Cytokines participate in neuronal death induced by trimethyltin in the rat hippocampus via type II glucocorticoid receptors

Microglial- and Astrocyte-Specific Expression of Purinergic Signaling Components and Inflammatory Mediators in the Rat Hippocampus During Trimethyltin-Induced Neurodegeneration

The present study examined the involvement of purinergic signaling components in the rat model of hippocampal degeneration induced by trimethyltin (TMT) intoxication (8 mg/kg, single intraperitoneal injection), which results in behavioral and neurological dysfunction similar to neurodegenerative disorders. We investigated spatial and temporal patterns of ecto-nucleoside triphosphate diphosphohydrolase 1 (NTPDase1/CD39) and ecto-5′ nucleotidase (eN/CD73) activity, their cell-specific localization, and analyzed gene expression pattern and/or cellular localization of purinoreceptors and proinflammatory mediators associated with reactive glial cells. Our study demonstrated that all Iba1+ cells at the injured area, irrespective of their morphology, upregulated NTPDase1/CD39, while induction of eN/CD73 has been observed at amoeboid Iba1+ cells localized within the hippocampal neuronal layers with pronounced cell death. Marked induction of P2Y₁₂R, P2Y₆R, and P2X₄-messenger RNA at the early stage of TMT-induced neurodegeneration might reflect the functional properties, migration, and chemotaxis of microglia, while induction of P2X₇R at amoeboid cells probably modulates their phagocytic role. Reactive astrocytes expressed adenosine A₁, A_2A, and P2Y₁ receptors, revealed induction of complement component C3, inducible nitric oxide synthase, nuclear factor-kB, and proinflammatory cytokines at the late stage of TMT-induced neurodegeneration. An increased set of purinergic system components on activated microglia (NTPDase1/CD39, eN/CD73, and P2X₇) and astrocytes (A₁R, A_2AR, and P2Y₁), and loss of homeostatic glial and neuronal purinergic pathways (P2Y₁₂ and A₁R) may shift purinergic signaling balance toward excitotoxicity and inflammation, thus favoring progression of pathological events. These findings may contribute to a better understanding of the involvement of purinergic signaling components in the progression of neurodegenerative disorders that could be target molecules for the development of novel therapies.

In vitro models to study insulin and glucocorticoids modulation of trimethyltin (TMT)-induced neuroinflammation and neurodegeneration, and in vivo validation in db/db mice

Brain susceptibility to a neurotoxic insult may be increased in a compromised health status, such as metabolic syndrome. Both metabolic syndrome and exposure to trimethyltin (TMT) are known to promote neurodegeneration. In combination the two factors may elicit additive or compensatory/regulatory mechanisms. Combined effects of TMT exposure (0.5-1 μM) and mimicked metabolic syndrome-through modulation of insulin and glucocorticoid (GC) levels-were investigated in three models: tridimensional rat brain cell cultures for neuron-glia effects; murine microglial cell line BV-2 for a mechanistic analysis of microglial reactivity; and db/db mice as an in vivo model of metabolic syndrome. In 3D cultures, low insulin condition significantly exacerbated TMT's effect on GABAergic neurons and promoted TMT-induced neuroinflammation, with increased expression of cytokines and of the regulator of intracellular GC activity, 11β-hydroxysteroid dehydrogenase 1 (11β-Hsd1). Microglial reactivity increased upon TMT exposure in medium combining low insulin and high GC. These results were corroborated in BV-2 microglial cells where lack of insulin exacerbated the TMT-induced increase in 11β-Hsd1 expression. Furthermore, TMT-induced microglial reactivity seems to depend on mineralocorticoid receptor activation. In diabetic BKS db mice, a discrete exacerbation of TMT neurotoxic effects on GABAergic neurons was observed, together with an increase of interleukin-6 (IL-6) and of basal 11β-Hsd1 expression as compared to controls. These results suggest only minor additive effects of the two brain insults, neurotoxicant TMT exposure and metabolic syndrome conditions, where 11β-Hsd1 appears to play a key role in the regulation of neuroinflammation and of its protective or neurodegenerative consequences.

