Spatiotemporal changes in hippocampal NMDA receptor binding as a consequence of trimethyltin neurotoxicity in the rat

Enhanced expression of immediate-early genes in mouse hippocampus after trimethyltin treatment

Immediate-early genes (IEGs) are transiently and rapidly activated in response to various cellular stimuli. IEGs mediate diverse functions during pathophysiologic events by regulating cellular signal transduction. We investigated the temporal expression of several IEGs, including c-fos, early growth response protein-1 (Egr-1), and activity-regulated cytoskeleton-associated protein (Arc), in trimethyltin (TMT)-induced hippocampal neurodegeneration. Mice (7 weeks old, C57BL/6) administered TMT (2.6mg/kg intraperitoneally) presented severe neurodegenerative lesions in the dentate gyrus (DG) and showed behavioral seizure activity on days 1-4 post-treatment, after which the lesions and behavior recovered spontaneously over time. c-fos, Egr-1, and Arc mRNA and protein levels significantly increased in the mouse hippocampus after TMT treatment. Immunohistochemical analysis showed that nuclear c-fos expression increased mainly in the DG, whereas nuclear Egr-1 expression was increased extensively in cornu ammonis (CA) 1, CA3, and the DG after TMT treatment. Increased Arc levels were detected in the cellular somata/dendrites of the hippocampal subregions after TMT treatment. Therefore, we suggest that increased IEGs are associated with TMT-induced pathological events in mouse hippocampus.

Identification of differentially expressed genes in the brain of Sebastiscus marmoratus in response to tributyltin exposure

Tributyltin (TBT), a ubiquitous marine environmental contaminant, has been reported to affect functioning of the central nervous system. However, the mechanism of its neurotoxicity remains unknown. In this study, an Anneal Control Primer-differential display Reverse Transcription-PCR method was employed to investigate differentially expressed genes in the brain of Sebastiscus marmoratus in response to acute TBT exposure. A total of 18 gene sequences were identified as having the potential for being differentially expressed, of which 9 could be identified with homologous database sequences. The expression profiles of 4 genes, namely cytochrome c oxidase subunit II, GRB2-associated binding protein 2, adaptor-related protein complex 2, and guanine nucleotide exchange factor p532, were analyzed in the brain using real time fluorescence quantitative PCR after treatment with 10, 100 and 1000 ng/L of TBT for 50 days. The results showed that chronic exposure to TBT induced down-regulation of these genes in a dose dependent manner. The present study provided a basis for studying the response of fish to TBT exposure and allowed the characterization of new potential neurotoxic biomarkers of TBT contamination in seawater.

Acute administration of tributyltin and trimethyltin modulate glutamate and N-methyl-D-aspartate receptor signaling pathway in Sebastiscus marmoratus

Tributyltin (TBT), widely used as an antifouling biocide, is the most abundant pesticide in coastal environments. Trimethyltin (TMT) is a potent neurotoxicant of a mechanism of action yet to be uncovered. The neurotoxicity of TBT and TMT on the brain of marine fish Sebastiscus marmoratus was investigated in this study. The results showed that TBT and TMT can modulate amino acid neurotransmitters and N-methyl-D-aspartate receptor (NMDAR) signaling pathway in the brain of marine fish in a different manner. TBT did not increase the content of the amino acid neurotransmitters except gamma-aminobutyricd acid (GABA). TMT increased the content of aspartate (Asp), glutamate (Glu) and GABA in a dose-dependent manner. The expression of NADAR and components on its signaling pathway, such as calmodulin, calmodulin-dependent kinase II (CaMKII) and cAMP-response element-binding (CREB) protein was significantly decreased in a dose-dependent manner after TBT exposure. However, the low dose of TMT exposure up-regulate rather than down-regulate the expression of NMDAR and other genes of its pathway. It is suggested that the Glu-NMDAR pathway plays a role in the mechanism for the brain injury in marine fish after TBT or TMT exposure. The alteration of expression of glutamatergic receptor NMDAR and components on its signaling pathway accompanied with the change of total brain transmitter level indicated the importance of glutamatergic system in organotin toxicity.

Ascorbate attenuates trimethyltin-induced oxidative burden and neuronal degeneration in the rat hippocampus by maintaining glutathione homeostasis

