Abstracts

Rationale: Cortical dysplasias are a common epileptogenic pathology and are associated with disorganized cortical organization and characteristic histologic features including the presence of balloon cells and dysplastic neurons. While a number of hisotological studies have been performed on resected dysplastic cortex, little is known about the neuronal-glial relationship in this tissue. Previous work has shown that glutamate-glutamine cycling is impaired in MTS tissue, due in part to impaired glutamine synthetase (GS) activity. We hypothesized that a similar derangement of neuronal-glial metabolism would be seen in dysplastic tissue. Methods: We used dysplastic cortex (n=8) and sclerotic hippocampi (n=16) resected during the course of epilepsy surgery. Comparison material was obtained from the middle temporal gyrus of MTS patients, n= 16. Slices were prepared and maintained in a warm, oxygenated environment in normal ACSF for a two hour recovery period. They were then exposed to either 2-13C glucose or 1-13C glutamate for up two hours. The bath was sampled at 15 minute intervals and the tissue at the end of the two hour incubation. The samples were rapidly frozen and then prepared for mass spectrometry. The isotopic enrichment for both 13C glutamate (IE GLT) and 13C glutamine (IE GLN) were determined as the %13C compound/total compound. Results: When 13C glucose (10 mM) was the label source, there was a significant decrease in IE GLN, but not IE GLT in the bath relative to the overlying neocortex in MTS hippocampi; this effect was enhanced when GS activity was driven with NH3. However, in the dysplastic tissue, there were no significant differences in the IE GLN or in NH3-mediated enhancement of GS activity. In contrast, we found a significantly reduced IE GLT in the dysplastic tissue relative to the comparison neocortex and which was lower than that seen in MTS hippocampi. When 13C GLT (20 µM) was added to the bath as the label source, there was a significantly reduced isotopic dilution in the bath for both the MTS and dysplastic tissue vs. comparison material, i.e. the extracellular IE GLT remained very high in both epileptogenic substrates. In concordance with the 13C glucose data, there was robust IE GLN derived from 13C GLT in the dysplastic, but not the MTS tissue. Conclusions: These data indicate that different epileptogenic pathologies have distinct metabolic profiles and that neuronal-glial relationships are altered in both cases. In dysplastic tissue, there appears to be intact glial GS functioning with impaired neuronal mitochondrial metabolism as indicated by the reduced IE GLT generated from 13C glucose. In contrast, in MTS, there is evidence for significantly impaired glial function while neuronal mitochondria are not as substantially affected. Finally, glutamate uptake appears to be impaired in both substrates. Supported by a Pediatric Partnership Grant from AES