Abstracts

Rationale: While it has been suspected for decades that prenatal exposure to antiepileptic drugs (AEDs) may cause birth defects, data gathered by pregnancy registries over the last decade show exposure to common AEDs increased the major malformations observed at birth by 2 - 10 fold more than unexposed (control) children. In the case of valproic acid (VPA), nearly 10% of the children were born with major malformations. Verbal fluency and executive function, which are mediated by frontal cortical areas, were most affected. The primary mechanism of action of AEDs is to alter neurotransmitter systems. VPA increases inhibition by potentiating GABAergic transmission as a GABA transaminase inhibitor. VPA also acts as an inhibitor of histone deacetylase 1. Methods: Timed pregnant C57BL6/J mice were injected with a single dose of VPA (200 mg/kg) on embryonic day (E) 12.5. Offspring were harvested at E13.5, birth and postnatal day (P) 30, and adult ages. The brains were fixed in buffered paraformaldehyde, sectioned and processed using immunohistochemical techniques. In order to determine cell proliferation, dams were injected with bromodeoxyuridine (50 mg/kg) simultaneously with the VPA, and embryos were harvested 1 h and 24 h after injection. Results: Immunohistochemistry for interneuron markers demonstrated altered migration pathways from the ventral forebrain (ganglionic eminence) into the developing cerebral wall. Additionally, we observed increased incorporation of bromodeoxyuridine and exit from the proliferative zones, indicating premature neuronal differentiation. Exposure to VPA alters the neuronal lineages and promotes the formation of glial populations. Adult mice that were prenatally exposed to VPA exhibit regional deficits in specific cortical GABAergic interneurons. Conclusions: Previous toxicity studies failed to show effects on neural stem cell viability or proliferation. Our in vivo data in embryonic mice exposed to a single dose of VPA during mid-gestation demonstrate increased exit from the forebrain proliferative zones and migration to final destinations. The animal studies are supported by in vitro studies using human neural stem cells. Our studies begin to reveal the molecular mechanisms and developmental perturbations that occur with AEDs. By understanding the critical pathways that lead to birth defects in the children of mothers who suffer from seizure and mood disorders, we can start to develop better therapies designed to effective treat women and protect their children.