The specific role of endogenous glutathione in response to neuronal degeneration induced by trimethyltin (TMT) in the hippocampus was examined in rats. A single injection of TMT (8 mg/kg, i.p.) produced a rapid increase in the formation of hydroxyl radical and in the levels of malondialdehyde (MDA) and protein carbonyl. TMT-induced seizure activity significantly increased after this initial oxidative stress, and remained elevated for up to 2 weeks post-TMT. Although a significant loss of hippocampal Cornus Ammonis CA1, CA3 and CA4 neurons was observed at 3 weeks post-TMT, the elevation in the level of hydroxyl radicals, MDA, and protein carbonyl had returned to near-control levels at that time. In contrast, the ratio of reduced to oxidized glutathione remained significantly decreased at 3 weeks post-TMT, and the glutathione-like immunoreactivity of the pyramidal neurons was decreased. However glutathione-positive glia-like cells proliferated mainly in the CA1, CA3, and CA4 sectors and were intensely immunoreactive. Double labeling demonstrated the co-localization of glutathione-immunoreactive glia-like cells and reactive astrocytes, as indicated by immunostaining for glial fibrillary acidic protein. This suggests that astroglial cells were mobilized to synthesize glutathione in response to the TMT insult. The TMT-induced changes in glutathione-like immunoreactivity appear to be concurrent with changes in the expression levels of glutathione peroxidase and glutathione reductase. Ascorbate treatment significantly attenuated TMT-induced seizures, as well as the initial oxidative stress, impaired glutathione homeostasis, and neuronal degeneration in a dose-dependent manner. These results suggest that ascorbate is an effective neuroprotectant against TMT. The initial oxidative burden induced by TMT may be a causal factor in the generation of seizures, prolonged disturbance of endogenous glutathione homeostasis, and consequent neuronal degeneration.

Oxiracetam pre- but not post-treatment prevented social recognition deficits produced with trimethyltin in rats

The social recognition paradigm was used to investigate the effect of trimethyltin (TMT) in adult male rats. Consequently, the effect of chronic oxiracetam (OXI) treatment in TMT impaired animals was evaluated. In all experiments, a behavioural testing was performed 3 weeks after TMT administration. Experiment 1: A single TMT oral dose, 5 and 7.5 but not 2.5mg/kg, impaired the natural ability of the adults to recognize a juvenile conspecific that they encountered 30 min before. The dose of 5mg/kg TMT was chosen to be used in subsequent experiments. Experiment 2: Chronic OXI pre-+post-treatment, daily 3 or 30 mg/kg sc for 7 days before and 7 days after the insult, protected the adults against recognition deficit produced by TMT. Experiment 3: OXI pre- but not post-treatment (always 3 and 30 mg/kg) had beneficial effects on the social recognition. The findings suggest that social recognition ability of adult male rats pre-treated sufficiently long with OXI is resistant to the neurotoxicity effect of TMT.

Differential and irreversible CNS ontogenic reduction in maximal MK-801 binding site number in the NMDA receptor after acute hypoxic hypoxia

CNS exposure to hypoxia impairs excitatory and inhibitory neurotransmission. Our aim was to determine variations induced by normobaric acute hypoxic hypoxia (8% O(2) for 60 min) on the NMDA receptor complex, as well as their potential reversibility after normoxic recovery. To this end, [3H]MK-801 binding assays to a synaptic membrane fraction isolated from chick optic lobes were performed. Previous studies throughout development had disclosed a characteristic age-dependent pattern. Results at embryonic day (ED) 12 and 18 indicated two distinct MK-801 binding sites. Hypoxic treatment failed to alter either the high affinity site dissociation constant (K(d)) or its maximal binding capacity (B(max)), whereas the low affinity site B(max) was significantly decreased (50% and 30% at ED12 and 18, respectively), without alteration in its K(d) values. Hypoxic embryos restored for 48 h at ED12 to normoxic conditions displayed unchanged MK-801 binding reduction, unlike those treated likewise at ED18 whose values fully recovered control levels. To conclude, hypoxic treatment reduces low affinity MK-801 B(max) in the NMDA receptor which proves irreversible up to ED12. Such early neuronal vulnerability may be due to post-transcriptional changes, to endocytosis followed by receptor degradation, or alternatively to cell death.

Long-term microglial and astroglial activation in the hippocampus of trimethyltin-intoxicated rat: stimulation of NGF and TrkA immunoreactivities in astroglia but not in microglia

In the present study we investigated the microglial and astroglial response after trimethyltin (TMT) exposure over a prolonged period of time. Male Wistar rats were given a single dose of TMT (8 mg/kg, i.p.) and survived 4, 7, 21, 60 and 180 days after the administration of the toxin. Histochemistry (Griffonia simplicifolia lectin staining) and immunocytochemistry for GFAP were applied to identify micro- and astroglial cells, respectively. To assess the trophic response of glial cells (NGF and TrkA expression), single or double staining experiments were performed. In addition, the biochemical evaluation of GFAP and NGF were carried out at chosen timepoints using immunoblotting technique and ELISA, respectively. The main findings of our study were as follows. (1) A protracted activation of microglia (at least up to 2 months posttreatment). (2) A long-lasting expression of GFAP immunoreactivity (at least up to 6 months posttreatment) and a steady increase in GFAP content (at least up to 2 months posttreatment). (3) The appearance of enormously enlarged, round-shape astrocytes exclusively localized to CA1 and observed 2 months posttreatment. (4) The stimulation of NGF and TrkA expression in reactive astrocytes. (5) The strongest activation of micro- and astroglia coincided with the most prominent neurodegeneration in the hippocampus, i.e., in CA4/CA3c and CA1. It is tempting to assume that the activation of glial cells in the hippocampal areas particularly vulnerable to TMT may affect neuronal fate after neurotoxic insult.