The role of the substantia nigra pars reticulata in kindling resistance in rats with genetic absence epilepsy

Prolongation of absence seizures and changes in serotonergic and dopaminergic neurotransmission by nigrostriatal pathway degeneration in genetic absence epilepsy rats

OBJECTIVE

Basal ganglia structures play an important role in the pathophysiology of absence epilepsy, known as remote control of absence seizures. We examined the role of the nigrostriatal dopaminergic pathway in absence epilepsy through behavioral and electroencephalography (EEG) parameters, immunohistochemical, and biochemical characteristics of dopamine and serotonin in the genetic absence epilepsy rat model.

METHODS

The nigrostriatal pathway was degenerated by the injection of chemical 6-hydroxydopamine hydrobromide (6-OHDA) into the medial forebrain bundle (MFB) in Wistar and genetic absence epilepsy rats from Strasbourg (GAERS). On the 21st day after stereotaxic surgery, spike-and-wave discharges (SWDs) on EEG were recorded in GAERS groups. Thereafter, Wistar-Control, GAERS-Control, Wistar-6OHDA, GAERS-6OHDA rats were subjected to the cylinder and apomorphine-induced rotation tests. Dopaminergic or serotonergic immunoreactivity was examined in the cortex, striatum, and substantia nigra pars compacta. High-performance liquid chromatography method was used for biochemical analysis of dopamine and serotonin in the cortex and thalamus.

RESULTS

In behavioral analysis, the number of rotations in the GAERS-6OHDA group was significantly higher than in Wistar-6OHDA rats. The degeneration of the nigrostriatal dopaminergic pathway produced a significant increase in the cumulative duration of SWDs and the duration of each SWD in GAERS-6OHDA rats. GAERS-Control rats displayed significantly higher cortical and striatal serotonin immunoreactivity and cortical serotonin level compared to Wistar-Control animals. Moreover, cortical and striatal serotonin immunoreactivity and cortical serotonin levels increased in Wistar-6OHDA and GAERS-6OHDA groups compared to their control groups.

SIGNIFICANCE

The effect of 6-OHDA-induced MFB lesion on absence epilepsy was examined for the first time by comparing Wistar and GAERS rats. The nigrostriatal dopaminergic pathway as a part of the remote-control system is likely to participate in the seizure network.

Morphological changes in the substantia nigra pars reticulata of the mice during kindling

Substantia nigra pars reticulata (SNpr) has been implicated in modulation, propagation and cessation of seizures. This study aimed to determine whether structural changes occur in SNpr during kindling. Male mice were randomly divided into four groups including early and late-phase kindled groups and their time-matched controls. Kindling was induced by every other day administration of a subconvulsive dose of PTZ (40 mg/kg, i.p.). The first occurrence of seizure behaviors was used to categorize the early and late phases of kindling. There was no significant difference in the volume of SNpr between the early- and late-phase kindled groups. The diameter of SNpr was significantly increased in the early phase group and decreased in the late phase group as compared to their matched controls (p < 0.05). Reduced neural cells and increased dead cell numbers were observed in the SNpr of the late-phase group in comparison to its control group (p < 0.05). These findings suggest that SNpr is a sensitive and vulnerable structure involving seizure propagation in the processes of epileptogenesis.

Ultrastructural GABA immunogold labeling in the substantia nigra pars reticulata of kindled genetic absence epilepsy rats

Genetic Absence Epilepsy Rats from Strasbourg (GAERS) is a well-known animal model of absence epilepsy and they are resistant to electrical kindling stimulations. The present study aimed to examine possible differences in gamma-aminobutyric acid (GABA) levels and synapse counts in the substantia nigra pars reticulata anterior (SNRa) and posterior (SNRp) regions between GAERS and Wistar rats receiving kindling stimulations. Animals in the kindling group either received six stimulations in the amygdala and had grade 2 seizures or they were kindled, having grade five seizures. Rats were decapitated one hour after the last stimulation. SNR regions were obtained after vibratome sectioning of the brain tissue. GABA immunoreactivity was detected by immunogold method and synapses were counted. Sections were observed by transmission electron microscope and analyzed by Image J program. GABA density in the SNRa region of fully kindled GAERS and Wistar groups increased significantly compared to that of their corresponding grade 2 groups. The number of synapses increased significantly in kindled and grade 2 GAERS groups, compared to kindled and grade 2 Wistar groups, respectively, in the SNRa region. GABA density in the SNRp region of kindled GAERS group increased significantly compared to that of GAERS grade 2 group. In the SNRp region, both kindled and grade 2 GAERS groups were found to have increased number of synapses compared to that of GAERS control group. We concluded that both SNRa and SNRp regions may be important in modulating resistance of GAERS to kindling stimulations.

