Domoic acid-induced neuronal damage in the rat hippocampus: Changes in apoptosis related genes (bcl-2, bax, caspase-3) and microglial response

Domoic acid (DA), a potent neurotoxin, administered intravenously (0.75 mg/ kg body weight) in adult rats evoked seizures accompanied by nerve cell dam age 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 ad ministration of DA. Nissl staining showed darkly stained degenerating neuro ns 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, cel ls that exhibited swollen morphology and shrunken electron-dense cells. Imm unoreactivity of Bcl-2 and Bax was increased considerably at 16 hr and 24 h r in the neurons of the hippocampus following DA administration. No signifi cant change was observed in the immunoreactivity of caspase-3 in the contro ls and DA-treated rats at any time interval. Microglial cells in the hippoc ampus showed intense immunoreaction with the antibodies OX-42 and OX-6 at 1 -21 days after DA administration, indicating the up-regulation of complemen t type 3 receptors and major histocompatibility complex type II antigens fo r increased phagocytic activity and antigen presentation, respectively. Ter minal deoxynucleotidyl-transferase-mediated dUTP nick end labeling (TUNEL) showed occasional positive neurons in the CA1 and CA3 regions at 5 days aft er DA administration, with no positive cells in the controls. RT-PCR analys is revealed that bcl-2 and bax mRNA transcripts in the hippocampus were sig nificantly increased at 16 hr and gradually decreased at 24 hr following th e administration of DA. Although bax and bcl-2 mRNA expression is rapidly i nduced at early stages, in situ hybridization analysis revealed complete lo ss of bcl-2, bax, and caspase-3 mRNA at 24 hr after DA administration in th e region of neuronal degeneration in the hippocampus. These results indicat e that the pattern of neuronal degeneration observed during DA-induced exci totoxic damage is mostly necrotic. (C) 2001 Wiley-Liss, Inc.