Co-injection of β-amyloid with ibotenic acid induces synergistic loss of rat hippocampal neurons

Abstract

Senile plaques are a pathological hallmark of Alzheimer's disease. The major component of senile plaques is β-amyloid which consists of approximately 4000 mol. wt of peptide. Accumulating evidence suggests that β-amyloid may represent the underlying cause of Alzheimer's disease. In vitro, β-amyloid has been shown either to be directly neurotoxic or to potentiate neurotoxic effects of excitatory amino acids. However, β-amyloid toxicity in vivo has not always been reproducible. In this study, we injected β-amyloid fragment 1–40 or 25–35 alone or in combination with a small amount of ibotenic acid, an excitatory amino acid, into rat hippocampus, and examined the histological and immunohistochemical changes two weeks after injection. Although β-amyloid alone or ibotenic acid alone exerted only minimal degenerating effects on neurons just around the injection site, the co-injection of β-amyloid 1–40 or β-amyloid 25–35 with ibotenic acid produced drastic neuronal loss; the haematoxylin–eosin staining revealed that most neurons not only around the injection site but also in distant areas including CA1, CA4 and dentate gyrus were depleted. The neuronal loss occurred in a dose-dependent manner with respect to ibotenic acid. Immunohistochemical analysis showed that β-amyloid with ibotenic acid induced great depletion of microtubule-associated protein-2 immunoreactivity and infiltration of astrocytes and microglia on neuronal loss. In addition, some apoptotic neuronal death indicated by DNA fragmentation and nucleic condensation was observed. β-amyloid depositions detected by two different types of anti-human β-amyloid antibodies were limited to the injection site. Dizocilpine maleate (MK-801), an antagonist for an excitatory amino acid receptor, completely inhibited the neuronal death in rat hippocampus. These results suggest that the co-injection of β-amyloid with a small amount of ibotenic acid provides a useful model for investigation of the pathogenetic mechanisms leading to Alzheimer's disease.