Neuropathology of trimethyltin intoxication. II. Electron microscopic study on the hippocampus

Neurotoxicity of metals

Metals are frequently used in industry and represent a major source of toxin exposure for workers. For this reason governmental agencies regulate the amount of metal exposure permissible for worker safety. While essential metals serve physiologic roles, metals pose significant health risks upon acute and chronic exposure to high levels. The central nervous system is particularly vulnerable to metals. The brain readily accumulates metals, which under physiologic conditions are incorporated into essential metalloproteins required for neuronal health and energy homeostasis. Severe consequences can arise from circumstances of excess essential metals or exposure to toxic nonessential metal. Herein, we discuss sources of occupational metal exposure, metal homeostasis in the human body, susceptibility of the nervous system to metals, detoxification, detection of metals in biologic samples, and chelation therapeutic strategies. The neurologic pathology and physiology following aluminum, arsenic, lead, manganese, mercury, and trimethyltin exposures are highlighted as classic examples of metal-induced neurotoxicity.

Trimethyltin-induced neurotoxicity: gene expression pathway analysis, q-RT-PCR and immunoblotting reveal early effects associated with hippocampal damage and gliosis

Damage to the CNS results in a complex series of molecular and cellular changes involving the affected targets and the ensuing glial reaction. The initial gene expression events that underlie these cellular responses may serve as early biomarkers of neurotoxicity. Here, we examined gene expression profiles during the initial phase of hippocampal damage resulting from systemic exposure of rats to the organometallic neurotoxicant, trimethyltin (TMT, 8.0 mg/kg, i.p.). Using TMT as a neurodegeneration tool confers several advantages for evaluating molecular events associated with neural damage: 1) regional and cellular targets and time course of damage are known, 2) the blood-brain barrier is not compromised, which limits the contribution of blood-borne factors, e.g. immune, to neural injury responses and 3) known protein and mRNA signatures of TMT-induced neurotoxicity can be used as positive controls to validate novel expression events associated with exposure to this neurotoxicant. Using Affymetrix Gene Chip® to assess gene expression after TMT, combined with Ingenuity Pathway Analysis®, we observed changes consistent for genes known to be affected in hippocampus, while corresponding changes were not detected in cerebellum, a non-target region. In agreement with previous observations, limited changes in expression of inflammation-related genes were observed. Correlated expression profiles were found after exposure to TMT, including changes in gene ontologies associated with neurological disease, cellular assembly and maintenance, as well as signaling pathways associated with cellular stress, energy metabolism and glial activation. Selected gene changes were confirmed from each category by q-RT-PCR and immunoblot analysis. The canonical relationships identified implicate molecular pathways and processes relevant to detection of early stages of hippocampal damage in the TMT model. These observations provide new insight into early events associated with neuronal degeneration and associated glial activation that may serve as the basis for discovery and development of biomarkers of neurotoxicity.

Dentate gyrus: alterations that occur with hippocampal injury

Injury to the brain usually manifests not in a diffuse uniform manner but rather with selective sites of damage indicative of differential vulnerability. This question of neuronal susceptibility has been one of major interest both in disease processes as well as damage induced by environmental factors. For experimental examination, brain structures with obvious neuronal subpopulations and organization such as the cerebellum and the hippocampus have offered the most promise. In the hippocampus distinct neuronal populations exist that demonstrate differential vulnerability to various forms of insult including ischemia, excitotoxicity, and environmental factors. The more recent data regarding the presence of neuronal progenitor cells in the subgranular zone of the dentate offers the opportunity to expand such experimental examination to the process of injury-induced neurogenesis. Thus, more recent studies have expanded the examination of the hippocampus to include models of damage to the dentate neurons in addition to the highly vulnerable pyramidal neurons. A number of these models are presented for both human disease and experimental animal conditions. Examination of the responses between these distinct cell populations offers the potential for understanding factors that are critical in neuronal death and survival.

