Brain energy metabolism: A roadmap for future research.

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+ , as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Spreading depolarizations pose critical energy challenges in acute brain injury.

Spreading depolarization (SD) is an electrochemical wave of neuronal depolarization mediated by extracellular K+ and glutamate, interacting with voltage-gated and ligand-gated ion channels. SD is increasingly recognized as a major cause of injury progression in stroke and brain trauma, where the mechanisms of SD-induced neuronal injury are intimately linked to energetic status and metabolic impairment. Here, I review the established working model of SD initiation and propagation. Then, I summarize the historical and recent evidence for the metabolic impact of SD, transitioning from a descriptive to a mechanistic working model of metabolic signaling and its potential to promote neuronal survival and resilience. I quantify the energetic cost of restoring ionic gradients eroded during SD, and the extent to which ion pumping impacts high-energy phosphate pools and the energy charge of affected tissue. I link energy deficits to adaptive increases in the utilization of glucose and O2 , and the resulting accumulation of lactic acid and CO2 downstream of catabolic metabolic activity. Finally, I discuss the neuromodulatory and vasoactive paracrine signaling mediated by adenosine and acidosis, highlighting these metabolites' potential to protect vulnerable tissue in the context of high-frequency SD clusters.

Patient-derived SLC6A1 variant S295L results in an epileptic phenotype similar to haploinsufficient mice.

The solute carrier family 6 member 1 (SLC6A1) gene encodes GAT-1, a γ-aminobutyric acid transporter expressed on astrocytes and inhibitory neurons. Mutations in SLC6A1 are associated with epilepsy and developmental disorders, including motor and social impairments, but variant-specific animal models are needed to elucidate mechanisms. Here, we report electrocorticographic (ECoG) recordings and clinical data from a patient with a variant in SLC6A1 that encodes GAT-1 with a serine-to-leucine substitution at amino acid 295 (S295L), who was diagnosed with childhood absence epilepsy. Next, we show that mice bearing the S295L mutation (GAT-1S295L/+ ) have spike-and-wave discharges with motor arrest consistent with absence-type seizures, similar to GAT-1+/- mice. GAT-1S295L/+ and GAT-1+/- mice follow the same pattern of pharmacosensitivity, being bidirectionally modulated by ethosuximide (200 mg/kg ip) and the GAT-1 antagonist NO-711 (10 mg/kg ip). By contrast, GAT-1-/- mice were insensitive to both ethosuximide and NO-711 at the doses tested. In conclusion, ECoG findings in GAT-1S295L/+ mice phenocopy GAT-1 haploinsufficiency and provide a useful preclinical model for drug screening and gene therapy investigations.

Thalamocortical circuits in generalized epilepsy: Pathophysiologic mechanisms and therapeutic targets.

Generalized epilepsy affects 24 million people globally; at least 25% of cases remain medically refractory. The thalamus, with widespread connections throughout the brain, plays a critical role in generalized epilepsy. The intrinsic properties of thalamic neurons and the synaptic connections between populations of neurons in the nucleus reticularis thalami and thalamocortical relay nuclei help generate different firing patterns that influence brain states. In particular, transitions from tonic firing to highly synchronized burst firing mode in thalamic neurons can cause seizures that rapidly generalize and cause altered awareness and unconsciousness. Here, we review the most recent advances in our understanding of how thalamic activity is regulated and discuss the gaps in our understanding of the mechanisms of generalized epilepsy syndromes. Elucidating the role of the thalamus in generalized epilepsy syndromes may lead to new opportunities to better treat pharmaco-resistant generalized epilepsy by thalamic modulation and dietary therapy.

Neuroworsening in the Emergency Department Is a Predictor of Traumatic Brain Injury Intervention and Outcome: A TRACK-TBI Pilot Study.

