Early-Onset Convulsive Seizures Induced by Brain Hypoxia-Ischemia in Aging Mice: Effects of Anticonvulsive Treatments

Aging is associated with an increased risk of seizures/epilepsy. Stroke (ischemic or hemorrhagic) and cardiac arrest related brain injury are two major causative factors for seizure development in this patient population. With either etiology, seizures are a poor prognostic factor. In spite of this, the underlying pathophysiology of seizure development is not well understood. In addition, a standardized treatment regimen with anticonvulsants and outcome assessments following treatment has yet to be established for these post-ischemic seizures. Previous studies have modeled post-ischemic seizures in adult rodents, but similar studies in aging/aged animals, a group that mirrors a higher risk elderly population, remain sparse. Our study therefore aimed to investigate early-onset seizures in aging animals using a hypoxia-ischemia (HI) model. Male C57 black mice 18-20-month-old underwent a unilateral occlusion of the common carotid artery followed by a systemic hypoxic episode (8% O2 for 30 min). Early-onset seizures were detected using combined behavioral and electroencephalographic (EEG) monitoring. Brain injury was assessed histologically at different times post HI. Convulsive seizures were observed in 65% of aging mice post-HI but not in control aging mice following either sham surgery or hypoxia alone. These seizures typically occurred within hours of HI and behaviorally consisted of jumping, fast running, barrel-rolling, and/or falling (loss of the righting reflex) with limb spasms. No evident discharges during any convulsive seizures were seen on cortical-hippocampal EEG recordings. Seizure development was closely associated with acute mortality and severe brain injury on brain histological analysis. Intra-peritoneal injections of lorazepam and fosphenytoin suppressed seizures and improved survival but only when applied prior to seizure onset and not after. These findings together suggest that seizures are a major contributing factor to acute mortality in aging mice following severe brain ischemia and that early anticonvulsive treatment may prevent seizure genesis and improve overall outcomes.


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(mean±SE) from individual animals were grouped and plotted vs. time. A, intra-peritoneal temperature was measured continuously and averaged every 10 seconds. B, heart rates were calculated from ECG signals and averaged every 30 seconds. Insert, a representative ECG segment collected from one aging mouse at the end of the hypoxia. C, regional cerebral blood flow (rCBF) was measured from the ipsilateral hemisphere, averaged every 30 seconds and then normalized as % of the baseline in individual animals.

Data details and Methods
Intra-peritoneal temperature and ECG signals were recorded telemetrically in freemoving aging mice as previously described [1]. Measured temperatures were 36.6±0.24°C during baseline monitoring and 37.1±0.01°C at 1.5 hours after occlusion of the common carotid artery and before hypoxia (n=11). There was a small but significant drop from the baseline temperatures by approximately 0.5°C during the hypoxia (S1 FigA). Lower body temperatures were also observed 1 hour later (35.4±0.33°C; p<0.05, post hypoxia vs. baseline, one way ANOVA). However, all the temperature measures following the hypoxia were within physiological temperature range (34-38°C) as per previous telemetric studies in adult mice [1][2][3][4].
Post-HI CS were observed in 6/11 animals examined. Regular ECG signals were observed during baseline monitoring and following hypoxia (n=5 aging mice; S1 FigB). Calculated heart rate was 527.1±22.9/min during baseline monitoring and 578.4±3.4/min after occlusion of the common carotid artery and before hypoxia.
Heart rate was significantly increased from baseline to 622.9±2.2/min at the end of hypoxia and 559.7±3.9/minute at 1 hour later (p<0.05, one way ANOVA). These increases might reflect a compensatory reaction of aging mice to increase cardiac output following hypoxic stress. Post-HI CS were observed in 3/5 animals examined. 3 A laser Doppler system (PF5010, Perimed, Järfälla, Sweden) was used to measure regional cerebral blood flow (rCBF) through the skull1. Ipsilateral rCBF signals were recorded from free-moving aging mice using a protocol modified from a previous study [5]. Briefly, the skull surface was surgically exposed under 2% isoflurane anesthesia, and a mouse-specific probe (MTB 500) was glued onto it in an area from bregma -2.2 to -2.8 mm and lateral 1.5 to 2.2 mm.
Baseline rCBF signals were collected before occlusion of the common carotid artery. The rCBF signals at 1.5-hours after the common carotid artery occlusion was 93.4±6.5% of the baseline level. A modified hypoxic episode with 15% O2 in the first 5 min and 8% O2 in next 30 min was used. This modification was to reduce animal movement and thereby also rCBF signal fluctuations in the early phase of the hypoxic challenge. The rCBF signals collected during 8% O2 hypoxia were analyzed as animals were largely immobile during this period. Ipsilateral rCBF signals declined substantially during the hypoxia (n=8 aging mice; S1 FigC). Measured at the end of hypoxia and at 15 min and 1 hour later, ipsilateral rCBF signals were 46.7±9.1%, 52.2±13.5%, and 48.0±6.3% of the baseline level, significantly lower than the level measured after the common carotid artery occlusion and prior to hypoxia (p<0.05, one way ANOVA).
Post-HI CS were observed in 6/8 animals examined.