Elsevier

Epilepsy Research

Volume 32, Issues 1–2, 1 September 1998, Pages 24-33
Epilepsy Research

Glutamate receptor mechanisms in human epileptic dysplastic cortex

https://doi.org/10.1016/S0920-1211(98)00037-0Get rights and content

Abstract

Developmental disorders of neuronal migrations in the human brain are referred to as `cortical dysplasia', and current knowledge of cortical dysplasia is limited to varied pathologic descriptions which lack specific investigations of glutamate receptor mechanisms. In this study, immunocytochemistry was used to study the expressions of glutamate receptor subunit proteins for NMDAR2A/B, NMDAR1 and AMPA Glu-R2/3 in human brain resected for intractable epilepsy associated with cortical dysplasia. Seventeen patients were studied with batch-matched glutamate subunit reagents on adjacent 30-μm sections. The most striking microscopic abnormalities identified in cresylecht violet stains were cortical dyslaminations, disoriented neurons, and unexpectedly, very dark Nissl body staining of those dysplastic neurons. NMDAR2A/B intensely labeled dysplastic neurons, showing staining in both the cell bodies and dendritic profiles. However, non-dysplastic neurons were not immunoreactive to NMDAR2A/B. Dysplastic neurons were also labeled by antibodies selective to NMDAR1. Both dysplastic neurons and non-dysplastic neurons were immunoreactive to AMPA GluR2/3. Our results suggest that the epileptic hyperexcitability of dysplastic cortical regions may result, at least in part, from the heteromeric coassembly and expressions of NMDAR2A/B subunits with selectively expressed NMDAR1 splice variants in dysplastic neurons. AMPA receptors are probably also essential but not sufficient to explain the `epileptic' properties of these dysplastic neurons. A longer, detailed report of some of these findings have been previously published (Ying et al., 1998. J. Neuropathol. Exp. Neurol. 57, 47–62).

Introduction

Misplaced cortical neurons in autopsied epileptics were described by Alzheimer (1907)and Rancke (1910); however, neither attributed them to causing specific seizure types. Jakob (1914)insisted that an epileptic tendency is based on the `aberrant differentiation' of neurons in the cerebral cortex, and during the past 20 years, investigators of epileptic brains have reported relatively high incidences of microdysgenesis: (1) 37% in patients with all types of epilepsy versus 4% in healthy individuals (Veith and Wicke, 1968); (2) 42% in those with temporal lobe epilepsy versus 0% in controls (Hardimann et al., 1988); and (3) 88% in patients with primary generalized epilepsy versus 0% in controls (Meencke and Janz, 1984). This high incidence of dystopic neurons in primary generalized epilepsy was confirmed in seven more well-documented patients with similar developmental abnormalities (Meencke and Janz, 1985). More recent studies of cortical dysplasias in both children and adults with epilepsy show that congenital, genetic, or postnatal migration defects are associated with a broad spectrum of seizures, from infantile (Vinters et al., 1992, Vinters et al., 1993) to focal neocortical seizures (Palmini et al., 1995). While neuronal migration disorders or cortical dysplasia (CD) can be frequently found in neocortical epilepsies, it is not necessarily known if these small or even larger morphological abnormalities are sufficient to induce chronic seizure foci. For example, malformations of cortical neuron laminations, abnormal neuron sizes, their locations and clusters, and varied gyral patterns are all termed `cortical dysplasia' or disorders of development in the central nervous system occurring prenatally or early postnatally. A unique form of cortical dysplasias involving giant neurons misplaced in disordered cortex was first described in adult temporal epilepsy (Taylor et al., 1971). In subsequent decades, microscopic studies of epileptic dysplastic neocortex resected from children and adults have confirmed that there are many features of migrational disorders (Mischel et al., 1995). Our study is limited to the mature neocortex, i.e. from non-infants with no gross structural malformations. The term `cortical dysplasia' used in our study is defined as microscopically identified abnormal cortical cytoarchitecture such as dislaminated cortical layers and aberrantly shaped neurons. These cortical abnormalities cannot be identified by palpation or by direct vision. The adult cortical dysplasia epileptic population is of particular interest because the pathology appears to be non-destructive, non-progressive, not unique to one lobe, and may be limited to only a few cortical layers in one lobe.

