Quisqualic acid-induced seizures during development: a behavioral and EEG study

Selective reductions in subpopulations of GABAergic neurons in a developmental rat model of epilepsy

In the rat, early postnatal development is a critical period for neuronal migration, differentiation and network formation, requiring appropriate and timely glutamate and gamma-aminobutyric acid (GABA) signaling. Insults that affect either of these systems may result in increased excitatory activity, potentially leading to changes in neuronal proliferation and/or connectivity. We have previously shown that postnatal administration of low doses of domoic acid (DOM) can produce many of the behavioral and morphological changes found in current animal models of temporal lobe epilepsy (TLE), as well as the human condition. Using immunohistochemical techniques, we sought to characterize alterations in specific hippocampal GABAergic subpopulations at various locations along the septo-temporal axis in the DOM model. Results show decreased levels of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) in the ventral hilus and region- and sex-specific reductions in parvalbumin (PV)-containing immunoreactivity, but no alterations in somatostatin (SST) expression. These regional and sex-dependent changes in specific subpopulations of GABAergic interneurons may contribute to seizure development in this slowly progressing developmental model of TLE, and highlight how even subtle intervention may alter the interplay between glutamate and GABA systems during critical developmental stages.

Cortical Excitatory Amino Acid Release and Cell Function during Hypotension in Near-Term Born Lambs

BACKGROUND

Energy failure due to insufficient cerebral O2-supply leads to excess accumulation of calcium ions in presynaptic neurons, followed by excess release of excitatory amino acids, which are potent neurotoxins, into the synaptic cleft.

AIM

To investigate whether electrocortical brain activity (ECBA) can provide an adequate measure for excitatory amino acid release due to hemorrhagic hypotension.

METHODS

Ten near-term lambs were delivered at 127 days of gestation (term: 147 days). After a stabilization period, hypotension was induced by stepwise withdrawal of blood. Cerebral microdialysis was used to measure the concentrations of glutamate and aspartate.

RESULTS

During hypotension, mean arterial blood pressure, cerebral O2-supply and ECBA decreased and the extracellular concentration of glutamate increased significantly. ECBA was significantly related to glutamate (R2: 0.67, p < 0.001) and aspartate (R2: 0.57, p < 0.001) concentrations.

CONCLUSION

The extracellular release of glutamate and aspartate in the cerebral cortex increases after hemorrhagic hypotension in near-term born lambs. The extracellular overflow of glutamate and aspartate were significantly inversely related to ECBA.

Overview of the Current Animal Models for Human Seizure and Epileptic Disorders

A diversity of animal models are available for the study of epilepsy and these models have a proven history in advancing our understanding of basic mechanisms underlying epileptogenesis and have been instrumental in the screening of novel antiepileptic drugs. This review addresses the criteria that should be met in a valid animal model and provides an overview of current animal models that are relevant to human conditions. In addition, models not specific for any one human condition but rather exhibiting partial or generalized seizures are discussed. While most human disorders are without any animal model, those models that are clinically relevant have strengths and weaknesses. Finally, although few relevant, well-characterized animal models have been added to the list over recent years, major advancements in molecular genetics are contributing to the discovery of novel pathways involved in epileptogenesis.

Long-term effects of neonatal seizures: a behavioral, electrophysiological, and histological study

Previous studies have demonstrated that recurrent seizures during the neonatal period lead to permanent changes in seizure threshold and learning and memory. The pathophysiological mechanisms for these changes are not clear. To determine if neonatal seizures cause changes in hippocampal excitability or inhibition, we subjected rats to 50 flurothyl-induced seizures during the first 10 days of life (five seizures per day). When the rats were adults, we examined seizure threshold using flurothyl inhalation, and learning and memory in the water maze. In separate groups of animals, we evaluated in vivo paired-pulse facilitation and inhibition in either CA1 with stimulation of the Schaffer collaterals or dentate gyrus with stimulation of the perforant path. Following these studies, the animals were sacrificed and the brains evaluated for mossy fiber sprouting with the Timm stain. Compared to control animals, rats with 50 flurothyl seizures had a reduced seizure threshold, impaired learning and memory in the water maze, and sprouting of mossy fibers in the CA3 pyramidal cell layer and molecular layer of the dentate gyrus. No significant differences in impaired paired-pulse inhibition was noted between the flurothyl-treated and control rats. This study demonstrates that recurrent neonatal seizures result in changes of neuronal connectivity and alterations in seizure susceptibility, learning and memory. However, the degree of impairment following 50 seizures was modest, demonstrating that the immature brain is remarkably resilient to seizure-induced damage.

