Effect of cortical spreading depression on synaptic transmission of rat hippocampal tissues

Spreading Depolarization Induces a Transient Potentiation of Excitatory Synaptic Transmission

Spreading depolarization (SD) is a slowly propagating wave of prolonged activation followed by a period of synaptic suppression. Some prior reports have shown potentiation of synaptic transmission after recovery from synaptic suppression and noted similarities with the phenomenon of long-term potentiation (LTP). Since SD is increasingly recognized as participating in diverse neurological disorders, it is of interest to determine whether SD indeed leads to a generalized and sustained long-term strengthening of synaptic connections. We performed a characterization of SD-induced potentiation, and tested whether distinctive features of SD, including adenosine accumulation and swelling, contribute to reports of SD-induced plasticity. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the hippocampal CA1 subregion of murine brain slices, and SD elicited using focal microinjection of KCl. A single SD was sufficient to induce a consistent potentiation of slope and amplitude of fEPSPs. Both AMPA- and NMDA-receptor mediated components were enhanced. Potentiation peaked ∼20 min after SD recovery and was sustained for ∼30 min. However, fEPSP amplitude and slope decayed over an extended 2-hour recording period and was estimated to reach baseline after ∼3 h. Potentiation was saturated after a single SD and adenosine A1 receptor activation did not mask additional potentiation. Induction of LTP with theta-burst stimulation was not altered by prior induction of SD and molecular mediators known to block LTP induction did not block SD-induced potentiation. Together, these results indicate an intermediate duration potentiation that is distinct from hippocampal LTP and may have implications for circuit function for 1-2 h following SD.

Inhibition of astroglial hemichannels prevents synaptic transmission decline during spreading depression

BACKGROUND

Spreading depression (SD) is an intriguing phenomenon characterized by massive slow brain depolarizations that affect neurons and glial cells. This phenomenon is repetitive and produces a metabolic overload that increases secondary damage. However, the mechanisms associated with the initiation and propagation of SD are unknown. Multiple lines of evidence indicate that persistent and uncontrolled opening of hemichannels could participate in the pathogenesis and progression of several neurological disorders including acute brain injuries. Here, we explored the contribution of astroglial hemichannels composed of connexin-43 (Cx43) or pannexin-1 (Panx1) to SD evoked by high-K+ stimulation in brain slices.

RESULTS

Focal high-K+ stimulation rapidly evoked a wave of SD linked to increased activity of the Cx43 and Panx1 hemichannels in the brain cortex, as measured by light transmittance and dye uptake analysis, respectively. The activation of these channels occurs mainly in astrocytes but also in neurons. More importantly, the inhibition of both the Cx43 and Panx1 hemichannels completely prevented high K+-induced SD in the brain cortex. Electrophysiological recordings also revealed that Cx43 and Panx1 hemichannels critically contribute to the SD-induced decrease in synaptic transmission in the brain cortex and hippocampus.

CONCLUSIONS

Targeting Cx43 and Panx1 hemichannels could serve as a new therapeutic strategy to prevent the initiation and propagation of SD in several acute brain injuries.

Astrocytes as critical players of the fine balance between inhibition and excitation in the brain: spreading depolarization as a mechanism to curb epileptic activity

Spreading depolarizations (SD) are slow waves of complete depolarization of brain tissue followed by neuronal silencing that may play a role in seizure termination. Even though SD was first discovered in the context of epilepsy research, the link between SD and epileptic activity remains understudied. Both seizures and SD share fundamental pathophysiological features, and recent evidence highlights the frequent occurrence of SD in experimental seizure models. Human data on co-occurring seizures and SD are limited but suggestive. This mini-review addresses possible roles of SD during epileptiform activity, shedding light on SD as a potential mechanism for terminating epileptiform activity. A common denominator for many forms of epilepsy is reactive astrogliosis, a process characterized by morphological and functional changes to astrocytes. Data suggest that SD mechanisms are potentially perturbed in reactive astrogliosis and we propose that this may affect seizure pathophysiology.

Transient Global Amnesia - Risk Factors and Putative Background

Objectives

Transient global amnesia (TGA) is a temporary short-term reversible memory loss. Etiology of TGA remains unclear with various hypotheses. We analyzed clinical characteristics, neuroimaging, and electrophysiological findings as well as comorbidities and seasonal variation in TGA patients with regard to possible background of the syndrome.

Materials and Methods

A total of 56 patients (42 women and 14 men) with TGA hospitalized from 2008 to 2016 in the Department of Neurology, Wrocław Medical University.

