Temporospatial patterns of COX-2 expression and pyramidal cell degeneration in the rat hippocampus after trimethyltin administration

Inhibitors of Lipoxygenase and Cyclooxygenase-2 Attenuate Trimethyltin-Induced Neurotoxicity through Regulating Oxidative Stress and Pro-Inflammatory Cytokines in Human Neuroblastoma SH-SY5Y Cells

Trimethyltin (TMT) is an environmental neurotoxin that mediates dopaminergic neuronal injury in the brain. In this study, we characterized the toxic mechanism and possible protective compounds against TMT-induced neurotoxicity in human dopaminergic neuroblastoma SH-SY5Y cells. Antioxidants such as melatonin, N-acetylcysteine (NAC), α-tocopherol, and allopurinol alleviated TMT toxicity. Apoptosis induced by TMT was identified by altered expression of cleaved caspase-3, Bax, Bcl-2, and Bcl-xL through Western blot analysis. The iron chelator deferoxamine ameliorated the alteration of apoptosis-related proteins through TMT exposure. TMT also induced delayed ultrastructural necrotic features such as mitochondrial swelling and cytoplasmic membrane rupture; NAC reduced these necrotic injuries. Esculetin, meloxicam, celecoxib, and phenidone decreased TMT toxicity. Elevation of the pro-inflammatory cytokines IL-1β, TNF-α, and NF-ĸB and reduction of the antioxidant enzymes catalase and glutathione peroxidase-1 (GPx-1) were induced by TMT and ameliorated by inhibitors of LOX and COX-2 enzymes. Both NMDA and non-NMDA antagonists attenuated TMT toxicity. The free calcium ion modulators nimodipine and BAPTA/AM contributed to neuronal survival against TMT toxicity. Inhibitors of the phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin pathway, an autophagy regulator, decreased TMT toxicity. These results imply that TMT neurotoxicity is the chief participant in LOX- and COX-2-mediated apoptosis, partly via necrosis and autophagy in SH-SY5Y cells.

Toxin-Induced Experimental Models of Learning and Memory Impairment

Animal models for learning and memory have significantly contributed to novel strategies for drug development and hence are an imperative part in the assessment of therapeutics. Learning and memory involve different stages including acquisition, consolidation, and retrieval and each stage can be characterized using specific toxin. Recent studies have postulated the molecular basis of these processes and have also demonstrated many signaling molecules that are involved in several stages of memory. Most insights into learning and memory impairment and to develop a novel compound stems from the investigations performed in experimental models, especially those produced by neurotoxins models. Several toxins have been utilized based on their mechanism of action for learning and memory impairment such as scopolamine, streptozotocin, quinolinic acid, and domoic acid. Further, some toxins like 6-hydroxy dopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and amyloid-β are known to cause specific learning and memory impairment which imitate the disease pathology of Parkinson's disease dementia and Alzheimer's disease dementia. Apart from these toxins, several other toxins come under a miscellaneous category like an environmental pollutant, snake venoms, botulinum, and lipopolysaccharide. This review will focus on the various classes of neurotoxin models for learning and memory impairment with their specific mechanism of action that could assist the process of drug discovery and development for dementia and cognitive disorders.

Possible role of the glycogen synthase kinase-3 signaling pathway in trimethyltin-induced hippocampal neurodegeneration in mice

Trimethyltin (TMT) is an organotin compound with potent neurotoxic effects characterized by neuronal destruction in selective regions, including the hippocampus. Glycogen synthase kinase-3 (GSK-3) regulates many cellular processes, and is implicated in several neurodegenerative disorders. In this study, we evaluated the therapeutic effect of lithium, a selective GSK-3 inhibitor, on the hippocampus of adult C57BL/6 mice with TMT treatment (2.6 mg/kg, intraperitoneal [i.p.]) and on cultured hippocampal neurons (12 days in vitro) with TMT treatment (5 µM). Lithium (50 mg/kg, i.p., 0 and 24 h after TMT injection) significantly attenuated TMT-induced hippocampal cell degeneration, seizure, and memory deficits in mice. In cultured hippocampal neurons, lithium treatment (0–10 mM; 1 h before TMT application) significantly reduced TMT-induced cytotoxicity in a dose-dependent manner. Additionally, the dynamic changes in GSK-3/β-catenin signaling were observed in the mouse hippocampus and cultured hippocampal neurons after TMT treatment with or without lithium. Therefore, lithium inhibited the detrimental effects of TMT on the hippocampal neurons in vivo and in vitro, suggesting involvement of the GSK-3/β-catenin signaling pathway in TMT-induced hippocampal cell degeneration and dysfunction.

