The action of triethyltin on the respiration of rat brain cortex slices

Toxic Peripheral Neuropathies: Agents and Mechanisms

Toxic peripheral neuropathies are an important form of acquired polyneuropathy produced by a variety of xenobiotics and different exposure scenarios. Delineating the mechanisms of neurotoxicants and determining the degenerative biological pathways triggered by peripheral neurotoxicants will facilitate the development of sensitive and specific biochemical-based methods for identifying neurotoxicants, designing therapeutic interventions, and developing structure-activity relationships for predicting potential neurotoxicants. This review presents an overview of the general concepts of toxic peripheral neuropathies with the goal of providing insight into why certain agents target the peripheral nervous system and produce their associated lesions. Experimental data and the main hypotheses for the mechanisms of selected agents that produce neuronopathies, axonopathies, or myelinopathies including covalent or noncovalent modifications, compromised energy or protein biosynthesis, and oxidative injury and disruption of ionic gradients across membranes are presented. The relevance of signaling between the main components of peripheral nerve, that is, glia, neuronal perikaryon, and axon, as a target for neurotoxicants and the contribution of active programmed degenerative pathways to the lesions observed in toxic peripheral neuropathies is also discussed.

Mitochondrial oxygen consumption inhibition importance for TMT-dependent cell death in undifferentiated PC12 cells

The evolving role of mitochondria as a target for different death-inducing noxae prompted us to investigate trimethyltin (TMT)-dependent effects on mitochondrial functionality. For this purpose, we used a homogeneous cell culture model represented by undifferentiated PC12 cells. Mitochondria isolated from PC12 cells treated with TMT for 6, 12 and 24h, showed a time-dependent inhibition of ADP-stimulated oxygen consumption using succinate or glutamate/malate as substrate. Using a fluorescent assay, the effect of TMT on mitochondrial membrane potential (delta Psi) in PC12 cells was also determined. After 24h in culture, a strong loss of mitochondrial membrane potential (delta Psi) was observed in TMT-treated cells. Collapse of mitochondrial membrane potential correlated with an increased expression of bax/bcl-2 ratio, as evaluated by polymerase chain reaction. Western blotting and spectrophotometric analysis showed that cytochrome c release and activation of caspase 3 were concurrently induced. Our findings suggest that inhibition of mitochondrial respiration represents the early toxic event for cell death in PC12 due to trimethyltin.

Hypoxia-like transcriptional activation in TMT-induced degeneration: microarray expression analysis on PC12 cells

To more clearly elucidate the complete network of molecular mechanisms induced by trimethyltin (TMT) toxicity, we used a homogeneous cell culture model represented by PC12 cells treated with 1 and 5 micromol/L TMT for 24 h. The gene expression profile was performed by microarray analysis, enabling us to identify 189 genes that were significantly modulated in treated cells, compared with controls. The main effects of TMT on gene expression seem to be related to the activation of metabolic processes (glycolysis and lipogenesis) along with cell death pathways, membrane remodeling and intracellular biomolecules trafficking. These alterations are triggered by the neurotoxicant earlier than a strong decrease in cell viability, which occurs at higher TMT concentrations or at later time points. Some aspects of the transcriptional modulation observed in this study resemble the gene activation known to occur during cell response to hypoxia. Other cell toxicants have also been reported to exert similar effects on gene expression. Therefore, our data help to delineate general basic adaptive mechanisms possibly shared by cells responding to different death-inducing noxae, such as TMT.