Organotins in Neuronal Damage, Brain Function, and Behavior: A Short Review

The consequences of exposure to environmental contaminants have shown significant effects on brain function and behavior in different experimental models. The endocrine-disrupting chemicals (EDC) present various classes of pollutants with potential neurotoxic actions, such as organotins (OTs). OTs have received special attention due to their toxic effects on the central nervous system, leading to abnormal mammalian neuroendocrine axis function. OTs are organometallic pollutants with a tin atom bound to one or more carbon atoms. OT exposure may occur through the food chain and/or contaminated water, since they have multiple applications in industry and agriculture. In addition, OTs have been used with few legal restrictions in the last decades, despite being highly toxic. In addition to their action as EDC, OTs can also cross the blood-brain barrier and show relevant neurotoxic effects, as observed in several animal model studies specifically involving the development of neurodegenerative processes, neuroinflammation, and oxidative stress. Thus, the aim of this short review is to summarize the toxic effects of the most common OT compounds, such as trimethyltin, tributyltin, triethyltin, and triphenyltin, on the brain with a focus on neuronal damage as a result of oxidative stress and neuroinflammation. We also aim to present evidence for the disruption of behavioral functions, neurotransmitters, and neuroendocrine pathways caused by OTs.

Increased pro-inflammatory cytokines, glial activation and oxidative stress in the hippocampus after short-term bilateral adrenalectomy

Background

Bilateral adrenalectomy has been shown to damage the hippocampal neurons. Although the effects of long-term adrenalectomy have been studied extensively there are few publications on the effects of short-term adrenalectomy. In the present study we aimed to investigate the effects of short-term bilateral adrenalectomy on the levels of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α; the response of microglia and astrocytes to neuronal cell death as well as oxidative stress markers GSH, SOD and MDA over the course of time (4 h, 24 h, 3 days, 1 week and 2 weeks) in the hippocampus of Wistar rats.

Results

Our results showed a transient significant elevation of pro-inflammatory cytokines IL-1β and IL-6 from 4 h to 3 days in the adrenalectomized compared to sham operated rats. After 1 week, the elevation of both cytokines returns to the sham levels. Surprisingly, TNF-α levels were significantly elevated at 4 h only in adrenalectomized compared to sham operated rats. The occurrence of neuronal cell death in the hippocampus following adrenalectomy was confirmed by Fluoro-Jade B staining. Our results showed a time dependent increase in degenerated neurons in the dorsal blade of the dentate gyrus from 3 days to 2 weeks after adrenalectomy. Our results revealed an early activation of microglia on day three whereas activation of astroglia in the hippocampus was observed at 1 week postoperatively. A progression of microglia and astroglia activation all over the dentate gyrus and their appearance for the first time in CA3 of adrenalectomized rats hippocampi compared to sham operated was seen after 2 weeks of surgery. Quantitative analysis revealed a significant increase in the number of microglia (3, 7 and 14 days) and astrocytes (7 and 14 days) of ADX compared to sham operated rats. Our study revealed no major signs of oxidative stress until 2 weeks after adrenalectomy when a significant decrease of GSH levels and SOD activity as well as an increase in MDA levels were found in adrenalectomized compared to sham rats.

Conclusion

Our study showed an early increase in the pro-inflammatory cytokines followed by neurodegeneration and activation of glial cells as well as oxidative stress. Taking these findings together it could be speculated that the early inflammatory components might contribute to the initiation of the biological cascade responsible for subsequent neuronal death in the current neurodegenerative animal model. These findings suggest that inflammatory mechanisms precede neurodegeneration and glial activation.

Trimethyltin-induced hippocampal neurodegeneration: A mechanism-based review

Trimethyltin (TMT), a toxic organotin compound, induces neurodegeneration selectively involving the limbic system and especially prominent in the hippocampus. Neurodegeneration-associated behavioral abnormalities, such as hyperactivity, aggression, cognitive deficits, and epileptic seizures, occur in both exposed humans and experimental animal models. Previously, TMT had been used generally in industry and agriculture, but the use of TMT has been limited because of its dangers to people. TMT has also been used to make a promising in vivo rodent model of neurodegeneration because of its region-specific characteristics. Several studies have demonstrated that TMT-treated animal models of epileptic seizures can be used as tools for researching hippocampus-specific neurotoxicity as well as the molecular mechanisms leading to hippocampal neurodegeneration. This review summarizes the in vivo and in vitro underlying mechanisms of TMT-induced hippocampal neurodegeneration (oxidative stress, inflammatory responses, and neuronal death/survival). Thus, the present review may be helpful to provide general insights into TMT-induced neurodegeneration and approaches to therapeutic interventions for neurodegenerative diseases, including temporal lobe epilepsy.

