Morphometric investigation of the hippocampal formation in Ihara's genetically epileptic rats using magnetic resonance images and histopathology

The newly developed Ihara's genetically epileptic rat (IGER) is thought to be a useful tool for studying epilepsy of the limbic type. Morphological changes in the hip-pocampus, which is one of the characteristic findings in the brains of patients with temporal lobe epilepsy, were presumed to occur also in IGER during the course of epileptic activities. We investigated chronological changes in volume of the IGER hippocampal formation using mag-netic resonance (MR) imaging. The experiment consisted of two protocols. First, MR imaging was performed for volumetric measurement of the cerebrum and the hippocampus in four male IGER, each at 2, 4, 6, 8, 10 and 12 months of age. After MR examination, the brains were subjected to histological and morphometric examinations. Second, changes in the hippocampal volume were traced chronologically in three IGER at the ages of 2, 4, 6, 8 and 10 months. Age-matched Wistar rats were used as controls. In IGER, the volume of the cerebrum, especially of the hippocampus, and the ratio of the hippocampal to the cerebral volume increased, in concert with the increase in seizure activities. Histological and morphometric invest-igations revealed that there was mild astrogliosis and marked microgliosis with hypertrophic change in the hippocampal formation with aging, but no loss of neurons was observed. Neither an increase in volume nor gliosis was found in the brains of control rats. The enlargement of the hippocampal formation in IGER brains was assumed to be partially due to gliosis of astrocyte and microglia with hypertrophy.