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

Mechanisms of injury in the central nervous system

Neurotoxicants with similar structural features or common mechanisms of chemical action frequently produce widely divergent neuropathologic outcomes. Methylmercury (MeHg) produces marked cerebellar dysmorphogenesis during critical periods of development. The pathologic picture is characterized by complete architectural disruption of neuronal elements within the cerebellum. MeHg binds strongly to protein and soluble sulphydryl groups. Binding to microtubular -SH groups results in catastrophic depolymerization of immature tyrosinated microtubules. However, more mature acetylated microtubules are resistant to MeHg-induced depolymerization. In contrast to MeHg, the structurally similar organotin trimethyltin (TMT) elicits specific apoptotic destruction of pyramidal neurons in the CA3 region of the hippocampus and in other limbic structures. Expression of the phylogenetically conserved protein stannin is required for development of TMT-induced lesions. Inhibition of expression using antisense oligonucleotides against stannin protects neurons from the effects of TMT, suggesting that this protein is required for expression of neurotoxicity. However, expression of stannin alone is insufficient for induction of apoptotic pathways in neuronal populations. The aromatic nitrocompound 1,3-dinitrobenzene (DNB) has 2 independent nitro groups that can redox cycle in the presence of molecular oxygen. Despite its ability to deplete neural glutathione stores, DNB produces edematous gliovascular lesions in the brain stem of rats. Glial cells are susceptible despite high concentrations of reduced glutathione compared with neuronal somata in the central nervous system (CNS). The severity of lesions produced by DNB is modulated by the activity of neurons in the affected pathways. The inherent discrepancy between susceptibility of neuronal and glial cell populations is likely mediated by differential control of the mitochondrial permeability transition in astrocytes and neurons. Lessons learned in the mechanistic investigation of neurotoxicants suggest caution in the evaluation and interpretation of structure-activity relationships, eg, TMT, MeHg, and DNB all induce oxidative stress, whereas TMT and triethyltin produce neuronal damage and myelin edema, respectively. The precise CNS molecular targets of cell-specific lipophilic neurotoxicants remain to be determined.

Odor-induced fast waves in the dentate gyrus depend on a pathway through posterior cerebral cortex: effects of limbic lesions and trimethyltin

Previous research has shown that the odor of a variety of organic solvents and of components of the anal scent gland excretions of various predators will elicit a burst of fast waves of about 20 Hz in the olfactory bulb, pyriform cortex, and dentate gyrus of the rat. The present experiments show that large lesions of the caudal cerebral cortex, involving particularly the entorhinal and subicular cortices and the angular bundle, abolish the olfactory fast wave response of the dentate gyrus, but not the similar response of the olfactory bulb. In confirmation of previous work, such a lesion also abolishes an average evoked response elicited in the dentate gyrus by electrical stimulation of the olfactory bulb. Systemic treatment with the neurotoxin trimethyltin abolished the olfactory fast wave response in the olfactory bulb and both the olfactory fast wave and the olfactory evoked potential in the dentate gyrus. Large lesions of the amygdala or the septal nuclei did not eliminate either the dentate olfactory evoked potential or the odor-induced dentate fast wave response. However, the septal lesion reduced the amplitude of both spontaneous and odor-induced dentate fast wave activity. It is suggested that olfactory stimuli elicit 20 Hz dentate fast waves via a pathway from the olfactory bulb through the entorhinal cortex and, further, that cholinergic interneurons in the dentate gyrus may be essential to the dentate fast wave response.

Calcitonin gene-related peptide immunoreactivity in the hippocampus and its relationship to cellular changes following exposure to trimethyltin

