreled pH microelectrode and a distal potential electrode. In Ringer solution of pH 7.1 containing picrotoxin (but not at a bath pH of 7.4), addition of 10 microM benzolamide increased the SD alkaline shift from 0.20 +/- 0.07 to 0.38 +/- 0.17 unit pH (means +/- SE). This was correlated with a significant shortening of the latency and an increase in the conduction velocity by 26 +/- 16%. In the presence of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), benzolamide still amplified the alkaline transient, however, its effect on the SD latency and propagation velocity was abolished. The intrinsic modulation of SD by its alkaline transient may play an important role under focal ischemic conditions by removing the proton block of NMDA receptors where interstitial acidosis would otherwise limit NMDA receptor activity.

Propagation of spreading depression among dendrites and somata of the same cell population

The propagation of sustained potential shifts associated with spreading depression (SD) was studied by microelectrodes placed in diverse layers at different locations in hippocampus of anesthetized rats. SD was induced by raising interstitial potassium concentration ([K +] 0) focally in the CA1 region of the dorsal hippocampus either by microdialysis or by microinjection. Recurrent waves of SD propagated from the dialysis site throughout the hippocampus. Potential shifts (ΔV 0) associated with SD usually began earlier and were always of larger amplitude and longer duration in stratum (st.) radiatum (layer of apical dendrites) than in st. pyramidale (layer of pyramidal cell somata). The velocity of propagation in the two layers differed and varied independently one from the other. When SD was provoked by orthodromic train stimuli, the apparent direction of propagation in st. pyramidale was opposite that in st. radiatum. Microinjection of high K + solution was more likely to provoke SD when placed in the st. radiatum, 50–100 μm ventral to st. pyramidale, than in other cytoarchitectonic layers. In about half the trials after 30 to 90 min of high K + dialysis, a prolonged depressed state developed during which the potential in st. radiatum shifted at irregular intervals between near-rest level and a strongly negative level, while ΔV 0 shifts in st. pyramidale were smaller and more irregular in amplitude. This state in termed prolonged unstable SD/2'. When the NMDA receptor antagonist CPP was dialyzed together with high K +, the onset of SD was postponed and ΔV 0 waves propagated in st. pyramidale without corresponding waves in st. radiatum; less frequently the other way around. It is concluded that the layer containing proximal apical dendrites is more prone to SD than the layers containing cell somata, basilar dendrites, or the distal apical dendritic branches. The SD process can occur semi-independently in different regions of the same cells. SD waves can propagate among processes of cells and probably also from one region to another within the same cells.

Role of calcium channels in spreading depression in rat hippocampal slices

In the CA1 region of rat hippocampal slices, spreading depression (SD) was provoked by a brief period of hypoxia or by localized application of high potassium solution. We measured extracellular DC voltage ( V o), extracellular potassium concentration ([K +] o) and/or extracellular Ca 2+ concentration ([Ca 2+]0). SD was provoked under control conditions and also when voltage-gated Ca 2+ channels were blocked by application of 2 mM Ni 2+ or Co 2+. In some experiments, CPP, DNQX, or the two together were also applied to block glutamate receptor-coupled channels. When SD was provoked by hypoxia, these treatments significantly increased the latency of SD onset and decreased the amplitudes of the accompanying δ V o δ[Ca 2+] o. Hypoxia-induced SD was never blocked completely, however and δ[Ca 2+] o was reduced at most by 50%. When SD was provoked by application of high K + solution near the recording site, Ni 2+ or Co 2+ partially suppressed the V o and [Ca 2+] o shifts but did not block SD altogether. When high K + solution was applied at a distance, Ni 2+ or Co 2+ blocked the propagation of SD to the recording site. We conclude that during SD, a significant proportion of the calcium ions flowing into neurons does not pass through voltage-gated or glutamate receptor-linked channels.

Lactate and pH change in close correlation in the extracellular space of the rat brain during cortical spreading depression.

pH sensitive microcelectrodes were used in combination with microdialysis (MD) technique to measure extracellular pH (pHe) and extracellular lactate (lace) within the cortex of rat brains during cortical spreading depression (SD). SD was induced by local K(+)-application and identified by DC recordings. It was accompanied by an extracellular acidification of 0.34 +/- 0.06 pH units and by a 2.8 +/- 0.80 fold increase of lace; the recovery of pHe took place within three phases, that of lace within 2 phases. The recovery of both parameters was complete about 45 min after the onset of SD. We conclude that the changes of lace and pHe are closely related. This indicates both lactate and protons to be transported in parallel.