Behavioural, biochemical and histological effects of trimethyltin (TMT) induced brain damage in the rat

To assess the nature and extent of behavioural, biochemical and histological changes induced by trimethyltin (TMT), rats were treated with a single injection of TMT over a dose range of 6, 7 and 8 mg/kg i.p. Behavioural observations were performed at a minimum of 21 days after the administration of TMT. The behavioural consequences of TMT were hyperactivity in the open-field test, increased locomotor activity and deficits in passive and active avoidance behaviour, T-maze alternation and Morris Water Maze behaviour. The behavioural changes were dose dependent and were accompanied by a degree of pathological damage to the hippocampal pyramidal cells which was particularly apparent at the highest dose. The main biochemical effects of TMT involved deficits in the serotonergic and GABA-ergic systems and a decrease in M1 and M2 binding sites in the hippocampus. These results suggest that the toxic interaction of TMT with the hippocampus and other limbic brain regions may be responsible for its effect on learning and memory.

Rapid disruption of cochlear function and structure by trimethyltin in the guinea pig

Trimethyltin (TMT) is a potent ototoxicant which acutely disrupts generation of the action potential evoked by a broad range of tone frequencies and subsequently produces selective high frequency impairment and outer hair cell (OHC) damage in the extreme basal turn of the cochlea. We investigated the development of TMT ototoxicity in the guinea pig 6-48 h following treatment using the compound action potential (CAP), cochlear microphonic (CM), endocochlear potential (EP) and light and electron microscopic examinations. At all time intervals studied, TMT reduced CAP sensitivity and CM amplitude. The effect was relatively broad across test frequencies at 6 h and subsequently became restricted to higher frequencies. No disruption of the EP was observed between 6 and 24 h following TMT. OHC pathology in the basal turn of the cochlea 12 h following TMT consisted of vacuolization in the supranuclear region and disruption of the cuticular plate; some mitochondria exhibited dark inclusions. Type 1 spiral ganglion cells appeared swollen at 24 h with separation of myelin from the cell bodies. No pathological changes were observed in the inner hair cells (IHC). The present data identify the OHC as targets responsible for the loss of CM sensitivity after TMT as the EP was unaffected. These data suggest that CAP and CM recovery at low and middle frequencies following acute TMT administration is accompanied by recovery of neurotransmission at the IHC or Type 1 SGC level and OHC recovery at apical regions of the cochlea.

Trimethyltin disrupts auditory function and cochlear morphology in pigmented rats

Trimethyltin (TMT) produces auditory deficits, presumably of cochlear origin, in rats. The present study identified pathological changes in the cochlea following treatment with TMT and correlated them with auditory threshold changes. Thresholds were determined by reflex-modulation audiometry, before and after treatment with TMT or with saline vehicle. Animals were then perfused and their cochleas embedded for examination as block-surface preparations or radial sections. In the first week following treatment, all TMT-treated rats showed threshold shifts of 40 to 60 dB at 40 kHz, and smaller threshold shifts (10-25 dB) at 2.5 and 10 kHz. At 3 weeks they showed threshold shifts similar to those identified one week following treatment, but with some recovery at 10 kHz. At 10 weeks, one animal showed complete recovery and three showed recovery of function at 10 but not at 40 kHz. TMT-treated animals showed losses of outer hair cells (OHC) in the basal turn of the cochlea as early as 48 hours following exposure. Comparable OHC pathology was seen at 9 days, along with some losses of inner hair cells. More extensive pathology occurred at longer survival times including the loss of type 1 spiral ganglion cells. The loss of auditory sensitivity at high frequencies was closely related to the loss of outer hair cells in the basal turn of the cochlea.

Organometal-induced increases in oxygen reactive species: the potential of 2',7'-dichlorofluorescin diacetate as an index of neurotoxic damage

The effects of the neurotoxic metals methylmercury (MeHg) and trimethyltin (TMT) on oxygen reactive species formation within a crude synaptosomal fraction (P2), using the probe 2',7'-dichlorofluorescin diacetate (DCFH-DA), and intracellular calcium ([Ca2+]i), with the fluorescent indicator fluo-3, have been investigated. Two and seven days after a single injection of MeHg (1 mg/kg) the formation rate of cerebellar oxygen reactive species was significantly increased. Hippocampal and frontocortical oxygen reactive species were elevated 2 days after TMT injection (3 mg/kg). In vitro exposure to MeHg (10-20 microM) increased the formation rate of oxygen reactive species, while TMT (5-40 microM) was without effect. Levels of [Ca2+]i were unaltered in P2 fractions from cerebellum and hippocampus of animals treated with either organometal. The data demonstrate that oxygen reactive species are elevated in brain regions, cerebellum (MeHg) and hippocampus (TMT), believed to be selectively vulnerable to these toxic agents. Findings suggest that oxidative damage may be a mechanism underlying the toxicity of both organometals. The use of DCFH-DA may have potential in the nervous system as an indicator of neurotoxic damage.