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is regionally regulated following peripheral insult and in central nervous system (CNS) damage models targeting limbic structures. Functional studies have shown this neuropeptide to be involved in neuronal protection and remodeling, vasodilation, immunomodulation, and apoptosis, thus making it an important constituent of the acute phase response. In the present study, we characterized the anatomic expression and distribution of CGRP immunoreactivity (CGRP-IR) after exposure to the toxin, trimethyltin (TMT). We chose this model because TMT causes dramatic changes in the endocrine system, the limbic system, particularly the hippocampus, as well as in the immune response. We have specifically focused on comparing the changes in CGRP-IR with the pattern of apoptosis (via TUNEL staining), cell-cycle activation (Ki67-IR), and in alteration in microglia (OX-42-IR) and astrocyte (gGFAP-IR) immunocytochemistry in TMT-treated hippocampus. Our results show a marked change in CGRP-IR in regions of the hippocampus that are temporally and anatomically correlated with the induction of apoptosis and activation of microglia, astrocyte, and the cell-cycle marker. Given the known effects of CGRP on these cell types and on programmed cell death elsewhere, these findings are consistent with a regional immunoregulatory/injury response role for CGRP following organotin poisoning.

Trimethyltin-induced expression of neuropeptide Y Y2 receptors in rat dentate gyrus

Trimethyltin (TMT) causes prominent neuronal damage and enhanced expression of neuropeptide Y in the hippocampus. We investigated expression of neuropeptide Y Y2 receptors after TMT intoxication. Markedly elevated (by 470%) concentrations of Y2 receptor mRNA were found in the suprapyramidal blade of the dentate granule cell layer after 5 days. Increases in the infrapyramidal blade were less prominent (by 198%). After 16 days, mRNA levels in both blades of the granule cell layer showed no significant difference from those in controls. Quantification of Y2 receptor-specific binding revealed no significant change at both 5 and 16 days after TMT intoxication. It is suggested, together with a previous report describing a similar increase of neuropeptide Y expression, that a transient expression of Y2 receptors in the dentate gyrus in the initial phase of TMT intoxication may be involved in mediating TMT-induced hippocampal damage.

Systemic administration of atipamezole, a selective antagonist of alpha-2 adrenoceptors, facilitates behavioural activity but does not influence short-term or long-term memory in trimethyltin-intoxicated and control rats

The present study used trimethyltin (TMT)-intoxicated rats as a model for the behavioural syndrome seen after neuronal damage to the limbic system. Behavioural assessments indicated increased locomotor activity and reduced number of groomings in an open-arena task in TMT-intoxicated (6.6 mg/kg as a free base) rats, as has been found previously. A novel finding was the severe deficit in swimming to a visible platform in the water maze task, with reduced swimming speed at the beginning of the training period. During the reacquisition phase of a radial arm maze task, TMT-intoxicated rats made more short-term and long-term memory errors, and their behavioural activity was increased in comparison with controls. The administration of atipamezole (300 micrograms/kg), a selective antagonist of alpha 2-adrenoceptors, enhanced locomotor activity compared to saline-treated rats, but these effects did not differ between the TMT group and their controls. Atipamezole did not enhance short-term or long-term memory in either TMT or control groups. Taken together, the present data indicate that TMT intoxication is a model for global dementia rather than for a specific loss of relational memory. Previous studies on the neurochemical effects of TMT and the alleviation or prevention of neurotoxicity of TMT are reviewed.

Trimethyltin intoxication induces marked changes in neuropeptide expression in the rat hippocampus

In situ hybridization and immunocytochemistry were applied to investigate changes in the expression of somatostatin, neuropeptide Y, neurokinin B, cholecystokinin, dynorphin, and Met-enkephalin in the rat hippocampus after administration of a single peroral dose of trimethyltin hydroxide (9 mg/kg). Two time intervals were investigated: 5 days after trimethyltin treatment, when CA3 damage becomes manifest and is associated with increased aggression, seizure susceptibility, and memory deficit, and 16 days after trimethyltin, when neuronal damage is almost maximal and seizure susceptibility is declining. Robust but transient increases of neuropeptide Y, neurokinin B, and Met-enkephalin mRNA levels were revealed in the granule cell layer of the dentate gyrus and increased neuropeptide Y and neurokinin B immunoreactivities were found in mossy fibers. In reverse, dynorphin mRNA and immunoreactivity were decreased transiently in the dentate gyrus and mossy fibers, respectively. Strong over-expression of NPY mRNA was also observed in hilar interneurons and in CA1 and CA3 pyramidal cells as well as in the cortex at 5 days postdosing. Cholecystokinin- or neurokinin B-containing basket cells were preserved, while somatostatin-bearing interneurons were damaged by trimethyltin exposure. These neurochemical changes induced by trimethyltin intoxication strikingly parallel to those observed in animal models of temporal lobe epilepsy and may reflect activation of endogenous protective mechanisms. It is also suggested that hilar interneurons respond differently to trimethyltin exposure, for which neuropeptides are valuable markers.

