Inhibitory networks in epilepsy-associated gangliogliomas and in the perilesional epileptic cortex
Introduction
Gangliogliomas (GG) are an increasingly recognized cause of epilepsy and represent the most common tumor entity in young patients undergoing surgery for chronic focal intractable epilepsy (Aronica et al., 2001b, Blumcke and Wiestler, 2002, Farrell and Vinters, 1997). GG consist of a mixture of dysplastic neurons and neoplastic astroglial cell elements and they are most commonly located in the temporal lobe. This histological composition, together with the expression of stem cell markers (such as CD34 and nestin) and the association with cortical dysplasia, has attracted considerable interest with respect to the origin of these lesions (Blumcke and Wiestler, 2002). A recent classification scheme for malformations of cortical development (MCD) includes GG among the disorders of proliferation (with abnormal cell types) together with dysembryoplastic neuroepithelial tumors, focal cortical dysplasia, tuberous sclerosis and hemimegalencephaly (Barkovich et al., 2005).
The cellular mechanism(s) underlying the epileptogenicity of GG remain largely unknown. Particular attention has been focussed on the neuronal glutamatergic population, consisting of highly differentiated cells expressing different glutamate receptor subtypes (Aronica et al., 2001c, Wolf et al., 1995). However, compelling evidence in human epileptogenic tissue and in different experimental models of seizures indicates that anatomical and functional alterations of the GABA-ergic networks may also critically contribute to epileptogenesis (for review see Avoli et al., 2005). Dysfunction of the inhibitory system may result from loss of the GABA-ergic interneurons (Avoli et al., 2005). In contrast to the glutamatergic neurons, these inhibitory cells are also generated in the ganglionic eminences and migrate tangentially to the cerebral cortex (Kriegstein and Noctor, 2004). Disruption of GABA-ergic interneuron development may represent a common mechanism underlying the pathogenesis and epileptogenesis of different MCD (Cobos et al., 2005, Levitt, 2005). Interestingly, reduction in the density of the interneuron population has been observed in tissue specimens from patients with focal cortical dysplasia (FCD) (Alonso-Nanclares et al., 2005, Garbelli et al., 1999, Spreafico et al., 1998, Spreafico et al., 2000, Thom et al., 2003). In addition, recent findings suggest that alterations in GABA-mediated synaptic inhibition in MCD could be secondary to reductions in GABA transporter expression (Calcagnotto et al., 2005, Spreafico et al., 2000) or changes in the expression and/or function of specific GABA receptor subtypes (Cepeda et al., 2005, Crino et al., 2001, D’Antuono et al., 2004).
In the present immunocytochemical study we investigated the expression and distribution of components of the GABA-ergic system in GG specimens from patients surgically treated for medically intractable epilepsy. In addition, we analyzed the morphological aspects of the GABA-ergic network in the perilesional tissue, which is of particular interest for its possible contribution to generation and/or propagation of seizures. Our major aim was to define abnormal patterns which may provide better insights into the mechanisms underlying the intrinsic and high epileptogenicity of these glioneuronal lesions.
Section snippets
Subjects
The cases included in this study were obtained from the files of the Departments of Neuropathology of the Academic Medical Center (University of Amsterdam) and the University Medical Center in Utrecht. A total of 30 specimens removed from patients undergoing surgery for GG were examined. Informed consent was obtained for the use of brain tissue and for access to medical records for research purposes. Tissue was obtained and used in a manner compliant with the Declaration of Helsinki. Two
Case material and histological features
The clinical features of the cases included in this study are summarized in Table 1. All patients had a history of chronic pharmacoresistant epilepsy. Postoperatively, 27 patients (90%) were completely seizure free. GG were composed histologically of a mixture of atypical neuronal cells and neoplastic astrocytes and showed a broad spectrum of histopathological features (Blumcke and Wiestler, 2002). Below, we first describe the staining pattern of each Ab in control tissue, followed by the
Discussion
The present study describes the immunocytochemical expression and cellular distribution of components of the GABA-ergic system in GG from patients with intractable epilepsy. The following observations were made:
- (1)
GG contain inhibitory neurons of different subtypes. Calcium-binding protein-positive interneurons represent a small fraction of the total neuronal population.
- (2)
Both GABAAR and GABABR (R1 and R2) subtypes were detected within the neuronal component of GG specimens; GABABR2 was also
Acknowledgements
This work was supported by the “Christelijke Vereniging voor de Verpleging van Lijders aan Epilepsie”, Stichting Epilepsie Instellingen Nederland and the National Epilepsy Fund, “Power of the Small” and Hersenstichting Nederland (NEF 02-10; NEF 05-11, E. Aronica and K. Boer).
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