Maternal Exposure to Valproic Acid Primarily Targets Interneurons Followed by Late Effects on Neurogenesis in the Hippocampal Dentate Gyrus in Rat Offspring

Valproic acid (VPA) is used to establish models of experimental autism. The present study investigated the developmental exposure effect of VPA on postnatal hippocampal neurogenesis in accordance with the exposure scheme of OECD Test Guideline 426 adopted for developmental neurotoxicity. Pregnant rats were administered drinking water containing 0, 667, or 2000 ppm VPA from gestational day 6 until day 21 post-delivery. In the subgranular zone (SGZ) and granule cell layer (GCL) of offspring, the number of granule cell lineage subpopulations remained unchanged upon weaning. However, in the hilus of the dentate gyrus, the number of reelin⁺ interneurons decreased at ≥667 ppm, and the number of PVALB⁺ or GAD67⁺ interneurons decreased at 2000 ppm. Conversely, Reln and Gad1 transcript levels increased at 2000 ppm, but Pvalb and Grin2d decreased, in the dentate gyrus. At the adult stage, PCNA⁺ proliferating SGZ cells, NeuN⁺ postmitotic SGZ/GCL neurons, and ARC⁺ or COX2⁺ GCL neurons increased at ≥667 ppm. In the dentate hilus, decreases in GAD67⁺ interneuron subpopulations and Grin2d transcript levels sustained at 2000 ppm. These results suggested that VPA primarily targets interneurons by developmental exposure, and this is followed by late effects on granule cell lineages, likely by influencing SGZ cell proliferation and synaptic plasticity. A reduced population of reelin⁺ or PVALB⁺ interneurons did not affect distribution of granule cell lineage subpopulations upon weaning. The late effect on neurogenesis, which resulted in increased GCL neurons, might be the result of a sustained decrease in GAD67⁺ interneurons expressing NR2D encoded by Grin2d.