Effects of in vivo treatment of rats with trimethyltin chloride on respiratory properties of rat liver mitochondria

Liver mitochondria isolated from rats treated in vivo with trimethyltin chloride show stimulation of respiration using glutamate/malate as substrate, and a transient inhibition on rates of respiration using palmitoyl-L-carnitine as substrate. This phenomenon was observed with both ADP- and FCCP-stimulated respiration. In contrast, rates of respiration by liver mitochondria isolated from rats treated in vivo with trimethyltin chloride, following prior treatment with clofibrate, were inhibited when glutamate/malate was respiratory substrates. With palmitoyl-L-carnitine no effect of trimethyltin chloride was observed. In vitro treatment of rat liver mitochondria, or of rat liver homogenates, led to the expected, powerful inhibition of respiration. The synthesis of ATP by liver mitochondria isolated from rats treated in vivo with trimethyltin chloride was not inhibited compared to mitochondria isolated from control rats. Similarly, ATP synthesis by mitochondria isolated from rats treated with clofibrate, before treatment with trimethyltin chloride, was not inhibited. We, therefore, conclude that the powerful inhibitory effects of trimethyltin found in vitro, is not expressed in vivo during the first 36 hr following administration. In vivo treatment of rats with trimethyltin chloride caused a marked increase in hepatic levels of taurine and glycine, while levels of glutathione and glutamine were diminished. This is consistent with an enhanced oxidative stress in the liver. Our findings lead to the conclusion that increased oxidative stress, rather than inhibition of the mitochondrial ATPase, is a likely major cause of the in vivo toxic effects due to trimethyltin chloride.

Cellular and molecular effects of trimethyltin and triethyltin: relevance to organotin neurotoxicity

Many of the neurotoxic aspects of organotin exposure have been described. Organotin exposure culminates in its accumulation in the CNS and PNS. The clinical picture is dominated by neurological disturbances; yet, the primary basis for their neurotoxicity is unknown. Trimethyltin (TMT) is primarily a CNS neurotoxin affecting neurons within the hippocampal pyramidal band and the fascia dentata. Triethyltin (TET) is a neurotoxin that produces a pathological picture dominated by brain and spinal cord edema. The first part of this review summarizes the current understanding of the interaction of TMT and TET with biologically active sites in the induction of neurotoxicity. In the second part, several hypotheses for the differential neurotoxic effects of these organotins and their shortcomings are discussed.

Chloride-dependent uncoupling of oxidative phosphorylation by triethyllead and triethyltin increases cytosolic free calcium in guinea pig cerebral cortical synaptosomes

Metabolically competent isolated cerebral cortical nerve terminals were used to determine the effects of triethyllead (TEL) and triethyltin (TET) on cytosolic free calcium ([Ca2+]c), on plasma and mitochondrial membrane potentials, and on oxidative metabolism. In the presence of physiological concentrations of extracellular ions, 20 microM TEL and 20 microM TET increase [Ca2+]c from 185 nM to 390 and 340 nM, respectively. A simultaneous depolarization of plasma membrane potential (delta psi p) by only 3-4 mV occurs, a drop which is insufficient to open the voltage-sensitive Ca2+ channels. In contrast, an instant and substantial depolarization of mitochondrial membrane potential (delta psi m) upon addition of TEL and TET is evident, as monitored with safranine O fluorescence. At the same concentration, TEL and TET stimulate basal respiration of synaptosomes by 45%, induce oxidation of endogenous NAD(P)H, and reduce the terminal ATP/ADP ratio by 45%. Thus, TEL and TET inhibit ATP production of intrasynaptosomal mitochondria by a mechanism consistent with uncoupling of oxidative phosphorylation. This bioenergetic effect by TEL and TET can be prevented by omitting external chloride, and a concomitant reduction of the increase in [Ca2+]c by about 60% is observed. Uncoupling of mitochondrial ATP synthesis from oxidation by TEL and TET, [corrected] a process that is dependent on external chloride, is the main mechanism by which they [corrected] increase [Ca2+]c.

