The effects of anesthetics on cortical spreading depression elicitation and c-fos expression in rats

Stroke and Translational Research - Review of Experimental Models with a Focus on Awake Ischaemic Induction and Anaesthesia

Animal models are an indispensable tool in the study of ischaemic stroke with hundreds of drugs emerging from the preclinical pipeline. However, all of these drugs have failed to translate into successful treatments in the clinic. This has brought into focus the need to enhance preclinical studies to improve translation. The confounding effects of anaesthesia on preclinical stroke modelling has been raised as an important consideration. Various volatile and injectable anaesthetics are used in preclinical models during stroke induction and for outcome measurements such as imaging or electrophysiology. However, anaesthetics modulate several pathways essential in the pathophysiology of stroke in a dose and drug dependent manner. Most notably, anaesthesia has significant modulatory effects on cerebral blood flow, metabolism, spreading depolarizations, and neurovascular coupling. To minimise anaesthetic complications and improve translational relevance, awake stroke induction has been attempted in limited models. This review outlines anaesthetic strategies employed in preclinical ischaemic rodent models and their reported cerebral effects. Stroke related complications are also addressed with a focus on infarct volume, neurological deficits, and thrombolysis efficacy. We also summarise routinely used focal ischaemic stroke rodent models and discuss the attempts to induce some of these models in awake rodents.

Propofol-Based Anesthesia Maintenance and/or Volatile Anesthetics during Intracranial Aneurysm Repair: A Comparative Analysis of Neurological Outcomes

BACKGROUND

Volatile and intravenous anesthetics have substantial effects on physiological functions, notably influencing neurological function and susceptibility to injury. Despite the importance of the anesthetic approach, data on its relative risks or benefits during surgical clipping or endovascular treatments for unruptured intracranial aneurysms (UIAs) remains scant. We investigated whether using volatile anesthetics alone or in combination with propofol infusion yields superior neurological outcomes following UIA obliteration.

METHODS

We retrospectively reviewed 1001 patients who underwent open or endovascular treatment for UIA, of whom 596 had short- and long-term neurological outcome data (modified Rankin Scale) recorded. Multivariable ordinal regression analysis was performed to examine the association between the anesthetic approach and outcomes.

RESULTS

Of 1001 patients, 765 received volatile anesthetics alone, while 236 received propofol infusion and volatile anesthetics (combined anesthetic group). Short-term neurological outcome data were available for 619 patients and long-term data for 596. No significant correlation was found between the anesthetic approach and neurologic outcomes, irrespective of the type of procedure (open craniotomy or endovascular treatment). The combined anesthetic group had a higher rate of ICU admission (p < 0.001) and longer ICU and hospital length of stay (LOS, p < 0.001). Similarly, a subgroup analysis revealed longer ICU and hospital LOS (p < 0.0001 and p < 0.001, respectively) in patients who underwent endovascular UIA obliteration under a combined anesthetic approach (n = 678).

CONCLUSIONS

The addition of propofol to volatile anesthetics during UIA obliteration does not provide short- or long-term benefits to neurologic outcomes. Compared to volatile anesthetics alone, the combination of propofol and volatile anesthetics may be associated with an increased rate of ICU admission, as well as longer ICU and hospital LOS.

The NMDA receptor antagonists memantine and ketamine as anti-migraine agents

Migraine is a debilitating disorder affecting females more frequently than males. There is some evidence that drugs targeting glutamate receptors: memantine and ketamine might be beneficial in the therapy of this entity. Therefore, the purpose of this work is to present NMDA receptor antagonists, memantine and ketamine, as potential anti-migraine agents. We searched PubMed/MEDLINE, Embase, and clinical trials submitted to ClinicalTrials.gov to find publications describing eligible trials published between database inception and December 31, 2021. This comprehensive literature review summarizes data on the use of the NMDA receptor antagonists memantine and ketamine in the pharmacotherapy of migraine. Results from 20 previous and recent preclinical experiments are discussed and correlated with 19 clinical trials (including case series, open-label, and randomized placebo-controlled trials). For the purposes of this review, the authors hypothesized that the propagation of SD is a major mechanism in the pathophysiology of migraine. In several animal studies and in vitro studies, memantine and ketamine inhibited or reduced propagation of the SD. In addition, the results of clinical trials suggest that memantine or ketamine may be an effective treatment option for migraine. However, most studies on these agents lack control group. Although further clinical trials are needed, the results suggest that ketamine or memantine may be promising molecules for the treatment of severe migraine. Particular attention should be paid to people who have a treatment-resistant form of migraine with aura or have exhausted existing treatment options. For them, the drugs under discussion could represent an interesting alternative in the future.

Efficacy profile of noninvasive vagus nerve stimulation on cortical spreading depression susceptibility and the tissue response in a rat model

Background

Noninvasive vagus nerve stimulation (nVNS) has recently emerged as a promising therapy for migraine. We previously demonstrated that vagus nerve stimulation inhibits cortical spreading depression (CSD), the electrophysiological event underlying migraine aura and triggering headache; however, the optimal nVNS paradigm has not been defined.

Methods

Various intensities and doses of nVNS were tested to improve efficacy on KCl-evoked CSD frequency and electrical threshold of CSD in a validated rat model. Chronic efficacy was evaluated by daily nVNS delivery for four weeks. We also examined the effects of nVNS on neuroinflammation and trigeminovascular activation by western blot and immunohistochemistry.

Results

nVNS suppressed susceptibility to CSD in an intensity-dependent manner. Two 2-minute nVNS 5 min apart afforded the highest efficacy on electrical CSD threshold and frequency of KCl-evoked CSD. Daily nVNS for four weeks did not further enhance efficacy over a single nVNS 20 min prior to CSD. The optimal nVNS also attenuated CSD-induced upregulation of cortical cyclooxygenase-2, calcitonin gene-related peptide in trigeminal ganglia, and c-Fos expression in trigeminal nucleus caudalis.

Conclusions

Our study provides insight on optimal nVNS parameters to suppress CSD and suggests its benefit on CSD-induced neuroinflammation and trigeminovascular activation in migraine treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01384-1.

