A chronic histopathological and electrophysiological analysis of a rodent hypoxic–ischemic brain injury model and its use as a model of epilepsy

Abstract

Ischemic brain injury is one of the leading causes of epilepsy in the elderly, and there are currently no adult rodent models of global ischemia, unilateral hemispheric ischemia, or focal ischemia that report the occurrence of spontaneous motor seizures following ischemic brain injury. The rodent hypoxic-ischemic (H-I) model of brain injury in adult rats is a model of unilateral hemispheric ischemic injury. Recent studies have shown that an H-I injury in perinatal rats causes hippocampal mossy fiber sprouting and epilepsy. These experiments aimed to test the hypothesis that a unilateral H-I injury leading to severe neuronal loss in young-adult rats also causes mossy fiber sprouting and spontaneous motor seizures many months after the injury, and that the mossy fiber sprouting induced by the H-I injury forms new functional recurrent excitatory synapses. The right common carotid artery of 30-day old rats was permanently ligated, and the rats were placed into a chamber with 8% oxygen for 30 min. A quantitative stereologic analysis revealed that the ipsilateral hippocampus had significant hilar and CA1 neuronal loss compared to the contralateral and sham-control hippocampi. The septal region from the ipsilateral and contralateral hippocampus had small but significantly increased amounts of Timm staining in the inner molecular layer compared to the sham-control hippocampi. Three of 20 lesioned animals (15%) were observed to have at least one spontaneous motor seizure 6-12 months after treatment. Approximately 50% of the ipsilateral and contralateral hippocampal slices displayed abnormal electrophysiological responses in the dentate gyrus, manifest as all-or-none bursts to hilar stimulation. This study suggests that H-I injury is associated with synaptic reorganization in the lesioned region of the hippocampus, and that new recurrent excitatory circuits can predispose the hippocampus to abnormal electrophysiological activity and spontaneous motor seizures. KeywordsEpilepsy; sprouting; recurrent excitation; population spikes; epileptiform bursts Correspondence to: F. Edward Dudek, Ph.D. Department of Physiology, University of Utah School of Medicine, 420 Chipeta Way, Suite 1700, Salt Lake City, UT 84108, (801) 587-5880, ed.dudek@hsc.utah.edu. 1 Current address: Department of Neurosciences, Case Western Reserve University School of Medicine, Cleveland, OH; 2 Current address: Department of Physiology, University of Utah School of Medicine, Salt Lake City, UT.Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; ...

Models of hypoxia and ischemia-induced seizures

“…A detailed histological study of the brains of rats 6 months following HI treatment revealed features such as porencephalic cysts and cortical microgyri which are similar to findings following perinatal HI in humans (Kadam and Dudek, 2007;Marin-Padilla, 2000;Villani et al, 2003). Interestingly, mossy fiber sprouting into the molecular layer of the dentate gyrus has been documented as well (Kadam and Dudek, 2007;Williams et al, 2004;Williams and Dudek, 2007). This type of anatomic plasticity is of particular interest because it is known to occur in human temporal lobe epilepsy and models of that disorder, and furthermore it has been suggested to possibly play are role in epileptogenesis Frotscher and Zimmer, 1983;Tauck and Nadler, 1985).…”

“…Moreover, release of caged glutamate in the granule layer resulted in repetitive EPSCs in granule cells in patch clamp experiments. These findings are suggestive of excitatory feedback in the HI treated animals (Williams and Dudek, 2007). Although these experiments were carried out in rats treated at P30 they are possibly quite relevant to the neonatal HI model in that rats treated at P7 also undergo mossy fiber sprouting (Kadam and Dudek, 2007;Williams et al, 2004).…”

“…Histological examination of the brains revealed that the animals with discharges had suffered more substantial damage than the treated, non-seizing animals (Romijn et al, 1994). A group of more recent papers has confirmed that early hypoxia-ischemia in rat pups results in later seizures and that the occurrence of the epileptic state is related to the presence of a cerebral infarct (Kadam and Dudek, 2007;Kadam et al, 2010;Williams et al, 2004;Williams and Dudek, 2007). Some of the reports however relied solely on behavioral monitoring for seizure detection and thus would not detect subclinical or even subtle behavioral seizures and thus would result in an underestimation of seizure frequency.…”

Posttraumatic seizures and epilepsy in adult rats after controlled cortical impact

“…In addition, excitatory drive to surviving hilar GABAergic interneurons after CCI may be enhanced by convergent input from both pyramidal and granule cells resulting in network destabilization (Hunt et al, 2011). The presence of aberrant mossy fiber sprouting in the IML of the hippocampus is typically considered as a marker for temporal lobe epilepsy and as one of several abnormal excitatory networks that may underlie seizure generation (Gorter et al, 2001;Williams and Dudek, 2007).…”

Optimized methods for epilepsy therapy development using an etiologically realistic model of focal epilepsy in the rat

“…To this end, new syndrome-specific models have been developed based on etiologically realistic epileptogenic insults such as viral encephalitis, stroke, febrile seizures, perinatal hypoxia and head trauma (D’Ambrosio et al, 2004; Dubé et al, 2010; Kelly et al, 2001; Rakhade et al, 2011; Williams and Dudek, 2007). These models have the advantages that: 1) they feature chronic spontaneous recurrent seizures (CSRSs), which are the hallmark of epilepsy, 2) they include focal non-convulsive CSRSs, which are known to be difficult to treat with current ASDs, 3) the realism of the initiating injury ensures that mechanisms that operate in the corresponding patient populations are recruited, and 4) the patient populations best suited for clinical tests of treatments discovered using these models are readily identifiable.…”