Results

A total of 52 patients (92.9%) underwent their first-ever episode of TGA. The potential triggers or events before episode could be recognized in 22 patients (39.3%). 35.7% patients had TGA in summer and 26.8% in winter months. In 92.9% patients chronic diseases were found, included: Hypertension (60.7%), dyslipidemia (48.2%), autoimmune thyroiditis (17.9%), and ischemic heart disease (14.3%). One patient (1,8%) suffered from migraine. Doppler ultrasonography of carotid arteries revealed abnormalities in 29 patients (51.8%). Electroencephalography abnormalities were observed in 10 (17.6%) of patients.

Conclusion

Our findings suggest a putative cerebrovascular background of transient global amnesia. No evidence has been provided for the association between TGA and epilepsy or migraine. Among comorbidities, autoimmune thyroiditis deserves further investigation with regard to its potential links with TGA.

Remote and Persistent Alterations in Glutamate Receptor Subunit Composition Induced by Spreading Depolarizations in Rat Brain

Spreading depolarizations (SDs) are massive breakdowns of ion homeostasis in the brain's gray matter and are a necessary pathologic mechanism for lesion development in various injury models. However, injury-induced SDs also propagate into remote, healthy tissue where they do not cause cell death, yet their functional long-term effects are unknown. Here we induced SDs in uninjured cortex and hippocampus of Sprague-Dawley rats to study their impact on glutamate receptor subunit expression after three days. We find that both cortical and hippocampal tissue exhibit changes in glutamate receptor subunit expression, including GluA1 and GluN2B, suggesting that SDs in healthy brain tissue may have a role in plasticity. This study is the first to show prolonged effects of SDs on glutamate signaling and has implications for neuroprotection strategies aimed at SD suppression.

Effect of Nortriptyline on Spreading Depolarization

Background: Spreading depolarization is associated with the extension of lesion size and complications in some important neurological diseases such as stroke, epilepsy, migraine, and traumatic brain injury. Objectives: This study aimed to reveal some molecular aspects of spreading depolarization and suggesting new therapeutic targets for its control by changing the function of different astrocytic and neuronal ion channels. Methods: The effects of nortriptyline on spreading depolarization in cortical and hippocampal tissues and on the electrophysiological properties of CA1 hippocampal pyramidal neurons were assessed by extra- and intracellular recording, following washing rat brain slices by the drug. Results: Nortriptyline made a significant increase in the amplitude of spreading depolarization in cortical and hippocampal tissues relative to control but did not change the duration significantly in each of the tissues. No significant difference was found in the effects of spreading depolarization on the electrophysiological properties of the CA1 pyramidal neurons between nortriptyline and control groups. Conclusions: The stimulating effect of nortriptyline on spreading depolarization is probably related to the augmentation of extracellular potassium collection in the cortex and hippocampus due to inhibition of astrocytic potassium scavenging function. This change can make more neurons prone to depolarization and increase the overall amplitude of spreading depolarization waves. Further studies should assess the effect of enhancing the clearance function of astrocyte-specific inwardly rectifying potassium channels, Kir4.1, or preventing other factors contributing to spreading depolarization on control of the process.

Neuroimaging investigation of memory changes in migraine: a systematic review

BACKGROUND

Individuals with migraine usually complain about lower memory performance. Diagnostic methods such as neuroimaging may help in the understanding of possible morphologic and functional changes related to the memory of those individuals. Therefore, the aim of this review is to analyze the available literature on neuroimaging changes related to memory processing in migraine.

METHODS

We searched the following databases: Pubmed/Medline, Psycinfo, Science Direct, Cochrane and Web of Science. We used articles without restriction of year of publication. The combination of descriptors used for this systematic review of literature were Neuroimaging OR Imaging OR Brain AND Migraine OR Chronic Migraine AND Memory.

RESULTS

Of the 306 articles found, nine were selected and all used magnetic resonance imaging (MRI). The studies used structural and functional MRI techniques with a predominance of 3 Tesla equipment and T1-weighted images. According to the results obtained reported by these studies, migraine would alter the activity of memory-related structures, such as the hippocampus, insula and frontal, parietal and temporal cortices, thereby suggesting a possible mechanism by which migraine would influence memory, especially in relation to the memory of pain.

CONCLUSIONS

Migraine is associated to global dysfunction of multisensory integration and memory processing. This condition changes the activity of structures in various regions related to memory of pain, prospective memory, as well as in short- and long-term verbal and visuospatial memories. However, it is necessary to perform studies with larger samples in association with cognitive tests, and without the interference of medications to verify possible alterations and to draw more concrete conclusions.

