Effects of Light Exposure duration on Severity and Long-Term Neurodevelopment following Photothrombotic Stroke in a Neonate.

Introduction: Perinatal stroke causes lasting neurological deficits and there are currently no effective treatment options. Established animal models of perinatal stroke do not always mimic the clinical presentation of neonatal injury or are technically challenging to perform. The photothrombotic (PT) stroke model is a minimally invasive method that replicates focal ischaemic injury. Few studies have applied the PT model in neonatal contexts, and none have examined both short- and long-term effects across varying injury severities. This study aimed to optimize a protocol to create a mild model of perinatal stroke and subsequently characterize injury progression, neuropathological impact, and motor deficits over time.

Methods: On postnatal day 10 we used the PT method to induce perinatal stroke in rat pups. Pups were exposed to various light exposure times (10, 20, or 30 min) to determine the optimal time needed to produce a mild and reproducible cortical stroke injury. Behavioural assessments were conducted on days 4, 10, 20, and 30 post-injury. Brains were collected for analysis on days 3 and 40 post-injury.

Results: Three days post-injury, the 20 and 30 min group had significant focal lesions and microbleeds were present in each of the PT groups. All PT groups showed significant neuron loss in the peri-infarct region and the thalamus, and microglia activation in multiple brain regions. As 30 min of light exposure showed extensive cortical tissue loss (>70%), we excluded the 30-min group from long-term assessment. 40 days post-injury, the 10 and 20 min groups demonstrated significant tissue loss and neuronal loss in the peri-infarct region and thalamus, but only the 20 min group showed neuron loss in the hippocampus. The 10 and 20 min groups both demonstrated ongoing motor deficits.

Conclusion: Our results demonstrate that increasing light exposure time in PT stroke results in a more severe stroke phenotype. 30 min of light exposure resulted in a severe injury at only 3 days post insult, therefore, was not further investigated. 10 and 20 min of light exposure had a similar effect at 3 days, however, after 40 days the 20 min exposure time created a moderate injury phenotype. From this study, we propose that 10 min of light exposure is optimal to create a mild stroke phenotype and is associated with motor deficits and altered neuropathology. This injury phenotype provides a focal and reproducible insult, while still being mild enough to feasibly test therapeutics.