Alzheimer's disease: environmental factors and etiologic hypotheses

A review of published reports on conventional and unconventional viruses, aluminum, neurotoxic metals and trace elements, neurotoxins of biological origin and immune systems, suggest that environmental factors, possibly multiple ones, play a significant role in the etiology of Alzheimer's disease. A complex interaction between genetic predisposition to this illness, natural aging processes, environmental factors over a life-time exposure and pathological alterations of the host immune system is proposed.

Neurotoxicological effects of trimethyltin on the stellate ganglion

Hamsters treated with trimethyltin (TMT), 3 or 4 mg/kg IP, developed neurological symptoms, including tremor, within 24 hours. Postganglionic action potentials were recorded from isolated stellate ganglia of untreated hamsters (control ganglia) and TMT-treated hamsters (TMT ganglia). Compound action potentials (nicotinic transmission) of control and TMT ganglia were not significantly different. The afterdischarges induced by preganglionic stimulation at 30 Hz for 2 sec in the presence of 10(-3) M hexamethonium (muscarinic transmission) were significantly smaller in TMT ganglia than in control ganglia. The discharges induced by the muscarinic cholinoceptor agonist. McN-A-343, were also smaller in the TMT ganglia. Two other muscarinic processes, posttetanic potentiation and potentiation of the compound action potential by McN-A-343, were not significantly reduced in the TMT ganglia. Morphological studies of the ganglia revealed marked changes in the TMT ganglia with severe neuronal degeneration including vacuole formations and accumulations of lysosomes in the cytoplasm. These results demonstrate that TMT has marked anatomical effects on the stellate ganglion that may lead to the reduction in muscarinic cholinergic transmission.

Reversible neuronal damage in hippocampal pyramidal cells with triethyllead: the role of astrocytes

A single dose (19 mg kg-1) of triethyllead given to weanling rats produces necrosis in a small number of hippocampal pyramidal (CA3) and hilar neurons with reversible changes in the remaining neurons of this region. The sequence of events has been studied by light and electron microscopy over a period from 12 h to 14 days after dosing. Early changes resemble those previously described for trimethyltin, with the formation of characteristic tubulo-vesicular dense bodies by 12 h accompanied by vacuolation of Golgi and smooth surfaced endoplasmic reticulum (SER) elements which became generalized by 24 h. Large numbers of secondary dense bodies, formed from tubulo-vesicular dense bodies as well as from autophagosomes, were present by 48 h, whilst very little rough surfaced endoplasmic reticulum (RER) and few polyribosomes remained and vacuolation was much reduced. In those animals which did not die from seizures, the majority of hippocampal pyramidal cells were able to recover from these changes with astrocytes playing a significant role in the elimination of the dense bodies. This involved astrocytes inserting processes into the neuronal perikaryon from where the secondary dense bodies were selectively transferred into the astrocyte cytoplasm. This activity was first seen at 48 h, reached a peak at 4 days, when most CA3 neurons contained one or more astroglial intrusions and subsided soon after. The surviving neurons returned to apparent normality over the period from 3 to 7 days with a gradual return of polyribosomes. Golgi elements and RER.

Degeneration of neuronal processes in rats induced by a protease inhibitor, leupeptin

Severe degeneration of neuronal processes, including axons and dendrites, as well as accumulation of lipofuscin-like dense bodies have been induced in rats by continuous intraventricular administration by infusion of a protase inhibitor, leupeptin. The aggregation of degenerated processes in neuropils mingled with glial cells and their processes resembled the aggregation of degenerated neurites that are important constituents of the senile plaque of Alzheimer's disease. The present findings provide morphological evidence supporting the hypothesis that protease inhibitors participate in the process of senile plaque formation.