Spatiotemporal changes in hippocampal NMDA receptor binding as a consequence of trimethyltin neurotoxicity in the rat

In the present study we examined the presumable changes in the distribution of N-methyl-D-aspartate (NMDA) receptors in the hippocampus of rat exposed to a potent neurotoxic drug, trimethyltin (TMT). Using in vitro receptor binding autoradiography, [3H]MK801 labelling was determined at 7, 14, 21, 30 and 60 days after treatment with TMT (single dose of 8 mg/kg, i.p.) in various hippocampal areas thought to be affected by the neurotoxin. At 21-60 days after exposure, a decrease in receptor binding was observed in CA1 hippocampal subfield (10-20%, P< 0.05). A reduction in binding density also occurred in CA4/ CA3c, where labelling vanished completely at longer times. In the molecular layer (ML) of the dentate gyrus (DG), however, 16-37% (P<0.05) increase in receptor binding was found at 14-60 days postexposure. These results suggest that exposure to TMT leads to an altered topography of NMDA receptor density sites in the rat hippocampus. Dynamics of the reduction in receptor binding in CA4/CA3c and CA1 followed the development of the well-known degenerative effects induced by the neurotoxin. In contrast, the enhanced binding density in the ML of the DG may be a part of a mechanism of plastic response of granule cells to denervation/reinnervation.

Immunohistochemical localization and quantification of glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25000) in the ageing hippocampus following administration of 5,7-dihydroxytryptamine

Responses to injury in the ageing hippocampus were assessed utilizing the synaptic markers glial fibrillary acidic protein and synaptosomal-associated protein (mol. wt 25,000) following administration of the neurotoxin, 5,7-dihydroxytryptamine, into the fimbria-fornix and cingulum bundle to denervate serotonergic afferent input to the dorsal hippocampus. Age-dependent alterations in hippocampal immunohistochemical localization of glial fibrillary acidic protein and synaptosomal-associated protein were evaluated in female Fischer 344 rats following serotonergic deafferentation with 5,7-dihydroxytryptamine. Across the lifespan, as indicated by measurements taken at three, 18, 21 and 29 months, marked increases in glial fibrillary acidic protein, but not synaptosomal-associated protein immunoreactivity, occurred throughout the hippocampus at 21 and 29 months compared to three and 18 months. Following three weeks pretreatment with 5,7-dihydroxytryptamine (20 microg total dose) or vehicle (0.1% ascorbic saline; 2 microl total volume) infused in the fimbria-fornix/cingulum bundle, immunohistochemical analysis demonstrated marked increases of glial fibrillary acidic protein, but not synaptosomal-associated protein, in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and 3-month 5,7-dihydroxytryptamine groups. Additionally, a significant increase in glial fibrillary acidic protein concentration was found by enzyme-linked immunosorbent assay in the 18-month 5,7-dihydroxytryptamine group compared to the 18-month vehicle and three-month 5,7-dihydroxytryptamine groups. These results demonstrate that selective neurotoxicant damage of the hippocampal serotonergic system differentially alters the expression of glial fibrillary acidic protein. This approach may provide a valuable tool to determine the ability of the hippocampus to respond to age-related neurodegenerative injury.

GM1 ganglioside potentiates trimethyltin-induced expression of interleukin-1 beta and the nerve growth factor in reactive astrocytes in the rat hippocampus: an immunocytochemical study

This study demonstrates potentiation by GM1 ganglioside treatment of trimethyltin (TMT) induced reactivity of astrocytes, and the expression of astroglial interleukin-1 beta (IL-1 beta) and nerve growth factor (NGF) immunoreactivities in the rat hippocampus. GM1 treatment also results in an increase of the number of IL-1 beta and NGF immunoreactive astrocytes. Both the intensity of gliosis and stimulation of IL-1 beta and NGF expression in astrocytes mostly occurs in the regions of heaviest neurodegeneration in the hippocampus (CA4/CA3c and CA1). It is tempting to assume that enhancement of astroglial NGF expression by GM1 ganglioside may play a role in the protective action of GM1 against neurotoxic insult.

