Abstracts

Rationale: Malformations of cortical development have been found in epilepsy, Alzheimer s, and learning disabilities. Experiments in rodents have suggested that released reelin controls the positions of pyramidal neurons. For example, Reeler knock-out or mutant reeler mice have neurons with inverted laminations. However, in human neuropathologies of these neurologic disorders, the cortical malformations are not primarily laminar inversions; rather the neurons have abnormal positioning, with oblique and transverse polarities often forming clusters in every layer.In rats radiated for 2 minutes at E17, the prenatal day of greatest numbers of pyramidal cell migrations, tested the hypothesis that the 2 minute interruption in neural stem cell cycling would be significant enough to interfere with normal mature neuron migration and positioning of all types of cerebral neurons. The second hypothesis was that reelin-secreting cells would be disorganized and spatially related to the pyramidal cell clusters. Methods: 14 dams with age matched pregnancies were given 145 rads at E17 and 7 dams had no radiation for controls. With 10 pups per pregnancy, postnatal (P) developmental comparisons were birth age-matched for 140 radiated pups and 70 control pups for postnatal development at P2, 4, 16, 24, 40 or 90. At these days the radiated and control rats were studied for neuron positions with Cresylviolet (CV), the extracellular matrix protein Reelin (immunofluorescence :IF), and intracellular tubulin Nestin (IF), and at P24, 40, and 60 memory and retention tests (Morris water maze) at P24, 40, and epidural EEG recordings at P60, 90. Results: Brain tissue sections showed cortical malformations were in neocortex but not in the subcortex. High magnification microscopy and fluoroscopy were necessary to relate neuronal clusters to other proteins. See the figure for comparing CV with adjacent reelin in Control and Radiated. To test the spatial relation of Reelin to mature disorganized neurons (which continued to produce Nestin), adjacent digital images were overlayed and semi-quantified measures were made relating neuron clusters to densest Reelin accumulations. The controls had no neuronal malformations and Reelin was distributed as expected. Post-weaning (P24, 40) learning and retention in the Morris water maze was defective in radiated rats compared to controls. Only in radiated rats did the EEG recordings show epileptiform spikes and mild clinical seizures, never in controls. Conclusions: In utero radiation at different gestational ages resulted in postnatal neuronal malformations only in the subcortex (E14, 15) or only in the neocortex (E17, 18). Since postmitotic mature neurons are not affected by radiation, the prenatal insults to neural stem cells cycling into mature neurons explains these results. Also, radiation at only E17 showed that Reelin expression was spatially related to the mature but anomalous neocortical neuronal clusters, which all produced heavy molecular weight tubulin(Nestin). Such malformations are comparable to human neurologic disorders.