Trimethyltin-induced neuronal damage in the rat brain: comparative studies using silver degeneration stains, immunocytochemistry and immunoassay for neuronotypic and gliotypic proteins

Trimethyltin is a neurotoxicant which produces a distinct pattern of neuronal cell death following peripheral administration of a single dose (8 mg/kg, i.p.) in rats. The cupric-silver degeneration stain was used to produce an atlas documenting the distribution and time course of trimethyltin-induced neuronal damage in adult, male Long-Evans rats. Animals were examined at survival times of 1, 2, 3, 4, 5, 7, 10 and 18 days after intoxication. The earliest degeneration was observed at day 1 in the intermediate and ventral divisions of the lateral septal nucleus, followed by development of degeneration on days 2-4 in neuron populations including the septohippocampal nucleus, septohypothalamic nucleus, anterior olfactory nucleus, bed nucleus of the stria terminalis, endopiriform nucleus, parafascicular nucleus, superior colliculus, interstitial nucleus of the posterior commissure, inferior colliculus, pontine nuclei, raphe nuclei, pars caudalis of the spinal trigeminal nucleus, the caudal aspect of nucleus tractus solitarius, dorsal vagal motor nucleus, granule cells in the dentate gyrus, pyramidal cells in CA fields of the hippocampus, and of neurons in the subiculum, pyriform cortex, entorhinal cortex and neocortex (mainly layer Vb and VI). This was followed by degenerative changes on days 5-7 in other structures, including the amygdaloid nuclei, the ventral posterolateral and ventral posteromedial thalamic nuclei and the periaqueductal gray. The distribution of terminal degeneration from these neurons indicate that specific pools of cells are affected in each structure, and the time course suggests somatofugal degeneration. The trimethyltin damage was also assessed with immunocytochemical visualization of a neuronotypic protein, protein-O-carboxyl methyltransferase and a radioimmunoassay for glial fibrillary acidic protein. Protein-O-carboxyl methyltransferase immunoreactivity was altered in neuronal populations damaged by trimethyltin, but did not appear to be either as sensitive or selective an assay of neuronal damage as the silver stain, especially at short survival times. Glial fibrillary acidic proteins were dramatically elevated 21 days after trimethyltin intoxication, particularly in areas of extensive damage. These studies revealed advantages and problems encountered in the use of each technique in assessing neurotoxic effects, forming a basis for discussion of the relative merits of using a battery of specific molecular probes for neurotoxicity evaluations.

Biological activity of organotin compounds--an overview

As a consequence of the rapid expansion of the uses and applications of the organotin compounds, the concern about their environmental and health effects is increasing. The main subject of this overview is the current understanding of the mammalian toxicity of the organotin compounds. Four different types of target organ toxicity, namely neurotoxicity, hepatoxicity, immunotoxicity, and cutaneous toxicity, are discussed in more detail. The effects of the organotin compounds on the mitochondrial and cellular level are summarized and discussed in relation to the mode of action of these compounds on the central nervous system, the liver and bile duct, the immune system, and the skin.

Effects of trimethyltin on the mouse hippocampus and adrenal cortex

The effects of trimethyltin (TMT) on the mouse adrenal histology and its relationship with neuropathology occurrence was studied. Young, male CD-1 mice were divided into three groups: group I, injected on 3 consecutive days with 1.0 mg TMT/kg body weight (b.w.); group II, injected on 2 consecutive days with 1.5 mg TMT/kg b.w.; and group III, injected with a single acute dose of 3.0 mg TMT/kg b.w. Control animals were injected with saline solution. The brain and adrenal glands were sampled for light-microscopic examination. Although all animals received the same total amount of TMT, pathological changes in the granule cells of the fascia dentate appeared to be group III greater than group II greater than group I, suggesting that acute exposures produced a more severe damage to the fascia dentate neurons. Likewise, the adrenal weights of the animals were group III greater than group II greater than group I greater than or equal to control. Significant proliferation and enlargement of the eosinophilic or the "X zone" were observed in the TMT-treated, particularly groups II and III, animals. The expansion of the eosinophilic cell layer (X zone) was accomplished at the expense of the cortical fasciculata cells. Transformation of fasciculata cells into eosinophilic cells could also be demonstrated. As the eosinophilic cells are known to be active in corticosterone production as seen in stress situations, the proliferation of these cells may reflect a feedback response to the hippocampal hyperexcitation.

