Mass spectrometry imaging as a novel approach to measure hippocampal zinc

Zinc (Zn²⁺) is an essential trace element that plays crucial roles in the functioning of hundreds of enzymes and DNA binding transcription factors. Zinc is also an essential neuromodulator and can act as a potent neurotoxin in excitotoxic brain injury after seizures, strokes, and brain trauma where high levels of Zn²⁺ can cause irreparable brain damage in certain brain regions. However, the mechanism of neurotoxicity has not been fully understood yet and is still under debate. In the present study, we have developed a time of flight secondary ion mass spectrometry (ToF-SIMS) imaging method to investigate the distribution of zinc in the rat brain. The zinc distribution in hippocampus sections from healthy rats and rats exposed to traumatic brain injury was imaged and the results were compared to those from conventional zinc-probe based fluorescence microscopy. Two related zinc species, ZnOH₃⁺ and ZnO₂H⁺, can successfully be visualized by ToF-SIMS in the rat hippocampus. Statistical data analysis of the image data demonstrated a substantial increase of both ZnOH₃⁺ and ZnO₂H⁺ in the zinc related species in the acute brain injury tissue. Our findings positively support the fact that toxic vesicular zinc accumulation might not be the sole source for neuronal degeneration following traumatic brain injuries. Also, we could successfully apply ToF-SIMS imaging for the first time to visualize the zinc content and distribution across hippocampus sections. Consequently, ToF-SIMS is a powerful method to further investigate biological phenomena such as seizures, ischemia, and strokes and also other forms of cellular damage in the central nervous system.