The continuum of spreading depolarizations in acute cortical lesion development: Examining Leão's legacy

Abstract

A modern understanding of how cerebral cortical lesions develop after acute brain injury is based on Aristides Leão's historic discoveries of spreading depression and asphyxial/anoxic depolarization. Treated as separate entities for decades, we now appreciate that these events define a continuum of spreading mass depolarizations, a concept that is central to understanding their pathologic effects. Within minutes of acute severe ischemia, the onset of persistent depolarization triggers the breakdown of ion homeostasis and development of cytotoxic edema. These persistent changes are diagnosed as diffusion restriction in magnetic resonance imaging and define the ischemic core. In delayed lesion growth, transient spreading depolarizations arise spontaneously in the ischemic penumbra and induce further persistent depolarization and excitotoxic damage, progressively expanding the ischemic core. The causal role of these waves in lesion development has been proven by real-time monitoring of electrophysiology, blood flow, and cytotoxic edema. The spreading depolarization continuum further applies to other models of acute cortical lesions, suggesting that it is a universal principle of cortical lesion development. These pathophysiologic concepts establish a working hypothesis for translation to human disease, where complex patterns of depolarizations are observed in acute brain injury and appear to mediate and signal ongoing secondary damage.

Keywords: Spreading depression; brain edema; brain ischemia; brain trauma; cardiac arrest; cerebral blood flow; cerebrovascular disease; diffusion weighted MRI; electrophysiology; focal ischemia; global ischemia; neurocritical care; neuroprotection; neurovascular coupling; selective neuronal death; stroke; subarachnoid hemorrhage; system biology; two photon microscopy; vasospasm.

Figure 1.

Direct-current potential shifts in spreading depression and after ischemia are of the same nature. In all traces, negative is up, amplitude divisions are 3 mV, and time divisions are 1 min. (a) The original tracing published by Leão showing the initially negative slow voltage variation that accompanies spreading depression. Spreading depression was elicited by application of a tetanic current to the cortex at time marked “S.” Inset shows recording and stimulation schematic. (b) In the same article, he showed a similar but persistent negative slow voltage variation several minutes after clamping (denoted by * in the trace) of the common carotid and basilar arteries. (c) The similar nature of these phenomena was further demonstrated by inducing spreading depression with electrical stimulation as in (a) and then clamping the arteries for 1 min (from * to #) as soon as the slow voltage variation of spreading depression was observed. In this case, the negative slow voltage variation of spreading depression was prolonged. Subsequent studies showed that both spreading depression and the response to ischemia are spreading mass depolarizations that are dependent on energy supply for reversal. In the experiment shown in (c), we further find a prediction of how spreading depolarization causes infarct growth: if the depolarization itself, rather than an investigator’s hand, can worsen ischemia, the tissue should remain depolarized. Adapted from Leão with permission.

Figure 2.

Persistent depolarization defines the ischemic core and causes initial and delayed secondary infarct development. Schematic diagrams illustrate various zones of focal ischemia, such as middle cerebral artery occlusion (MCAO), defined by electrophysiologic function and regional cerebral blood flow (rCBF). Direct current (DC) potential recordings (black) of spreading depolarizations and rCBF (red) are shown below for the three monitoring locations indicated by stars. The diagram in (a) depicts the status of ischemic zones after passage of the initial spreading depolarization (Time A). Within seconds of ischemic onset, a broad area with rCBF <15–23 ml/100 g/min simultaneously develops (in a nonspreading manner) complete suppression of the electrocorticogram (electrical silence). After 2–5 min, spreading depolarization then develops, as shown for instance by the cross-hairs, in a more central zone where rCBF is <5–10 ml/100 g/min. Here, the depolarization is persistent and will be terminal if perfusion is not restored. The zone of persistent depolarization and rCBF <5–10 ml/100 g/min is defined as the ischemic core, and the zone just beyond the core with electrical silence is the classical (inner) penumbra. The initial depolarization spreads through penumbra (arrows) and into normally perfused tissue as a transient wave with decreasing duration determined by local rCBF. (b) In the subsequent course of minutes, hours, or even days, spreading depolarizations arise repeatedly in penumbral hot zones as a result of local energy supply-demand mismatch. These hot zones may be located in the outer penumbra (oligemic tissue with preserved spontaneous activity) or in the inner penumbra. In the former case, as illustrated, the depolarization propagates not only outward, but also inward toward the ischemic core, inducing persistent depolarization and further perfusion decrease in the innermost penumbra (recording location 2). Thus, the ischemic core and region of developing infarction increase in a step-wise manner after the passage of each spreading depolarization (Time B). Note that spreading depolarizations that are prolonged or recur in high numbers can have the same cumulative effect as a single persistent depolarization, reaching the threshold duration of ionic derangement and metabolic impairment that commits neurons to cell death.

Figure 3.