Gene expression profiling as a tool to investigate the molecular machinery activated during hippocampal neurodegeneration induced by trimethyltin (TMT) administration

Trimethyltin (TMT) is an organotin compound exhibiting neurotoxicant effects selectively localized in the limbic system and especially marked in the hippocampus, in both experimental animal models and accidentally exposed humans. TMT administration causes selective neuronal death involving either the granular neurons of the dentate gyrus or the pyramidal cells of the Cornu Ammonis, with a different pattern of localization depending on the different species studied or the dosage schedule. TMT is broadly used to realize experimental models of hippocampal neurodegeneration associated with cognitive impairment and temporal lobe epilepsy, though the molecular mechanisms underlying the associated selective neuronal death are still not conclusively clarified. Experimental evidence indicates that TMT-induced neurodegeneration is a complex event involving different pathogenetic mechanisms, probably acting differently in animal and cell models, which include neuroinflammation, intracellular calcium overload, and oxidative stress. Microarray-based, genome-wide expression analysis has been used to investigate the molecular scenario occurring in the TMT-injured brain in different in vivo and in vitro models, producing an overwhelming amount of data. The aim of this review is to discuss and rationalize the state-of-the-art on TMT-associated genome wide expression profiles in order to identify comparable and reproducible data that may allow focusing on significantly involved pathways.

The neuroprotective effect of gugijihwang-tang on trimethyltin-induced memory dysfunction in the rat

Gugijihwang-Tang (the herbal formula PM012), a decoction consisting of several herbs including Rehmanniae Radix Preparata, has been widely used as herbal treatment for dementia. In order to investigate the neuroprotective action of this prescription, we examined the effect of Gugijihwang-Tang on learning and memory using the Morris water maze and [F-18]FDG micro PET neuroimaging technique. After injection of trimethyltin (TMT, 8.0 mg/kg, i.p.), which is a potent toxicant that selectively kills cells in the central nervous system, rats were administered Gugijihwang-Tang (100 mg/kg, p.o.) daily for two weeks, followed by the Morris water maze tasks and [F-18]FDG micro PET neuroimaging. In Gugijihwang-Tang administered TMT-treated rats, they showed improved learning and memory abilities in water maze tasks and glucose metabolism, suggesting that Gugijihwang-Tang plays effectively positive role in the improvement of brain function including learning and memory after TMT-induced neurodegeneration. Taken together, our results suggested that the Gugijihwang-Tang should be useful for developing strategies protecting nervous system and improving brain function.

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.

Opposite effect of inflammation on subventricular zone versus hippocampal precursors in brain injury

OBJECTIVE

Inflammation promotes epidermal wound healing but is considered detrimental to recovery from central nervous system injury. Sick infants have increased levels of cytokines in their cerebrospinal fluid that correlate with poor neurological outcome. In this study, we investigated the role of neuroinflammation and more specifically interleukin 6 (IL-6) in the amplification of subventricular zone (SVZ) and subgranular zone (SGZ) neural precursors after neonatal brain injury.

METHODS

Neonatal hypoxia/ischemia (H/I) was induced in P6 rat pups, and IL-6 was quantified with or without indomethacin administration. Neural precursor responses were evaluated by neurosphere assays as well as by stereological analyses. Studies were performed to determine how IL-6 and leukemia-inhibiting factor (LIF) affect SVZ cell expansion, proliferation, and self-renewal.

RESULTS

Consistent with earlier studies, medially situated SVZ cells expanded after H/I. Contrary to our expectations, indomethacin significantly decreased both the initial reactive increase in these precursors and their ability to self-renew. By contrast, indomethacin increased proliferation in the SGZ and lateral SVZ. Indomethacin diminished the accumulation of microglia/macrophages and IL-6 production after H/I. In vitro IL-6 enhanced neurosphere growth, self-renewal, and tripotentiality and was more effective than LIF in promoting self-renewal. Enhanced precursor self-renewal also was obtained using prostaglandin E2, which is downstream of cyclooxygenase 2 and a target of indomethacin.