Characterization of optogenetically-induced cortical spreading depression in awake mice using graphene micro-transistor arrays

Objective.The development of experimental methodology utilizing graphene micro-transistor arrays to facilitate and advance translational research into cortical spreading depression (CSD) in the awake brain.Approach.CSDs were reliably induced in awake nontransgenic mice using optogenetic methods. High-fidelity DC-coupled electrophysiological mapping of propagating CSDs was obtained using flexible arrays of graphene soultion-gated field-effect transistors (gSGFETs).Main results.Viral vectors targetted channelrhopsin expression in neurons of the motor cortex resulting in a transduction volume ⩾1 mm³. 5-10 s of continous blue light stimulation induced CSD that propagated across the cortex at a velocity of 3.0 ± 0.1 mm min^-1. Graphene micro-transistor arrays enabled high-density mapping of infraslow activity correlated with neuronal activity suppression across multiple frequency bands during both CSD initiation and propagation. Localized differences in the CSD waveform could be detected and categorized into distinct clusters demonstrating the spatial resolution advantages of DC-coupled recordings. We exploited the reliable and repeatable induction of CSDs using this preparation to perform proof-of-principle pharmacological interrogation studies using NMDA antagonists. MK801 (3 mg kg^-1) suppressed CSD induction and propagation, an effect mirrored, albeit transiently, by ketamine (15 mg kg^-1), thus demonstrating this models' applicability as a preclinical drug screening platform. Finally, we report that CSDs could be detected through the skull using graphene micro-transistors, highlighting additional advantages and future applications of this technology.Significance.CSD is thought to contribute to the pathophysiology of several neurological diseases. CSD research will benefit from technological advances that permit high density electrophysiological mapping of the CSD waveform and propagation across the cortex. We report anin vivoassay that permits minimally invasive optogenetic induction, combined with multichannel DC-coupled recordings enabled by gSGFETs in the awake brain. Adoption of this technological approach could facilitate and transform preclinical investigations of CSD in disease relevant models.

Effects of Volatile Anesthetics on Postoperative Ischemic Stroke Incidence

Background

Preclinical studies suggest that volatile anesthetics decrease infarct volume and improve the outcome of ischemic stroke. This study aims to determine their effect during noncardiac surgery on postoperative ischemic stroke incidence.

Methods and Results

This was a retrospective cohort study of surgical patients undergoing general anesthesia at 2 tertiary care centers in Boston, MA, between October 2005 and September 2017. Exclusion criteria comprised brain death, age <18 years, cardiac surgery, and missing covariate data. The exposure was defined as median age‐adjusted minimum alveolar concentration of all intraoperative measurements of desflurane, sevoflurane, and isoflurane. The primary outcome was postoperative ischemic stroke within 30 days. Among 314 932 patients, 1957 (0.6%) experienced the primary outcome. Higher doses of volatile anesthetics had a protective effect on postoperative ischemic stroke incidence (adjusted odds ratio per 1 minimum alveolar concentration increase 0.49, 95% CI, 0.40–0.59, P<0.001). In Cox proportional hazards regression, the effect was observed for 17 postoperative days (postoperative day 1: hazard ratio (HR), 0.56; 95% CI, 0.48–0.65; versus day 17: HR, 0.85; 95% CI, 0.74–0.99). Volatile anesthetics were also associated with lower stroke severity: Every 1‐unit increase in minimum alveolar concentration was associated with a 0.006‐unit decrease in the National Institutes of Health Stroke Scale (95% CI, −0.01 to −0.002, P=0.002). The effects were robust throughout various sensitivity analyses including adjustment for anesthesia providers as random effect.

Conclusions

Among patients undergoing noncardiac surgery, volatile anesthetics showed a dose‐dependent protective effect on the incidence and severity of early postoperative ischemic stroke.

Therapeutic implications of cortical spreading depression models in migraine

Migraine is among the most common and disabling neurological diseases in the world. Cortical spreading depression (CSD) is a wave of near-complete depolarization of neurons and glial cells that slowly propagates along the cortex creating the perception of aura. Evidence suggests that CSD can trigger migraine headache. Experimental models of CSD have been considered highly translational as they recapitulate migraine-related phenomena and have been validated for screening migraine therapeutics. Here we outline the essential components of validated experimental models of CSD and provide a comprehensive review of potential modulators and targets against CSD. We further focus on novel interventions that have been recently shown to suppress CSD susceptibility that may lead to therapeutic targets in migraine.

Systematic review of the pharmacological agents that have been tested against spreading depolarizations

Spreading depolarization (SD) occurs alongside brain injuries and it can lead to neuronal damage. Therefore, pharmacological modulation of SD can constitute a therapeutic approach to reduce its detrimental effects and to improve the clinical outcome of patients. The major objective of this article was to produce a systematic review of all the drugs that have been tested against SD. Of the substances that have been examined, most have been shown to modulate certain SD characteristics. Only a few have succeeded in significantly inhibiting SD. We present a variety of strategies that have been proposed to overcome the notorious harmfulness and pharmacoresistance of SD. Information on clinically used anesthetic, sedative, hypnotic agents, anti-migraine drugs, anticonvulsants and various other substances have been compiled and reviewed with respect to the efficacy against SD, in order to answer the question of whether a drug at safe doses could be of therapeutic use against SD in humans.

Novel Therapeutic Targets Against Spreading Depression

Migraine is among the most prevalent and disabling neurological diseases in the world. Cortical spreading depression (SD) is an intense wave of neuronal and glial depolarization underlying migraine aura, and a headache trigger, which has been used as an experimental platform for drug screening in migraine. Here, we provide an overview of novel therapeutic targets that show promise to suppress SD, such as acid-sensing ion channels, casein kinase Iδ, P2X7-pannexin 1 complex, and neuromodulation, and outline the experimental models and essential quality measures for rigorous and reproducible efficacy testing.

