Neuropeptide Y and somatostatin participate differently in the seizure-generating mechanisms following trimethyltin-induced hippocampal damage

Trimethyltin Increases Intracellular Ca2+ Via L-Type Voltage-Gated Calcium Channels and Promotes Inflammatory Phenotype in Rat Astrocytes In Vitro

Astrocytes are the first responders to noxious stimuli by undergoing cellular and functional transition referred as reactive gliosis. Every acute or chronic disorder is accompanied by reactive gliosis, which could be categorized as detrimental (A1) of beneficial (A2) for nervous tissue. Another signature of pathological astrocyte activation is disturbed Ca2+ homeostasis, a common denominator of neurodegenerative diseases. Deregulation of Ca+ signaling further contributes to production of pro-inflammatory cytokines and reactive oxygen species. Trimethyltin (TMT) intoxication is a widely used model of hippocampal degeneration, sharing behavioral and molecular hallmarks of Alzheimer's disease (AD), thus representing a useful model of AD-like pathology. However, the role of astrocyte in the etiopathology of TMT-induced degeneration as well as in AD is not fully understood. In an effort to elucidate the role of astrocytes in such pathological processes, we examined in vitro effects of TMT on primary cortical astrocytes. The application of a range of TMT concentrations (5, 10, 50, and 100 μM) revealed changes in [Ca2+]i in a dose-dependent manner. Specifically, TMT-induced Ca2+ transients were due to L-type voltage-gated calcium channels (VGCC). Additionally, TMT induced mitochondrial depolarization independent of extracellular Ca2+ and disturbed antioxidative defense of astrocyte in several time points (4, 6, and 24 h) after 10 μM TMT intoxication, inducing oxidative and nitrosative stress. Chronic exposure (24 h) to 10 μM TMT induced strong upregulation of main pro-inflammatory factors, components of signaling pathways in astrocyte activation, A1 markers, and VGCC. Taken together, our results provide an insight into cellular and molecular events of astrocyte activation in chronic neuroinflammation.

Organotins in obesity and associated metabolic disturbances

The objective of the present study was to review the mechanisms of organotin-induced adipogenesis, obesity, and associated metabolic disturbances. Peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor α (RXRα) activation is considered as the key mechanism of organotin-induced adipogenesis. Particularly, organotin exposure results in increased adipogenesis both in cell and animal models. Moreover, transgenerational inheritance of organotin-induced obese phenotype was demonstrated in vivo. At the same time, the existing data demonstrate that organotin compounds (OTCs) induces aberrant expression of PPARγ-targeted genes, resulting in altered of adipokine, glucose transporter, proinflammatory cytokines levels, and lipid and carbohydrate metabolism. The latter is generally characterized by hyperglycemia and insulin resistance. Other mechanisms involved in organotin-induced obesity may include estrogen receptor and corticosteroid signaling, altered DNA methylation, and gut dysfunction. In addition to cellular effects, organotin exposure may also affect neural circuits of appetite regulation, being characterized by neuropeptide Y (NPY) up-regulation in parallel with of pro-opiomelanocortin (POMC), Agouti-related protein (AgRP), and cocaine and amphetamine regulated transcript (CART) down-regulation in the arcuate nucleus. These changes result in increased orexigenic and reduced anorexigenic signaling, leading to increased food intake. The existing data demonstrate that organotins are potent adipogenic agents, however, no epidemiologic studies have been performed to reveal the association between organotin exposure and obesity and the existing indirect human data are contradictory.

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.

In vivo magnetic resonance approach to trimethyltin induced neurodegeneration in rats

Trimethyltin (TMT) is commonly used to induce neurodegeneration in mice and rats; however, only scarce data of in vivo magnetic resonance (MR) spectroscopy and imaging characterizing TMT neurotoxicity are available. Our aim was to assess brain metabolite changes and brain atrophy by in vivo MR in the rat model of neurodegeneration induced by TMT. Adult male Wistar rats exposed to TMT (8mg/kg, i.p.) were used in the study. Proton MRS was applied on the dorsal hippocampus to reveal changes in neurochemical profile, and MR imaging was used to assess the volume of the entire hippocampus, ventricles and whole brain. Hippocampal levels of N-acetylaspartate (NAA), glutamate (Glu), total creatine (tCr) and taurine (Tau) significantly decreased, while the levels of myo-Inositol (mIns) and glutamine (Gln) significantly increased in TMT treated rats compared to controls. No changes in choline metabolites (tCho), glutathione (GSH), and GABA were observed. MR volumetry revealed a substantial loss of hippocampal mass, cerebral volume shrinkage and ventricular enlargement in the TMT treated group in comparison to the control group. To the best of our knowledge, this is the first study characterizing TMT induced neurodegeneration in the rat by in vivo MRS. Our findings suggest that TMT exposed rats may serve as a reliable animal model of neurodegeneration and MR based parameters could serve as potential in vivo biomarkers of therapeutic response.

