Abstracts

Rationale: Cortical and hippocampal dysgenesis is often related to drug-resistant epilepsy. Recent studies suggest an impairment of the inhibitory system in dysplastic brain of irradiated (XRT) rats, a model for malformation of brain development. Calbindin (CB) and parvalbumin (PV) are two markers for inhibitory neurons in the central nervous system. We studied the distribution of CB- and PV-positive cells and fibers in the various regions of dysplastic hippocampus (HIPP) from in utero XRT animals.Methods: Three 45-55 day-old rats exposed to 145 cGy on E 17 were used. The animals were anesthetized and sacrificed by decapitation. Their brains were fixed, cryoprotected and cut (30 μm thick slices) for histological and immunohistochemical processing (CB and PV antibodies). The slides were analyzed under light microscopy. One normal adult rat brain was processed in the same way as control and as a means to examine agreement with previously published data.Results: HIPP of XRT animals showed areas of severely dispersed pyramidal cell layer, located mainly in CA1 region and subiculum (SUB). Groups of ectopic neurons at the stratum oriens next to CA1/SUB-dysplastic areas were observed. As previously described, CB positively stained processes from granule cells of dentate gyrus (DG), extending from hilus to the stratum radiatum of CA3 region. Some of the interneurons of the molecular layer and hilus, and several of the granule cells were strongly CB positive. In the CA1 and CA2 areas, many pyramidal cells were CB-positive, as were rare interneurons of all hippocampal layers. In contrast to normal animals, dysplastic HIPP show an additional area of CB staining at the internal part of CA3’s stratum radiatum. Several CB-positive cells could be seen dispersed in the dysplastic CA1/SUB areas, as well as in ectopic neuronal collections. PV stains a fiber plexus that surrounded dentate granule cells, CA1-CA3 pyramidal cells and SUB cells. PV was present in interneurons located in all layers of all areas of DG and CA3-1 areas, especially in the pyramidal cell layers. Dysplastic HIPP showed PV-positive interneurons that were dispersed in dysplastic CA1 areas. Ectopic cell collections contained several PV-positive neurons. Variable size areas lacking CB- and PV-positive neurons were also observed in all HIPP regions.Conclusions: Our data show important alterations in the distribution of inhibitory neurons in morphologically disrupted areas in the hippocampus of radiated rats. The general pattern of CB and PV in areas of HIPP not affected by dysplasia was similar to that found in normal rat brains. Disrupted inhibitory hippocampal neurons may contribute directly to the higher susceptibility of dysplastic rats to seizures, both in vivo and in vitro. Our results are concordant with the previously reported impaired inhibitory physiology in dysplastic cortex of the same model, suggesting that morphological alterations may result in underlying electrophysiological abnormalities. The altered morphology of the inhibitory system in the XRT model of dysplasia may be a pivotal component of the epileptogenic capacity of those brains.