Abstracts

RATIONALE: Malformations of the cerebral cortex, including neuronal heterotopias and focal cortical dysplasias, are a major cause of human epilepsy. In previous work in the rodent, we have demonstrated that prenatal exposure to neurotrophin-4 (NT4) causes heterotopic accumulations of neurons in the embryonic marginal zone (MZ). In this study we analyzed the brains of postnatal animals, exposed to NT4 in utero, in order to produce a potential animal model for epilepsy.
METHODS: Embryonic rats were injected intraventricularly with NT4 or PBS at E16 and E18. Brains were examined histologically at either E19 or at birth (P0) and immunolabeled for reelin and gamma aminobutyric acid (GABA). We also observed the behaviors of animals that survived postnatally.
RESULTS: At E19, we found the same NT4-induced MZ abnormalities noted previously, including band-like heterotopias that cover entire hemispheres and focal collections that indent the underlying cortical plate. These heterotopias are composed of increased numbers of neurons that express either GABA and/or reelin. At birth (P0) rats injected with NT4, but not PBS, have abnormal behaviors that resemble seizures. They have feeding difficulties and frequently die within 12-24 hours after birth. Histological examination of the brains of NT4-treated pups (P0) reveals a very different malformation from that seen at E19. Scattered accumulations of neurons are present in the MZ. There are also distinct abnormalities of the upper cortical layers that are not present at E19: These include multiple collections of radially-alligned cells located at the top of the cortical plate (CP) that resemble CP neurons but are separated from other CP neurons by large rifts and cracks. Immunolabeling demonstrates that there are several cell types containing GABA and/or reelin that are located in and around the CP abnormalities. Additionally, exuberant reelin-positive processes are present in the underlying rifts and within the collections. Many of the rifts surrounding the collections also contain tangled cell bodies and processes that immunolabel for GABA.
CONCLUSIONS: Exposure of the developing neocortex to NT4, prenatally, induces distinct embryonic and postnatal malformations of the MZ and upper cortical layers. We suggest that excess production of or hypersensitivity to NT4 may play a role in certain focal cortical dysplasias found in humans. Furthermore, our work provides evidence that cortical dysplasias can arise as a primary disturbance in the normal development and behaviour of layer 1 neurons, causing a secondary disruption of cortical lamination. Finally, our work provides a promising rodent model for the study of mechanisms underlying epilepsy in cortical dysplasias.
Support: Epilepsy Foundation of America, NINDS, FAPESP