Consequences of recurrent seizures during early brain development

It is well documented that prolonged seizures (status epilepticus) can cause neuronal injury and result in synaptic reorganization in certain brain regions. However, the effect of recurrent, relatively short seizures in young animals on subsequent brain development is not known. To study the consequences of recurrent seizures on the developing brain, we subjected immature rats to a total of 50 flurothyl-induced seizures from postnatal day 11 until day 23. Immunohistochemistry for c-fos was performed to characterize the pattern of neuronal activation following the seizures. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, using unbiased stereological methods, and the silver impregnation method were used to evaluate neuronal death following the recurrent seizures. Timm and Golgi staining were performed four weeks after the 50th seizure to evaluate the effects of recurrent seizures on synaptic organization. Our results show that recurrent flurothyl-induced seizures progressively increased excitability of the brain, as revealed by a dramatic increase in the extent and intensity of c-fos immunostaining. While no cell loss was detected in the hippocampus with either Cresyl Violet or silver stains, animals experiencing multiple daily seizures developed increased mossy fiber sprouting in both the supragranular layer of the dentate gyrus and the infrapyramidale layer of the CA3 region. Golgi staining confirmed that there was an increase in mossy fibers in the pyramidal cell layer. Our results suggest that serial recurrent seizures in the immature brain can lead to significant changes in mossy fiber distribution even though the seizures do not cause significant hippocampal cell loss.

Acute and chronic effects of seizures in the developing brain: lessons from clinical experience

Seizures in the neonate are often considered a form of status epilepticus (SE) because they are relatively prolonged, difficult to control with antiepileptic drugs (AEDs), and may be associated with significant morbidity and mortality. Despite their clinical importance, there is still no clear understanding of how seizures may affect the developing brain. Although both basic neuroscience and clinical research have addressed these issues, there are difficulties in the design and analysis of each type of investigation. Animal studies should reflect the human condition, the most relevant studies being those that consider neocortical rather than hippocampal seizures. Clinical investigations should be based on precise, age-specific definitions of seizures of epileptic origin and of SE. Treatment strategies should be standardized with defined rationale and end points. Outcome measures are best when defined and quantifiable. The relative effects of underlying CNS injuries that coexist with the onset of neonatal seizures may be difficult to differentiate from the effects of the seizures themselves or their treatment. Current clinical studies suggest that the overriding factors in determining the outcome of neonates with seizures are the cause, the degree, and the distribution of brain injury at the time of seizure occurrence. However, such studies have limitations and may not yet employ methodology sensitive enough to detect a full range of adverse effects of seizures themselves.

Mossy fiber sprouting after recurrent seizures during early development in rats

In some children, epilepsy is a catastrophic condition, leading to significant intellectual and behavioral impairment, but little is known about the consequences of recurrent seizures during development. In the present study, we evaluated the effects of 15 daily pentylenetetrazol-induced convulsions in immature rats beginning at postnatal day (P) 1, 10, or 60. In addition, we subjected another group of P10 rats to twice daily seizures for 15 days. Both supragranular and terminal sprouting in the CA3 hippocampal subfield was assessed in Timm-stained sections by using a rating scale and density measurements. Prominent sprouting was seen in the CA3 stratum pyramidale layer in all rats having 15 daily seizures, regardless of the age when seizures began. Based on Timm staining in control P10, P20, and P30 rats, the terminal sprouting in CA3 appears to be new growth of axons and synapses as opposed to a failure of normal regression of synapses. In addition to CA3 terminal sprouting, rats having twice daily seizures had sprouting noted in the dentate supragranular layer, predominately in the inferior blade of the dentate, and had a decreased seizure threshold when compared with controls. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, with unbiased stereological methods demonstrated no differences from controls in rats with daily seizures beginning at P1 or P10, whereas adult rats with daily seizures had a significant decrease in CA1 neurons. Rats that received twice daily seizures on P10-P25 had an increase in dentate granule cells. This study demonstrates that, like the mature brain, immature animals have neuronal reorganization after recurrent seizures, with mossy fiber sprouting in both the CA3 subfield and supragranular region. In the immature brain, repetitive seizures also result in granule cell neurogenesis without loss of principal neurons. Although the relationship between these morphological changes after seizures during development and subsequent cognitive impairment is not yet clear, our findings indicate that during development recurrent seizures can result in significant alterations in cell number and axonal growth.