Cortical spreading depolarization increases adult neurogenesis, and alters behavior and hippocampus-dependent memory in mice

Cortical spreading depolarizations are an epiphenomenon of human brain pathologies and associated with extensive but transient changes in ion homeostasis, metabolism, and blood flow. Previously, we have shown that cortical spreading depolarization have long-lasting consequences on the brains transcriptome and structure. In particular, we found that cortical spreading depolarization stimulate hippocampal cell proliferation resulting in a sustained increase in adult neurogenesis. Since the hippocampus is responsible for explicit memory and adult-born dentate granule neurons contribute to this function, cortical spreading depolarization might influence hippocampus-dependent cognition. To address this question, we induced cortical spreading depolarization in C57Bl/6 J mice by epidural application of 1.5 mol/L KCl and evaluated neurogenesis and behavior at two, four, or six weeks thereafter. Congruent with our previous findings in rats, we found that cortical spreading depolarization increases numbers of newborn dentate granule neurons. Moreover, exploratory behavior and object location memory were consistently enhanced. Reference memory in the water maze was virtually unaffected, whereas memory formation in the Barnes maze was impaired with a delay of two weeks and facilitated after four weeks. These data show that cortical spreading depolarization produces lasting changes in psychomotor behavior and complex, delay- and task-dependent changes in spatial memory, and suggest that cortical spreading depolarization-like events affect the emotional and cognitive outcomes of associated brain pathologies.

GluN2A/B ratio elevation induced by cortical spreading depression: electrophysiological and quantitative studies of the hippocampus

Cortical spreading depression (CSD), an underlying mechanism of migraine aura, propagates to the hippocampus, and might explain hippocampusassociated symptoms during migraine attack. We hypothesised that this process is, some parts, mediated by NMDA receptors. By using a rat model, CSD was elicited by solid KCl for 45 minutes prior to electrophysiological and quantitative analyses. The result from electrophysiological study was the ratio of glutamate NMDA receptor 2A and 2B subunits (GluN2A/B). Total NMDA receptor response was isolated using an AMPA antagonist, prior to a GluN2B receptor antagonist. The GluN2A/B ratio was calculated by dividing the remaining NMDA-mediated field-excitatory synaptic potentials (fEPSP) with the subtracted difference of NMDAmediated fEPSP. Western blot analysis of the hippocampus was performed to confirm the quantitative change of GluN2A/B ratio. In hippocampal slice study (n = 12), the GluN2A/B ratio of hippocampal fEPSP was significantly increased in CSD group. Western blot analysis (n = 30) revealed an increase in GluN2A subunits and a decrease in GluN2B subunits in the hippocampus ipsilateral to the CSD induction. Our current study revealed that GluN2A/B ratio was shown to be elevated following CSD stimulation by increasing the total number of GluN2A while reducing the total number of GluN2B subunits. This ratio was demonstrated to be associated with synaptic plasticity of the hippocampus in numerous studies. In conclusion, we showed that CSD increased GluN2A/B ratio, in turn, would result in altered synaptic plasticity. Our findings provide a probable implication on the correlation of migraine aura and hippocampusassociated symptoms.

Vascular Ischemia as a Cause of Transient Global Amnesia: A Patient Series

INTRODUCTION

Epileptic, migrainous, and vascular pathologies may cause transient global amnesia (TGA); however, the mechanism of causation remains unclear. We investigated possible vascular causes of TGA.

METHODS

We retrospectively evaluated the clinical and radiologic studies of 13 patients with TGA. On admission, patients underwent diffusion-weighted imaging (DWI) and intra- and extracranial magnetic resonance angiography (MRA); vascular risk factor profiles for diabetes, hypertension, and hyperlipidemia; electroencephalography; and neuropsychological tests. Seven patients underwent control DWIs 24 h after symptom onset.

RESULTS

One patient had two punctiform acute infarcts in the left hippocampus, and one had a left pontine paramedian acute infarct. In the second patient, control DWI showed additional left hippocampal and right frontal acute infarcts. None of the patients had electroencephalographic evidence of epileptic activity. All patients except for one had at least one vascular risk factor. The second patient was shown to have paroxsysmal atrial fibrillation during follow-up.

CONCLUSION

Minor posterior circulation ischemic stroke appears to cause TGA in some patients. Evaluations such as DWI and vascular risk factor assessment may be helpful in making the diagnosis.