Influence of cadmium chloride, mercuric chloride, and sodium vanadate on the glutathione-conjugating enzyme system in liver, kidney, and brain of mice

Sublethal doses of CdCl2 (3 mg/kg iv), HgCl2 (2 mg/kg iv), or NaVO3 (6 mg/kg iv) did not alter the content of reduced glutathione (GSH) in the livers of mice during the 24-h observation period. In the kidneys, a tendency to increased GSH content was seen, especially after HgCl2 treatment; in lung and brain the GSH levels were significantly lowered upon the treatment with all three metals. The activities of GSH S-transferase toward an aryl substrate (CDNB; 1-chloro-2,4-dinitrobenzene) was enhanced in all tissues by the administration of HgCl2 greater than NaVO3 greater than CdCl2. The activity of GSH S-transferase toward an epoxide substrate [1,2-epoxy-3-(p-nitrophenoxy)propane was only measurable in the livers and was inhibited 1 and 2 h after the administration of HgCl2 and NaVO3. It is concluded that sublethal doses of CdCl2, HgCl2, or NaVO3 do not impair the GSH concentration and GSH-conjugating enzyme activities toward the aryl substrate in different target organs of their toxicity, which is in contrast to results obtained in vitro.

Increased free intrasynaptosomal Ca2+ by neurotoxic organometals: distinctive mechanisms

Effects of several alkylmetals on free intrasynaptosomal Ca2+ concentration, [Ca2+]i, were studied in vitro using the fluorescent Ca2+ indicator fura-2. Neurotoxic alkylmetals methylmercury (Met-Hg), triethyllead (TEL), triethyltin (TET), and trimethyltin (TMT) (at 2.5-30 microM) increased [Ca2+]i to different degrees. Met-Hg was the most potent, elevating [Ca2+]i 100-800 nM, dose dependently and significantly more than high K+ (150 nM) or veratridine (350 nM). The effect of Met-Hg could not be inhibited with a Ca2+ channel blocker, verapamil, nor with a Na+ channel blocker, tetrodotoxin. Inhibition of the mitochondrial Ca2+ uptake in situ with rotenone + oligomycin decreased the potency of Met-Hg to elevate [Ca2+]i but did not change the resting [Ca2+]i. Met-Hg also slightly decreased synaptosomal ATP. TEL and TET elevated [Ca2+]i by 100-200 nM. The effect of TEL, but not that of TET, could be blocked with verapamil (36%) and veratridine (67%). TEL was less efficient in the presence of ouabain. Neither TEL nor TET had significant mitochondrial effects in situ contributing to [Ca2+]i. TMT increased [Ca2+]i less than TET while dimethyltin and methyltin were inactive. These results indicate that neurotoxic derivatives of alkylmetals studied increase [Ca2+]i. This occurs mainly either by nonspecific increase (Met-Hg, TET) of Ca2+ leakage through the plasma membrane and/or specific interference with the mechanisms regulating Ca2+ fluxes through the plasma membrane (TEL).

Inhibition of protein synthesis by trimethyltin

The effects of trimethyltin chloride (TMT) on protein synthesis, measured as the incorporation of [3H]valine into trichloroacetic acid-precipitable material, were investigated in mice. One hour after intraperitoneal administration of a 3.0 mg/kg dose, TMT decreased brain protein synthesis by 47% and also caused a significant decrease (4.2 degrees C) in body temperature. When hypothermia was prevented by maintaining the animals at 35 degrees C, TMT decreased protein synthesis by 20%. Twenty-four hours following administration of TMT, protein synthesis was decreased in brain and liver; however, only a reduction of brain protein synthesis was observed at 48 hr. No hypothermia was present at either time point. A regional study in brain showed that at 24 and 48 hr after TMT administration, protein synthesis was decreased by 18-23% in cerebral cortex and hippocampus but not in cerebellum. TMT also inhibited protein synthesis in vitro in mouse brain homogenates with an IC50 of about 100 microM. Neither SnCl2, nor dimethyltin or monomethyltin had any effect on protein synthesis in vitro. These results suggest that, as for other neurotoxicants such as methyl mercury or acrylamide, inhibition of protein synthesis might be involved in TMT neurotoxicity.

