Ingested d-Aspartate Facilitates the Functional Connectivity and Modifies Dendritic Spine Morphology in Rat Hippocampus

Abstract

d-Aspartate (d-Asp), the stereoisomer of l-aspartate, has a role in memory function in rodents. However, the mechanism of the effect of d-Asp has not been fully understood. In this study, we hypothesized that ingested d-Asp directly reaches the hippocampal tissues via the blood circulation and modifies the functional connectivity between hippocampus and other regions through spinogenesis in hippocampal CA1 neurons. The spinogenesis induced by the application of d-Asp was investigated using rat acute hippocampal slices. The density of CA1 spines was increased following 21 and 100 μM d-Asp application. The nongenomic spine increase pathway involved LIM kinase. In parallel to the acute slice study, brain activation was investigated in awake rats using functional MRI following the intragastric administration of 5 mM d-Asp. Furthermore, the concentration of d-Asp in the blood serum and hippocampus was significantly increased 15 min after intragastric administration of d-Asp. A functional connectivity by awake rat fMRI demonstrated increased slow-frequency synchronization in the hippocampus and other regions, including the somatosensory cortex, striatum, and the nucleus accumbens, 10-20 min after the start of d-Asp administration. These results suggest that ingested d-Asp reaches the brain through the blood circulation and modulates hippocampal neural networks through the modulation of spines.