Related Experiment Video
Updated: May 9, 2025
Continuous Video Electroencephalogram during Hypoxia-Ischemia in Neonatal Mice
Published on: June 11, 2020
Ongoing loss of viable neurons for weeks after mild hypoxia-ischaemia
Melanie A McNally1, Lauren A Lau1, Simon Granak1
1Department of Neurology, Harvard Medical School and Massachusetts General Hospital, Boston, MA 02114, USA.
Insights
Mild hypoxic-ischaemic encephalopathy in neonates causes delayed brain injury and neuronal loss, even with minimal initial damage. This suggests potential for neuroprotection interventions targeting later stages of brain injury.
Area of Science:
- Neuroscience
- Neonatal research
- Brain injury mechanisms
Background:
- Mild hypoxic-ischaemic encephalopathy (HIE) is common in newborns, leading to significant neurodevelopmental issues by school age.
- Current therapies for mild HIE lack evidence-based guidelines.
- The progression and cellular mechanisms of mild perinatal brain injury remain poorly understood.
Purpose of the Study:
- To investigate the evolution of mild perinatal brain injury using advanced imaging techniques.
- To characterize the cellular and network-level changes following mild hypoxia-ischaemia.
- To identify potential therapeutic windows for neuroprotection.
Main Methods:
- Longitudinal two-photon imaging of transgenic mice with fluorescent calcium indicators.
- In vitro studies using organotypic hippocampal cultures subjected to oxygen-glucose deprivation.
- In vivo studies involving mild hypoxia-ischaemia in P10 mice (carotid ligation and hypoxia).
- Assessment of neuronal viability, activity, and volumetric changes via imaging and MRI.
Main Results:
- Mild hypoxia-ischaemia induced delayed and progressive neuronal loss in the hippocampus and neocortex within two weeks, despite minimal immediate cell death.
- Cortical network activity was transiently suppressed for over 2 hours post-injury but recovered within 24 hours.
- Neurons destined for delayed death showed normal function for days after injury, indicating a novel pathophysiology.
- No seizures or long-term disruption of cortical activity development were observed.
Conclusions:
- Mild perinatal brain injury leads to delayed neuronal death, challenging traditional views of acute injury.
- Neurons surviving the initial insult may be vulnerable to later, progressive loss.
- The findings suggest that neuroprotective strategies could be effective if targeted at specific biomarkers of neuronal viability after the acute phase.
Abstract:
Mild hypoxic-ischaemic encephalopathy is common in neonates, and there are no evidence-based therapies. By school age, 30-40% of those patients experience adverse neurodevelopmental outcomes. The nature and progression of mild injury is poorly understood. We studied the evolution of mild perinatal brain injury using longitudinal two-photon imaging of transgenic fluorescent calcium-sensitive and insensitive proteins to provide a novel readout of neuronal viability and activity at cellular resolution in vitro and in vivo. In vitro, perinatal organotypic hippocampal cultures underwent 15-20 min of oxygen-glucose deprivation. In vivo, mild hypoxia-ischaemia was completed at post-natal day 10 with carotid ligation and 15 min of hypoxia (FiO2, 0.08). Consistent with a mild injury, minimal immediate neuronal death was seen in vitro or in vivo, and there was no volumetric evidence of injury by ex vivo MRI 2.5 weeks after injury (n = 3 pups/group). However, in both the hippocampus and neocortex, these mild injuries resulted in delayed and progressive neuronal loss by the second week after injury compared to controls; measured by fluorophore quenching (n = 6 slices/group in vitro, P < 0.001; n = 8 pups/group in vivo, P < 0.01). Mild hypoxia-ischaemia transiently suppressed cortical network calcium activity in vivo for over 2 h after injury (versus sham, n = 13 pups/group; P < 0.01). No post-injury seizures were seen. By 24 h, network activity fully recovered, and there was no disruption in the development of normal cortical activity for 11 days (n = 8 pups/group). The participation in network activity of individual neurons destined to die in vivo was indistinguishable from those that survived up to 4 days post-injury (n = 8 pups/group). Despite a lack of significant immediate neuronal death and only transient disruptions of network activity, mild perinatal brain injury resulted in a delayed and progressive increase of neuronal death in the hippocampus and neocortex. Neurons that died late were functioning normally for days after injury, suggesting a new pathophysiology of neuronal death after mild injury. Critically, the neurons destined to die late demonstrated multiple biomarkers of viability long after mild injury, suggesting their later death may be modified with neuroprotective interventions.