Role of Na-K pump potassium regulation and IPSPs in seizures and spreading depression in immature rabbit hippocampal slices

1. Using the immature (8-12 days postnatal) rabbit hippocampal slice preparation, we investigated regional extracellular potassium concentration [( K+]o) changes that occur during spontaneous and evoked spreading depression (SD) episodes. We report here a difference between the CA1 and CA3 cell populations in the immature hippocampus with regard to 1) resting [K+]o, 2) magnitude of the [K+]o change during seizurelike events and SDs, and 3) susceptibility to SD episodes. Experiments were also performed to elucidate the roles that the Na-K pump and synaptic inhibition play in controlling SD onset, duration, and recovery. We demonstrated a major role for potassium regulation by the Na-K pump and a lesser modulatory role for inhibitory postsynaptic potentials (IPSPs) in preventing SD in the CA3 region. 2. Simultaneous intra- and extracellular recordings were made in the CA1 and CA3 regions of the immature rabbit hippocampus during spontaneous or evoked SD, while potassium ion-sensitive microelectrodes (K-ISMs) monitored changes in [K+]o. The CA1 region had 1) a higher frequency of spontaneous SD episodes than CA3, 2) a lower threshold to potassium-triggered SD, 3) a longer duration SD episode, and 4) smaller post-SD membrane potential and [K+]o undershoots (below the original resting membrane potential and resting [K+]o). 3. During the onset of a SD episode in the CA1 region, the local [K+]o rose either before or at the same time as the membrane potential depolarization. 4. In the CA3 region, spontaneous ictallike events consisting of tonic cell depolarization with repetitive activity followed by clonic afterdischarges were more likely to occur than SD episodes. During these ictallike episodes, [K+]o rose above the 10- to 12-mM ceiling level reported for adult CNS tissue during seizures. Increases in [K+]o evoked by repetitive stimulation were regulated at a lower level in CA3 (average [K+]o rise to 11.4 mM) than in CA1 (average [K+]o rise to 18.3 mM). 5. In CA3, bath application of 10 microM bicuculline or 3.4 mM penicillin did not change the frequency of spontaneously occurring SDs or the SD response threshold to local pressure ejection of 2 M KCl. However, blockade of IPSPs did lead to lower thresholds for SD or seizurelike episodes elicited by stimulation of the mossy fibers. 6. A single application of ouabain (10 microM) to CA3 by local pressure ejection caused a slow rise in local [K+]o measured with K-ISMs. The ouabain treatment also increased the frequency of spontaneous postsynaptic potential activity and decreased the amplitude and duration of CA3 pyramidal cell afterhyperpolarizations (AHPs).(ABSTRACT TRUNCATED AT 400 WORDS)

Quisqualate, kainate and NMDA can initiate spreading depression in the turtle cerebellum

This study evaluated the role of excitatory amino acid (EAA) receptor activation in spreading depression (SD), using the in vitro turtle cerebellum as a model system. SD was triggered by electrical stimulation or by elevated K + after the cerebellum had been conditioned for at least 30 min with physiological saline in which most of the chloride had been replaced by propionate. SD was recognized as a transient (1–3 min) negative shift of extracellular potential accompanied by depression of evoked potentials (15–30 min) and an increase of extracellular K + up to 60 mM, which spread across the cerebellum at rates of 1–7 mm/min. SD usually commenced in the granular layer, which apparently contains the 3 major EAA receptor subtypes, quisqualate, kainate and N-methyl- d-aspartate (NMDA), then subsequently spread to the molecular layer, which is largely free of NMDA receptors. Glutamate, aspartate, NMDA, kainate and quisqualate all triggered SD. Kynurenic acid and 2-aminophosphonovaleric acid (APV) inhibited SD under certain conditions further suggesting involvement of EAA receptors. The initiation of SD was blocked by high Mg 2+ and facilitated in low extracellular Mg 2+, which also eliminated the delay in molecular layer SD onset. Our data suggest that no one EAA receptor subtype is singly responsible for SD.

Transient alkaline shift in cortical tissue pH during the onset of spreading depression and of anoxic negative DC-potential shift

Transient alkaline shift in cortical tissue pH during the onset of spreading depression and of anoxic negative DC-potential shift

Spreading depression in elasmobranch cerebellum

Spreading depression (SD) occurs in the cerebellum of an elasmobranch fish, the skate (Raja erinacea, Raja ocellata). The elasmobranch cerebellum, because of its unique separation of granular form molecular layer, provides an excellent opportunity to study the characteristics of SD in two distinct neuronal populations. Both the DC potential shifts and changes in neuronal activity effected by SD were analyzed. The SD DC potential shifts in both layers closely resembled those in mammalian cerebral cortices. Consisting of a predominantly negative extracellular potential shift, they were typically 1--10 min in duration and reached 5--40 mV peak amplitudes. The largest negative shifts were found in the granular layer, without any consistent positive phases in the white matter, molecular, or granular layers. The SD propagated radially from surface electrical stimulation at 0.78 mm/min (+/- 0.16, n = 8) in the molecular layer and 0.43 mm/min (+/- 0.17, n = 8) in the granular layer at 15 degrees C. At 18 degrees C, the molecular layer propagatory velocity was 1.1 mm/min (+/- 0.12, n = 20) while, at 10 degrees C, it was 0.52 mm/min (+/- 0.21, n = 20), suggesting a temperature-dependent Q10 factor of 2. A profound depression of both spontaneous and evoked neuronal activity accompanied the DC potential shift. Activation of Purkinje cells antidromically, white matter, and granular layer neurons was typically abolished by the peak of the negative DC shift. However, a significant increase in granular layer excitability often followed the neuronal depression, remaining so far up to an hour. Repeated waves of SD sometimes occurred in the absence of neuronal recovery. A similar post-SD excitability increase was not seen in molecular layer neurons. Intracellular recordings from Purkinje cells revealed a spontaneous burst of action potentials at the onset of SD, closely followed by a depolarization of membrane potential from an average of -64 mV (+/- 12 mV, n = 3) to -11 mV (+/- 5 mV, n = 3).