Immunomodulatory Effect of Toll-Like Receptor-3 Ligand Poly I:C on Cortical Spreading Depression

The release of inflammatory mediators following cortical spreading depression (CSD) is suggested to play a role in pathophysiology of CSD-related neurological disorders. Toll-like receptors (TLR) are master regulators of innate immune function and involved in the activation of inflammatory responses in the brain. TLR3 agonist poly I:C exerts anti-inflammatory effect and prevents cell injury in the brain. The aim of the present study was to examine the effect of systemic administration of poly I:C on the release of cytokines (TNF-α, IFN-γ, IL-4, TGF-β1, and GM-CSF) in the brain and spleen, splenic lymphocyte proliferation, expression of GAD65, GABAAα, GABAAβ as well as Hsp70, and production of dark neurons after induction of repetitive CSD in juvenile rats. Poly I:C significantly attenuated CSD-induced production of TNF-α and IFN-γ in the brain as well as TNF-α and IL-4 in the spleen. Poly I:C did not affect enhancement of splenic lymphocyte proliferation after CSD. Administration of poly I:C increased expression of GABAAα, GABAAβ as well as Hsp70 and decreased expression of GAD65 in the entorhinal cortex compared to CSD-treated tissues. In addition, poly I:C significantly prevented production of CSD-induced dark neurons. The data indicate neuroprotective and anti-inflammatory effects of TLR3 activation on CSD-induced neuroinflammation. Targeting TLR3 may provide a novel strategy for developing new treatments for CSD-related neurological disorders.

Propagation of cortical spreading depression into the hippocampus: The role of the entorhinal cortex

Propagation of cortical spreading depression (CSD) to the subcortical structures could be the underlying mechanism of some neurological deficits in migraine with aura. The entorhinal cortex (EC) as a gray matter bridge between the neocortex and subcortical regions plays an important role in this propagation. In vitro combined neocortex-hippocampus brain slices were used to study the propagation pattern of CSD between the neocortex and the hippocampus. The effects of different compounds as well as tetanic electrical stimulations in the EC on propagation of CSD to the hippocampus were investigated. Repetitive induction of CSD by KCl injection in the somatosensory cortex enhanced the probability of CSD entrance to the hippocampus via EC. Local application of AMPA receptor blocker CNQX and cannabinoid receptor agonist WIN 55212-2 in EC facilitated the propagation of CSD to the hippocampus, whereas application of NMDA receptor blocker APV and GABA_A receptor blocker bicuculline in this region reduced the probability of CSD penetration to the hippocampus. Application of tetanic stimulation in EC also facilitated the propagation of CSD entrance to the hippocampus. Our data suggest the importance of synaptic plasticity of EC in filtering the propagation of CSD into subcortical structures and possibly the occurrence of concomitant neurological deficits. Synapse 68:574-584, 2014. © 2014 Wiley Periodicals, Inc.

Molecular alterations in the hippocampus after experimental subarachnoid hemorrhage

Patients with aneurysmal subarachnoid hemorrhage (SAH) frequently have deficits in learning and memory that may or may not be associated with detectable brain lesions. We examined mediators of long-term potentiation after SAH in rats to determine what processes might be involved. There was a reduction in synapses in the dendritic layer of the CA1 region on transmission electron microscopy as well as reduced colocalization of microtubule-associated protein 2 (MAP2) and synaptophysin. Immunohistochemistry showed reduced staining for GluR1 and calmodulin kinase 2 and increased staining for GluR2. Myelin basic protein staining was decreased as well. There was no detectable neuronal injury by Fluoro-Jade B, TUNEL, or activated caspase-3 staining. Vasospasm of the large arteries of the circle of Willis was mild to moderate in severity. Nitric oxide was increased and superoxide anion radical was decreased in hippocampal tissue. Cerebral blood flow, measured by magnetic resonance imaging, and cerebral glucose metabolism, measured by positron emission tomography, were no different in SAH compared with control groups. The results suggest that the etiology of loss of LTP after SAH is not cerebral ischemia but may be mediated by effects of subarachnoid blood such as oxidative stress and inflammation.