Peri-infarct depolarizations during focal ischemia in the awake Spontaneously Hypertensive Rat. Minimizing anesthesia confounds in experimental stroke

Anesthesia profoundly impacts peri-infarct depolarizations (PIDs), but only one prior report has described their monitoring during experimental stroke in awake animals. Since temporal patterns of PID occurrence are model specific, the current study examined PID incidence during focal ischemia in the awake Spontaneously Hypertensive Rat (SHR), and documented the impact of both prior and concurrent isoflurane anesthesia. For awake recordings, electrodes were implanted under isoflurane anesthesia 1day to 5weeks prior to occlusion surgery. Rats were then subjected to permanent or transient (2h) tandem occlusion of the middle cerebral and ipsilateral common carotid arteries, followed by PID monitoring for up to 3days. Comparison perfusion imaging studies evaluated PID-associated hyperemic transients during permanent ischemia under anesthesia at varied intervals following prior isoflurane exposure. Prior anesthesia attenuated PID number at intervals up to 1week, establishing 2weeks as a practical recovery duration following surgical preparation to avoid isoflurane preconditioning effects. PIDs in awake SHR were limited to the first 4h after permanent occlusions. Maintaining anesthesia during this interval reduced PID number, and prolonged their occurrence through several hours following anesthesia termination. Although PID number otherwise correlated with infarct size, PID suppression by anesthesia was not protective in the absence of reperfusion. PIDs persisted up to 36h after transient occlusions. These results differ markedly from the one previous report of such monitoring in awake Sprague-Dawley rats, which found an extended biphasic PID time course during 24h after both permanent and transient filament occlusions. PID occurrence closely reflects the time course of infarct progression in the respective models, and may be more useful than absolute PID number as an index of ongoing pathology.

NR2A contributes to genesis and propagation of cortical spreading depression in rats

Cortical spreading depression (CSD) is a transient propagating excitation of synaptic activity followed by depression, which is implicated in migraine. Increasing evidence points to an essential role of NR2A-containing NMDA receptors in CSD propagation in vitro; however, whether these receptors mediate CSD genesis in vivo requires clarification and the role of NR2A on CSD propagation is still under debate. Using in vivo CSD in rats with electrophysiology and in vitro CSD in chick retina with intrinsic optical imaging, we addressed the role of NR2A in CSD. We demonstrated that NVP-AAM077, a potent antagonist for NR2A-containing receptors, perfused through microdialysis probes, markedly reduced cortex susceptibility to CSD, but also reduced magnitude of CSD genesis in rats. Additionally, NVP-AAM077 at 0.3 nmol perfused into the contralateral ventricle, considerably suppressed the magnitude of CSD propagation wave and propagation rate in rats. This reduction in CSD propagation was also observed with TCN-201, a negative allosteric modulator selective for NR2A, at 3 μM, in the chick retina. Our data provides strong evidence that NR2A subunit contributes to CSD genesis and propagation, suggesting drugs selectively antagonizing NR2A-containing receptors might constitute a highly specific strategy treating CSD associated migraine with a likely better safety profile.

Dynamics of Ionic Shifts in Cortical Spreading Depression

Cortical spreading depression is a slowly propagating wave of near-complete depolarization of brain cells followed by temporary suppression of neuronal activity. Accumulating evidence indicates that cortical spreading depression underlies the migraine aura and that similar waves promote tissue damage in stroke, trauma, and hemorrhage. Cortical spreading depression is characterized by neuronal swelling, profound elevation of extracellular potassium and glutamate, multiphasic blood flow changes, and drop in tissue oxygen tension. The slow speed of the cortical spreading depression wave implies that it is mediated by diffusion of a chemical substance, yet the identity of this substance and the pathway it follows are unknown. Intercellular spread between gap junction-coupled neurons or glial cells and interstitial diffusion of K(+) or glutamate have been proposed. Here we use extracellular direct current potential recordings, K(+)-sensitive microelectrodes, and 2-photon imaging with ultrasensitive Ca(2+) and glutamate fluorescent probes to elucidate the spatiotemporal dynamics of ionic shifts associated with the propagation of cortical spreading depression in the visual cortex of adult living mice. Our data argue against intercellular spread of Ca(2+) carrying the cortical spreading depression wavefront and are in favor of interstitial K(+) diffusion, rather than glutamate diffusion, as the leading event in cortical spreading depression.

Stress hormone corticosterone enhances susceptibility to cortical spreading depression in familial hemiplegic migraine type 1 mutant mice

Stress is a putative migraine trigger, but the pathogenic mechanisms involved are unknown. Stress and stress hormones increase neuronal excitability by enhancing glutamatergic neurotransmission, but inhibitory effects have also been reported. We hypothesise that an acute rise in stress hormones, such as corticosteroids which are released after stress, increase neuronal excitability and thereby may increase susceptibility to cortical spreading depression (CSD), the mechanism underlying the migraine aura. Here we investigated effects of acute restraint stress and of the stress hormone corticosterone on CSD susceptibility as surrogate migraine marker, in a transgenic mouse model of familial hemiplegic migraine type 1 (FHM1), which displays increased glutamatergic cortical neurotransmission and increased propensity for CSD. We found that 20-min and 3-h restraint stress did not influence CSD susceptibility in mutant or wild-type mice, despite elevated levels of plasma corticosterone. By contrast, subcutaneous administration of 20mg/kg corticosterone increased CSD frequency exclusively in mutant mice, while corticosterone plasma levels were similarly elevated in mutants and wild types. The effect of corticosterone on CSD frequency was normalised by pre-administration of the glucocorticoid receptor (GR) antagonist mifepristone. These findings suggest that corticosteroid-induced GR activation can enhance susceptibility to CSD in genetically susceptible individuals, and may predispose to attacks of migraine. Although corticosterone levels rise also during acute stress, the latter likely triggers a spatiotemporally more complex biological response with multiple positive and negative modulators which may not be adequately modeled by exogenous administration of corticosterone alone.

