Intact functional inhibition in the surround of experimentally induced focal cortical dysplasias in rats

Early postnatal injections of ibotenate into the rat neopallium induce cort ical dysplasias mimicking human polymicrogyria which often goes along with seizure disorders. Under in vitro conditions these experimentally induced d ysplasias cause widespread hyperexcitability. The underlying mechanisms are as yet not fully understood. Electrophysiologically there is clear evidenc e of widespread alterations of the excitatory system. Intracellular recordi ngs also showed some changes of the inhibitory system but have concentrated on recordings from focal areas close to the microgyrus. We investigated th e integrity of functional inhibition using a paired-pulse paradigm to map t he whole ipsilateral hemisphere. In rat cortical slices double-pulses were applied in layer VI/white matter and field potentials recorded in layer II/ III. The ratio of the field potential amplitude did not show significant al terations in the dysplasias or their surround as compared with control and sham-injected animals. This result was obtained with two different location s of the dysplasias, excluding a mere areal specific effect. Our results sh ow that despite prominent hyperexcitability in the surround of ibotenate-in duced cortical dysplasias the inhibitory network appears to be functionally intact.