INTRODUCTION: Neuroworsening may be a sign of progressive brain injury and is a factor for treatment of traumatic brain injury (TBI) in intensive care settings. The implications of neuroworsening for clinical management and long-term sequelae of TBI in the emergency department (ED) require characterization. METHODS: Adult TBI subjects from the prospective Transforming Research and Clinical Knowledge in Traumatic Brain Injury Pilot Study with ED admission and disposition Glasgow Coma Scale (GCS) scores were extracted. All patients received head computed tomography (CT) scan <24 h post-injury. Neuroworsening was defined as a decline in motor GCS at ED disposition (vs. ED admission). Clinical and CT characteristics, neurosurgical intervention, in-hospital mortality, and 3- and 6-month Glasgow Outcome Scale-Extended (GOS-E) scores were compared by neuroworsening status. Multivariable regressions were performed for neurosurgical intervention and unfavorable outcome (GOS-E ≤ 3). Multivariable odds ratios (mOR) with [95% confidence intervals] were reported. RESULTS: In 481 subjects, 91.1% had ED admission GCS 13-15 and 3.3% had neuroworsening. All neuroworsening subjects were admitted to intensive care unit (vs. non-neuroworsening: 26.2%) and were CT-positive for structural injury (vs. 45.4%). Neuroworsening was associated with subdural (75.0%/22.2%), subarachnoid (81.3%/31.2%), and intraventricular hemorrhage (18.8%/2.2%), contusion (68.8%/20.4%), midline shift (50.0%/2.6%), cisternal compression (56.3%/5.6%), and cerebral edema (68.8%/12.3%; all p < 0.001). Neuroworsening subjects had higher likelihoods of cranial surgery (56.3%/3.5%), intracranial pressure (ICP) monitoring (62.5%/2.6%), in-hospital mortality (37.5%/0.6%), and unfavorable 3- and 6-month outcome (58.3%/4.9%; 53.8%/6.2%; all p < 0.001). On multivariable analysis, neuroworsening predicted surgery (mOR = 4.65 [1.02-21.19]), ICP monitoring (mOR = 15.48 [2.92-81.85], and unfavorable 3- and 6-month outcome (mOR = 5.36 [1.13-25.36]; mOR = 5.68 [1.18-27.35]). CONCLUSIONS: Neuroworsening in the ED is an early indicator of TBI severity, and a predictor of neurosurgical intervention and unfavorable outcome. Clinicians must be vigilant in detecting neuroworsening, as affected patients are at increased risk for poor outcomes and may benefit from immediate therapeutic interventions.

Which Spreading Depolarizations Are Deleterious To Brain Tissue?

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.

Evidence that adenosine contributes to Leao's spreading depression in vivo.

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 A1 receptor (A1R) 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 A1R 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 A1R activation, in part, contributes to the persistent secondary phase of Leao's cortical spreading depression.

Spreading depolarization-induced adenosine accumulation reflects metabolic status in vitro and in vivo.

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 O2 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.

Intracellular Zn2+ accumulation enhances suppression of synaptic activity following spreading depolarization.

Spreading depolarization (SD) is a feed-forward wave that propagates slowly throughout brain tissue and recovery from SD involves substantial metabolic demand. Presynaptic Zn(2+) release and intracellular accumulation occurs with SD, and elevated intracellular Zn(2+) ([Zn(2+) ]i ) can impair cellular metabolism through multiple pathways. We tested here whether increased [Zn(2+) ]i could exacerbate the metabolic challenge of SD, induced by KCl, and delay recovery in acute murine hippocampal slices. [Zn(2+) ]i loading prior to SD, by transient ZnCl2 application with the Zn(2+) ionophore pyrithione (Zn/Pyr), delayed recovery of field excitatory post-synaptic potentials (fEPSPs) in a concentration-dependent manner, prolonged DC shifts, and significantly increased extracellular adenosine accumulation. These effects could be due to metabolic inhibition, occurring downstream of pyruvate utilization. Prolonged [Zn(2+) ]i accumulation prior to SD was required for effects on fEPSP recovery and consistent with this, endogenous synaptic Zn(2+) release during SD propagation did not delay recovery from SD. The effects of exogenous [Zn(2+) ]i loading were also lost in slices preconditioned with repetitive SDs, implying a rapid adaptation. Together, these results suggest that [Zn(2+) ]i loading prior to SD can provide significant additional challenge to brain tissue, and could contribute to deleterious effects of [Zn(2+) ]i accumulation in a range of brain injury models.