Congenital brain dysplasias of different genesis can differently affect susceptibility to pilocarpine- or kainic acid-induced seizures in the rat

Structural changes in the neocortex as correlates of variations in EEG spectra and seizure susceptibility in rat brains with different degrees of dysplasia

Disturbances of the early stages of neurogenesis lead to irreversible changes in the structure of the mature brain and its functional impairment, including increased excitability, which may be the basis for drug‐resistant epilepsy. The range of possible clinical symptoms is as wide as the different stages of disturbed neurogenesis may be. In this study, we used a quadruple model of brain dysplasia by comparing structural and functional disorders in animals whose neurogenesis was disturbed with a single dose of 1 Gy of gamma rays at one of the four stages of neurogenesis, that is, on days 13, 15, 17, or 19 of prenatal development.

When reached adulthood, the prenatally irradiated rats received EEG teletransmitter implantation. Thereafter, pilocarpine was administered and significant differences in susceptibility to seizure behavioral symptoms were detected depending on the degree of brain dysplasia. Before, during, and after the seizures significant correlations were found between the density of parvalbumin‐immunopositive neurons located in the cerebral cortex and the intensity of behavioral seizure symptoms or increases in the power of particular EEG bands. Neurons expressing calretinin or NPY showed also dysplasia‐related increases without, however, correlations with parameters of seizure intensity. The results point to significant roles of parvalbumin‐expressing interneurons, and also to expression of NPY—an endogenous anticonvulsant and neuroprotectant reducing susceptibility to seizures and supporting neuronal survival.

Changes of EEG spectra in rat brains with different patterns of dysplasia in response to pilocarpine-induced seizures

Disorders of neurogenesis at early developmental stages lead to irreversible structural and functional impairments of the brain. As further their consequences, increases in brain excitability and the development of drug-resistant epilepsy can frequently be observed in clinical cases. Mechanisms underlying these phenomena can also be examined on animal models of brain dysplasia. This study was conducted on rats with four degrees of brain dysplasia following exposure to gamma radiation on days 13, 15, 17, or 19 of prenatal development. When reached adulthood, the rats received electroencephalographic (EEG) transmitter implantation. Thereafter, pilocarpine was administered, and significant differences in susceptibility to seizures were detected depending on the degree of brain dysplasia. Before, during, and after the seizures, EEG was recorded in free moving animals. Additionally, the intensity of seizure behavioral symptoms was assessed. Strong and moderate correlations were found between the intensity of seizure behavioral symptoms, the power of particular EEG bands, and volumes of dysplastic brains and their regions. The data drew particular attention to correlations between variations in EEG spectra and changes in the midbrain and pons volumes. The results point to possible significant roles of these regions in the observed changes of susceptibility to seizures. Consequently, the frequently used experimental model was considered here not only as representing cases of cortical dysplasia but also of generalized, diffuse dysplasia of the whole brain.

Review: Roles for astrocytes in epilepsy: insights from malformations of cortical development

Malformations of cortical development (MCDs), such as cortical dysplasia and tuberous sclerosis complex, are common causes of intractable epilepsy, especially in paediatric patients. Recently, mounting evidence points to a common pathology of these disorders. Hyperactivation of mammalian target of rapamycin (mTOR) has been proposed as a central mechanism in most, if not all, MCDs. The transition from mTOR hyperactivation and cellular abnormalities to large-scale functional changes and seizure is, however, not fully understood. In this article we set out to review currently available information regarding MCD pathology, focusing on glial cells - especially astrocytes - and their interactions with the brain vascular system. A large body of evidence points to these elements as potential targets in MCD. Here, we attempt to provide a review of this evidence and propose some hypotheses regarding the possible chain of events linking primary glial dysfunction and epilepsy. We focus on extracellular matrix remodelling, blood-brain barrier leakage and failure of astrocyte-dependent removal of extracellular debris. We posit that the failure of these systems results in a chronically pro-inflammatory environment, maintaining local astrocytes in a state of gliosis, with increased susceptibility to seizures as a consequence.