Trimethyltin as a selective adrenal chemosympatholytic agent in vivo: effect precedes both clinical and histopathological evidence of toxicity

Trimethyltin (TMT) is a potent neuronotoxiciant but there is little data regarding its systemic effects. In this study, female BALB/c mice were administered either 0.9% saline or 2.75 mg TMT/kg intraperitoneally (i.p.). The animals were then housed in room air or in glass chambers flushed with either 10%, 40%, or 100% oxygen. Mice were sacrificed at 4, 8, 24, and 48 h after treatment and adrenals analyzed for various neurotransmitters by ion-pairing HPLC with electrochemical detection. In addition, adrenal S-adenosylmethionine (SAM) and blood ketone bodies were determined Sections of adrenals were evaluated by electron microscopy for histopathological changes. In vivo treatment with the toxicant resulted in a significant decrease in adrenal epinephrine and norepinephrine levels as early as 8 h following treatment. This effect preceded the appearance of both clinical signs and histopathological changes in the hippocampus by 12-24 h. With exposure to TMT in room air, mouse adrenal content of epinephrine fell from 1861.3 +/- 97.3 ng/4 mg to 1493.3 +/- 137.0 ng/4 mg while norepinephrine levels fell from 779.6 +/- 32.3 ng/4 mg to 503.4 +/- 44.3 ng/4 after 8 h. Supplementation with 40% oxygen did not attenuate this effect but in the case of mice treated with TMT and housed in 100% oxygen for 48 h, actually exacerbated the adrenal epinephrine depletion. Housing in approximately half normal atmospheric oxygen (10%) neither prevented nor enhanced the effects of TMT. The epinephrine/norepinephrine ratios were: control, 2.44; TMT (room air), 1.56; TMT (10% O2), 1.72; TMT (40% O2), 1.44; TMT (100% O2), 1.07. None of the conditions used in this study caused a decrease in adrenal dopamine, 5-hydroxyindole acetic acid (5-HIAA), 5-hydroxytryptamine (5-HT) or in the level of SAM. TMT treatment significantly increased blood ketone bodies indicating additional metabolic dysfunction. The significance of these findings in relation to TMT neuronotoxicity and fatty liver syndrome are discussed.

Developmental toxicology of trimethyltin in the rat

Pregnant rats were treated on either gestational day (GD) 7, 12, or 17 with single doses of trimethyltin chloride (TMT) ip at either 0, 5, 7, or 9 mg/kg. A significant effect of dose was manifest as decreased maternal weight at term, which persisted during lactation until postnatal day (PND) 15 in some groups. For all treatments combined, term weights of dams exposed on GDs 7 and 12 were greater than those treated on GD 17. Litter sizes were decreased for groups treated on GD 17 with 9 mg/kg TMT. Pups treated in utero and exhibiting treatment-induced decreases in weight at or near birth remained smaller than untreated animals into adulthood (PND 280). By PND 20, weights of pups treated on GD 7 greater than GD 12 greater than GD 17. Neuropathology of pups sacrificed on PND 1 was minimal in all animals with lesions only identified in animals treated on GDs 12 or 17 which consisted of subtle degenerative changes in the CA3 and CA4 regions of Ammon's horn of the hippocampus. Muscarinic cholinergic receptor binding in whole brains from pups on PND 1 did not show any significant changes compared to controls for any dose or day of exposure. These data indicate that prenatal TMT exposure results in postnatal toxicity in treated pups but only in the presence of maternal toxicity.

Acute trimethyltin intoxication in the monkey (Macaca fascicularis)

Adult cynomolgus monkeys were administered trimethyltin (TMT) iv in dosages ranging from 0.75 to 4.0 mg TMT/kg and observed for behavioral changes. Animals were subsequently killed for light and electron microscopic examination. TMT showed a dose-related toxicity, with high dose animals (4.0 and 3.0 mg/kg) dying within 24 hr, and low dose animals (0.75 mg/kg) surviving without morphological effects. Animals given 1.10 mg TMT/kg displayed a reproducible clinical course, characterized by tremor, hyperactivity, and ataxia which progressed to stupor and finally unconsciousness. By light microscopy, neuropathology was most pronounced in the CA-3 and CA-4 regions of Ammon's horn. Degenerating pyramidal neurons, micro- and astrogliosis, and neuronophagia were commonly observed. Mild degenerative changes were identified in amygdala, medulla, spinal cord, and Purkinje cells. The fascia dentata remained intact. Ultrastructurally, injured neurons contained accumulations of lysosomes and lysosome-like structures within perikarya and neurites. Demyelination or vascular damage was not observed. Data indicate the monkey to be highly sensitive to TMT, with morphological injury most severe in limbic structures.

Does phenobarbital protect against trimethyltin-induced neuropathology of limbic structures?