INTERPRETATION

These data implicate neuroinflammation and in particular IL-6 as a positive effector of primitive neural precursor expansion after neonatal brain injury. These findings have important clinical implications, as indomethacin and other anti-inflammatory agents are administered to premature infants for a variety of reasons.

Down-regulation of microglia and NG2-positive cells reaction in trimethyltin-injured hippocampus of rats treated with myelin basic protein-reactive T cells: possible contribution to the neuroprotective effect of T cells

In our previous investigations, we demonstrated that CD4(+) antimyelin basic protein (MBP) T cells protect hippocampal neurons against trimethyltin-induced damage. We hypothesized involvement of T cells, interacting with the various glial populations activated during the neurodegeneration process. In this study, we employ immunocytochemical methods to investigate the influence of administration of T cells on the response of microglia and of NG2(+) cells to trimethyltin (TMT)-induced damage. Female Lewis rats were treated with anti-MBP CD4(+) T cells (4 million per animal, i.v) 24 hr after TMT (8 mg/kg, i.p) intoxication. TMT caused degeneration of CA4 hipppocampal neurons and evoked an abundant reaction of microglial and NG2(+) cells in the injured region. The cells changed morphology into the activated state, and the number of OX42(+) and NG2(+) cells increased about 4.5-fold and 3-fold, respectively, relative to controls as assessed on day 21 after TMT treatment. Additionally, the cells of ameboid morphology, which expressed NG2 or microglial antigens, appeared in the zone of neurodegeneration. Furthermore, certain cells of ameboid phenotype shared both antigens. In rats treated with T cells, down-regulation of the activation of both glial classes and reduction of formation of their ameboid forms was observed. The number of the total OX42(+) and NG2(+) cells decreased by 21% and 54%, respectively, and the number of their ameboid forms decreased by 46% and 73%, respectively. Our data suggest that the diminished activation of microglia and NG2(+) cells, particularly the reduced number of their ameboid forms, may contribute to the neuroprotective effect of T cells.

[Protective effects of buckwheat hull extract against experimental hippocampus injury induced by trimethyltin in rats]

OBJECTIVES

The main objective of this study is to clarify the protective effects of buckwheat hull extract (BWHE) against toxicant-induced spatial memory impairment and hippocampal neuron injury in rats.

METHODS

Male Sprague-Dawley (Jsl: SD) rats were fed chow containing 0.75% (w/w) BWHE during the experimental period. Two weeks after the start of the experiment, trimethyltin (TMT) (8 mg/kg bw) was administered orally to 6-week-old rats. After another two weeks, the rats were subjected to the Morris water maze task, which was used to determine spatial memory impairment. On the day after the Morris water maze task was performed, the right hemi-hippocampi were removed from the right half of the brain and weighed. Coronal sections of the left half of the brain were cut into 16-mum sections using a cryostat, and the number of neurons in each hippocampal region was evaluated by counting the surviving neurons using a light microscope.

RESULTS

The impairment of spatial memory and the decrease in the hippocampal weight were observed after the TMT administration. Prolonged supplementation of BWHE seemed to reverse these TMT-induced toxic effects, and also improved the spatial memory of rats.

CONCLUSIONS

The present results suggest that the BWHE supplementation of foods enhanced the spatial memory of rats and may have protective effects against hippocampal neurodegeneration accompanied by spatial memory impairment.

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.

Role of maternal corticosterone in the development and maturation of the aminoacidergic systems of the rat brain

Previous studies have suggested an important role for maternal glucocorticoids in the development of the aminoacidergic systems of the rat brain. This study examines the effect of metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), i.p.-administered to gestating mothers, on the maturation of the aminoacidergic systems of their offsprings' brains. gamma-Aminobutyric acid (GABA) and glutamate concentrations were determined in male and female offspring at postnatal days (PN) 23 and 90 in four brain areas: the hippocampus, hypothalamus, striatum and cortex. The activity of glutamic acid decarboxylase (GAD), the most important enzyme in the synthesis of GABA, was also analysed. The results show that a reduction in maternal corticosterone during gestation leads to a reduced GABAergic content in all brain areas studied at PN23; permanent organizational changes occurred in the cortex, striatum and hypothalamus. Maternal metyrapone treatment also affected the development of the glutamatergic systems, females being more affected than males at both PN23 and PN90 particularly in the hypothalamus and cortex. The metyrapone treatment produced no changes in GAD activity at PN23, but induced an important increase in this activity at PN90.