Brain dysplasia evoked by gamma irradiation at different stages of prenatal development leads to different tonic and clonic seizure reactivity

Rats with brain dysplasia evoked by interruption of different stages of prenatal neurogenesis show characteristic variations in susceptibility to seizures depending on the neurochemical specificity of pharmacological agents used to evoke seizures. To verify a discrepancy between the data obtained using different pharmacological models, neurochemically neutral electroshocks were applied here. To produce brain dysplasia of different degrees, pregnant Wistar rats were exposed to a single 1.0Gy dose of gamma rays on gestation days 13, 15, 17 or 19. From the postnatal day 60, their male offspring (E13s, E15s, E17s and E19s, respectively) were subjected to 21 daily electrical stimulations to evoke seizures. Profiles of tonic and clonic reactivity to electrical stimulation significantly differed from those observed following pilocarpine or kainic acid administration. E17s showed minimal intensity of tonic but maximal of clonic responses. On the contrary, very high tonic and low clonic reactivity was observed in E13s and E15s. Periventricular nodular heterotopias (PNHs) were observed exclusively in E15s and E17s. Generally, the size of PNHs was correlated positively with susceptibility to tonic seizures but negatively with susceptibility to clonic seizures. Analogous correlations with the size of the neocortex were opposite. E13s and E19s had brains devoid PNHs but showed high tonic seizure susceptibility similar to that in E15s. It can therefore be concluded that PNHs modified the type of seizure reactivity from tonic to clonic, depending of their size, but the presence of PNHs was not necessary for the development of seizure susceptibility itself.

Novel animal models of pediatric epilepsy

When mimicking epileptic processes in a laboratory setting, it is important to understand the differences between experimental models of seizures and epilepsy. Because human epilepsy is defined by the appearance of multiple spontaneous recurrent seizures, the induction of a single acute seizure without recurrence does not constitute an adequate epilepsy model. Animal models of epilepsy might be useful for various tasks. They allow for the investigation of pathophysiological mechanisms of the disease, the evaluation, or the development of new antiepileptic treatments, and the study of the consequences of recurrent seizures and neurological and psychiatric comorbidities. Although clinical relevance is always an issue, the development of models of pediatric epilepsies is particularly challenging due to the existence of several key differences in the dynamics of human and rodent brain maturation. Another important consideration in modeling pediatric epilepsy is that "children are not little adults," and therefore a mere application of models of adult epilepsies to the immature specimens is irrelevant. Herein, we review the models of pediatric epilepsy. First, we illustrate the differences between models of pediatric epilepsy and models of the adulthood consequences of a precipitating insult in early life. Next, we focus on new animal models of specific forms of epilepsies that occur in the developing brain. We conclude by emphasizing the deficiencies in the existing animal models and the need for several new models.

Irradiation exacerbates cortical cytopathology in the Eker rat model of tuberous sclerosis complex, but does not induce hyperexcitability

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by multi-organ pathologies. Most TSC patients exhibit seizures, usually starting in early childhood. The neuropathological hallmarks of the disease - cortical tubers, containing cytopathological neuronal and glial cell types - appear to be the source of seizure initiation. However, the contribution of these aberrant cell populations to TSC-associated epilepsies is not fully understood. To gain further insight, investigators have attempted to generate animal models with TSC-like brain abnormalities. In the current study, we focused on the Eker rat, in which there is a spontaneous mutation of the TSC2 gene (TSC2+/-). We attempted to exacerbate TSC-like brain pathologies with a "second-hit" strategy - exposing young pups to ionizing irradiation of different intensities, and at different developmental timepoints (between E18 and P6). We found that the frequency of occurrence of dysmorphic neurons and giant astrocytes was strongly dependent on irradiation dose, and weakly dependent on timing of irradiation in Eker rats, but not in irradiated normal controls. The frequency of TSC-like pathology was progressive; there were many more abnormal cells at 3 months compared to 1 month post-irradiation. Measures of seizure propensity (flurothyl seizure latency) and brain excitability (paired-pulse and post-tetanic stimulation studies in vitro), however, showed no functional changes associated with the appearance of TSC-like cellular abnormalities in irradiated Eker rats.