The spreading depolarization continuum unites anoxic (persistent) depolarization and spreading depression. (a) After middle cerebral artery occlusion, simultaneous suppression of the electrocorticogram (AC-ECoG, 0.5–70 Hz) develops within seconds in the ischemic core and classical penumbra. This is followed ∼2 min later by persistent depolarization that develops in a spreading manner, as revealed by a negative DC shift of the cortical potential (DC-ECoG). (b) The same sequence of events occurs after asphyxiation, as shown in global ischemia by Leão. (c) In focal ischemia, the initial spreading depolarization propagates into the normally perfused periphery, where it induces spreading depression of spontaneous AC-ECoG activity. Here, asphyxiation was induced by reducing inhaled oxygen to 0% and focal ischemia was induced by injecting a blood clot near the origin of the middle cerebral artery. ECoG was recorded from two glass micropipette electrodes separated by 2 mm and cerebral blood flow was monitored by laser Doppler flowmetry.

Figure 4.

Changes in extracellular ion concentrations in spreading depression and asphyxial depolarization. Extracellular ion concentrations change to a similar degree and with similar time course during spreading depression (historical use, Table 1) in the normal brain and during the spreading depolarization that develops after asphyxiation. In the latter case, depolarization persists until cerebral perfusion is restored. A notable exception between the two cases is the gradual rise in [K⁺]_e and decline of pH prior to the abrupt shifts during asphyxial depolarization. Adapted from Hansen and Lauritzen.

Figure 5.

Spreading depolarizations evoked in hippocampal slices by oxygen-glucose deprivation and by high K⁺. CA1 neurons were loaded with Ca²⁺ indicator fura-6F. The first image panel shows raw 380 nm flourescence and subsequent panels are pseudocolor images that represent [Ca²⁺]_i. (a) Following oxygen-glucose deprivation, there is no increase in Ca²⁺ in ∼11.5 min prior to onset of spreading depolarization (SD). After SD, there is a large irrecoverable Ca²⁺ increase (∼24 µM) that originates in the soma and progresses toward apical dendrites, resulting in rapid neuronal injury. (b) SD evoked by high K⁺, by contrast, produces a transient Ca²⁺ elevation in distal dendrites that propagates toward, but never fully involves the soma, and [Ca²⁺]_i returns to basal levels in <2 min without neuronal injury. SD propagation rates are similar in the two conditions. Initiation and propagation of SD evoked by high K⁺, but not by oxygen-glucose deprivation, is dependent on the intracellular Ca²⁺ influx and can be prevented by Ca²⁺ removal from the bath, illustrating a mechanistic difference that arises along the continuum. Reproduced from Dietz et al.

Figure 6.

Cytotoxic edema consequent to persistent depolarization is the basis of diffusion lesions in clinical imaging of stroke. (a) and (b) show two-photon images of apical dendrites in superficial layers of mouse cerebral cortex following bilateral common carotid artery occlusion (BCCAO) at times shown in DC potential recording above. The recording shows the onset of spreading depolarization in the imaging field 4 min 30 sec after BCCAO. Normal dendritic morphology with spines, which are sites of excitatory synaptic transmission, is observed for the first minutes after BCCAO, but spines disappear and dendrites become beaded after the tissue depolarizes. Image in (b) is taken 26 sec after depolarization onset. (c) and (d) show diffusion-weighted MRI scans from patients with ischemic stroke and aneurysmal subarachnoid hemorrhage. The scan in (c) was taken within 48 h of cardioembolic stroke in the right middle and anterior cerebral artery territory of a 41-year old woman with initial NIH stroke scale of 16. The scan in (d) was obtained 3 days following rupture of a basilar tip aneurysm in a 57-year old man who presented as Hunt-Hess grade 4 and modified Fisher grade 2. At this time, transcranial Doppler showed moderate-severe vasospasm in the right and mild-moderate vasospasm in the left middle cerebral arteries. Hyperintense regions reflect decreases in apparent diffusion coefficient caused by cytotoxic edema, dendritic beading and restricted intracellular diffusion of water. Two-photon imaging studies suggest that these changes reflect the occurrence of persistent tissue depolarization.

Figure 7.

The continuum of spreading depolarizations in a rat model of acute subdural hematoma. Electrophysiologic recordings of spontaneous electrocorticographic activity (top traces; 0.5–50 Hz) and DC potential (bottom traces) from two micropipette electrodes in cerebral cortex. Slow infusion of 0.4 ml of arterial blood into the subdural space (large arrow, left) causes an immediate depolarization that begins at electrode 2 and spreads to electrode 1 at 4 mm/min. At electrode 2, the depolarization is persistent through 4 h of monitoring and TTC-staining confirms tissue infarction at this location. The initial depolarization at electrode 1 is prolonged with recovery after 15 min, and spontaneous short-lasting depolarizations are subsequently observed. When the animal is killed by asphyxiation after 4 h (large arrow, right), terminal depolarization is observed in viable cortex (electrode 1) but not in the infarct core. Scale bars are 1 and 20 mV for top and bottom traces, respectively, and apply to both electrodes.