Sustained NMDA receptor activation by spreading depolarizations can initiate excitotoxic injury in metabolically compromised neurons

Authors


C. W. Shuttleworth: Department of Neurosciences, University of New Mexico School of Medicine, Albuquerque, NM 87131-0001, USA. Email: bshuttleworth@salud.unm.edu

Key points

  • • Spreading depolarization (SD) is a profound neuronal and glial depolarization implicated in the progression of acute brain injury.
  • • In metabolically compromised tissue, SD can trigger irrecoverable injury; however, the underlying cellular mechanisms are not well established.
  • • We investigated consequences of SD in hippocampal brain slices, using a combination of electrophysiological and single-cell imaging methods.
  • • We characterized a brief period of prolonged NMDA receptor (NMDAR) activation after SD onset, which appeared to underlie extended Ca2+ elevations in dendrites.
  • • Prolonged NMDAR activation was sufficient to cause injury after SD in metabolically compromised neurons, and therefore may contribute to deleterious consequences of SD in pathological conditions.

Abstract  Spreading depolarizations (SDs) are slowly propagating waves of near-complete neuronal and glial depolarization. SDs have been recorded in patients with brain injury, and the incidence of SD significantly correlates with outcome severity. Although it is well accepted that the ionic dyshomeostasis of SD presents a severe metabolic burden, there is currently limited understanding of SD-induced injury processes at a cellular level. In the current study we characterized events accompanying SD in the hippocampal CA1 region of murine brain slices, using whole-cell recordings and single-cell Ca2+ imaging. We identified an excitatory phase that persisted for approximately 2 min following SD onset, and accompanied with delayed dendritic ionic dyshomeostasis. The excitatory phase coincided with a significant increase in presynaptic glutamate release, evidenced by a transient increase in spontaneous EPSC frequency and paired-pulse depression of evoked EPSCs. Activation of NMDA receptors (NMDARs) during this late excitatory phase contributed to the duration of individual neuronal depolarizations and delayed recovery of extracellular slow potential changes. Selectively targeting the NMDAR activation following SD onset (by delayed pressure application of a competitive NMDAR antagonist) significantly decreased the duration of cellular depolarizations. Recovery of dendritic Ca2+ elevations following SD were also sensitive to delayed NMDA antagonist application. Partial inhibition of neuronal energy metabolism converted SD into an irrecoverable event with persistent Ca2+ overload and membrane compromise. Delayed NMDAR block was sufficient to prevent these acute injurious events in metabolically compromised neurons. These results identify a significant contribution of a late component of SD that could underlie neuronal injury in pathological circumstances.

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