Recording, analysis, and interpretation of spreading depolarizations in neurointensive care: Review and recommendations of the COSBID research group.

Spreading depolarizations (SD) are waves of abrupt, near-complete breakdown of neuronal transmembrane ion gradients, are the largest possible pathophysiologic disruption of viable cerebral gray matter, and are a crucial mechanism of lesion development. Spreading depolarizations are increasingly recorded during multimodal neuromonitoring in neurocritical care as a causal biomarker providing a diagnostic summary measure of metabolic failure and excitotoxic injury. Focal ischemia causes spreading depolarization within minutes. Further spreading depolarizations arise for hours to days due to energy supply-demand mismatch in viable tissue. Spreading depolarizations exacerbate neuronal injury through prolonged ionic breakdown and spreading depolarization-related hypoperfusion (spreading ischemia). Local duration of the depolarization indicates local tissue energy status and risk of injury. Regional electrocorticographic monitoring affords even remote detection of injury because spreading depolarizations propagate widely from ischemic or metabolically stressed zones; characteristic patterns, including temporal clusters of spreading depolarizations and persistent depression of spontaneous cortical activity, can be recognized and quantified. Here, we describe the experimental basis for interpreting these patterns and illustrate their translation to human disease. We further provide consensus recommendations for electrocorticographic methods to record, classify, and score spreading depolarizations and associated spreading depressions. These methods offer distinct advantages over other neuromonitoring modalities and allow for future refinement through less invasive and more automated approaches.

Multi-modal assessment of neurovascular coupling during cerebral ischaemia and reperfusion using remote middle cerebral artery occlusion.

Hyperacute changes in cerebral blood flow during cerebral ischaemia and reperfusion are important determinants of injury. Cerebral blood flow is regulated by neurovascular coupling, and disruption of neurovascular coupling contributes to brain plasticity and repair problems. However, it is unknown how neurovascular coupling is affected hyperacutely during cerebral ischaemia and reperfusion. We have developed a remote middle cerebral artery occlusion model in the rat, which enables multi-modal assessment of neurovascular coupling immediately prior to, during and immediately following reperfusion. Male Wistar rats were subjected to remote middle cerebral artery occlusion, where a long filament was advanced intraluminally through a guide cannula in the common carotid artery. Transcallosal stimulation evoked increases in blood flow, tissue oxygenation and neuronal activity, which were diminished by middle cerebral artery occlusion and partially restored during reperfusion. These evoked responses were not affected by administration of the thrombolytic alteplase at clinically used doses. Evoked cerebral blood flow responses were fully restored at 24 h post-middle cerebral artery occlusion indicating that neurovascular dysfunction was not sustained. These data show for the first time that the rat remote middle cerebral artery occlusion model coupled with transcallosal stimulation provides a novel method for continuous assessment of hyperacute neurovascular coupling changes during ischaemia and reperfusion, and offers unique insight into hyperacute ischaemic pathophysiology.

Neocortical gamma oscillations in idiopathic generalized epilepsy.

OBJECTIVE: Absence seizures in patients with idiopathic generalized epilepsy (IGE) may in part be explained by a decrease in phasic GABAA (type-A γ-aminobutyric acid) receptor function, but the mechanisms are only partly understood. Here we studied the relation between ictal and interictal spike-wave discharges (SWDs) and electroencephalography (EEG) gamma oscillatory activity (30-60 Hz) in patients with IGE. METHODS: EEG recordings were obtained of 14 children with IGE (mean age, 8.5 ± 5 years) and 14 age- and sex-matched controls. Time-frequency analysis of each seizure and seizure-free control epochs was performed and cross-coherences of neocortical gamma oscillations were calculated to describe interictal and ictal characteristics of generalized seizures. RESULTS: SWDs were characterized with an abrupt increase of oscillatory activity of 3-4 and 13-60 Hz, peaking at 3-4 and 30-60 Hz, and with a simultaneous decrease in the 8-12 Hz frequency band. The rise in EEG gamma oscillations was short-lasting and decreased before activity declined at lower frequency ranges. Compared to control patients, patients with epilepsy also showed higher interictal values of mean coherence of gamma activity, but this interictal increase was not significant after post hoc analysis. SIGNIFICANCE: Our data support the hypothesis that gamma oscillatory activity increase concomitantly with rises in activity of lower EEG frequencies during absence seizures and that the activity starts to cease earlier than lower EEG frequencies. The data did not support a change in gamma activity preceding the 3-4 Hz SWDs. SWDs are hypothetically generated by the synchronous interaction between the thalamus and the cortex, whereas the production of gamma activity is the result of activity in local inhibitory networks. Thus, the modification of SWD by gamma activity may be understood in terms of the cellular and synaptic mechanisms involved.

