Altered spatial distribution of PV-cortical cells and dysmorphic neurons in the somatosensory cortex of BCNU-treated rat model of cortical dysplasia

Altered functional connectivity and network excitability in a model of cortical dysplasia

Focal cortical dysplasias (FCDs) are malformations of cortical development that often result in medically refractory epilepsy, with a greater incidence in the pediatric population. The relationship between the disturbed cortical morphology and epileptogenic activity of FCDs remains unclear. We used the BCNU (carmustine 1-3-bis-chloroethyl-nitrosourea) animal model of cortical dysplasia to evaluate neuronal and laminar alterations and how these result in altered activity of intracortical networks in early life. We corroborated the previously reported morphological anomalies characteristic of the BCNU model, comprising slightly larger and rounder neurons and abnormal cortical lamination. Next, the neuronal activity of live cortical slices was evaluated through large field-of-view calcium imaging as well as the neuronal response to a stimulus that leads to cortical hyperexcitability (pilocarpine). Examination of the joint activity of neuronal calcium time series allowed us to identify intracortical communication patterns and their response to pilocarpine. The baseline power density distribution of neurons in the cortex of BCNU-treated animals was different from that of control animals, with the former showing no modulation after stimulus. Moreover, the intracortical communication pattern differed between the two groups, with cortexes from BCNU-treated animals displaying decreased inter-layer connectivity as compared to control animals. Our results indicate that the altered anatomical organization of the cortex of BCNU-treated rats translates into altered functional networks that respond abnormally to a hyperexcitable stimulus and highlight the role of network dysfunction in the pathophysiology of cortical dysplasia.

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.

Effect of nicorandil on the spatial arrangement of primary motor cortical neurons in the sub-acute phase of stroke in a rat model

INTRODUCTION

Ischemic stroke remains a major cause of disability and death worldwide. The density and the spatial distribution of the primary motor (M1) cortical neurons are important in signal transmission and control the movement-related functions. Recently, the neuroprotective effect of nicorandil in cerebral ischemia was described through its anti-apoptosis, antioxidant and anti-inflammatory properties. This study aimed to determine the effects of nicorandil on the neurobehavioral outcome, infarct size, and density, and spatial distribution of M1 cortical neurons after cerebral ischemia.

METHODS

Thirty Sprague-Dawley rats were randomly divided into three groups. Sham underwent surgery without middle cerebral artery occlusion (MCAO) and drug. The MCAO and treatment groups after MCAO received saline or nicorandil 2, 24, 48, and 72 h after the induction of brain ischemia. Neurobehavioral tests were performed, brains removed, sectioned, and stained by 2,3,5-triphenyltetrazolium chloride (TTC) to estimate the size of the infarction and Nissl staining to evaluate the numerical density, mean area, and the distribution pattern of M1 cortical neurons, using Voronoi spatial tessellation.

RESULTS

Although nicorandil treatment significantly decreased the neurological deficits and density of neuronal neighbors, it could not preserve the normal regular spatial distributions of M1 cortical neurons after MCAO. It also could not significantly improve motor function or reduce ischemic lesion size.

CONCLUSIONS

Treatment using the present dose of nicorandil during sub-acute ischemic stroke could not increase neuronal density or preserve the normal regular spatial distributions after MCAO. However, it had beneficial effects on neurobehavioral and motor function and somewhat reduced ischemic lesion size.

Morphological and Calcium Signaling Alterations of Neuroglial Cells in Cerebellar Cortical Dysplasia Induced by Carmustine

Cortical dysplasias are alterations in the organization of the layers of the brain cortex due to problems in neuronal migration during development. The neuronal component has been widely studied in experimental models of cortical dysplasias. In contrast, little is known about how glia are affected. In the cerebellum, Bergmann glia (BG) are essential for neuronal migration during development, and in adult they mediate the control of fine movements through glutamatergic transmission. The aim of this study was to characterize the morphology and intracellular calcium dynamics of BG and astrocytes from mouse cerebellum and their modifications in a model of cortical dysplasia induced by carmustine (BCNU). Carmustine-treated mice were affected in their motor coordination and balance. Cerebellar dysplasias and heterotopias were more frequently found in lobule X. Morphology of BG cells and astrocytes was affected, as were their spontaneous [Ca²⁺]ⁱ transients in slice preparation and in vitro.

Effect of Dimethyl Fumarate on the Motor Function and Spatial Arrangement of Primary Motor Cortical Neurons in the Sub-Acute Phase of Stroke in a Rat Model

BACKGROUND

The therapeutic effects of dimethyl fumarate (DMF) in patients with multiple sclerosis and animal models of neurologic disease were reported. The density and the distribution pattern of motor neurons are important in transmitting the signal and controlling the movement-related functions. The present study evaluated the effects of DMF treatment on the neurological functions, infarct volume, and spatial distribution of the neurons in the primary motor cortex after cerebral ischemia.