To date, most studies on cortical dysplasias remain limited to varied pathological descriptions, which lack specific investigations of cellular mechanisms at the level of axons, receptors or transmitters. For example, cortical dysplasia exhibit a spectrum of pathologic changes; however, probably not all of these microscopically identified pathologic abnormalities are necessarily required to generate seizures. Some of the pathologic abnormalities are not related to epilepsy and should be regarded as merely markers of abnormal cortical neuron development. A recent study (Ying et al., 1998), demonstrated for the first time that there is EEG-verified hyperexcitability near regions of dysplastic cortex, and a selective expression of NR2 subunit proteins in the dysplastic neurons. Rapid excitatory signal transduction is mediated by activating the postsynaptic glutamate receptors. Electrophysiologic and pharmacologic studies (for reviews see Watkins and Evans, 1981, Monaghan, 1989, Fagg and Massieu, 1991) have identified several different classes of glutamate receptors based on their selective agonist activation and selective antagonist blockade. Ionotrophic AMPA receptors transmit these fast excitatory synaptic potentials, while NMDA receptors, which have ligand-gated ion channels with voltage-dependent properties, mediate prolonged neuronal depolarization. Recent molecular techniques for functional receptor expression have identified receptor subunits that comprise specific functional glutamate receptors (for reviews see Hollmann and Heinemann, 1994, Nakanishi and Masu, 1994). To date, four receptor subunit proteins, termed GluR1 to GluR4, have been identified for the AMPA receptor. The NMDA receptor has two families of subunit proteins, NMDAR1 and NMDAR2. The NMDAR1 gene generates eight alternate splice variants; the NMDAR2 gene has four gene products generating NR2A-NR2D subunits. In this study, we used immunocytochemistry to examine the expressions of various glutamate receptor subunit proteins in surgically resected human epileptic dysplastic cortex.

Section snippets

Materials and methods

The cortical dysplasias described in this study were obtained from 17 patients who underwent partial lobectomies, including frontal lobe, central region, motor strip, parietal lobe, temporal lobe, and occipital lobe. The patients' ages ranged from 5 to 65 years, (mean=26, median=30, mode=19); cortical dysplasias have been identified in all of the patients. Normal temporal neocortex was found to be free of dysplasia in a series of standard en bloc resections of the hippocampus which included

Characteristics of dysplastic neocortex

Fig. 1 contrasts the main features of normal temporal neocortex to dysplastic temporal neocortex. Non-dysplastic cortex is defined as those regions with well-preserved vertical and horizontal laminations. Such normal regions were identified in the temporal cortex of patients who had typical hippocampal sclerosis and no other pathologies, especially no evidence of cortical dysplasia by careful histological examination of the en bloc resected temporal lobe. Other control comparisons were possible

Histologic features of epileptic cortical dysplasias

The term cortical dysplasia used in our study is defined as microscopically identified abnormal cortical cytoarchitecture such as dislaminated cortical layers and aberrantly shaped neurons. The most reliable microscopic abnormalities identified in CV stains were cortical dyslaminations, disoriented neurons, and very dark Nissl body staining of those dysplastic neurons. Those disoriented, misshapen, often enlarged neurons had darkly stained long apical branches and basilar dendritic processes.

Acknowledgements

The authors thank the Comprehensive Epilepsy Surgery Program and the Clinical Neurophysiology lab of the Cleveland Clinic Foundation for the precise preoperative diagnoses made on the patients reported in this paper. Thanks also goes to the Department of Pathology for independent analyses and diagnoses. The research on resected human brain was approved by the Institutional Review Board (Approval number IRB 1263). This work was supported by National Institutes of Health Grant NS-31655 to T.L.

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