Characterization of inhibitory GABA-A receptor activation during spreading depolarization in brain slice

Spreading depolarization (SD) is a slowly propagating wave of near complete depolarizations of neurons and glia. Previous studies have reported large GABA releases during SD, but there is limited understanding of how GABA release and receptor activation are regulated and influence the propagating SD wavefront, as well as an excitatory phase immediately following the passage of SD. The present study characterized GABA-A type receptor (GABA_AR) currents during SD generated by KCl microinjection in acute hippocampal slices from adult mice. Spontaneous GABA_AR-mediated currents (sIPSCs) were initially enhanced, and were followed by a large outward current at the wavefront. sIPSC were then transiently supressed during the late SD phase, resulting in a significant reduction of the sIPSC/sEPSC ratio. The large outward current generated during SD was eliminated by the GABA_AR antagonist gabazine, but the channel potentiator/agonist propofol failed to potentiate the current, likely because of a ceiling effect. Extracellular Cl⁻ decreases recorded during SD were reduced by the antagonist but were not increased by the potentiator. Together with effects of GABA_AR modulators on SD propagation rate, these results demonstrate a significant inhibitory role of the initial GABA_AR activation and suggest that intracellular Cl⁻ loading is insufficient to generate excitatory GABA_AR responses during SD propagation. These results provide a mechanistic explanation for facilitating effects of GABA_AR antagonists, and the lack of inhibitory effect of GABA_AR potentiators on SD propagation. In addition, selective suppression of GABA transmission in the late SD period and the lack of effect of GABA_A modulators on the duration of SD suggests that GABA modulation may not be effective approach to protect neurons during the vulnerable phase of SD.

Pearls and pitfalls in experimental models of spreading depression

BACKGROUND

Spreading depression (SD) is the electrophysiological substrate of migraine aura and a potential trigger for headache. Since its discovery by Leão in 1944, SD has transformed from being viewed as an epiphenomenon into a therapeutic target relevant in the pathophysiology of migraine and brain injury.

AIM

Despite decades of research, the underpinnings of SD are still poorly understood, hampering our efforts to selectively block its initiation and spread. Experimental models have nevertheless been useful to measure the likelihood of SD occurrence (i.e. SD susceptibility) and characterize genetic, physiological and pharmacological modulation of SD in search of potential therapies, such as in migraine prophylaxis and stroke. Here, I review experimental SD susceptibility endpoints and surrogates, and minimum essential model requirements to improve their utility in drug screening.

CONCLUSION

A critical reappraisal of strengths and caveats of experimental models of SD susceptibility is needed to set standards and improve data quality, interpretation and reconciliation.

Isoflurane suppresses cortical spreading depolarizations compared to propofol--implications for sedation of neurocritical care patients

Sedatives in the neurointensive care unit can strongly influence patients' risks of developing secondary brain damage. In particular, isoflurane, a volatile anesthetic, has been recently re-introduced to the neurointensive care unit, and first clinical studies suggest beneficial effects due to elevation of cerebral blood flow and reduction of metabolism. In contrast, propofol is a commonly used intravenous sedative that reduces cerebral blood flow and intra-cranial pressure. We have here studied the influence of these two sedatives on the occurrence of cortical spreading depolarizations (CSDs), which have emerged over the last decade as a major mechanism of delayed brain injury in stroke and brain trauma, constituting a substantial vascular and metabolic threat to peri-infarct tissue and being associated with poor patient outcome. Two experimental models were tested in Wistar rats anesthetized either with isoflurane or with propofol: KCl-evoked CSDs (n=10) and spontaneous CSDs after occlusion of the middle cerebral artery (n=14). Spatiotemporal patterns of CSD waves were observed by real-time laser speckle imaging of regional cerebral blood flow changes associated with the CSDs. During 30 min of cortical KCl application, 5.2±0.7 CSDs were induced under isoflurane compared to 10.2±1.8 CSDs under propofol (p<0.001). After focal ischemia, 2.43±1.0 CSDs/h emerged spontaneously under isoflurane versus 6.83±2.5 CSDs/h under propofol (p<0.001). Furthermore, baseline blood flow and glycemia were much higher under isoflurane compared to propofol, which may set the tissue in better metabolic conditions to recover from the occurrence of CSD waves. We conclude that isoflurane, in comparison to propofol, decreases the occurrence of CSDs and may improve recovery from these metabolically demanding waves. To reduce CSD induced secondary tissue damage, we suggest isoflurane to be favored over propofol to sedate acute stroke and trauma patients in the neurointensive care unit.

Extendable, miniaturized multi-modal optical imaging system: cortical hemodynamic observation in freely moving animals

Observation of brain activities in freely moving animals has become an important approach for neuroscientists to understand the correlation between brain function and behavior. We describe an extendable fiber-optic-based multi-modal imaging system that can concurrently carry out laser speckle contrast imaging (LSCI) of blood flow and optical intrinsic signal (OIS) imaging in freely moving animals, and it could be extended to fluorescence imaging. Our imaging system consists of a multi-source illuminator, a fiber multi-channel optical imaging unit, and a head-mounted microscope. The imaging fiber bundle delivers optical images from the head-mounted microscope to the multi-channel optical imaging unit. Illuminating multi-mode fiber bundles transmit light to the head-mounted microscope which has a mass of less than 1.5 g and includes a gradient index lens, giving the animal maximum movement capability. The internal optical components are adjustable, allowing for a change in magnification and field of view. We test the system by observing hemodynamic changes during cortical spreading depression (CSD) in freely moving and anesthetized animals by simultaneous LSCI and dual-wavelength OIS imaging. Hemodynamic parameters were calculated. Significant differences in CSD propagation durations between the two states were observed. Furthermore, it is capable of performing fluorescence imaging to explore animal behavior and the underlying brain functional activity further.

Anesthetic effects on susceptibility to cortical spreading depression

Cortical spreading depression (CSD) is a transient neuronal and glial depolarization and disruption of membrane ionic gradients that propagates slowly across the cerebral cortex. Recent clinical and experimental evidence has implicated CSD in the pathophysiology of migraines and neuronal injury states. In the current study, we examined the influence of four different anesthetics (propofol, dexmedetomidine, isoflurane, pentobarbital) on CSD susceptibility in a KCl application animal model. We found that isoflurane and dexmedetomidine suppressed CSD frequency, and tended to reduce the CSD propagation speed. Our data suggest that these anesthetics may be therapeutically beneficial in preventing CSD in diverse neuronal injury states.