METHODS

Thirty-three Sprague-Dawley rats were randomly divided into three groups: The sham group underwent surgery without middle cerebral artery occlusion (MCAO) and drug. The vehicle and treatment groups after MCAO received a vehicle or DMF for three consecutive days. Post-stroke neurological and motor functions were assessed. At the end of the third day, the brains were removed, and the cerebral infarct volume was evaluated. We used cresyl violet staining to analyze the density and the spatial arrangement of motor cortical neurons using Voronoi tessellation.

RESULTS

Treatment of the brain ischemia for three days with DMF could not significantly reduce the neurological and motor function deficits and infarct volume. However, it reduced the neuronal area and death and preserved their spatial distribution in the normal regular pattern.

CONCLUSION

Cerebral ischemia decreased the neuronal density of the primary motor cortex and changed their distributions to a random pattern. DMF treatment during sub-acute ischemic stroke did not significantly improve the neurological deficit scores. However, it could prevent neuronal swelling and death and preserved the spatial distribution of the cortical neurons in their normal pattern.

Age-dependent changes in the medial prefrontal cortex and medial amygdala structure, and elevated plus-maze performance in the healthy male Wistar rats

Aging affects different parts of the brain structure and function. These changes are associated with several age-related emotional alterations like anxiety that is regulated by the amygdala and medial prefrontal cortex (mPFC). Thus, this study aimed to explore the effects of aging on the morphology changes in these regions. Twenty male Wistar rats were assigned to young and old groups. The anxiety level was evaluated by elevated plus-maze. Then, their brains were removed, fixed, cut, and stained with Cresyl Violet or Golgi-Cox. In addition to the estimation of stereological parameters, dendrite complexity, and spatial distribution of the neurons in the mPFC and amygdala were evaluated. Aging increased the medial amygdala volume and its total number of neurons, but it did not have a significant effect on these parameters in the mPFC. Furthermore, the size of the neurons in the mPFC increased, whereas the total length of the dendrite and its complexity significantly decreased with aging in this structure and increased in the amygdala. Although aging did not significantly change the dendritic spine density in both regions, old rats showed a more mature spine in the mPFC and more anxiety-like behavior. In conclusion, the increase of anxiety in the old individuals could be attributed to structural changes in the morphology of the dendrite and neuron and its spatial distribution in the mPFC and amygdala. The findings of this study partly support this hypothesis.

Axon Initial Segment Structural Plasticity is Involved in Seizure Susceptibility in a Rat Model of Cortical Dysplasia

Cortical dysplasia is the most common etiology of intractable epilepsy. Both excitability changes in cortical neurons and neural network reconstitution play a role in cortical dysplasia epileptogenesis. Recent research shows that the axon initial segment, a subcompartment of the neuron important to the shaping of action potentials, adjusts its position in response to changes in input, which contributes to neuronal excitability and local circuit balance. It is unknown whether axon initial segment plasticity occurs in neurons involved in seizure susceptibility in cortical dysplasia. Here, we developed a "Carmustine"- "pilocarpine" rat model of cortical dysplasia and show that it exhibits a lower seizure threshold, as indicated by behavior studies and electroencephalogram monitoring. Using immunofluorescence, we measured the axon initial segment positions of deep L5 somatosensory neurons and show that it is positioned closer to the soma after acute seizure, and that this displacement is sustained in the chronic phase. We then show that Nifedipine has a dose-dependent protective effect against axon initial segment displacement and increased seizure susceptibility. These findings further our understanding of the pathophysiology of seizures in cortical dysplasia and suggests Nifedipine as a potential therapeutic agent.

The effect of carmustine on Bergmann cells of the cerebellum

Administration of the alkylating agent carmustine to pregnant mice induces hyperlocomotion in the offspring. Motor performance was evaluated by the rotarod task, which revealed that these animals have diminished Grab Frequency and a higher Performance Index, whereas Error of Latency and Latency to Fall were unaffected. Considering the recently revealed role of Bergmann cells of cerebellum in the control of motor activity, we used the transgenic mice GFAP-GFP to explore the impact of carmustine on the organization of these glial cells. Multiple examples of cell layer disorganization were detected; many soma of Bergmann cells were displaced to the external cell layer, and their processes were not well defined until young adulthood. In addition, the roof of the fourth ventricle was convoluted. These observations suggest that the exacerbated locomotion induced by carmustine may be due, in part, to the altered organization of the cell layers of cerebellum.