A transcranial sonography study of brainstem and its association with depression in idiopathic generalized epilepsy with tonic-clonic seizures

Brainstem raphe (BR) hypoechogenicity in transcranial sonography (TCS) has been depicted in patients with depression. But, up to date, the association of BR alterations in TCS with depression in patients with epilepsy has never been reported. This study was to investigate the possible role of BR examination via TCS in patients with idiopathic generalized epilepsy with tonic-clonic seizures (IGE-TCS) and depression. Forty-six patients with IGE-TCS and 45 healthy controls were recruited. Echogenicity of the caudate nuclei (CN), lentiform nuclei (LN), substantia nigra (SN), and BR and widths of the lateral ventricle (LV) frontal horns and the third ventricle (TV) were assessed via TCS. The determination of depression was based on the criteria of the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV), and depression severity measured by Chinese version Neurological Disorders Depression Inventory for Epilepsy (C-NDDI-E) and Beck Depression Inventory-II (BDI-II). The width of TV in patients with epilepsy was found significantly larger than that in healthy controls (p = 0.001), but there was no significant difference in TV width between patients with IGE-TCS with and without depression. There were no significant differences between patients with IGE-TCS and healthy controls in LV frontal horn width, as well as in SN, CN, LN, and BR echogenicity. Here, it seems that patients with IGE-TCS were detected with smaller SN echogenic area compared with controls though they had no statistical significance. Patients with IGE-TCS with hypoechogenic BR had significantly higher C-NDDI-E and BDI-II scores than those with normal BR signal, and most patients with IGE-TCS with depression exhibited hypoechogenic BR, but few patients with IGE-TCS without depression exhibited hypoechogenic BR. In conclusion, BR echogenic signal alterations in TCS can be a biomarker for depression in epilepsy, but it might not be associated with epilepsy itself. The alterations of SN echogenic area and TV width in TCS may reflect a potential role of SN and diencephalon structure in the pathogenesis of epilepsy, which needs to be further elucidated.

Abnormalities of diffusional kurtosis imaging and regional homogeneity in idiopathic generalized epilepsy with generalized tonic-clonic seizures

Neuroimaging techniques have been used to investigate idiopathic generalized epilepsy with generalized tonic-clonic seizures (IGE-GTCS) and different studies employing these methods have produced varying results. However, there have been few studies exploring diffusional kurtosis imaging (DKI) and regional homogeneity (ReHo) techniques in patients with IGE-GTCS. In the current study, resting-state functional magnetic resonance imaging (fMRI) and DKI data were collected from 28 patients with IGE-GTCS and 28 healthy controls. The ReHo method and tract-based spatial statistical (TBSS) analysis were performed to compare differences between the groups. Compared with healthy controls, patients with IGE-GTCS exhibited markedly increased ReHo in the bilateral putamen, the thalamus, right pallidum, right supplementary motor area and the bilateral paracentral lobules. Compared with healthy controls, patients with IGE-GTCS also exhibited markedly decreased ReHo in the posterior cingulate/precuneus, left angular gyrus and dorsolateral prefrontal cortex. In patients with IGE-GTCS, DKI revealed lower fractional anisotropy in the left anterior/superior corona radiata, left superior longitudinal fasciculus and genu/body of the corpus callosum. Higher mean diffusivity was detected in the bilateral anterior corona radiata, left superior corona radiata, left cingulum, and genu/body/splenium of the corpus callosum. Furthermore, reduced mean kurtosis values were identified over the bilateral superior/posterior corona radiate, left anterior corona radiata, right superior longitudinal fasciculus, left posterior thalamic radiation and the genu/body/splenium of the corpus callosum. Therefore, the results of the current study revealed abnormalities in spontaneous activity in the gray and white matter tracts in patients with IGE-GTCS. These results suggest that novel MRI technology may be useful to help determine the pathogenesis of IGE-GTCS.