Adenosine receptor activation is responsible for prolonged depression of synaptic transmission after spreading depolarization in brain slices

Spreading depolarization (SD) is a slowly propagating, coordinated depolarization of brain tissue, which is followed by a transient (5-10min) depression of synaptic activity. The mechanisms for synaptic depression after SD are incompletely understood. We examined the relative contributions of action potential failure and adenosine receptor activation to the suppression of evoked synaptic activity in murine brain slices. Focal micro-injection of potassium chloride (KCl) was used to induce SD and synaptic potentials were evoked by electrical stimulation of Schaffer collateral inputs to hippocampal area Cornu Ammonis area 1 (CA1). SD was accompanied by loss of both presynaptic action potentials (as assessed from fiber volleys) and field excitatory postsynaptic potentials (fEPSPs). Fiber volleys recovered rapidly upon neutralization of the extracellular direct current (DC) potential, whereas fEPSPs underwent a secondary suppression phase lasting several minutes. Paired-pulse ratio was elevated during the secondary suppression period, consistent with a presynaptic mechanism of synaptic depression. A transient increase in extracellular adenosine concentration was detected during the period of secondary suppression. Antagonists of adenosine A1 receptors (8-cyclopentyl-1,3-dipropylxanthine [DPCPX] or 8-cyclopentyl-1,3-dimethylxanthine [8-CPT]) greatly accelerated fEPSP recovery and abolished increases in paired-pulse ratio normally observed after SD. The duration of fEPSP suppression was correlated with both the duration of the DC shift and the area of tissue depolarized, consistent with the model that adenosine accumulates in proportion to the metabolic burden of SD. These results suggest that in brain slices, the duration of the DC shift approximately defined the period of action potential failure, but the secondary depression of evoked responses was in large part due to endogenous adenosine accumulation after SD.

The effects of tetanic stimulation on plasticity of remote synapses in the hippocampus-perirhinal cortex-amygdala network

In the temporal lobe, multiple synaptic pathways reciprocally link different structures. These multiple pathways play an important role in the integrity of the function of the temporal lobe and malfunction in this network has been suggested to underlie some neurological disorders such as epilepsy. To test whether the induction of long-term potentiation (LTP) in one temporal lobe structure would modulate functional synaptic plasticity in other structures of this network, tetanic stimulation was applied to the white matter of the perirhinal cortex, Schaffer collaterals of the hippocampus, or the external capsule in combined rat amygdala-hippocampus-cortex slices. This tetanic stimulation was accompanied by enhancement of the evoked field potential slope in the third layer of perirhinal cortex, hippocampal CA1 area, and the lateral amygdala. Induction of LTP in each of these structures was concomitant with increased evoked field potentials in the neighboring structures. Surgical disconnection of anatomical pathways between these structures inhibited this concomitant enhancement of the evoked field potential slope. Both NMDA and AMPA glutamate sub-receptors were involved in changes of synaptic plasticity elicited by induction of LTP in the neighboring structures. The present data indicate a reciprocal control among the perirhinal cortex, the amygdala, and the hippocampus plasticity. This could be important for the formation and retention of the medial temporal lobe-dependent memory and may play a role in the involvement of all different regions of the temporal lobe in pathological conditions such as epilepsy that affect this brain structure.

Quantifying altered long-term potentiation in the CA1 hippocampus

Long-term potentiation (LTP) of synaptic transmission is a widely accepted model of learning and memory. In vitro brain slice techniques were used to investigate the effects of cortical-spreading depression and picrotoxin, an antagonist of the gamma-aminobutyric acid A (GABA(A)) receptor, on the tetanus-induced long-term potentiation of field excitatory postsynaptic potentials. Cortical-spreading depression is involved in glutamate desensitization; on the other hand, GABA(A) antagonists could increase postsynaptic excitability. This study shows that picrotoxin effectively induced long-term potentiation with 142.25 ± 4.18% of the baseline in the picrotoxin group (n = 8) versus 134.36 ± 2.35% of the baseline in the control group (n = 10). In group with picrotoxin applied to CSD, we obtained the smallest magnitude of LTP (120.15 ± 3.73% of the baseline, n = 8). These results suggest that picrotoxin could increase hippocampal activity and LTP; on the contrary, CSD reduced LTP magnitude. In addition, the results also suggest that the decay rate of post-tetanic potentiation has a direct relationship with LTP. Moreover, data were interpreted by nonlinear least squares quantifying, and LTP could also be quantified. The nonlinear attribute of LTP had an influence on the fitting, with respect to increasing the accuracy of the parameters and the compatibility of combination of stimuli that produce LTP.

Involvement of AMPA receptors in CSD-induced impairment of LTP in the hippocampus

OBJECTIVE

To investigate the alteration of hippocampal long-term plasticity and basal synaptic transmission induced by repetitive cortical spreading depressions (CSDs).

BACKGROUND

There is a relationship between migraine aura and amnesia attack. CSD, a state underlying migraine attacks, may be responsible for hippocampus-related symptoms. However, the precise role of CSD on hippocampal activity has not been investigated.

