Spreading depolarization (SD) is a slowly propagating, coordinated depolarization of brain tissue, which is followed by a transient (5–10 min) 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 KCl was used to induce SD and synaptic potentials were evoked by electrical stimulation of Schaffer collateral inputs to hippocampal area 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 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 (DPCPX or 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.

Leao's spreading depression of cortical activity is a propagating silencing of neuronal activity resulting from spreading depolarization (SD). We evaluated the contributions of action potential (AP) failure and adenosine A receptor (AR) activation to the depression of evoked and spontaneous electrocorticographic (ECoG) activity after SD in vivo, in anesthetized mice. We compared depression with SD-induced effects on AP-dependent transmission, and synaptic potentials in the transcallosal and thalamocortical pathways. After SD, APs recovered rapidly, within 1-2 min, as demonstrated by evoked activity in distant projection targets. Evoked corticocortical postsynaptic potentials recovered next, within ∼5 min. Spontaneous ECoG and evoked thalamocortical postsynaptic potentials recovered together, after ∼10-15 min. The duration of ECoG depression was shortened 20% by systemic (10 mg/kg) or focal (30 µM) administration of AR competitive antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). ECoG depression was also shortened by focal application of exogenous adenosine deaminase (ADA; 100 U/mL), and conversely, was prolonged 50% by the non-competitive ADA inhibitor deoxycoformycin (DCF; 100 µM). We concluded that while initial depolarization block is brief, adenosine AR activation, in part, contributes to the persistent secondary phase of Leao's cortical spreading depression.

Spreading depolarization (SD), a pathologic feature of migraine, stroke and traumatic brain injury, is a propagating depolarization of neurons and glia causing profound metabolic demand. Adenosine, the low-energy metabolite of ATP, has been shown to be elevated after SD in brain slices and under conditions likely to trigger SD in vivo. The relationship between metabolic status and adenosine accumulation after SD was tested here, in brain slices and in vivo. In brain slices, metabolic impairment (assessed by nicotinamide adenine dinucleotide (phosphate) autofluorescence and O 2 availability) was associated with prolonged extracellular direct current (DC) shifts indicating delayed repolarization, and increased adenosine accumulation. In vivo, adenosine accumulation was observed after SD even in otherwise healthy mice. As in brain slices, in vivo adenosine accumulation correlated with DC shift duration and increased when DC shifts were prolonged by metabolic impairment (i.e., hypoglycemia or middle cerebral artery occlusion). A striking pattern of adenosine dynamics was observed during focal ischemic stroke, with nearly all the observed adenosine signals in the periinfarct region occurring in association with SDs. These findings suggest that adenosine accumulation could serve as a biomarker of SD incidence and severity, in a range of clinical conditions. Keywords: acute stroke; adenosine; brain slice; electrophysiology; energy metabolism; spreading depression INTRODUCTION Spreading depolarization (SD) of brain tissue is a self-propagating wave of neuronal and glial activation, carried by large transmembrane currents that depolarize cells and disrupt ionic gradients. 1,2 Spreading depolarization imposes a large metabolic burden on brain tissue, evidenced by depletion of energy substrates and O 2 3,4 and by accumulation of metabolic byproducts such as lactate and H + . 5,6 ATP concentration drops by nearly 50% during SD in normoxic conditions, 7 and metabolic depletion is expected to be more severe when occurring in tissue with limited substrate availability.Recent clinical studies strongly suggest that SD is involved in the pathophysiology of migraine with aura, thromboembolic stroke, traumatic brain injury, and subarachnoid hemorrhage. 8 While normoxic, normoglycemic SD can be noninjurious, 9 SD in the setting of metabolic compromise exacerbates neuronal injury. 10,11 Work in animal models has identified delayed repolarization, marked by prolonged extracellular direct current (DC) shifts, as a hallmark of injurious SD in metabolically compromised tissue. 10 Observational studies in human subjects have confirmed the association between prolonged DC shifts and poor outcome after SD, 12 but human studies have so far been limited by the requirement for invasive electrical recordings in patients with craniotomies. It would be helpful to establish additional biomarkers of metabolic status, to assess vulnerability to SD in injured brain.Extracellular adenosine accumulation can serve as a global indicator of energy charge....

Spreading depolarizations (SDs) are profound disruptions of cellular homeostasis that slowly propagate through gray matter and present an extraordinary metabolic challenge to brain tissue. Recent work has shown that SDs occur commonly in human patients in the neurointensive care setting and have established a compelling case for their importance in the pathophysiology of acute brain injury. The International Conference on Spreading Depolarizations (iCSD) held in Boca Raton, Florida, in September of 2018 included a discussion session focused on the question of "Which SDs are deleterious to brain tissue?" iCSD is attended by investigators studying various animal species including invertebrates, in vivo and in vitro preparations, diseases of acute brain injury and migraine, computational modeling, and clinical brain injury, among other topics. The discussion included general agreement on many key issues, but also revealed divergent views on some topics that are relevant to the design of clinical interventions targeting SDs. A draft summary of viewpoints offered was then written by a multidisciplinary writing group of iCSD members, based on a transcript of the session. Feedback of all discussants was then formally collated, reviewed and incorporated into the final document. It is hoped that this report will stimulate collection of data that are needed to develop a more nuanced understanding of SD in different pathophysiological states, as the field continues to move toward effective clinical interventions.

No abstract