In vivo DTI tractography of the rat brain: an atlas of the main tracts in Paxinos space with histological comparison

Diffusion tensor imaging (DTI) is a magnetic resonance modality that permits to characterize the orientation and integrity of white matter (WM). DTI-based tractography techniques, allowing the virtual reconstruction of WM tract pathways, have found wide application in preclinical neurological research. Recently, anatomically detailed rat brain atlases including DTI data were constructed from ex vivo DTI images, but tractographic atlases of normal rats in vivo are still lacking. We propose here a probabilistic tractographic atlas of the main WM tracts in the healthy rat brain based on in vivo DTI acquisition. Our study was carried out on 10 adult female Sprague-Dawley rats using a 7T preclinical scanner. The MRI protocol permitted a reliable reconstruction of the main rat brain bundles: corpus callosum, cingulum, external capsule, internal capsule, anterior commissure, optic tract. The reconstructed fibers were compared with histological data, proving the viability of in vivo DTI tractography in the rat brain with the proposed acquisition and processing protocol. All the data were registered to a rat brain template in the coordinate system of the commonly used atlas by Paxinos and Watson; then the individual tracts were binarized and averaged, obtaining a probabilistic atlas in Paxinos-Watson space of the main rat brain WM bundles. With respect to the recent high-resolution MRI atlases, the resulting tractographic atlas, available online, provides complementary information about the average anatomical position of the considered WM tracts and their variability between normal animals. Furthermore, reference values for the main DTI-derived parameters, mean diffusivity and fractional anisotropy, were provided. Both these results can be used as references in preclinical studies on pathological rat models involving potential alterations of WM.

Altered migratory behavior of interneurons in a model of cortical dysplasia: the influence of elevated GABAA activity

Appropriate function of the neocortex depends on timely generation and migration of cells produced in the germinal zones of the neocortex and ganglionic eminence (GE). Failure to accurately complete migration results in cortical dysplasia, a developmental syndrome implicated in many neurologic disorders. We developed a model of cortical dysplasia in ferrets involving administration of methylaxozymethanol acetate (MAM), an antimitotic, to pregnant ferrets on gestational day 33, leading to dramatic reduction of layer 4 in the neocortex. Here, using time-lapse video imaging, we investigate dynamic behavior of migrating cells arising from the GE and cortical ventricular zone (CVZ) in ferrets and the role of GABAA activity. Treatment with MAM significantly reduced migration speed and the relative proportion of cells arising from the GE demonstrating exploratory behavior. To a lesser extent, the behavior of cells leaving the CVZ was affected. Pharmacologic inhibition of GABAA receptors (GABAAR) improved the speed of migration and exploratory ability of migrating MAM-treated cells arising from the GE. Additionally, the expression of α2 and α3 subunits of GABAAR and the potassium chloride co-transporter (KCC2) increased in the neocortex of MAM-treated animals. After MAM treatment, increases in endogenous KCC2 and GABAAR combine to alter the dynamic properties and exploratory behavior of migrating interneurons in ferrets. We show a direct correlation between increased GABAA and KCC2 expression with impaired migration and ability to explore the environment.

Genesis of heterotopia in BCNU model of cortical dysplasia, detected by means of in utero electroporation

Derangements of cortical development can cause a wide spectrum of malformations, generally termed 'cortical dysplasia' (CD), which are frequently associated with drug-resistant epilepsy and other neurological and mental disorders. 1,3-Bis-chloroethyl-nitrosurea (BCNU)-treated rats represent a model of CD due to the presence of histological alterations similar to those observed in human CD. BCNU is an alkylating agent that, administered at embryonic day 15 (E15), causes the loss of many cells destined to cortical layers; this results in cortical thinning but also in histological alterations imputable to migration defects, such as laminar disorganization and cortical and periventricular heterotopia. In the present study we investigated the genesis of heterotopia in BCNU-treated rats by labeling cortical ventricular zone (VZ) cells with a green fluorescent protein (GFP) expression vector by means of in utero electroporation. Here, we compared the migratory pattern and subsequent distribution of the GFP-labeled cells in the developing somatosensory cortex of control and BCNU-treated animals. To this aim, we investigated the expression of a panel of developmental marker genes which identified radial glia cells (Pax6), intermediate precursors cells (Tbr2), and postmitotic neurons destined to infragranular (Tbr1) or supragranular layers (Satb2). The VZ of BCNU-treated rats appeared disorganized since E18 and at E21 the embryos showed an altered migratory pattern: migration of superficial layers appeared delayed, with a number of migrating cells in the intermediate zone and some neurons destined to superficial layers arrested in the VZ, thus forming periventricular heterotopia. Moreover, neurons that reached their correct position did not extend their axons through the corpus callosum in the contralateral hemisphere as in the control, but toward the ipsilateral cingulated cortex. Our analysis sheds light on how a malformed cortex develops after a temporally discrete environmental insult.

