Mapping Brain Activations Induced by Exposure to Enriched Environment with Manganese-Enhanced Magnetic Resonance Imaging

Introduction It is well-known that exposure to an enriched environment (EE) can induce plastic changes in the hippocampus and improves learning and memory in hippocampus-dependent tasks. The commonly used techniques in the previous studies on the effects of EE on brain include behavioral tests, electrophysiological recording, microdialysis and morphological evaluations of neurons and synapses. Few previous studies have utilized neuroimaging techniques to map brain activities associated with EE exposure. In this study, we attempted to map accumulative brain activations in rats exposed to the EE for a period of 24 hrs with manganese-enhanced magnetic resonance imaging (MEMRI). Materials and methods Nine male SD rats, weighing 180-220 g, received a single dose of i.p. injection of MnCl2 solution (120 mmol/L, 3 ml/kg). The animals were then randomly assigned into two groups— the standard housing group (n=5) and the EE-treated group (n=4). The rats in the former group were returned to their original plastic cage (40 cm×30 cm×20 cm (height)) immediately after MnCl2 injection, while the rats in the later group were transferred into an EE cage (80 cm×60 cm×45 cm (height)), made of stainless steel, that they never encountered. The EE cage contained platforms at different levels, six toys of different shapes and colors, two wheels, and 3 tunnels with shading. All animals were imaged at 24 hrs after MnCl2 injection on a 4.7 T/30 cm Bruker Biospec scanner with a 5 cm-diameter volume coil. A 3D T1-weighted gradient echo pulse sequence was used with FOV 3 cm×3 cm×3 cm, matrix size 128×128×64, TR 35 ms, TE 5 ms, flip angle 30 ○