Because of the similarity in the pattern of limbic sites damaged by both compounds, it has been suggested that trimethyltin (TMT) may be an excitotoxin like kainic acid (KA). KA produces seizures which eventually result in neuronal damage similar to that found in epilepsy. Anticonvulsants reduce both the seizures and pathology associated with KA. Because TMT may also produce seizures, we undertook to determine whether or not some of the TMT-induced limbic neuropathology could result from seizure activity. To do this, a single dose of TMT chloride (either 7.5 or 15 mg/kg) was given per os to rats, and then phenobarbital (30 mg/kg) was administered subcutaneously in repeated doses. Treatment with phenobarbital did not prevent pathologic changes in the hippocampus, dentate gyrus, and pyriform or prepyriform cortex. Since phenobarbital did not protect against TMT-induced neuronal damage, as it has been reported by others to protect against KA-induced damage, the present findings suggest that these two toxicants probably produce hippocampal pathology via different mechanisms and that the TMT-induced pathologic changes do not require sustained electrical seizure activity.

Evolution of the intracellular changes in neurons caused by trimethyltin

Rats have been given a single dose of trimethyltin (10 mg/kg) and the intracellular events have been followed particularly in hippocampus, cerebral cortex, cerebellum and spinal ganglion cells. The earliest change visible occurs 12 h after this dose and is found to be dense membrane-bound bodies, probably derived from branching tubulo-vesicular smooth endoplasmic reticulum formations. These occur in close connection with rought endoplasmic reticulum and polyribosomes and appear also to have some association with the Golgi complex. At 24 h there is a general vacuolation of Golgi cisterns and SER membranes, and the membrane-bound dense body formation is greatly increased. SER abnormalities are particularly conspicuous in Purkinje cells. In spinal ganglion cells, while vacuolation of Golgi cisterns is intense, dense bodies are inconspicuous and are replaced by increased autophagosomes, often of great complexity. By 48 h vacuolation of Golgi cisterns has waned, but accumulation of dense bodies and secondary lysosomes has steadily increased. In spinal ganglion cells autophagosomes only are increased as the Golgi vacuolation declines. At later times steady increases of lysosomal dense bodies is seen generally accompanied in hippocampal pyramidal cells and dentate fascia cells by abundant cell death. The suggestion is put forward that the Golgi complex may be the seat of the critical metabolic lesion and disturbances to protein transfer and protein synthesis follow. No explanation for the selective loss of hippocampal h1-5 (CA1-CA4 except Sommer's sector) pyramidal cells and of small dentate fascia neurons can be derived from these conclusions.

Neuropathology of trimethyltin intoxication. IV. Changes in the spinal cord

Young C57BL/6N mice were injected (ip) with trimethyltin chloride at a dosage of 3.0 mg/kg body wt. Animals were sacrificed between 48 to 72 hr postinjection by means of intracardial perfusion of saline solution followed by 2.5% buffered glutaraldehyde. For light microscopy, the cords were further fixed in 10% buffered formalin and embedded in Paraplast. For electron microscopy, tissue samples were obtained from the cord levels at L1-L4, further fixed in glutaraldehyde and osmium tetroxide, and embedded in Epon. Chromatolytic and vacuolar changes involving neurons mainly in the medial and lateral motor nuclei of the anterior horns were observed. Electron microscopy revealed lysosomal accumulation and extensive dilatation of the cytoplasmic membrane systems (endoplasmic reticulum, Golgi complex). Large intraneuronal vacuoles were formed as a result of extensive intraneuronal edema. Progressive distention of the cytoplasmic membranes resulted in severe vacuolation, disintegration, and total breakdown of the neurons.

Behavioral effects of trimethyltin in two strains of mice. II. Multiple fixed ratio, fixed interval

Adult male C57BL/6N and BALB/c mice were trained to respond under a multiple fixed-ratio 30, fixed-interval 600-sec schedule of milk presentation. After performance had stabilized, each mouse received a single dose of trimethyltin X Cl (TMT) by ip administration. Three hours after administration, behavioral testing was started and continued at 24-hr intervals thereafter. The behavior of mice receiving 0.3 mg/kg was unaffected at 3, 27, and 51 hr after administration. Likewise, no effects were seen at 3 and 27 hr after 1 mg/kg. However, at 51 hr after administration there was a significant change in the temporal patterning of fixed-interval responding in the C57BL/6N strain, as shown by a decrease in the fixed-interval quarter-life. There was no significant change in the BALB/c strain at this dose. At 3 mg/kg the responding of both strains was greatly affected, and responding remained disrupted for at least 6 to 7 weeks. Neuropathological examination of the brains of both strains of mice showed no significant lesions (light microscopy) at 0.3 and 1 mg/kg. At 3 mg/kg, severe neuronal necrosis was observed in the fascia dentata region in both strains. Thus, in mice receiving TMT, the behavioral deficits were closely paralleled by the presence or absence of significant neuropathology.