Trimethyltin-induced cochlear degeneration in rat

Trimethyltin (TMT) is an occupational and environmental health hazard behaving as a potent neurotoxin known to affect the central nervous system as well as the peripheral auditory system. However, the mechanisms underlying TMT-induced ototoxicity are poorly understood. To elucidate the effects of TMT on the cochlea, a single injection of 4 or 8 mg/kg TMT was administered intraperitoneally to adult rats. The compound action potential (CAP) threshold was used to assess the functional status of the cochlea and histological techniques were used to assess the condition of the hair cells and auditory nerve fibers. TMT at 4 mg/kg produced a temporary CAP threshold elevation of 25–60 dB that recovered by 28 d post-treatment. Although there was no hair cell loss with the 4 mg/kg dose, there was a noticeable loss of auditory nerve fibers particularly beneath the inner hair cells. TMT at 8 mg/kg produced a large permanent CAP threshold shift that was greatest at the high frequencies. The CAP threshold shift was associated with the loss of outer hair cells and inner hair cells in the basal, high-frequency region of the cochlea, considerable loss of auditory nerve fibers and a significant loss of spiral ganglion neurons in the basal turn. Spiral ganglion neurons showed evidence of soma shrinkage and nuclear condensation and fragmentation, morphological features of apoptotic cell death. TMT-induced damage was greatest in the high-frequency, basal region of the cochlea and the nerve fibers beneath the inner hair cells were the most vulnerable structures.

Wogonin Attenuates Hippocampal Neuronal Loss and Cognitive Dysfunction in Trimethyltin-Intoxicated Rats

We examined whether wogonin (WO) improved hippocampal neuronal activity, behavioral alterations and cognitive impairment, in rats induced by administration of trimethyltin (TMT), an organotin compound that is neurotoxic to these animals. The ability of WO to improve cognitive efficacy in the TMT-induced neurodegenerative rats was investigated using a passive avoidance test, and the Morris water maze test, and using immunohistochemistry to detect components of the acetylcholinergic system, brain-derived neurotrophic factor (BDNF), and cAMP-response element-binding protein (CREB) expression. Rats injected with TMT showed impairments in learning and memory and daily administration of WO improved memory function, and reduced aggressive behavior. Administration of WO significantly alleviated the TMT-induced loss of cholinergic immunoreactivity and restored the hippocampal expression levels of BDNF and CREB proteins and their encoding mRNAs to normal levels. These findings suggest that WO might be useful as a new therapy for treatment of various neurodegenerative diseases.

Estrogen administration modulates hippocampal GABAergic subpopulations in the hippocampus of trimethyltin-treated rats

Given the well-documented involvement of estrogens in the modulation of hippocampal functions in both physiological and pathological conditions, the present study investigates the effects of 17-beta estradiol (E2) administration in the rat model of hippocampal neurodegeneration induced by trimethyltin (TMT) administration (8 mg/kg), characterized by loss of pyramidal neurons in CA1, CA3/hilus hippocampal subfields, associated with astroglial and microglial activation, seizures and cognitive impairment. After TMT/saline treatment, ovariectomized animals received two doses of E2 (0.2 mg/kg intra-peritoneal) or vehicle, and were sacrificed 48 h or 7 days after TMT-treatment. Our results indicate that in TMT-treated animals E2 administration induces the early (48 h) upregulation of genes involved in neuroprotection and synaptogenesis, namely Bcl2, trkB, cadherin 2 and cyclin-dependent-kinase-5. Increased expression levels of glutamic acid decarboxylase (gad) 67, neuropeptide Y (Npy), parvalbumin, Pgc-1α and Sirtuin 1 genes, the latter involved in parvalbumin (PV) synthesis, were also evident. Unbiased stereology performed on rats sacrificed 7 days after TMT treatment showed that although E2 does not significantly influence the extent of TMT-induced neuronal death, significantly enhances the TMT-induced modulation of GABAergic interneuron population size in selected hippocampal subfields. In particular, E2 administration causes, in TMT-treated rats, a significant increase in the number of GAD67-expressing interneurons in CA1 stratum oriens, CA3 pyramidal layer, hilus and dentate gyrus, accompanied by a parallel increase in NPY-expressing cells, essentially in the same regions, and of PV-positive cells in CA1 pyramidal layer. The present results add information concerning the role of in vivo E2 administration on mechanisms involved in cellular plasticity in the adult brain.