Targeted disruption of layer 4 during development increases GABAA receptor neurotransmission in the neocortex

Cortical dysplasia (CD) associates with clinical pathologies, including epilepsy and mental retardation. CD results from impaired migration of immature neurons to their cortical targets, leading to clustering of neural cells and changes in cortical properties. We developed a CD model by administering methylazoxymethanol (MAM), an anti-mitotic, to pregnant ferrets on embryonic day 33; this leads to reduction in cortical thickness in addition to redistribution and increased expression of GABAA receptors (GABAAR). We evaluated the impact of MAM treatment on GABAAR-mediated synaptic transmission in postnatal day 0-1 neurons, leaving the ganglionic eminence (GE) and in layer 2/3 pyramidal cells of postnatal day 28-38 ferrets. Embryonic day 33 MAM treatment significantly increases the amplitude and frequency of spontaneous GABAAR-mediated inhibitory postsynaptic currents (IPSCs) in the cells leaving the GE. In older MAM-treated animals, the amplitude and frequency of GABAAR-mediated spontaneous IPSCs in layer 2/3 pyramidal cells is increased, as are the amplitude and frequency of miniature IPSCs. The kinetics of GABAAR opening also altered following treatment with MAM. Western blot analysis shows that the expression of the GABAAα3R and GABAAγ2R subunits amplified in our model animals. We did not observe any significant change in the passive properties of either the layer 2/3 pyramidal cells or cells leaving the GE after MAM treatment. These observations reinforce the idea that synaptic neurotransmission through GABAAR enhances following treatment with MAM and coincides with our finding of increased GABAAαR expression within the upper cortical layers. Overall, we demonstrate that small amounts of toxins delivered during corticogenesis can result in long-lasting changes in ambient expression of GABAAR that influence intrinsic neuronal properties.

Cytoarchitectural, behavioural and neurophysiological dysfunctions in the BCNU-treated rat model of cortical dysplasia

Cortical dysplasias (CDs) include a spectrum of cerebral lesions resulting from cortical development abnormalities during embryogenesis that lead to cognitive disabilities and epilepsy. The experimental model of CD obtained by means of in utero administration of BCNU (1-3-bis-chloroethyl-nitrosurea) to pregnant rats on embryonic day 15 mimics the histopathological abnormalities observed in many patients. The aim of this study was to investigate the behavioural, electrophysiological and anatomical profile of BCNU-treated rats in order to determine whether cortical and hippocampal lesions can directly lead to cognitive dysfunction. The BCNU-treated rats showed impaired short-term working memory but intact long-term aversive memory, whereas their spontaneous motor activity and anxiety-like response were normal. The histopathological and immunohistochemical analyses, made after behavioural tests, revealed the disrupted integrity of neuronal populations and connecting fibres in hippocampus and prefrontal and entorhinal cortices, which are involved in memory processes. An electrophysiological evaluation of the CA1 region of in vitro hippocampal slices indicated a decrease in the efficiency of excitatory synaptic transmission and impaired paired pulse facilitation, but enhanced long-term potentiation (LTP) associated with hyperexcitability in BCNU-treated rats compared with controls. The enhanced LTP, associated with hyperexcitability, may indicate a pathological distortion of long-term plasticity. These findings suggest that prenatal developmental insults at the time of peak cortical neurogenesis can induce anatomical abnormalities associated with severe impairment of spatial working memory in adult BCNU-treated rats and may help to clarify the pathophysiological mechanisms of cognitive dysfunction that is often associated with epilepsy in patients with CD.

Methylnitrosourea induces neural progenitor cell apoptosis and microcephaly in mouse embryos

BACKGROUND

Prenatal exposure to methylnitrosourea (MNU), an alkylating agent, induces microcephaly in mice. However, its pathogenetic mechanism has not been clarified, especially that in the development of the cerebral cortex.

METHODS

ICR mice were treated with MNU at 10 mg/kg intraperitoneally on day 13.5 or 15.5 of gestation, and the embryos were observed histologically 24 hr after treatment with MNU or at term. To clarify the pathogenesis of microcephaly and histological changes, especially apoptosis, neurogenesis, and neural migration/positioning, we performed histological analysis employing a cell-specific labeling experiment using thymidine-like substances (BrdU, CldU, and IdU) and markers of neurons/neural stem cells.