Neuroprotective strategies in hippocampal neurodegeneration induced by the neurotoxicant trimethyltin

The selective vulnerability of specific neuronal subpopulations to trimethyltin (TMT), an organotin compound with neurotoxicant effects selectively involving the limbic system and especially marked in the hippocampus, makes it useful to obtain in vivo models of neurodegeneration associated with behavioural alterations, such as hyperactivity and aggression, cognitive impairment as well as temporal lobe epilepsy. TMT has been widely used to study neuronal and glial factors involved in selective neuronal death, as well as the molecular mechanisms leading to hippocampal neurodegeneration (including neuroinflammation, excitotoxicity, intracellular calcium overload, mitochondrial dysfunction and oxidative stress). It also offers a valuable instrument to study the cell-cell interactions and signalling pathways that modulate injury-induced neurogenesis, including the involvement of newly generated neurons in the possible repair processes. Since TMT appears to be a useful tool to damage the brain and study the various responses to damage, this review summarises current data from in vivo and in vitro studies on neuroprotective strategies to counteract TMT-induced neuronal death, that may be useful to elucidate the role of putative candidates for translational medical research on neurodegenerative diseases.

Possible involvement of galectin-3 in microglial activation in the hippocampus with trimethyltin treatment

Trimethyltin (TMT) is an organotin neurotoxicant with effects that are selectively localized to the limbic system (especially the hippocampus), which produces memory deficits and temporal lobe seizures. Galectin-3 (Gal-3) is a beta-galactoside-binding lectin that is important in cell proliferation and regulation of apoptosis. The present study evaluated the temporal expression of Gal-3 in the hippocampus of adult BALB/c mice after TMT treatment (i.p., 2.5mg/kg). Western blotting analyses showed that Gal-3 immunoreactivity began to increase days after treatment; the immunoreactivity peaked significantly within days after treatment but significantly declined between days 4 and 8. Immunohistochemical analysis indicated that Gal-3 expression was very rare in the hippocampi of vehicle-treated controls. However, Gal-3 immunoreactivity appeared between 2 and 8 days after TMT treatment and was primarily localized to the hippocampal dentate gyrus (DG), in which neuronal degeneration occurred. The immunoreactivity was detected predominantly in most of the Iba1-positive microglia and in some GFAP-positive astrocytes of the hippocampal DG. Furthermore, Gal-3 expression co-localized with the pro-inflammatory enzymes cyclooxygenase-2 and inducible nitric oxide synthase in the hippocampal DG. Therefore, we suggest that Gal-3 is involved in the inflammatory process of neurodegenerative disorder induced by organotin intoxication.

Endothelial microsomal prostaglandin E synthase-1 facilitates neurotoxicity by elevating astrocytic Ca2+ levels

Recurrent seizures may cause neuronal damage in the hippocampus. As neurons form intimate interactions with astrocytes via glutamate, this neuron-glia circuit may play a pivotal role in neuronal excitotoxicity following such seizures. On the other hand, astrocytes contact vascular endothelia with their endfeet. Recently, we found kainic acid (KA) administration induced microsomal prostaglandin E synthase-1 (mPGES-1) and prostaglandin E(2) (PGE(2)) receptor EP3 in venous endothelia and on astrocytes, respectively. In addition, mice deficient in mPGES-1 exhibited an improvement in KA-induced neuronal loss, suggesting that endothelial PGE(2) might modulate neuronal damage via astrocytes. In this study, we therefore investigated whether the functional associations between endothelia and astrocytes via endothelial mPGES-1 lead to neuronal injury using primary cultures of hippocampal slices. We first confirmed the delayed induction of endothelial mPGES-1 in the wild-type (WT) slices after KA-treatment. Next, we examined the effects of endothelial mPGES-1 on Ca(2+) levels in astrocytes, subsequent glutamate release and neuronal injury using cultured slices prepared from WT and mPGES-1 knockout mice. Moreover, we investigated which EP receptor on astrocytes was activated by PGE(2). We found that endothelial mPGES-1 produced PGE(2) that enhanced astrocytic Ca(2+) levels via EP3 receptors and increased Ca(2+)-dependent glutamate release, aggravating neuronal injury. This novel endothelium-astrocyte-neuron signaling pathway may be crucial for neuronal damage after repetitive seizures, and hence could be a new target for drug development.