Effects of tri-n-butyltin chloride on energy metabolism, macromolecular synthesis, precursor uptake and cyclic AMP production in isolated rat thymocytes

The inhibitor of oxidative phosphorylation tri-n-butyltin chloride (TBTC) causes membrane damage and disintegration of isolated rat thymocytes at concentrations higher than 1 microM. From a concentration of 0.1 microM, TBTC disturbs energy metabolism as indicated by an increase in methylglucose uptake, glucose consumption and lactate production and by a decrease in cellular ATP levels. Over the same TBTC concentration range, the incorporation of DNA, RNA and protein precursors are markedly reduced. Moreover the production of cyclic AMP upon stimulation of the cells with prostaglandin E1 is effectively inhibited. These effects cannot be explained by an inhibition of nucleoside kinase activity, amino acid uptake or adenylate cyclase activity. The effects of TBTC on macromolecular synthesis and cyclic AMP production are possibly due to a disturbance of the cellular energy state.

Protection with butylated hydroxytoluene and other compounds against intoxication and mortality caused by hexachlorophene

The antioxidants butylated hydroxytoluene (BHT) and ethoxyquin protected rats against intoxication and mortality normally produced by hexachlorophene (HCP, 100 mg/kg). BHT also prevented the elevation of cerebrospinal fluid pressure, a central nervous system effect of HCP poisoning. In addition, both phenobarbital and SKF-525A protected against HCP poisoning, with the barbiturate also offering significant protection against triethyltin. L-Ascorbic acid, vitamin E, N,N-diphenyl-p-phenylenediamine and reduced and oxidized glutathione over a range of doses were ineffective in preventing HCP lethality. The protective effect of phenobarbital against HCP and triethyltin intoxication further supports existing evidence of a common or similar mechanism of toxic action for these two structurally dissimilar compounds.

Inhibitory effect of triethyltin on taurine transport by glioma cells

The effects of triethyltin (TET) on the transport of taurine, glutamate, lysine, Na+, K+ (using 86Rb+ as tracer), and Cl- by LRM55 glioma cells were examined. Taurine transport was inhibited by TET at much lower concentrations (IC50 = 2.5 microM) than either glutamate or lysine transport (135 and 110 microM, respectively). TET had no significant effect on Na+, Cl-, or 86Rb+ influx at the low concentrations greater than 100 microM. The failure of low concentrations (less than or equal to 10 microM) of TET to affect ion transport indicated that inhibition of taurine transport was not secondary to effects of TET on ion movements or gradients. This conclusion was supported by the observation that neither ouabain nor furosemide, which do affect ion movement and gradients, strongly inhibited taurine transport. Uncouplers and inhibitors of oxidative phosphorylation (cyanide, 2,4-dinitrophenol, and carbonyl cyanide-m-chlorophenyl hydrazone) also had only small effects on taurine transport, suggesting that inhibition by TET was not secondary to possible effects on oxidative phosphorylation. TET had no effect on the efflux of taurine from LRM55 cells at the low concentrations that inhibit uptake, but it induced a nonspecific increase in membrane permeability at much higher concentrations (greater than 100 microM). Tri-n-propyltin and tri-n-butyltin were also potent inhibitors of taurine transport (IC50 = 2.3 and 11 microM, respectively), but trimethyltin was much less potent (144 microM).

Immediate and long-term alterations in maximal electroshock seizure responsiveness in rats neonatally exposed to triethyltin bromide

Neonatal rats were injected (sc) with 0, 0.25, 0.5, 1.0, 2.5, or 5.0 mg/kg triethyltin bromide (TET) in a 2% ethanol vehicle on Days 0, 5, 10, 15, and 20 postnatally. TET-exposed neonates exhibited a dose-dependent delay in the ontogenetic appearance of both the clonic and tonic maximal electroshock seizure (MES) responses which were apparent up to Day 45, as evaluated by the MES grade distributions and the durations of the individual phases of the MES. In marked contrast, adult rats (exposed to TET only as neonates) exhibited long-term increases in MES severity, as evaluated by the durations of the individual phases of the MES. At 75 days of age, a time when the 1.0 mg/kg developmentally TET-treated group exhibited an increased seizure severity, a single challenge dose of 1.0 mg/kg TET (ip) produced a proportional decrease in MES severity in both developmentally treated and control rats. No changes in preweaning or postweaning body weight were observed in the animals in the 0.25, 0.5, 1.0, or 2.5 mg/kg groups. A 30% decrease in weaning weight and an approximate 15% decrease in postweaning weight were observed in the 5.0 mg/kg group. These seizure results demonstrate that developmental exposure to TET produces immediate and long-term alterations in central nervous system functioning, which are of an opposite character. Interesting, we have previously shown that developmental lead exposure produces a similar developmental/adult dichotomy of effects with regard to the MES severity; however, the two patterns are reversed (D. A. Fox, S. R. Overmann, and D. E. Woolley (1979).