Delayed cerebral ischaemia after subarachnoid haemorrhage: looking beyond vasospasm

Despite improvements in the clinical management of aneurysmal subarachnoid haemorrhage over the last decade, delayed cerebral ischaemia (DCI) remains the single most important cause of morbidity and mortality in those patients who survive the initial bleed. The pathological mechanisms underlying DCI are still unclear and the calcium channel blocker nimodipine remains the only therapeutic intervention proven to improve functional outcomes after SAH. The recent failure of the drug clazosentan to improve functional outcomes despite reducing vasoconstriction has moved the focus of research into DCI away from cerebral artery constriction towards a more multifactorial aetiology. Novel pathological mechanisms have been suggested, including damage to cerebral tissue in the first 72 h after aneurysm rupture ('early brain injury'), cortical spreading depression, and microthrombosis. A greater understanding of the significance of these pathophysiological mechanisms and potential genetic risk factors is required, if new approaches to the prophylaxis, diagnosis, and treatment of DCI are to be developed. Furthermore, objective and reliable biomarkers are needed for the diagnosis of DCI in poor grade SAH patients requiring sedation and to assess the efficacy of new therapeutic interventions. The purpose of this article is to appraise these recent advances in research into DCI, relate them to current clinical practice, and suggest potential novel avenues for future research.

Altered resting-state functional connectivity after cortical spreading depression in mice

Cortical spreading depression (CSD) underlies some neurological disorders. Previous imaging work suggests that CSD is associated with functional and structural alterations in the cerebral cortex. However, the changes in cortical functional network following CSD are poorly understood. The present study examines the changes in resting-state function connectivity (RSFC) of the mouse sensorimotor cortex after the onset of CSD by using optical intrinsic signal imaging. Our results show that RSFC between ipsilateral sensorimotor cortex (the cortex where CSD spreads) and contralateral sensorimotor cortex (the cortex where CSD does not spread) was significantly reduced after CSD. Moreover, a marked connectivity increase was found after CSD not only within contralateral somatosensory cortex and contralateral motor cortex themselves, but also between contralateral somatosensory cortex and contralateral motor cortex. Amplitude of low-frequency fluctuation (ALFF) analysis revealed an increase in ALFF in the ipsilateral cortex but a decrease in the contralateral cortex after CSD, indicating different effects of CSD on the neural activity in the ipsilateral and contralateral sensorimotor cortexes. These results suggest that CSD would alter the RSFC in the sensorimotor cortexes, and functional connectivity analysis may help to understand the effect of CSD on the cerebral functional network.

Pharmacological targeting of spreading depression in migraine

Migraine, particularly with aura, is a genetically heterogeneous disorder of ion channels, pumps or transporters associated with increased cortical excitability. Spreading depression, as one reflection of hyperexcitability, is the electrophysiological event underlying aura symptoms and a trigger for headache. Endogenous (e.g., genes and hormones) and exogenous factors (e.g., drugs) modulating migraine susceptibility have also been shown to modulate spreading depression susceptibility concordantly, suggesting that spreading depression can be a relevant therapeutic target in migraine. In support of this, several migraine prophylactic drugs used in clinical practice have been shown to suppress spreading depression susceptibility as a probable mechanism of action, despite belonging to widely different pharmacological classes. Hence, susceptibility to spreading depression can be a useful preclinical model with good positive and negative predictive value for drug screening.

New directions in migraine

Migraine is a highly prevalent neurological disorder imparting a major burden on health care around the world. The primary pathology may be a state of hyperresponsiveness of the nervous system, but the molecular mechanisms are yet to be fully elucidated. We could now be at a watershed moment in this respect, as the genetic loci associated with typical forms of migraine are being revealed. The genetic discoveries are the latest step in the evolution of our understanding of migraine, which was initially considered a cerebrovascular condition, then a neuroinflammatory process and now primarily a neurogenic disorder. Indeed, the genetic findings, which have revealed ion channels and transporter mutations as causative of migraine, are a powerful argument for the neurogenic basis of migraine. Modulations of ion channels leading to amelioration of the migraine 'hyperresponsive' brain represent attractive targets for drug discovery. There lies ahead an exciting and rapidly progressing phase of migraine translational research, and in this review we highlight recent genetic findings and consider how these may affect the future of migraine neurobiology and therapy.

Enhanced subcortical spreading depression in familial hemiplegic migraine type 1 mutant mice

Familial hemiplegic migraine type 1, a monogenic migraine variant with aura, is linked to gain-of-function mutations in the CACNA1A gene encoding Ca(V)2.1 channels. The S218L mutation causes severe channel dysfunction, and paroxysmal migraine attacks can be accompanied by seizures, coma, and hemiplegia; patients expressing the R192Q mutation exhibit hemiplegia only. Familial hemiplegic migraine knock-in mice expressing the S218L or R192Q mutation are highly susceptible to cortical spreading depression, the electrophysiological surrogate for migraine aura, and develop severe and prolonged motor deficits after spreading depression. The S218L mutants also develop coma and seizures and sometimes die. To investigate underlying mechanisms for these symptoms, we used multielectrode electrophysiological recordings, diffusion-weighted magnetic resonance imaging, and c-fos immunohistochemistry to trace spreading depression propagation into subcortical structures. We showed that unlike the wild type, cortical spreading depression readily propagated into subcortical structures in both familial hemiplegic migraine type 1 mutants. Whereas the facilitated subcortical spread appeared limited to the striatum in R192Q, hippocampal and thalamic spread was detected in the S218L mutants with an allele-dosage effect. Both strains exhibited increased susceptibility to subcortical spreading depression and reverberating spreading depression waves. Altogether, these data show that spreading depression propagates between cortex, basal ganglia, diencephalon, and hippocampus in genetically susceptible brains, which could explain the prolonged hemiplegia, coma, and seizure phenotype in this variant of migraine with aura.