METHODS

Male Wistar rats were divided into CSD and control groups. Repetitive CSDs were induced in vivo by topical application of solid KCl. Forty-five minutes later, the ipsilateral hippocampus was removed, and hippocampal slices were prepared for a series of electrophysiological studies.

RESULTS

Repetitive CSDs led to a decrease in the magnitude of long-term potentiation in the hippocampus. CSD also reduced hippocampal synaptic efficacy, as shown by a reduction in post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses. In contrast, the post-synaptic N-methyl-d-aspartate receptor responses remained unchanged. In addition, there were no changes in paired-pulse profiles between the groups, indicating that CSD did not induce any presynaptic alterations.

CONCLUSION

These findings suggest that a reduction of post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses is the mechanism responsible for impaired hippocampal long-term potentiation induced by CSD.

Sequential changes in neuronal activity in single neocortical neurons after spreading depression

BACKGROUND

Cortical spreading depression (CSD) has an important role in migraine with aura. Prolonged neuronal depression is followed by a late excitatory synaptic plasticity after CSD.

METHOD

Intra- and extracellular recordings were performed to investigate the effect of CSD on intracellular properties of mouse neocortical tissues in the late excitatory period.

RESULTS

During CSD, changes in the membrane potentials usually began with a relatively short hyperpolarization followed by an abrupt depolarization. These changes occurred roughly at the same time point after CSD as the beginning of the negative extracellular deflection. Forty-five minutes after CSD, neurons showed significantly smaller amplitude of afterhyperpolarization and a reduced input resistance. Depolarization and hyperpolarization of the cells by constant intracellular current injections in this period significantly changed the frequency of the action potentials.

CONCLUSION

These data indicate higher excitability of the neocortical neurons after CSD, which can be assumed to contribute to hyperexcitability of neocortical tissues in patients suffering from migraine.

Changes in membrane potential and the intracellular calcium concentration during CSD and OGD in layer V and layer II/III mouse cortical neurons

Cortical spreading depression (CSD) is an episode of electrical silence following intense neuronal activity that propagates across the cortex at ∼3-6 mm/min and is associated with transient neuronal depolarization. CSD is benign in normally perfused brain tissue, but there is evidence suggesting that repetitive CSD contributes to infarct growth following focal ischemia. Studies to date have assumed that the cellular responses to CSD are uniform across neuronal types because there are no data to the contrary. In this study, we investigated the effect of CSD on membrane potential and the intracellular calcium concentration ([Ca(2+)](i)) of mouse layer V and layer II/III pyramidal neurons in brain slices. To place the data in context, we made similar measurements during anoxic depolarization induced by oxygen and glucose deprivation (OGD). The [Ca(2+)](i) was quantified using the low-affinity ratiometric indicator Fura-4F. During both CSD- and OGD-induced depolarization, the membrane potential approached 0 mV in all neurons. In layer V pyramids OGD resulted in an increase in [Ca(2+)](i) to a maximum of 3.69 ± 0.73 (SD) μM (n = 12), significantly greater than the increase to 1.81 ± 0.70 μM in CSD (n = 34; P < 0.0001). Membrane potential and [Ca(2+)](i) returned to nearly basal levels following CSD but not OGD. Layer II/III neurons responded to CSD with a greater peak increase in [Ca(2+)](i) than layer V neurons (2.88 ± 0.6 μM; n = 9; P < 0.01). We conclude there is a laminar difference in the response of pyramidal neurons to CSD; possible explanations are discussed.

The effect of repetitive spreading depression on neuronal damage in juvenile rat brain

Spreading depression (SD) is pronounced depolarization of neurons and glia that travels slowly across brain tissue followed by massive redistribution of ions between intra- and extracellular compartments. There is a relationship between SD and some neurological disorders. In the present study the effects of repetitive SD on neuronal damage in cortical and subcortical regions of juvenile rat brain were investigated. The animals were anesthetized and the electrodes as well as cannula were implanted over the brain. SD-like event was induced by KCl injection. The brains were removed after 2 or 4 weeks after induction of 2 or 4 SD-like waves (with interval of 1 week), respectively. Normal saline was injected instead of KCl in sham group. For stereological study, paraffin-embedded brains were cut in 5 microm sections. The sections were stained with Toluidine Blue to measure the volume-weighted mean volume of normal neurons and the numerical density of dark neurons. The volume-weighted mean volume of normal neurons in the granular layer of the dentate gyrus and layer V of the temporal cortex in SD group were significantly decreased after four repetitive SD. Furthermore, densities of dark neurons in the granular layer of the dentate gyrus (after 2 weeks), the caudate-putamen, and layer V of the temporal cortex (after 4 weeks) were significantly increased in SD group. Repetitive cortical SD in juvenile rats may cause neuronal damage in cortical and subcortical areas of the brain. This may important in pathophysiology of SD-related neurological disorders.

