Regulatory mechanism of synaptic plasticity in the abnormally developed visual cortex

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We have used experimental hydrocephalus induced by Keolin injection to study the functional reorganization of the cerebral cortex. We confirmed enlargement of cerebral ventricles and thinner cortical layers by several imaging methods and maintained model animals in good conditions by performing cerebral ventricular-peritoneal cavity shunting operation. By using an intrinsic optical imaging method, we found that the orientation maps in areas 17 and 18 were formed almost in normal manner in these model animals. By introducing a microelectrode in the abnormally thinner visual cortex, we succeeded to record single unit activities and found that some neurons showed normal receptive field properties. However, most of them were monocular, indicating the neuronal circuits required for binocular vision were poorly developed in these animals. In vitro slice experiments showed that visuocortical neurons in hydrocephalus cats could generate normal action potentials responding to the depolarizing currents injected into the cells. Their dendritic structures were poorly developed in these cells. We further examined metabolic activity of the visual cortex of hydrocephalus cats by use of positron emission tomography. PET imaging using F18-labelled deoxyglucose showed that medial bank of the visual cortex has relatively higher uptake activity of the tracer, however general metabolic activity in the visual cortex was clearly lower in these animals. These results indicate that the functional reorganization was induced during development to compensate the structural and metabolic abnormality.