Effects of organotin compounds on maximal electroshock seizure (MES) responsiveness in mice. I. TRI(n-alkyl)tin compounds

Male mice (25-30 g) were injected (ip) with 0, 3.5 X 10(-6), or 17.5 X 10(-6) mol trimethyltin bromide (TMT), triethyltin bromide (TET), tri-n-propyltin chloride (TPT), or tri-n-butyltin bromide (TBT) per kg. Additional groups of mice were also injected (ip) with either 0 or 17.5 X 10(-6) mol sodium bromide (NaBr) or 17.5 X 10(-6) mol stannic bromide (SnBr4) per kg. The mice were tested with maximal electroshock seizure (MES) at 0.5, 4, 21-24, and 96 h following exposure to the organotin compounds. Mice exposed to TMT, TET, TPT, or TBT exhibited dose-dependent decreases in MES severity as evaluated by seizure-grade distributions and duration of tonic seizure phases. The tri-n-alkyltin compounds exhibited a structure-activity relationship in their ability to decreased maximal responsiveness to the MES test. In order of decreasing ability they were: TMT greater than TET greater than TPT greater than TBT. Administration of NaBr and SnBr4 did not alter MES responsiveness, indicating the essential role of the alkyl moieties of the tri-n-alkyltin compounds in producing alterations in central nervous system function.

Resistance of quaking mouse CNS to triethyl tin edema

Intraperitoneal injection of triethyl tin (TET) sulfate, 5 or 10 mg/kg body weight did not induce intramyelinic edema without altering water content in quaking mice while in C57BL/6J and littermate control mice, water content was increased and typical intramyelinic edema was induced following TET injection. Even among control mice, however, there were some strain differences in the histological severity of the edema, which were in precise agreement with the quantitative alterations in water content. These observations suggest that CNS myelin in quaking may differ qualitatively from that in controls and the mode of response to TET is under genetic control.

Kinetics of and rate equations for the uptake of alpha-amino-isobutyric aicd and gamma-aminobutyric acid by mouse brain slices incubated in a glucose-free medium containing pyruvate as the energy source

Mouse cerebrum slices were incubated in a medium containing pyruvate instead of glucose as the energy source. After a preincubation period alpha-aminoisobutyric acid (AIB) or gamma-aminobutyric acid (GABA) was added, and the rate of uptake by the slices was measured. AIB and GABA are taken up to above their concentration in the medium. Although influx is slower, the rate equation is the same as that for influx from a glucose medium; namely v = Vmax/1 x Kt/S) + kuS, where v = rate of uptake, S = concentration of AIB or GABA in the medium, and Vmax, Kt, and Ku are constants. The equation shows two parallel pathways for concentrative uptake, one saturable and one unsaturable. The uptake systems for AIB and GABA are qualitatively different. The maximum rate of uptake of AIB by the saturable component is the same in both media even though, in the pyruvate medium, AIB is bound less strongly to the 'carriers'. The maximum rate of uptake of GABA by the saturable component is less in the pyruvate medium although GABA is bound somewhat more strongly to the 'carriers'. The temperature coefficients of the kinetic parameters and their corresponding energies were determined for GABA. Going from glucose to pyruvate medium has little effect on the Arrhenius activation energy (Ea) associated with Vmax and the heat of reaction (delta H) associated with Kt but increases Ea associated with Ku by 1140%.