Persistent zinc depletion in the mossy fiber terminals in the intrahippocampal kainate mouse model of mesial temporal lobe epilepsy

Purpose: Zinc is released in synaptic vesicles with glutamate, and modulates glutamatergic neurotransmission. In brain, the highest amount of zinc, detected by Timm staining, is in the mossy fiber (MF) system in the hippocampus. In the intrahippocampal kainate (KA) mouse model of mesial temporal lobe epilepsy, which is elicited by intrahippocampal KA, prominent MF sprouting develops rapidly within 2 weeks post-KA. However, the intensity of Timm staining is reduced gradually thereafter. The present study is designed to determine the mechanisms underlying this reduction of Timm staining. Methods: The changes in Timm staining, and VGluT1, Synapsin-1, and zinc transporter 3 (ZnT3) immunoreactivity (IR) were examined from 4-56 days post-KA. An analysis of glutamate release in the KA-injected hippocampus was conducted by microdialysis before and during the continuous injection of midazolam (MDZ). Results: At 56 days post-KA, Timm staining disappeared completely, whereas VGluT-1-, Synapsin-1-, and ZnT3-IR were increased in the sprouted MF boutons. However, when the seizures were suppressed by a continuous perfusion of MDZ, the glutamate release in the hippocampus decreased and Timm staining was recovered. Discussion: This study showed that the reduction of Timm staining is the result of decreased zinc content but not the loss of MF itself. The reduction is the result of the enhanced release of zinc relative to storage, and it should facilitate the glutamate excitation that might be related to the epileptogenesis and rapid advancement of the morphologic changes in this model.