Cortical hyperexcitability and mechanism of medication-overuse headache

The present study was conducted to determine the effect of acute (1 h) and chronic (daily dose for 30 days) paracetamol administration on the development of cortical spreading depression (CSD), CSD-evoked cortical hyperaemia and CSD-induced Fos expression in cerebral cortex and trigeminal nucleus caudalis (TNC). Paracetamol (200 mg/kg body weight, intraperitonealy) was administered to Wistar rats. CSD was elicited by topical application of solid KCl. Electrocorticogram and cortical blood flow were recorded. Results revealed that acute paracetamol administration substantially decreased the number of Fos-immunoreactive cells in the parietal cortex and TNC without causing change in CSD frequency. On the other hand, chronic paracetamol administration led to an increase in CSD frequency as well as CSD-evoked Fos expression in parietal cortex and TNC, indicating an increase in cortical excitability and facilitation of trigeminal nociception. Alteration of cortical excitability which leads to an increased susceptibility of CSD development can be a possible mechanism underlying medication-overuse headache.

Acute neuroprotection to pilocarpine-induced seizures is not sustained after traumatic brain injury in the developing rat

Following CNS injury there is a period of vulnerability when cells will not easily tolerate a secondary insult. However recent studies have shown that following traumatic brain injury (TBI), as well as hypoxic-ischemic injuries, the CNS may experience a period of protection termed "preconditioning." While there is literature characterizing the properties of vulnerability and preconditioning in the adult rodent, there is an absence of comparable literature in the developing rat. To determine if there is a window of vulnerability in the developing rat, post-natal day 19 animals were subjected to a severe lateral fluid percussion injury followed by pilocarpine (Pc)-induced status epilepticus at 1, 6 or 24 h post TBI. During the first 24 h after TBI, the dorsal hippocampus exhibited less status epilepticus-induced cell death than that normally seen following Pc administration alone. Instead of producing a state of hippocampal vulnerability to activation, TBI produced a state of neuroprotection. However, in a second group of animals evaluated 20 weeks post injury, double-injured animals were statistically indistinguishable in terms of seizure threshold, mossy fiber sprouting and cell survival when compared to those treated with Pc alone. TBI, therefore, produced a temporary state of neuroprotection from seizure-induced cell death in the developing rat; however, this ultimately conferred no long-term protection from altered hippocampal circuit rearrangements, enhanced excitability or later convulsive seizures.

Periodic fasting alters neuronal excitability in rat neocortical and hippocampal tissues

Dietary restriction has been shown to be associated with marked changes in brain function. Periodic fasting was suggested to be beneficial in reducing both the incidence and severity of some neurological disorders. The aim of this investigation was to study the effect of periodic fasting on the neuronal network excitability in the neocortex and hippocampus and its possible influence on the brain under pathological conditions. Direct current (DC) recordings in the somatosensory neocortex of fasting rats (15 h water and food deprivation per day) during drinking revealed a negative potential shift. Using voltage sensitive dye imaging and tetanus-induced long-term potentiation (LTP) in ex vivo/in vitro experiments, neuronal network activities as well as synaptic efficacy were investigated in rat neocortical and hippocampal slices after 4 weeks of periodic fasting. Stimulus-induced patterns of bioelectric activity showed enhanced neuronal network excitability in the neocortex and decreased bioelectric activity in the hippocampus. LTP was significantly increased in neocortical slices and inhibited in hippocampal tissues. Both hippocampal and neocortical tissues exhibited a higher tolerance to hypoxic stress but not to 0-Mg(2+)-eliciting epileptiform field potentials. Neocortical slices also exhibited a higher threshold for the initiation of spreading depression. These experiments indicate that repetitive DC potential shifts occurring in fasting rats change the pattern of bioelectrical activities in cortical and subcortical regions. Through these alterations, the neocortex and hippocampus may become tuned for the efficient regulation of consummatory behaviour.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Sorting out astrocyte physiology from pharmacology

A number of exciting findings have been made in astrocytes during the past 15 years that have led many researchers to redefine how the brain works. Astrocytes are now widely regarded as cells that propagate Ca(2+) over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca(2+)-dependent manner to modulate a host of important brain functions. Although these discoveries have been very exciting, it is essential to place them in the proper context of the approaches used to obtain them to determine their relevance to brain physiology. This review revisits the key observations made in astrocytes that greatly impact how they are thought to regulate brain function, including the existence of widespread propagating intercellular Ca(2+) waves, data suggesting that astrocytes signal to neurons through Ca(2+)-dependent release of glutamate, and evidence for the presence of vesicular machinery for the regulated exocytosis of gliotransmitters.