Imaging blood-brain barrier dysfunction as a biomarker for epileptogenesis

A biomarker that will enable the identification of patients at high-risk for developing post-injury epilepsy is critically required. Microvascular pathology and related blood-brain barrier dysfunction and neuroinflammation were shown to be associated with epileptogenesis after injury. Here we used prospective, longitudinal magnetic resonance imaging to quantitatively follow blood-brain barrier pathology in rats following status epilepticus, late electrocorticography to identify epileptic animals and post-mortem immunohistochemistry to confirm blood-brain barrier dysfunction and neuroinflammation. Finally, to test the pharmacodynamic relevance of the proposed biomarker, two anti-epileptogenic interventions were used; isoflurane anaesthesia and losartan. Our results show that early blood-brain barrier pathology in the piriform network is a sensitive and specific predictor (area under the curve of 0.96, P < 0.0001) for epilepsy, while diffused pathology is associated with a lower risk. Early treatments with either isoflurane anaesthesia or losartan prevented early microvascular damage and late epilepsy. We suggest quantitative assessment of blood-brain barrier pathology as a clinically relevant predictive, diagnostic and pharmaco!dynamics biomarker for acquired epilepsy.

Evaluation of GAD67 immunoreactivity in the region of substantia nigra pars reticulata in resistance to development of convulsive seizure in genetic absence epilepsy rats

OBJECTIVE:

Nonconvulsive absence epilepsy and convulsive epilepsy seizures are rarely seen in the same patient. It has been demonstrated that there is a resistance to development of convulsive seizures in genetic absence epilepsy models. The present study investigated glutamic acid decarboxylase (GAD) immunoreactivity in the brain region related to the interaction of these two seizure types, namely substantia nigra pars reticulata (SNR) subregions, SNR_anterior and SNR_posterior.

METHODS:

Nonepileptic adult male Wistar rats and Genetic Absence Epilepsy Rats from Strasbourg (GAERS) were used. Experimental groups of Wistar and GAERS were electrically stimulated for kindling model to induce convulsive epileptic seizures. An electrical stimulation cannula was stereotaxically implanted to the basolateral amygdala and recording electrodes were placed on the cortex. Sagittal sections of SNR were used to evaluate immunohistochemical reaction. Sections were incubated with anti-GAD67 antibody. Densitometric analysis of GAD67 immunoreactive neurons was performed using photographs of stained sections. One-way analysis of variance and post hoc Bonferroni test were used for statistical analysis of the data.

RESULTS:

There was no difference in GAD67 immunoreactivity of SNR subregions of control Wistar and control GAERS. An increase in GAD67 immunoreactivity was detected in SNR_posterior subregion of stimulated Wistar rats, whereas there was a decrease in GAD67 immunoreactivity in SNR_posterior of stimulated GAERS. The difference in GAD67 immunoreactivity between these two groups was statistically significant.

CONCLUSION:

Level of synthetized gamma-aminobutyric acid in SNR_posterior subregion plays an important role in the interaction of nonconvulsive absence epilepsy seizures and convulsive epilepsy seizures.

Increased inhibitory synaptic activity in the hippocampus (CA1) of genetic absence epilepsy rats: Relevance of kindling resistance

PURPOSE

Genetic absence epilepsy rats from Strasbourg (GAERS), a well-validated genetic rat model for typical absence epilepsy, are known to manifest a resistance to secondary generalization of abnormal focal electrical activity evoked by kindling. The mechanism of this resistance is still unclear. In order to understand the possible mechanism of kindling resistance, we investigated for the first time, the differences of short-term synaptic plasticity by using a paired-pulse paradigm as an indicator of GABAergic activity in CA1 region of hippocampus in GAERS and non-epileptic Wistar rats in-vivo.

METHODS

Rats were subjected to kindling process, basolateral amygdala was stimulated twice a day, with a supra-threshold current, until they displayed limbic or convulsive seizures. One hour after the last kindling stimulation, evoked field potentials from CA1 pyramidal layer of hippocampus were recorded in-vivo under urethane anesthesia.

RESULTS

In response to supra-threshold kindling stimulations GAERS showed a significantly delayed kindling progression and displayed a significant increase in hippocampal excitability at early stages of kindling that is the critical for the development of convulsive seizures. In control rats that were not received kindling stimulation, paired-pulse depression (PPD) was significantly pronounced in GAERS with respect to the Wistar group. During the kindling course, PPD was gradually reduced in the Wistar rats as kindling progression was advanced. However in GAERS, PPD ratios were not significantly changed at early stages of kindling. When GAERS reached convulsive stage, their PPD ratios became similar to that of Wistar rats.

DISCUSSION

The increased inhibition in paired-pulse responses at early stages of kindling in GAERS suggests the role of augmented GABAergic activity as one of the underlying mechanisms of kindling resistance observed in genetic rat models of absence epilepsy.