Increased 20-HETE synthesis explains reduced cerebral blood flow but not impaired neurovascular coupling after cortical spreading depression in rat cerebral cortex.

Cortical spreading depression (CSD) is associated with release of arachidonic acid, impaired neurovascular coupling, and reduced cerebral blood flow (CBF), caused by cortical vasoconstriction. We tested the hypothesis that the released arachidonic acid is metabolized by the cytochrome P450 enzyme to produce the vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), and that this mechanism explains cortical vasoconstriction and vascular dysfunction after CSD. CSD was induced in the frontal cortex of rats and the cortical electrical activity and local field potentials recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension (tpO(2)) using polarographic microelectrodes. 20-HETE synthesis was measured in parallel experiments in cortical brain slices exposed to CSD. We used the specific inhibitor HET0016 (N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine) to block 20-HETE synthesis. CSD increased 20-HETE synthesis in brain slices for 120 min, and the time course of the increase in 20-HETE paralleled the reduction in CBF after CSD in vivo. HET0016 blocked the CSD-induced increase in 20-HETE synthesis and ameliorated the persistent reduction in CBF, but not the impaired neurovascular coupling after CSD. These findings suggest that CSD-induced increments in 20-HETE cause the reduction in CBF after CSD and that the attenuation of stimulation-induced CBF responses after CSD has a different mechanism. We suggest that blockade of 20-HETE synthesis may be clinically relevant to ameliorate reduced CBF in patients with migraine and acute brain cortex injuries.

Cyclosporine A, FK506, and NIM811 ameliorate prolonged CBF reduction and impaired neurovascular coupling after cortical spreading depression.

Cortical spreading depression (CSD) is associated with mitochondrial depolarization, increasing intracellular Ca(2+), and the release of free fatty acids, which favor opening of the mitochondrial permeability transition pore (mPTP) and activation of calcineurin (CaN). Here, we test the hypothesis that cyclosporine A (CsA), which blocks both mPTP and CaN, ameliorates the persistent reduction of cerebral blood flow (CBF), impaired vascular reactivity, and a persistent rise in the cerebral metabolic rate of oxygen (CMRO(2)) following CSD. In addition to CsA, we used the specific mPTP blocker NIM811 and the specific CaN blocker FK506. Cortical spreading depression was induced in rat frontal cortex. Electrocortical activity was recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension with polarographic microelectrodes. Electrocortical activity, basal CBF, CMRO(2), and neurovascular and neurometabolic coupling were unaffected by all three drugs under control conditions. NIM811 augmented the rise in CBF observed during CSD. Cyclosporine A and FK506 ameliorated the persistent decrease in CBF after CSD. All three drugs prevented disruption of neurovascular coupling after CSD; the rise in CMRO(2) was unchanged. Our data suggest that blockade of mPTP formation and CaN activation may prevent persistent CBF reduction and vascular dysfunction after CSD.

Principal cell spiking, postsynaptic excitation, and oxygen consumption in the rat cerebellar cortex.

One contention within the field of neuroimaging concerns the character of the depicted activity: Does it represent neuronal action potential generation (i.e., spiking) or postsynaptic excitation? This question is related to the metabolic costs of different aspects of neurosignaling. The cerebellar cortex is well suited for addressing this problem because synaptic input to and spiking of the principal cell, the Purkinje cell (PC), are spatially segregated. Also, PCs are pacemakers, able to generate spikes endogenously. We examined the contributions to cerebellar cortical oxygen consumption (CMRO2) of postsynaptic excitation and PC spiking during evoked and ongoing neuronal activity in the rat. By inhibiting excitatory synaptic input using ionotropic glutamate receptor blockers, we found that the increase in CMRO2 evoked by parallel fiber (PF) stimulation depended entirely on postsynaptic excitation. In contrast, PC spiking was largely responsible for the increase in CMRO2 when ongoing neuronal activity was increased by gamma-aminobutyric acid type A receptor blockade. In this case, CMRO2 increased equally during PC spiking with excitatory synaptic activity as during PC pacemaker spiking without excitatory synaptic input. Subsequent inhibition of action potential propagation and neurotransmission by blocking voltage-gated Na+-channels eliminated the increases in CMRO2 due to PF stimulation and increased PC spiking, but left a large fraction of CMRO2, i.e., basal CMRO2, intact. In conclusion, whereas basal CMRO2 in anesthetized animals did not seem to be related to neurosignaling, increases in CMRO2 could be induced by all aspects of neurosignaling. Our findings imply that CMRO2 responses cannot a priori be assigned to specific neuronal activities.