N-methyl-D-aspartate induces cortical hyperemia through cortical spreading depression-dependent and -independent mechanisms in rats

OBJECTIVE

N-methyl-d-aspartate (NMDA) is a powerful cerebrovascular dilator in vivo. Cortical spreading depression (CSD) has recently been shown to contribute to the pial arteriolar dilation in mice. Our main aim was to examine the participation of CSD in the overall cerebrovascular response to NMDA in the rat.

METHODS

Anesthetized Wistar rats (eight weeks old) were equipped with a closed cranial window to allow topical application of NMDA (10(-5)-10(-3) M) to the parietal cortex. Cortical blood flow (CoBF) under and outside the cranial window was simultaneously monitored by using a two-channel laser-Doppler flowmeter. CSDs were detected by recording the changes in the cortical DC potential.

RESULTS

Concentrations of 10(-4) and 10(-3) M of NMDA evoked single CSDs associated with rapid, transient hyperemia, followed by a sustained, but reduced, increase in CoBF. The latency and magnitude of the CoBF responses were dose dependent. The higher dose resulted in shorter latency (100+/-5* vs. 146+/-11 seconds, *P<0.05; mean+/-standard error of the mean) and larger overall flow response (77+/-12* vs. 28+/-3% from baseline) under, but not outside, the cranial window.

CONCLUSIONS

NMDA elicits dose-dependent increases in CoBF that are composed of CSD-dependent and -independent components in rats.

Unilateral cortical spreading depression affects sleep need and induces molecular and electrophysiological signs of synaptic potentiation in vivo

Cortical spreading depression (CSD) is an electrophysiological phenomenon first described by Leao in 1944 as a suppression of spontaneous electroencephalographic activity, traveling across the cerebral cortex. In vitro studies suggest that CSD may induce synaptic potentiation. One recent study also found that CSD is followed by a non-rapid eye movement (NREM) sleep duration increase, suggesting an increased need for sleep. Recent experiments in animals and humans show that the occurrence of synaptic potentiation increases subsequent sleep need as measured by larger slow wave activity (SWA) during NREM sleep, prompting the question whether CSD can affect NREM SWA. Here, we find that, in freely moving rats, local CSD induction increases corticocortical evoked responses and strongly induces brain derived neurotrophic factor (BDNF) in the affected cortical hemisphere but not in the contralateral one, consistent with synaptic potentiation in vivo. Moreover, for several hours after CSD, large slow waves occur in the affected hemisphere during rapid eye movement sleep and quiet waking but disappear during active exploration. Finally, we find that CSD increases NREM sleep duration and SWA, the latter specifically in the affected hemisphere. These effects are consistent with an increase in synaptic strength triggered by CSD, although nonphysiological phenomena associated with CSD may also play a role.

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.

New concepts regarding cerebral vasospasm: glial-centric mechanisms

PURPOSE

Poor outcome in patients with cerebral vasospasm following subarachnoid hemorrhage remains a serious clinical problem. The current management with focus on the cerebrovascular constriction accounts for the use of "triple-H" therapy (hypertension, hypervolemia, and hemodilution) to enhance cerebral blood flow through constricted vessels. Recent work suggests that spreading depression (a stereotypical response of cerebral cortical tissue to noxious stimuli with subsequent oligemic blood flow) occurs in patients with cerebral vasospasm. A narrative review was conducted to examine the relationship between spreading depression and subarachnoid hemorrhage and to identify the anesthetic effects on the propagation of spreading depression.

PRINCIPAL FINDINGS

Following review of the literature, an underlying mechanism is advanced that cerebral vasospasm is not primarily a problem of the cerebral vasculature but a consequence of glial cell dysfunction following spreading depression - a glial-centric cause for vasospasm. Such a mechanism for vasospasm becomes manifest when spreading depression waves transition to peri-infarct depolarization waves - with protracted ischemic blood flow in compromised tissue. The extracellular microenvironment with high potassium and low nitric oxide tension can account for conducting vessel narrowing.

CONCLUSIONS

The implication for clinical management is discussed supposing glial cell dysfunction is an underlying mechanism responsible for the vascular spasm.

Spreading depression: from serendipity to targeted therapy in migraine prophylaxis

Despite the relatively well-characterized headache mechanisms in migraine, upstream events triggering individual attacks are poorly understood. This lack of mechanistic insight has hampered a rational approach to prophylactic drug discovery. Unlike targeted abortive and analgesic interventions, mainstream migraine prophylaxis has been largely based on serendipitous observations (e.g. propranolol) and presumed class effects (e.g. anticonvulsants). Recent studies suggest that spreading depression is the final common pathophysiological target for several established or investigational migraine prophylactic drugs. Building on these observations, spreading depression can now be explored for its predictive utility as a preclinical drug screening paradigm in migraine prophylaxis.

Genetic and hormonal factors modulate spreading depression and transient hemiparesis in mouse models of familial hemiplegic migraine type 1

Familial hemiplegic migraine type 1 (FHM1) is an autosomal dominant subtype of migraine with aura that is associated with hemiparesis. As with other types of migraine, it affects women more frequently than men. FHM1 is caused by mutations in the CACNA1A gene, which encodes the alpha1A subunit of Cav2.1 channels; the R192Q mutation in CACNA1A causes a mild form of FHM1, whereas the S218L mutation causes a severe, often lethal phenotype. Spreading depression (SD), a slowly propagating neuronal and glial cell depolarization that leads to depression of neuronal activity, is the most likely cause of migraine aura. Here, we have shown that transgenic mice expressing R192Q or S218L FHM1 mutations have increased SD frequency and propagation speed; enhanced corticostriatal propagation; and, similar to the human FHM1 phenotype, more severe and prolonged post-SD neurological deficits. The susceptibility to SD and neurological deficits is affected by allele dosage and is higher in S218L than R192Q mutants. Further, female S218L and R192Q mutant mice were more susceptible to SD and neurological deficits than males. This sex difference was abrogated by ovariectomy and senescence and was partially restored by estrogen replacement, implicating ovarian hormones in the observed sex differences in humans with FHM1. These findings demonstrate that genetic and hormonal factors modulate susceptibility to SD and neurological deficits in FHM1 mutant mice, providing a potential mechanism for the phenotypic diversity of human migraine and aura.