Induction of neurogenesis in the adult dentate gyrus by cortical spreading depression

BACKGROUND AND PURPOSE

Spreading depression (SD) is an epiphenomenon of neurological disorders, like stroke or traumatic brain injury. These diseases have been associated with an increased neurogenesis in the adult rodent dentate gyrus. Such proliferative activity can also be induced by conditions that--like SD--coincide with a disturbed neuronal excitability, eg, epilepsy. Thus we hypothesized that SD might likewise influence hippocampal neurogenesis and potentially act as mediator of injury-induced neurogenesis.

METHODS

Repetitive cortical SD were induced by epidural application of 3 mol/L KCl. At different time points thereafter dentate gyrus neurogenesis was investigated by means of intraperitoneal bromodeoxyuridine injections and immunocytochemistry. Spatial learning and memory was tested in a Morris water maze.

RESULTS

Cortical SD significantly increased proliferative activity in the ipsilateral subgranular zone on days 2 and 4. We detected about 280% more newborn cells in the dentate gyrus of rats that received bromodeoxyuridine during the first week after SD and were allowed to recover for 6 weeks. Most of these cells expressed the mature neuronal marker NeuN. The mitogenic action of SD was suppressed by systemic administration of the NMDA receptor antagonist MK-801. Behavioral performance of SD animals in the Morris water maze did not improve significantly.

CONCLUSIONS

From our data we postulate that the increased dentate gyrus neurogenesis observed after brain injury may at least partly be mediated by SD-like epiphenomena. Furthermore they indicate that even a strongly enhanced dentate gyrus neurogenesis may occur without significant improvements in hippocampus-dependent spatial learning and memory.

Cortical spreading depression modulates synaptic transmission of the rat lateral amygdala

Clinical and pathophysiological evidence connects migraine and the amygdala. Cortical spreading depression (CSD) plays a causative role in the generation of aura symptoms. However, the role of CSD in the pathophysiology of other symptoms of migraine needs to be investigated. An in vitro brain slice technique was used to investigate CSD effects on tetanus-induced long-term potentiation (LTP) in the lateral amygdala (LA) of the combined rat amygdala-hippocampus-cortex slices. More than 75% of CSD induced in temporal cortex propagated to LA. Induction of CSD in combined amygdala-hippocampus-cortex slices in which CSD propagated from neocortex to LA significantly augmented LTP in LA. LTP was inhibited when CSD travelled only in the neocortical tissues. Separation of the amygdala from the remaining neocortical part of the slice, in which CSD propagation was limited to the neocortex, increased LTP close to the control levels. Pharmacological manipulations of the slices, in which CSD reached LA, revealed the involvement of NMDA and AMPA glutamate subreceptors as well as dopamine D2 receptors in the enhancement of LTP in LA. However, neither blocking of GABA receptors nor activation of dopamine D1 receptors affected LTP in these slices. The results indicate the disturbances of LA synaptic transmission triggered by propagation of CSD. This perturbation of LA synaptic transmission induced by CSD may relate to some symptoms occurring during migraine attacks.

The effect of estrogen and progesterone on spreading depression in rat neocortical tissues

Although gender differences in the incidence of migraine with aura appear to be related to high circulating levels of ovarian hormones, the underlying mechanisms are not yet fully understood. Several studies have suggested a major role for spreading depression (SD) in the pathogenesis and symptomatology of migraine with aura. To investigate a possible role of SD in the association of high female hormones and attacks of migraine with aura, the effects of beta-estradiol and progesterone on SD were studied in rat neocortical tissues. Application of both hormones enhanced the repetition rate as well as the amplitude of SD in neocortical slices treated with hypotonic artificial cerebrospinal fluid. beta-Estradiol and progesterone also dose dependently increased the amplitude of SD induced by KCl microinjection. Both hormones exhibited a pronounced, persisting, and significant enhancement of long-term potentiation of the field excitatory postsynaptic potential in the neocortical tissues. The changes in SD characteristics in the presence of estrogen and progesterone may responsible for increased migraine with aura attacks associated by high female hormones. These hormones may exert their effects on SD via facilitation of synaptic transmission.