A systematic review of the causes and consequences of spreading depolarization in neuroinflammation; implications for neurovascular disorders.

Background: Spreading depolarization (SD) is a wave of neuronal and glial depolarization observed in various neurological conditions, including stroke, traumatic brain injury, subarachnoid haemorrhage, and migraine aura. This depolarization disrupts ion homeostasis, creating high energy demand for recovery. While healthy tissue can compensate, pathological tissue may develop ischemia, worsening brain injury and outcomes. Identifying inflammatory mediators that exacerbate neuroinflammation after SD could guide targeted therapies. This review aimed to explore both the neuroinflammatory effects of SD and the impact of experimentally induced inflammatory states on SD characteristics.

Methods: PubMed and Scopus were systematically searched for preclinical studies that examined the effects of SD on inflammation, and the effects of an inflammatory state on SD responses. Data extracted included authors, publication details, study type, animal characteristics, group sizes, exclusions, relevant findings, and limitations. Additional details were collected for studies on SD and neuroinflammation, including induction methods, inflammatory markers and SD characteristics in altered inflammatory states.

Results: Several studies indicated that SD triggered a robust neuroinflammatory response, marked by upregulation of cytokines-interleukin-1β, tumour necrosis factor-α, and interleukin-6-alongside transcription factors such as nuclear factor kappa B, and activation of astrocytes and microglia. Key mediators including toll-like receptors, cyclooxygenase-2 and high mobility group box 1 were also implicated, with evidence of neurogenic involvement via the release of calcitonin gene-related peptide. Differences in inflammatory responses were identified between single and multiple SD induction. Studies measuring the effect of altered inflammatory states on SD propagation were limited. Models of peripheral inflammation and non-demyelinating autoimmune encephalomyelitis did not lead to significant alterations in SD characteristics. However, administration of tumour necrosis factor was able to reduce SD amplitude, suggesting a possible neuroprotective effect.

Conclusion: This review suggests potential mechanisms underlying the role of SD in neurological disorders. While SD is associated with inflammatory markers, evidence for the impact of heightened inflammatory states on cortical susceptibility to SD remains limited. Significant methodological variability and inflammatory disease models underscores the need for standardization to validate these findings. Further research into these mechanisms could identify novel therapeutic targets to mitigate SD-related neuroinflammation in neurological disorders.