Persistent increase in oxygen consumption and impaired neurovascular coupling after spreading depression in rat neocortex.

Cortical spreading depression (CSD) is associated with a dramatic failure of brain ion homeostasis and increased energy metabolism. There is strong clinical and experimental evidence to suggest that CSD is the mechanism of migraine, and involved in progressive neuronal injury in stroke and head trauma. Here we tested the hypothesis that single episodes of CSD induced acute hypoxia, and prolonged impairment of neurovascular and neurometabolic coupling. Cortical spreading depression was induced in rat frontal cortex, whereas cortical electrical activity and local field potentials (LFPs) were recorded by glass microelectrodes, cerebral blood flow (CBF) by laser-Doppler flowmetry, and tissue oxygen tension (tpO(2)) with polarographic microelectrodes. Cortical spreading depression increased cerebral metabolic rate of oxygen (CMRO(2)) by 71%+/-6.7% and CBF by 238%+/-48.1% for 1 to 2 mins. For the following 2 h, basal tpO(2) and CBF were reduced whereas basal CMRO(2) was persistently elevated by 8.1%+/-2.9%. In addition, within first hour after CSD we found impaired neurovascular coupling (LFP versus CBF), whereas neurometabolic coupling (LFP versus CMRO(2)) remained unaffected. Impaired neurovascular coupling was explained by both reduced vascular reactivity and suppressed function of cortical inhibitory interneurons. The protracted effects of CSD on basal CMRO(2) and neurovascular coupling may contribute to cellular dysfunction in patients with migraine and acutely injured cerebral cortex.

Pathway-specific variations in neurovascular and neurometabolic coupling in rat primary somatosensory cortex.

Functional neuroimaging signals are generated, in part, by increases in cerebral blood flow (CBF) evoked by mediators, such as nitric oxide and arachidonic acid derivatives that are released in response to increased neurotransmission. However, it is unknown whether the vascular and metabolic responses within a given brain area differ when local neuronal activity is evoked by an activity in the distinct neuronal networks. In this study we assessed, for the first time, the differences in neuronal responses and changes in CBF and oxygen consumption that are evoked after the activation of two different inputs to a single cortical area. We show that, for a given level of glutamatergic synaptic activity, corticocortical and thalamocortical inputs evoked activity in pyramidal cells and different classes of interneurons, and produced different changes in oxygen consumption and CBF. Furthermore, increases in stimulation intensities either turned off or activated additional classes of inhibitory interneurons immunoreactive for different vasoactive molecules, which may contribute to increases in CBF. Our data imply that for a given cortical area, the amplitude of vascular signals will depend critically on the type of input, and that a positive blood oxygen level-dependent (BOLD) signal may be a consequence of the activation of both pyramidal cells and inhibitory interneurons.

Contribution of somatosensory cortex to evoked cerebellar blood flow responses.

Projections from the trigeminocerebellar pathway and the somatosensory cortex coincide spatially in the granule cell layer of Crus I/II of the cerebellar hemisphere. A biphasic field potential was seen: one peak at 10 ms (trigeminal input) and another at 20 ms (somatosensory input). Linear correlation analysis revealed only a weak coupling between somatosensory input and cerebellar blood flow responses to infraorbital nerve stimulation. In separate experiments, cortical spreading depression attenuated the field potential peak at 20 ms while blood flow responses remained unaltered. Thus, trigeminocerebellar activity explained the evoked blood flow responses. Our data provide further evidence that activity-dependent blood flow responses are context-sensitive and that interaction between excitatory neuronal circuits targeting the same cells may occlude vascular responses.