In vivo 3D MRI staining of the mouse hippocampal system using intracerebral injection of MnCl₂

The morphology and function of the hippocampal system of C57BL/6J mice (n = 8) was studied in vivo using T1-weighted 3D magnetic resonance imaging (MRI) (117 μm isotropic resolution) after bilateral injection of MnCl₂ (0.25 μl, 5 or 200 mM) into the posterior hippocampal formation. The neuronal uptake of the T1-shortening Mn²⁺ ions resulted in a pronounced MRI signal enhancement within the CA3 subfield and dentate gyrus with milder increases in CA1 and subiculum. This finding is in line with differences in the excitability of hippocampal neurons previously reported using electrophysiologic recordings. The subsequent axonal transport of Mn²⁺ highlighted the principal extrinsic projections from the posterior hippocampal formation via the fimbria and the precommissural fornix to the dorsal part of the lateral septal nucleus. A strong MRI signal enhancement was also observed in the ventral hippocampal commissure. A time-course analysis revealed unsaturated conditions of Mn²⁺ accumulation at about 2 h after injection and optimal contrast-to-noise ratios at about 6 h after injection. The present results using Mn²⁺-enhanced 3D MRI open new ways for studying the role of the hippocampal system in specific aspects of learning and memory in normal and mutant mice.