The role of diurnal fluctuations in excitatory amino acid carrier 1 levels in post-ischemic hippocampal Zn2+ accumulation

Abstract Accumulating evidence indicates time-of-day variations in ischemic neuronal injury. Under ischemic conditions, Zn2+ is massively released from hippocampal glutamatergic neurons, and intracellular Zn2+ accumulation results in neuron death. Notably, excitatory amino acid carrier 1 (EAAC1), known as a cysteine transporter, is involved in Zn2+ homeostasis, and its expressions exhibit a diurnal fluctuation. This study aimed to investigate whether time of day of an ischemic insult affects Zn2+ accumulation and neuronal injury and determine whether altered Zn2+ accumulation is modulated by EAAC1 diurnal fluctuation in the hippocampus in a mouse model of ischemic stroke. Mice subjected to transient global ischemia for 40 min at Zeitgeber time 18 (ZT18) (23:00) exhibited reduced Zn2+ accumulation and neuronal death in the hilar region of the hippocampus compared to those at ZT4 (09,00). The EAAC1 protein expression in the hippocampus was increased at ZT18 relative to ZT4. Intracerebroventricular injection of a non-selective excitatory amino acid transporter inhibitor, DL-threo-β-benzyloxyaspartate, or a selective EAAC1 inhibitor, L-aspartic acid β-hydroxamate, increased ischemia-induced Zn2+ accumulation and neuronal death in the hilus at ZT18. These findings suggest that ischemia-induced Zn2+ accumulation displays circadian fluctuations through diurnal variations in EAAC1 expressions and affects susceptibility to ischemic neuronal injury in the hippocampal hilar region.