The neurogenic effects of exogenous neuropeptide Y: early molecular events and long-lasting effects in the hippocampus of trimethyltin-treated rats

Modulation of endogenous neurogenesis is regarded as a promising challenge in neuroprotection. In the rat model of hippocampal neurodegeneration obtained by Trimethyltin (TMT) administration (8 mg/kg), characterised by selective pyramidal cell loss, enhanced neurogenesis, seizures and cognitive impairment, we previously demonstrated a proliferative role of exogenous neuropeptide Y (NPY), on dentate progenitors in the early phases of neurodegeneration. To investigate the functional integration of newly-born neurons, here we studied in adult rats the long-term effects of intracerebroventricular administration of NPY (2 µg/2 µl, 4 days after TMT-treatment), which plays an adjuvant role in neurodegeneration and epilepsy. Our results indicate that 30 days after NPY administration the number of new neurons was still higher in TMT+NPY-treated rats than in control+saline group. As a functional correlate of the integration of new neurons into the hippocampal network, long-term potentiation recorded in Dentate Gyrus (DG) in the absence of GABA_A receptor blockade was higher in the TMT+NPY-treated group than in all other groups. Furthermore, qPCR analysis of Kruppel-like factor 9, a transcription factor essential for late-phase maturation of neurons in the DG, and of the cyclin-dependent kinase 5, critically involved in the maturation and dendrite extension of newly-born neurons, revealed a significant up-regulation of both genes in TMT+NPY-treated rats compared with all other groups. To explore the early molecular events activated by NPY administration, the Sonic Hedgehog (Shh) signalling pathway, which participates in the maintenance of the neurogenic hippocampal niche, was evaluated by qPCR 1, 3 and 5 days after NPY-treatment. An early significant up-regulation of Shh expression was detected in TMT+NPY-treated rats compared with all other groups, associated with a modulation of downstream genes. Our data indicate that the neurogenic effect of NPY administration during TMT-induced neurodegeneration involves early Shh pathway activation and results in a functional integration of newly-generated neurons into the local circuit.

Alteration in glutathione homeostasis and oxidative stress during the sequelae of trimethyltin syndrome in rat brain

Trimethyltin (TMT), a by-product of tin, is used in a wide variety of industrial and agricultural purposes which serves as a model neurotoxicant in hippocampal neurodegeneration, and this could, in turn, be exploited for various therapeutic compounds essential for hippocampal neurodegeneration. Therefore, the present investigation explores the sequential changes in behavior, oxidative burden, and apoptosis following TMT administration in rat hippocampus. Male SD rats weighing 250 g were given single dose of 8.5 mg/kg TMT (i.p.) that resulted in "TMT syndrome" which begins at the third post-TMT exposure and continued till 21 days posttreatment. This resulted in behavioral alteration (aggression and spontaneous seizures), cognitive impairment as assessed by plus maze, and passive avoidance resulting in short-term memory deficits. These behavioral alterations were associated with an increase in oxidative stress. The levels of malondialdehyde, reactive oxygen species, and protein carbonyl were significantly increased (p < 0.001) in the TMT-treated rats after the third day of exposure and were maximum at day 14 postexposure. The glutathione system was not able to adapt rapidly in response to oxidative stress which resulted in imbalance in redox status. The imbalance in the redox state resulted in the death of neurons as seen by a significant increase in caspase activation at gene as well as protein level after TMT exposure on day 14, quoting an extent of changes. Therefore, it is proposed that behavioral deficits could be accounted by the impairment of endogenous glutathione homeostasis which resulted in death of neurons in the hippocampal region.

Endocrine disrupters as obesogens

The recent dramatic rise in obesity rates is an alarming global health trend that consumes an ever increasing portion of health care budgets in Western countries. The root cause of obesity is thought to be a prolonged positive energy balance. Hence, the major focus of preventative programs for obesity has been to target overeating and inadequate physical exercise. Recent research implicates environmental risk factors, including nutrient quality, stress, fetal environment and pharmaceutical or chemical exposure as relevant contributing influences. Evidence points to endocrine disrupting chemicals that interfere with the body's adipose tissue biology, endocrine hormone systems or central hypothalamic-pituitary-adrenal axis as suspects in derailing the homeostatic mechanisms important to weight control. This review highlights recent advances in our understanding of the molecular targets and mechanisms of action for these compounds and areas of future research needed to evaluate the significance of their contribution to obesity.