Species-difference of cyclooxygenase-2 in the hippocampus of rodents

Cyclooxygenase (COX) generates free radicals and it is important in inflammatory response. In this study, we observed the immunoreactivity in mice (ICR and C57BL/6 strain), rats and gerbils. In these animals, COX-2 immunoreactivity was mainly detected in pyramidal cells of the hippocampal CA2/3 region and in granule cells of the dentate gyrus. COX-2 immunoreactivity in the CA2/3 region was the highest in ICR mice, while in gerbils COX-2 immunoreactivity was the lowest; COX-2 immunoreactivity in the dentate gyrus was the highest in rats and the lowest in gerbils. The protein levels of COX-2 were similar to the immunohistochemical data. COX-2 mRNA transcript was the highest in the gerbil and the lowest in the rat. In brief, COX-2 protein, not mRNA, in the hippocampus is generally higher in mice (ICR and C57BL/6 strain) than rats and gerbils.

Trimethyltin intoxication up-regulates nitric oxide synthase in neurons and purinergic ionotropic receptor 2 in astrocytes in the hippocampus

Nitric oxide (NO) and purinergic ionotropic receptors (P2X) mediate cellular events in the central nervous system (CNS) under physiological conditions as well as during pathological events, and they have been recently proposed to interact in mediating CNS response to injury (Viscomi et al. [2004] Neuroscience 123:393-404; Florenzano et al. [2008] Pflugers Arch. 452:622-644). Trimethyltin (TMT) is an organotin compound that generates neurotoxic effects, and it has been used in a model of neurodegenerative disease and memory dysfunction. TMT causes neuronal death and reactive gliosis primarily in the hippocampus and other limbic regions. In the present study, we examined the degenerative events and the expression of nitric oxide synthase (NOS) and P2X receptor subtypes (P2X(1,2,4,7)Rs) that were induced by TMT administration at different time points (3, 7, 14, and 21 days) by conventional and confocal microscopy and Western blotting. Massive glial activation and neuronal death in the CA1 and CA3 regions were observed after TMT treatment. In these areas, astrocytic P2X(2)R and neuronal NOS were temporarily enhanced in association with the progression of neuronal death. In the hippocampus, the physiological expression of P2X(1)R, P2X(4)R, and P2X(7)R was not modified by TMT. The present data demonstrate that, as in other neurodegenerative models, TMT-induced hippocampal degeneration is associated with nitrergic and purinergic activations. Nevertheless, at odds with previous data, in this model the two systems are active in segregated cell populations, namely, P2XR in astrocytes and NOS in neurons. Finally, the temporal relations between P2XR and NOS expression and neuronal degeneration suggest interactions between P2XR/NO signaling and cell survival.

FK506-protective effects against trimethyltin neurotoxicity in rats: hippocampal expression analyses reveal the involvement of periarterial osteopontin

There is little information on the molecular mechanisms in FK506-mediated neuroprotection. In the present study, we investigated the protective effect of FK506, an immunosuppressant and neuroprotectant, on trimethyltin (TMT)-induced neurotoxicity in the rat hippocampus. Histologically, TMT-induced neuronal damage was partially prevented by FK506 in the hippocampal CA1 region, but not in CA3. FK506 treatment significantly reduced the number of apoptotic cells in CA1, but not in CA3, and also prevented induction of cognitive deficits by TMT. Microarray analysis of the rat hippocampus detected 14 genes with TMT-induced alteration of mRNA expression that was rescued by FK506 treatment. Subsequent quantitative RT-PCR analysis confirmed elevated mRNA levels for four inflammatory genes, glutathione S-transferase, lysozyme, matrix Gla protein, and osteopontin after TMT treatment. Upregulation of these genes was reversed by FK506 treatment at 5 days postgavage. Immunohistochemistry revealed that FK506 reduced osteopontin (OPN) induction by TMT in the periarterial area at 5 days postgavage. Our data suggest that inflammatory gene expression is involved in TMT-induced damage to the hippocampal CA1 region, resulting in apoptosis, and that this process is initiated by periarterial OPN activation, and can be alleviated by FK506.