RESULTS

Histological abnormalities of the dorsal telencephalon, and the excessive cell death of proliferative neural progenitor/stem cells were noted in the MNU-treated embryos. The highest frequencies of cell death occurred at 36 hr after MNU treatment, and little or no neurogenesis was observed in the ventricular zone of the dorsal telencephalon. Abnormality of the radial migration was caused by the reduction of radial fibers in the radial glias. Birth-date analysis revealed the abnormal positioning of neurons and aberrant lamination of the cerebral cortex.

CONCLUSIONS

Our data suggest that prenatal exposure to MNU induces the excessive cell death of neural precursor/stem cells, and the defective development of the cerebral cortex, resulting in microcephalic abnormalities.

N,N'-Bis(2-chloroethyl)-N-nitrosourea (BCNU)-induced Apoptosis of Neural Progenitor Cells in the Developing Fetal Rat Brain

N,N’-bis(2-chloroethyl)-N-nitrosourea (BCNU) is one of the major drugs used in chemotherapy against malignant gliomas due to its effects, such as induction of bifunctional alkylation of DNA and formation of interstrand DNA cross-linkages, and induces cortical malformations in the fetal and neonatal rat brain. In this study, pregnant rats were treated with 7.5 mg/kg of BCNU on gestational day 13 (GD 13), and their fetuses were collected from 12 to 72 hours after BCNU treatment in order to examine the timecourses of morphological and immunohistochemical changes in neural progenitor cells in the developing brain. The number of pyknotic cells in the telencephalon peaked at 24 h and then gradually decreased until 72 h. The majority of these pyknotic cells were positive for cleaved caspase-3, a key executioner of apoptosis. The pyknotic cells showed the ultrastructural characteristics of apoptosis. The number of p53-positive cells began to increase prior to the appearance of apoptotic cells and p21-positive cells. The number of phosphorylated-histone H3-positive cells (mitotic cells) decreased from 24 to 36 h. The number of Iba1-positive cells (microglial cells) in the telencephalon increased from 12 to 48 h. These results suggest that BCNU induces p53-dependent apoptosis and reduces proliferative activity, resulting in reduction of the weight of the telencephalon and the thickness of the telencephalic wall in the fetal brain. This study will help to clarify the mechanisms of BCNU-induced fetal brain toxicity.

Impact of ethanol on the developing GABAergic system

Alcohol intake during pregnancy has a tremendous impact on the developing brain. Embryonic and early postnatal alcohol exposures have been investigated experimentally to elucidate the fetal alcohol spectrum disorders' (FASD) milieu, and new data have emerged to support a devastating effect on the GABAergic system in the adult and developing nervous system. GABA is a predominantly inhibitory neurotransmitter that during development excites neurons and orchestrates several developmental processes such as proliferation, migration, differentiation, and synaptogenesis. This review summarizes and brings new data on neurodevelopmental aspects of the GABAergic system with FASD in experimental telencephalic models.

Expression of layer-specific markers in the adult neocortex of BCNU-Treated rat, a model of cortical dysplasia

The experimental model of cortical dysplasia (CD) obtained by administering carmustine (1-3-bis-chloroethyl-nitrosurea [BCNU]) in pregnant rat uterus mimics the histopathological abnormalities observed in human CD patients: altered cortical layering, and presence of heterotopia and dysmorphic/heterotopic neurons. To investigate further the cortical layering disruption and the neuronal composition of heterotopia in BCNU-exposed cortex, we analyzed the expression pattern of the transcription factors Nurr1, Er81, Ror-beta, and Cux2 (respectively specific markers of layers VI, V, IV and superficial layers) in the cortical areas of BCNU-treated rats by means of in situ hybridization, and compared the findings with those observed in adult control rats. Combining in situ hybridization and immunohistochemistry we also investigated the origin of dysmorphic or heterotopic neurons. The main results of the present study are (i) the analysis of cortical layer thickness revealed that the cortical thinning in the BCNU model was prevalently restricted to the superficial layers; (ii) in cortical and periventricular heterotopia, the prevalent presence of superficial layer neurons in the internal areas, and deeper layer neurons in a more peripheral region, demonstrated a rudimentary pattern of laminar organization in nodule formation; and (iii) the Er81 signal in the dysmorphic and heterotopic pyramidal neurons located in layers I/II showed that they belong to layer V. These results shed light on the disorganization of the laminar architecture of the BCNU model by providing correlations with normal cortical layering and revealing the ontogenesis of heterotopia and heterotopic/dysmorphic neurons. They also provide strong evidence of the usefulness of layer-specific markers in investigating the neuropathology of CD, thus opening up the possibility of expanding their application to human neuropathology.