BACKGROUND: Manganism may cause learning and memory impairment by influencing the normal function of the hippocampus, however, this effect requires further examination.OBJECTIVE: To investigate the effects of manganism on the rat hippocampus using immunohistochemistry and MRI examination.DESIGN, TIME AND SETTING: A randomized controlled study, performed in the School of Medicine and Life Science, Jianghan University and the State Key Laboratory of Atomic & Molecular Physics and Spectroscopy, Chinese Academy of Science, from July to September 2005.MATERIALS: Fourteen healthy SD rats aged two months were selected for this study. MnCl2 4H2O (BIID, UK) (batch number: 9791325); glial fibrillary acidic protein (GFAP) staining kit (Beijing Zhongshan Biotechnology); Biospec 4.7T/30 animal MRI formatter (Bruker, Germany).METHODS: Fourteen rats were randomly divided into a control group (n=7) and a manganism group (n=7). Rats in the manganism group Received intraperitoneal injection of MnCl2·4H2O (50mg/kg), once a day, for four successive days. Rats in the control group were injected according to the manganism regimen, but using saline instead of manganese solution.MAIN OUTCOME MEASURES: Twenty-four hours after the last injection, rats were examined using MRI. Immunohistochemically stained GFAP and hematoxylin-eosin stained hippocampal sections were observed under optical microscopy.RESULTS: Fourteen rats were included in the final analysis. After manganese treatment, T1 weighted image and inversion recovery MRI demonstrated that the signal intensity was significantly enhanced in hippocampus, compared to controls. Neuronal necrosis was not observed in the hippocampus after HE staining. As compared to the control group, GFAP expression was markedly enhanced in the hippocampus of the manganism group.CONCLUSION: Within the rat brain, manganese preferentially localizes to the hippocampus and can induce astroctye activation.