Expression and cell distribution of group I and group II metabotropic glutamate receptor subtypes in Taylor-type focal cortical dysplasia

Purpose: Focal cortical dysplasia (FCD) is known to be a major cause of intractable epilepsy. The cellular mechanism(s) underlying the epileptogenicity of FCD remain largely unknown. Because recent studies indicate that metabotropic glutamate receptor subtypes (mGluRs) play a role in epileptogenesis, we investigated the expression and cellular distribution pattern of mGluRs in FCD specimens. Methods: Immunocytochemical expression of group I and group II mGluR subtypes was investigated in 15 specimens of human FCD obtained during epilepsy surgery. Results: Strong mGluRlalpha and mGluR5 (group I mGluRs) immunoreactivity (IR) was observed in the majority of FCD specimens in dysplastic as well as in heterotopic neurons. mGluRla was expressed in a subpopulation of neurons (mainly large dysplastic cells), whereas mGluR5 was represented in a higher percentage of dysplastic neuronal cells. Group II mGluRs (mGluR2/3) IR was observed less frequently than that in group I mGluRs and generally appeared in <10% of the dysplastic neurons. IR for all three mGluR subtypes was observed in balloon cells. mGluR2/3 appeared to be most frequently expressed in glial fibrillary acidic protein (GFAP)-positive balloon cells (glial type), and mGluRlalpha, in microtubule-associated protein (MAP)2-positive cells (neuronal type). mGluR5 was present in the majority of balloon cells. Occasionally glial mGluRlalpha IR was observed in bizarre glial cells with di- or multinuclei. Reactive astrocytes were intensively stained, mainly with mGluR5 and mGluR2/3. Conclusions: The cellular distribution of mGluR subtypes, with high expression of mGluRlalpha and mGluR5 in dysplastic neurons, suggests a possible contribution of group I mGluRs to the intrinsic and high epileptogenicity of dysplastic cortical regions.