The impact of anesthetics and hyperoxia on cortical spreading depression

Cortical spreading depression (CSD), a transient neuronal and glial depolarization that propagates slowly across the cerebral cortex, is the putative electrophysiological event underlying migraine aura. It negatively impacts tissue injury during stroke, cerebral contusion and intracranial hemorrhage. Susceptibility to CSD has been assessed in several experimental animal models in vivo, such as after topical KCl application or cathodal stimulation. Various combinations of anesthetics and ambient conditions have been used by different laboratories making comparisons problematic and differences in data difficult to reconcile. We systematically studied CSD susceptibility comparing commonly used experimental anesthetics (isoflurane, alpha-chloralose, and urethane) with or without N(2)O or normobaric hyperoxia (100% O(2) inhalation). The frequency of evoked CSDs, and their propagation speed, duration, and amplitude were recorded during 2 h topical KCl (1 M) application. We found that N(2)O reduced CSD frequency when combined with isoflurane or urethane, but not alpha-chloralose; N(2)O also decreased CSD propagation speed and duration. Urethane anesthesia was associated with the highest CSD frequency that was comparable to pentobarbital. Inhalation of 100% O(2) did not alter CSD frequency, propagation speed or duration in combination with any of the anesthetics tested. Our data show anesthetic modulation of CSD susceptibility in an experimental model of human disease, underscoring the importance of proper study design for hypothesis testing as well as for comparing results between studies.

Role of cortical spreading depressions for secondary brain damage after traumatic brain injury in mice

In recent years, several studies have unequivocally shown the occurrence of cortical spreading depressions (CSDs) after stroke and traumatic brain injury (TBI) in humans. The fundamental question, however, is whether CSDs cause or result from secondary brain damage. The aim of the current study was, therefore, to investigate the role of CSDs for secondary brain damage in an experimental model of TBI. C57/BL6 mice were traumatized by controlled cortical impact. Immediately after trauma, each animal showed one heterogeneous direct current (DC) potential shift accompanied by a profound depression of electroencephalogram (EEG) amplitude, and a temporary decrease of ipsi- and contralateral regional cerebral blood flow (rCBF) suggesting bilateral CSDs. Within the next 3 h after TBI, CSDs occurred at a low frequency (0.38 CSD/h per animal, n=7) and were accompanied by rCBF changes confined to the ipsilateral hemisphere. No significant relationship between the number of SDs and lesion size or intracranial pressure (ICP) could be detected. Even increasing the number of posttraumatic CSDs by application of KCl by more than six times did not increase ICP or contusion volume. We therefore conclude that CSDs may not contribute to posttraumatic secondary brain damage in the normally perfused and oxygenated brain.

Cortical spreading depression confounds concentration-dependent pial arteriolar dilation during N-methyl-D-aspartate superfusion

Pial arterioles do not express N-methyl-D-aspartate (NMDA) receptors but dilate in response to topical NMDA application. We explored the mechanism underlying NMDA-mediated responses in murine pial arterioles (11-31 microm), using a closed cranial window preparation, and found that arteriolar dilation was not concentration dependent. Pial arteriolar diameter abruptly increased within 3 min of superfusing 50 or 100 microM NMDA. Dilation reached a peak within 1 min (46 +/- 14%) and then declined to a plateau (28 +/- 13%) for the duration of superfusion. Whereas a higher concentration (200 microM) did not produce further dilation, lower concentrations (1-10 microM) did not dilate the arterioles at all. MK-801 (10 microM) abrogated the dilation response, whereas Nomega-nitro-L-arginine (1 mM) attenuated the peak and abolished the sustained dilation during NMDA superfusion. We determined that NMDA-induced pial arteriolar responses were evoked by cortical spreading depression, because abrupt vasodilation during 50 or 100 microM NMDA superfusion was associated with a large negative slow potential shift and electrocorticogram suppression that spread from the superfusion window to distant cortical areas. Our data suggest that the responses of pial arterioles to NMDA are caused in part by neurovascular coupling due to cortical spreading depression.

Transient increases in extracellular K+ produce two pharmacological distinct cytosolic Ca2+ transients

Transient increases in extracellular K+ are observed under various conditions, including repetitive neuronal firing, anoxia, ischemia and hypoglycemic coma. We studied changes in cytoplasmic Ca2+ ([Ca2+]cyt) evoked by pulses of KCl in human neuroblastoma SH-SY5Y cells and rat dorsal root ganglia (DRG) neurons at 37 degrees C. A "pulse" of KCl evoked two transient increases in [Ca2+]cyt, one upon addition of KCl (K+on) and the other upon removal of KCl (K+off). The K+on transient has been described in many cell types and is initiated by the activation of voltage-dependent Ca2+ channels followed by Ca2+-evoked Ca2+ release from intracellular Ca2+ stores. The level of KCl necessary to evoke the K+off transient depends on the type of neuron, in SH-SY5Y cells it required 100 mM KCl, in most (but not all) of dorsal root ganglia neurons it could be detected with 100-200 mM KCl and in a very few dorsal root ganglia neurons it was detectable at 20-50 mM KCl. In SH-SY5Y cells, reduction of extracellular Ca2+ inhibited the K+on more strongly than the K+off and slowed the decay of K+off. Isoflurane (1 mM) reduced the K+on)- but not the K+off-peak. However, isoflurane slowed the decay of K+off. The nonspecific cationic channel blocker La3+ (100 microM) had an effect similar to that of isoflurane. Treatment with thapsigargin (TG) at a concentration known to only deplete IP3-sensitive Ca2+ stores did not affect K+on or K+off, suggesting that Ca2+ release from the IP3-sensitive Ca2+ stores does not contribute to K+on and K+off transients and that the thapsigargin-sensitive Ca2+ ATPases do not contribute significantly to the rise or decay rates of these transients. These findings indicate that a pulse of extracellular K+ produces two distinct transient increases in [Ca2+]cyt.