Behavioral effects of trimethyltin in two strains of mice. I. Spontaneous motor activity

Adult male mice (C57BL/6N and BALB/c) were administered single doses of trimethyltin X Cl (TMT) by the ip route. The effects of TMT administration were determined on lethality (3-6 mg/kg), spontaneous motor activity (SMA), and the physical appearance of the mice (0.3-3 mg/kg). The effects of TMT on lethality were strain dependent in that a single dose of 3 mg/kg, ip, produced approximately 35% lethality in the C57BL/6N strain during the first 72 hr following administration. Less than 15% lethality was observed at this dose in the BALB/c strain. In both strains, 3.5 mg/kg, ip, produced more than 70% lethality during the first 144 hr after administration. Higher doses produced proportionally greater lethality. The SMA of both strains was not affected significantly at doses below 1 mg/kg, ip. At 1 mg/kg a small decrease in activity was observed during the first 24 hr. At 3 mg/kg, SMA was initially decreased in both strains. However, the decrease was of smaller magnitude in the C57BL strain and was followed by a large increase in SMA which did not return to control levels for approximately 1 week. An increase in SMA was observed in the BALB/c strain on the fifth day following TMT but returned to control values by Day 6. At 3 mg/kg, ip, the C57BL mice were observed to have severe whole body tremors and were hypersensitive to external stimuli. The whole body tremor was not as marked in the BALB/c strain. Neuropathological studies on the treated mice indicated that the behavioral studies paralleled the pathology produced by TMT. These data confirm the initial observation of greater sensitivity of the mouse to toxic effects of TMT compared to the rat.

The neurotoxicity of trimethyltin chloride in hamsters, gerbils and marmosets

Trimethyltin chloride (TMT) was given to Syrian hamsters, gerbils and marmosets, and the changes in the brain were studied 1 day to 7 weeks later by light and electron microscopy. Within the marmoset brain, TMT was found to be uniformly distributed, similar to that in the rat. In all three species, signs of poisoning included whole-body tremors and prostration, while death might occur in 3-4 days; in marmosets ataxia, agitation, aggression and occasional fits were also observed. Bilateral symmetrical neuronal necrosis and chromatolysis were seen in the majority, which involved the hippocampus, pyriform cortex, amygdaloid nucleus, neocortex, various brain stem nuclei and in marmosets the retina. The probably lethal dose of TMT in all three species is approximately 3 mg kg-1, while the LD50 for the rat is 12.6 mg kg-1. The lower figure is probably related to lack of binding to haemoglobin in contrast to the binding in the rat. TMT does not bind to human haemoglobin and thus the predicted lethal dose for humans may be about 3 mg kg-1 (15.1 mumol kg-1), while the dose required to produce neuronal damage could well be less.

Neuropathology of trimethyltin intoxication. III. Changes in the brain stem neurons

Young adult BALB/c mice were administered trimethyltin chloride (TMT) at a dosage of 3.0 mg TMT/kg body wt. Animals displayed severe toxic signs (tremor and aggression) within 24 hr and were sacrificed at 48 and 72 hr postinjection. The brain stems of these animals were examined with light and electron microscopy. Degenerative and vacuolar changes were observed in many large brain stem neurons, especially those in the mesencephalic trigeminal nuclei. These neurons acquired a chromatolytic character with eccentric nuclei, loss of Nissl substance, and hyalinoid cytoplasm. Extensive vacuolation was also found in these nerve cells. Electron microscopy examination revealed progressive loss of the Nissl substance (rough endoplasmic reticulum) and distention of the cytoplasmic membranes (endoplasmic reticulum and Golgi complex). Severe distention of these membranes resulted in large membrane-limited vacuoles within these nerve cells. This intraneuronal vacuolation reflects an intracellular edema condition of these nerve cells and is potentially reversible. Mitochondrial damage in these neurons was only moderate. Further investigation is needed to elucidate the full toxic impact and pathogenetic mechanisms of TMT in the nervous system.