Excitatory amino acid receptors in brains of rats with methylazoxymethanol-induced microencephaly

Increased frequency of distinct TSC2 allelic variants in focal cortical dysplasias with balloon cells and mineralization

Focal cortical dysplasias with balloon cells (FCD(IIb)) usually present with characteristic imaging and molecular features, that is, a transmantle sign on fluid-attenuated inversion recovery MRI and abundance of allelic variants of the tuberous sclerosis gene 1 (TSC1). Recently, we observed several mineralized lesions (n = 5) lacking this MRI pattern and which surprisingly turned out as FCD(IIb) upon neuropathological examination. These mineralized FCD(IIb) revealed an increased frequency of TSC2 allelic variants but not TSC1 (intron 31: 60% vs. 11% in controls; P = 0.0164, exon 41: 40% vs. 6.5% in controls; P = 0.0441). Mineralized FCD(IIb) have a favorable postsurgical outcome and need consideration in the presurgical differential diagnosis of calcified lesions associated with pharmacoresistant focal epilepsies.

Altered glutamate receptor - transporter expression and spontaneous seizures in rats exposed to methylazoxymethanol in utero

PURPOSE

Brain malformations are a common cause of intractable epilepsy and cognitive dysfunction in children. Prenatal exposure to the teratogen methylazoxymethanol (MAM) is a rodent model of brain malformation featuring loss of lamination, clusters of displaced hippocampal cells, and pharmaco-resistance to antiepileptic drugs. In a normotopic hippocampus, expression of postsynaptic glutamate receptors and the transporters regulating neurotransmitter reuptake are critical factors modulating excitation and synaptic communication. Alterations in this system can have profound effects on overall excitability, cognitive function, and seizure thresholds.

METHODS

Immunohistochemical techniques were used to analyze the expression of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5 methylisoxazole-4-proprionic acid (AMPA) receptor subunits in rats exposed to MAM in utero (25 mg/kg, intraperitoneal injection). We also examined the expression of several glutamate transporters (EAAC1, vGLUT1, and vGLUT2). A video-electroencephalographic (video-EEG) system was used for long-term monitoring of adult MAM-exposed rats.

RESULTS

Heterotopic hippocampal neurons exhibited striking reductions in GluR1 and EAAC1 expression; vGlut2 expression was prominent in these regions. Spontaneous electrographic seizures were verified in two animals.

CONCLUSIONS

We conclude that glutamate receptor subunit and transporter expression are altered in animals exposed to MAM in utero. Further studies in the MAM model may provide greater insight into the potential disruptions in excitatory synaptic neurotransmission that can occur in a malformed brain.

Evidence of oxidative stress in the brains of fetuses with CNS anomalies and islet cell hyperplasia

Infants of diabetic mothers have an increased frequency of congenital anomalies, including CNS malformations. Fetal hyperglycemia may promote such damage via oxidative stress. Postmortem studies have shown that fetal hyperglycemia associated with maternal diabetes results in islet cell hyperplasia. Islet cell hyperplasia may correlate with the presence of oxidative stress injury in the CNS because of hyperglycemia and related metabolic derangement. This study examines 3-nitrotyrosine immunoreactivity as a marker of oxidative stress in the brains of fetuses stratified by the presence or absence of islet cell hyperplasia and CNS developmental anomalies. Fetuses with both islet cell hyperplasia and CNS developmental anomalies showed a 1.8-fold increase in semiquantitatively scored 3-nitrotyrosine immunostaining compared to negative controls. Fetuses with islet cell hyperplasia but no CNS anomalies demonstrated a 1.6-fold increase. Comparison between fetuses with islet cell hyperplasia which were stratified by presence or absence of CNS anomalies were not statistically different but did show more intense staining in those with CNS malformations. These results support the contention that hyperglycemia may contribute to CNS malformation via oxidative stress.

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

Absence of excitotoxic neuropathology in microencephalic rats after systemic kainic acid administration

We have s.c. injected with kainic acid (12 mg/kg) normal adult rats as well as rats rendered microencephalic by selectively timed administration of the DNA alkylating agent methylazoxymethanol acetate (MAM) to the mother during pregnancy. Histological examination of the brains revealed that normal animals underwent neurodegeneration in brain regions sensitive to kainic acid excitotoxicity, such as the olfactory cortex and the hippocampus, while no damage was apparent in the same regions of microencephalic rats. Evaluation of the neurotoxic outcome consequent to the excitotoxic stimulation, was quantitatively performed by measuring the levels of appropriate neurochemical markers 15 days after kainic acid injection. In normal animals, this resulted in significant decrease (up to 60% in the olfactory cortex and 30% in the hippocampus) of markers related to glutamatergic and GABAergic neurons, whereas in MAM-treated rats the same markers were not significantly affected, thus demonstrating a substantial protection against the excitotoxic insult in the microencephalic condition.

IT15 gene expression in fetal human brain

To examine the expression of the gene which causes Huntington's disease (HD), IT15, during development, in situ hybridization of radiolabeled riboprobes was performed in human fetal (gestational ages 20-23 weeks) and adult brain. Optical densities of autoradiographs were determined in various brain regions and compared to cell density in those regions. IT15 expression was found in all regions of the fetal and adult brain, and there was a high degree of correlation of autoradiographic signal with cell number in all regions but germinal matrix in fetal brain and white matter in adult brain. These two regions are notable for their significant proportion of glial cells, and suggest that IT15 expression is predominantly neuronal. There was no preponderance of IT15 expression in striatal compartments in fetal brain as demonstrated by acetylcholinesterase activity, nor was there differential expression of IT15 in brain regions known to be particularly affected in HD. IT15 gene expression is present by 20 weeks gestation in human brain, and at that stage of development exhibits a pattern of distribution which is similar to adult brain. If a developmentally-regulated role for IT15 exists in the pathogenesis of HD, it must occur prior to 20 weeks gestation.

The NMDA antagonist MK-801 can impair attention to exteroceptive stimuli

Using a modified open-field method, we assessed the effects of MK-801 (0.01 to 0.3 mg/kg) on locomotion and on attention to a stimulus object located in a computer-generated central zone (CZ). The CZ comprised 1/9 of the open-field floor area and was monitored independently from the rest of the area. Intermittently, a 4 x 4 x 2 cm block was placed in the CZ. In 10-min trials, non-drug tests showed that the presence of the stimulus object repeatedly and consistently increased the rats' visit duration in the CZ as compared with tests when the object was absent. Locomotor activity and entries to the CZ were unaffected by the object. MK-801 induced dose dependent hyperlocomotion and increased CZ entries and, most important, a dose dependent decrease in the animal's response to the stimulus object in the CZ. The present investigation suggests that MK-801 impacts upon two major functions; (a) a blockade of processing of attentional information from the external world and (b) activation of locomotor response systems. These findings are consistent with the hypothesis that blockade of glutamate neurotransmission by MK-801 impairs the flow of information from the external world to response mechanisms in the striatum. The present study also suggests that MK-801's potential as a therapeutic agent for motoric activation in the treatment for Parkinson's disease would be contraindicated by its disruptive influence upon attention processing functions.