Delayed secondary phase of peri-infarct depolarizations after focal cerebral ischemia: relation to infarct growth and neuroprotection

In focal cerebral ischemia, peri-infarct depolarizations (PIDs) cause an expansion of core-infarcted tissue into adjacent penumbral regions of reversible injury and have been shown to occur through 6 hr after injury. However, infarct maturation proceeds through 24 hr. Therefore, we studied PID occurrence through 72 hr after both transient and permanent middle cerebral artery occlusion (MCAo) via continuous DC recordings in nonanesthetized rats. PIDs occurred an average 13 times before reperfusion at 2 hr and then ceased for an average approximately 8 hr. After this quiescent period, PID activity re-emerged in a secondary phase, which reached peak incidence at 13 hr and consisted of a mean 52 PIDs over 2-24 hr. This phase corresponded to the period of infarct maturation; rates of infarct growth through 24 hr coincided with changes in PID frequency and peaked at 13 hr. In permanent MCAo, PIDs also occurred in a biphasic pattern with a mean of 78 events over 2-24 hr. Parameters of secondary phase PID incidence correlated with infarct volumes in transient and permanent ischemia models. The role of secondary phase PIDs in infarct development was further investigated in transient MCAo by treating rats with a high-affinity NMDA receptor antagonist at 8 hr after injury, which reduced post-treatment PID incidence by 57% and provided 37% neuroprotection. Topographic mapping with multielectrode recordings revealed multiple sources of PID initiation and patterns of propagation. These results suggest that PIDs contribute to the recruitment of penumbral tissue into the infarct core even after the restoration of blood flow and throughout the period of infarct maturation.

Preparation for the in vivo recording of neuronal responses in the visual cortex of anaesthetised marmosets (Callithrix jacchus)

The marmoset is becoming an important model for studies of primate vision, due to factors such as its small body size, lissencephalic brain, short gestational period and rapid postnatal development. For many studies of visual physiology (including single-cell recordings), it is a requirement that the animal is maintained under anaesthesia and neuromuscular block in order to ensure ocular stability. However, maintaining such a small animal (290-400 g) in good physiological condition for long periods requires expert attention. This becomes particularly important in the case of recordings from visual association cortex, where neuronal responses are known to be highly sensitive to factors such as the type and dose of anaesthetic, and the animal's physiological balance. The present protocol summarises our laboratory's experience over the last decade in developing a preparation for the study of marmoset visual cortex. It allows excellent recording from extrastriate areas for periods of at least 48 h, including the continuous study of isolated single cells for several hours.

Cortical spreading depression in the feline brain following sustained and transient stimuli studied using diffusion-weighted imaging

Cortical spreading depression (CSD) was induced by transient (10 min) applications of KCl in agar upon the cortical surface of alpha-chloralose anaesthetised cats. Its features were compared with CSD resulting from sustained applications of crystalline KCl through a mapping of the apparent diffusion coefficient (ADC) using diffusion-weighted echo planar imaging (DWI) over a poststimulus period of 60-100 min. Individual CSD events were computationally detected with the aid of Savitzky-Golay smoothing applied to critically sampled data derived from regions of interest (ROIs) made up of 2 x 2 pixel matrices. The latter were consistently placed at three selected sites on the suprasylvian gyrus (SG) and six sites on the marginal gyrus (MG). The CSD events thus detected were then quantitatively characterised for each ROI using the original time series. Both stimuli consistently elicited similar spreading patterns of initial, primary CSD events that propagated over the SG and marginal MG and were restricted to the hemispheres on which the stimuli were applied. There followed secondary events over smaller extents of cortical surface. Sustained stimuli elicited primary and secondary CSD events with similar amplitudes of ADC deflection that were distributed around a single mean. The ADC deflections were also conserved in peak amplitude throughout the course of their propagation. The initial primary event showed a poststimulus latency of 1.1 +/- 0.1 min. Successive secondary events followed at longer, but uniform, time intervals of around 10 min. Primary and secondary CSDs showed significantly different velocities of conduction (3.32 +/- 0.43 mm min(-1) vs. 2.11 +/- 0.21 mm min(-1), respectively; n = 5) across the cerebral hemisphere. In contrast, transient stimuli produced significantly fewer numbers of CSD events (3.8 +/- 0.5 events per animal, n = 5) than did sustained stimuli (7.4 +/- 0.5 events per animal, mean +/- S.E.M., n = 5, P = 0.002). The peak ADC deflection of their primary CSD events declined by approximately 30 % as they propagated from their initiation site to the interhemispheric boundary. The primary CSD event following a transient stimulus showed a latency of 1.4 +/- 0.1 min. It was followed by successive and smaller secondary ADC deflections that were separated by progressively longer time intervals. Conduction velocities of secondary events were similar to those of primary events. Conduction velocities of both primary and secondary events were slower than their counterparts following a sustained stimulus. ADC changes associated with CSD thus persist at times well after stimulus withdrawal and vary markedly with the nature of the initiating stimulus even in brain regions remote from the stimulus site.

Asymmetric propagation of spreading depression along the anteroposterior axis of the cerebral cortex in mice

The purpose of this study was to ascertain whether or not spreading depression (CSD) propagates symmetrically along the anteroposterior axis of the cortex of mice, and to determine where CSD should be elicited to achieve a uniform exposure of the cortex to this phenomenon. Experiments were performed in halothane-anesthetized mice, with three different locations aligned 1.5 mm from the midline used for either KCl elicitation of CSD or the recording of its propagation. Our results demonstrated that, at least in the mouse cortex, CSD propagated much more effectively from posterior to anterior regions than in the opposite direction. This feature was due to a different efficacy of propagation in the two opposite directions, and not to a reduced susceptibility of occipital regions to CSD elicitation. Heterogeneous CSD propagation constitutes a potential pitfall for neurochemical studies of post-CSD changes in mice, as brain tissue samples collected for this purpose should be uniformly exposed to CSD. Occipital sites for CSD induction are clearly optimal for this purpose. If CSD propagation is confirmed to be more effective from posterior to anterior regions in other species, this may be relevant to the pathophysiology of classical migraine because the most frequent aura symptoms (i.e., visual disturbances) originate in the occipital cortex.