Maternal exposure to low levels of corticosterone during lactation protects the adult offspring against ischemic brain damage

A growing body of evidence underscores the importance of early life events as predictors of health in adulthood. Abnormalities in maternal care or other forms of early postnatal stress induce long-term changes in behavior and influence the vulnerability to illnesses throughout life. Some of these changes may be produced by the activation of the hypothalamic-pituitary-adrenal (HPA) axis, which is invariably associated with stress. We used a model in which neonate rats are fed by mothers drinking water supplemented with 0.2 mg/ml corticosterone, the main glucocorticoid hormone in rodents. Plasma corticosterone levels increased in the dams to an extent similar to that induced by a mild stress. Corticosterone-treated dams also showed an increase in maternal care. Remarkably, adult rats that had been nursed by corticosterone-treated mothers were protected against neuronal damage and cognitive impairment produced by transient global brain ischemia. Neuroprotection was associated with a reduced HPA response to ischemia and was primarily decreased when corticosterone was injected at a dose that eliminated any difference in endogenous corticosterone levels between rats raised by mothers supplemented with corticosterone and their matched controls. These data suggest that an increased maternal care protects the offspring against ischemic neuronal damage and that at least a component of neuroprotection is mediated by a reduced response of the HPA axis to ischemia.

Recent Studies on the Trimethyltin Actions in Central Nervous Systems

Trimethyltin (TMT) is a toxic organotin compound that produces injury to the central nervous systems of mammals. Recently, high-dose TMT (2.8 mg/kg) has been shown to produce neurodegeneration and subsequent neurogenesis specifically in the hippocampal dentate gyrus of mice, indicating that mice injected with TMT serve as a useful in vivo model to study neurogenesis as well as neurodegeneration in this brain region. In addition, gene-engineered mice have allowed research to focuse on the mechanisms of TMT toxicity. These studies have revealed the involvement of stannin, nuclear factor kappa B (NF-kappaB), presenilin-1, apolipoprotein E, and pituitary adenylyl cyclase-activating polypeptide (PACAP) in TMT toxicity and suggested the relationship between genetic mutations and neuronal susceptibility to degeneration. In this review, we briefly summarize the previous studies and discuss the current status of research on TMT.

Disruption of glucocorticoid action by environmental chemicals: potential mechanisms and relevance

Glucocorticoids play an essential role in the regulation of key physiological processes, including immunomodulation, brain function, energy metabolism, electrolyte balance and blood pressure. Exposure to naturally occurring compounds or industrial chemicals that impair glucocorticoid action may contribute to the increasing incidence of cognitive deficits, immune disorders and metabolic diseases. Potentially, "glucocorticoid disruptors" can interfere with various steps of hormone action, e.g. hormone synthesis, binding to plasma proteins, delivery to target cells, pre-receptor regulation of the ratio of active versus inactive hormones, glucocorticoid receptor (GR) function, or export and degradation of glucocorticoids. Several recent studies indicate that such chemicals exist and that some of them can cause multiple toxic effects by interfering with different steps of hormone action. For example, increasing evidence suggests that organotins disturb glucocorticoid action by altering the function of factors that regulate the expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD) pre-receptor enzymes, by direct inhibition of 11beta-HSD2-dependent inactivation of glucocorticoids, and by blocking GR activation. These observations emphasize on the complexity of the toxic effects caused by such compounds and on the need of suitable test systems to assess their effects on each relevant step.

Occurrence, use and potential toxic effects of metals and metal compounds

Metals and metal compounds are constituents of our natural environment. Their distribution depends on the existence of natural sources (e.g. volcanoes or erosion) and their use in human's activity. They are transformed naturally (e.g. by bacterial activity) with formation of organic species that influence their mobility and accumulation in abiotic as well as biotic systems. Up to date metal species are released into the environment questioning their influence on human health. Due to their widespread use in human activities such as industry, agriculture and even as medicine (e.g. As, Se, Pt), numerous health risks may be associated with exposure to these substances. Different reports on metal intoxication are documented and studies especially on neurotoxicity, genotoxicity, or carcinogenicity, are previously published in numerous articles. This mini-review gives an overview on the use and the actions of selected metal species of actual scientific concern, with a focus on neuronal cells.