Ex vivo comparative investigation of suprachiasmatic nucleus excitotoxic resiliency

Background: Glutamate signaling in the brain is regulated by release, reuptake, and receptor responsiveness. In diseased conditions, glutamate signaling can exceed normal regulatory processes, giving rise to a condition called excitotoxicity. Although regional differences in the excitotoxic effects of glutamate in the brain have been reported, the extent and characteristics of these potential differences are not clear.  Here we compared the excitotoxic resiliency of the suprachiasmatic nucleus (SCN), anterior hypothalamus (AH) and cortex.  Methods: : We treated acute brain slices containing either the SCN and AH or the cortex from adult male mice at different times across the diurnal cycle with varying concentrations of N-methyl-D-aspartate (NMDA), NMDA+ α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or control medium. The extent of cell damage was assessed using propidium iodide (PI), a cell death marker. Results: : The results indicate that all three brain regions exhibited increasing cell damage/death when treated with increasing concentrations of NMDA. However, higher concentrations of NMDA were needed to significantly increase cell damage in the SCN compared to the cortex and AH.  All three brain regions also exhibited greater cell death/damage when treated in the nighttime compared to the daytime, although the SCN exhibited increased cell death during a more restricted time interval compared to the AH and cortex.  Conclusions: : Together, these data confirm previous studies showing excitotoxic resiliency in the SCN, while extending them in two ways.  First, we demonstrate a dose-dependency in excitotoxic susceptibility that differentiates the SCN from the surrounding AH and the cortex using a brain slice preparation. Second, we demonstrate a diurnal rhythm in excitotoxic susceptibility with a broadly similar phase across all three brain regions. These data increase our understanding of the extent and nature of the SCN excitotoxic resiliency, which will inform future studies on the cellular mechanisms underlying this phenomenon.