Pentylenetetrazol seizures induce cell suffering but not death in the immature rat brain

To investigate whether long-term functional consequences of status epilepticus (SE) induced by pentylenetetrazol in 10-day-old rats correlated with cell injury and/or death, acid fuchsin and TUNEL staining were performed between 4 to 144 h after SE. Acid fuchsin stained hippocampus, amygdala and cerebral cortex at 24 h but not at 72 and 144 h. No DNA fragmentation was apparent at any time. Thus, immature neurons subjected to sustained seizures suffer transiently but survive probably by activating repair processes.

Convulsant action of D,L-homocysteic acid and its stereoisomers in immature rats

PURPOSE

We wished to characterize the convulsant effect of homocysteic acid (HCA) in developing rats.

METHODS

Seizures were induced in 7-, 12-, 18-, and 25-day-old rats by intraperitoneal (i.p.) administration of D,L-HCA and in 12-day-old rats by i.p. injection of L- and D-stereoisomers of HCA. The animals were observed for 30 min after injection. The incidence, latencies, pattern of motor seizures, and all behavioral phenomena were noted. Fifty percent convulsant dose (CD50) values were calculated by probit analysis. Electrocorticograms (ECoG) were recorded after injection.

RESULTS

HCA did not elicit minimal clonic seizures whereas generalized tonic-clonic seizures (GTCS) occurred in all the age groups studied. Flexion (emprosthotonic) convulsions occurred to postnatal day 18. ECoG recordings exhibited delta activity in younger pups and sharp graphoelements in older pups, but electroclinical correlation was poor. Young animals were more sensitive to the convulsant effect of D,L-HCA. In addition, D-HCA was significantly more effective than L-HCA in inducing both flexion and generalized seizures.

CONCLUSIONS

Our data clearly indicate that seizures induced by HCA differ from those evoked by homocysteine. There are no qualitative differences in the motor pattern of seizures induced by the two stereoisomers of HCA, but marked differences were apparent in the very first signs of their action. These differences might be due to interaction with different glutamate receptor subtypes.

Epilepsy in the developing brain: lessons from the laboratory and clinic

Children with epilepsy present unique challenges to the clinician. In addition to having differences in clinical and EEG phenomena, children differ from adults in regard to etiological factors, response to antiepileptic drugs (AEDs), and outcome. It is now recognized that the immature brain also differs from the mature brain in the basic mechanisms of epileptogenesis and propagation of seizures. The immature brain is more prone to seizures due to an imbalance between excitation and inhibition. gamma-Aminobutyric acid (GABA), the major CNS inhibitory neurotransmitter in the mature brain, can lead to depolarization in the hippocampal CA3 region in very young rats. There are also age-related differences in response to GABA agonists and antagonists in the substantia nigra, a structure important in the propagation of seizures. These age-related differences in response to GABAergic agents provide further evidence that the pathophysiology of seizures in the immature brain differs from that in the mature brain. Although prolonged seizures can cause brain damage at any age, the extent of brain damage after prolonged seizures is highly age dependent. Far less histological damage and fewer disturbances in cognition result from prolonged seizures in the immature brain than from seizures of similar duration and intensity in mature animals. However, detrimental effects of AEDs may be greater in the immature brain, than in the mature brain. These lessons from the animal laboratory raise questions about the appropriateness of current therapeutic approaches to childhood seizure disorders.

Age-dependent effects of glutamate toxicity in the hippocampus

While prolonged seizures can cause brain damage at any age, the extent of brain damage following prolonged seizures is highly age-dependent. Seizures in the immature brain are followed by far less histological damage than seizures of similar duration and intensity in mature animals. The reasons for this age-related phenomenon are unclear. Seizure-induced cell death may be due to the neurotoxic effects of excessive glutamate release, we tested the hypothesis that the immature brain is less vulnerable to glutamate-induced neurotoxicity than the mature brain. We administered equal amounts of glutamate (0.5 mumol in 1.0 microliter) unilaterally into the CA1 subfield of the hippocampus of rats at postnatal (P) days 10, 20, 30, and 60. Equal volumes of saline were injected in the contralateral hippocampus. Rats were killed 7 days later and their brains were examined for hippocampal cell loss. The size of the resultant hippocampal lesion was highly age-dependent. Minimal cell loss was noted in the P10 rats, lesions in the P20 rats were smaller than those at P30 and P60, which were similar in extent. This study demonstrates that the extent of glutamate neurotoxicity in the hippocampus is highly age-dependent, with immature hippocampi relatively resistant to glutamate-induced cell death.