Domoic acid‐induced neuronal damage in the rat hippocampus: Changes in apoptosis related genes (Bcl‐2, Bax, caspase‐3) and microglial response

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Abstract
Domoic acid (DA), a potent neurotoxin, administered intravenously (0.75 mg/kg body weight) in adult rats evoked seizures accompanied by nerve cell damage in the present study. Neuronal degeneration and microglial reaction in the hippocampus were investigated, and the temporal profile of bcl‐2, bax, and caspase‐3 genes in cell death or survival was assessed following the administration of DA. Nissl staining showed darkly stained degenerating neurons in the hippocampus following the administration of DA at 1–21 days, the degeneration being most severe at 5 days. Ultrastructural study in CA1 and CA3 regions of hippocampus revealed two types of neuronal degeneration, cells that exhibited swollen morphology and shrunken electron‐dense cells. Immunoreactivity of Bcl‐2 and Bax was increased considerably at 16 hr and 24 hr in the neurons of the hippocampus following DA administration. No significant change was observed in the immunoreactivity of caspase‐3 in the controls and DA‐treated rats at any time interval. Microglial cells in the hippocampus showed intense immunoreaction with the antibodies OX‐42 and OX‐6 at 1–21 days after DA administration, indicating the up‐regulation of complement type 3 receptors and major histocompatibility complex type II antigens for increased phagocytic activity and antigen presentation, respectively. Terminal deoxynucleotidyl‐transferase‐mediated dUTP nick end labeling (TUNEL) showed occasional positive neurons in the CA1 and CA3 regions at 5 days after DA administration, with no positive cells in the controls. RT‐PCR analysis revealed that bcl‐2 and bax mRNA transcripts in the hippocampus were significantly increased at 16 hr and gradually decreased at 24 hr following the administration of DA. Although bax and bcl‐2 mRNA expression is rapidly induced at early stages, in situ hybridization analysis revealed complete loss of bcl‐2, bax, and caspase‐3 mRNA at 24 hr after DA administration in the region of neuronal degeneration in the hippocampus. These results indicate that the pattern of neuronal degeneration observed during DA‐induced excitotoxic damage is mostly necrotic. J. Neurosci. Res. 66:177–190, 2001.

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