Trimethyltin (TMT) neurotoxicity in organotypic rat hippocampal slice cultures

The neurotoxic effects of trimethyltin (TMT) on the hippocampus have been extensively studied in vivo. In this study, we examined whether the toxicity of TMT to hippocampal neurons could be reproduced in organotypic brain slice cultures in order to test the potential of this model for neurotoxicological studies, including further studies of neurotoxic mechanisms of TMT. Four-week-old cultures, derived from 7-day-old donor rats and grown in serum-free medium, were exposed to TMT (0.5-100 microM) for 24 h followed by 24 h in normal medium. TMT-induced neurodegeneration was then monitored by (a) propidium iodide (PI) uptake, (b) lactate dehydrogenase (LDH) efflux into the culture medium, (c) cellular cobalt uptake as an index of calcium influx, (d) ordinary Nissl cell staining, and (e) immunohistochemical staining for microtubule-associated protein 2 (MAP-2). Cellular degeneration as assessed by densitometric measurements of PI uptake displayed a dose and time-dependent increase, with the following ranking of vulnerability of the hippocampal subfields: FD>CA4>/=CA3c>CA1>CA3ab. This differential neuronal vulnerability observed by PI uptake was confirmed by MAP-2 immunostaining and corresponded to in vivo cell stain observations of rats acutely exposed to TMT. The mean PI uptake of the cultures and the LDH efflux into the medium were highly correlated. The combined results obtained by the different markers indicate that the hippocampal slice culture method is a feasible model for further studies of TMT neurotoxicity.

Trimethyltin syndrome as a hippocampal degeneration model: temporal changes and neurochemical features of seizure susceptibility and learning impairment

The effects of trimethyltin on the hippocampus were investigated in terms of changes in histology, depth electroencephalography, learning acquisition and memory retention, choline acetyltransferase and neuropeptides, and seizure-induced c-fos messenger RNA expression. The results were as follows. (1) Morphologically, trimethyltin produced a progressive loss of hippocampal CA3 and CA4 pyramidal cells, starting from four days after peroral treatment with trimethyltin hydroxide (9 mg/kg), as described previously. (2) Neurophysiologically, the increased seizure susceptibility to pentylenetetrazol treatment reached a maximum at four days post-trimethyltin and then declined after five days post-trimethyltin. The maximal seizure susceptibility at four days post-trimethyltin was confirmed by the immediate and long-lasting appearance of spike discharge in the hippocampus. However, this was not verified by the expression of c-fos messenger RNA in the hippocampus, which was comparable between trimethyltin-treated and control rats. (3) Behaviorally, the time-courses of aggression and learning impairment were similar to that of the seizure susceptibility. (4) Neurochemically, trimethyltin treatment caused changes of neurochemical markers, which were manifested by the elevation of neuropeptide Y content in the entorhinal cortex, and of choline acetyltransferase in the hippocampal CA3 subfield. Trimethyltin may offer potential as a tool for investigations on the relationship between neuronal death in the hippocampus and the development of seizure susceptibility and learning impairment. Alterations in glucocorticoids, glutamate and neuropeptides may all contribute to the manifestation of the trimethyltin syndrome.

Actin modifications and calcium homoeostasis in neurotoxicity. The case of organotin salts

The cytoskeleton is a major constituent of the neuronal cytoplasm; it controls cell shape and plays important roles in regulating various physiological processes. In neurons, actin filaments are involved in the growth of the neurite and the neurotransmitter release. Recent findings suggest that actin filaments play a role in modulating [Ca(2+)](i) responses to neurotoxic insults. The physiological functioning of the neural cell is critically dependent on the intracellular distribution of calcium. An increase of cytosolic free calcium can activate a number of intracellular reactions, including neurotransmitter release, protein phosphorylation, protease activity, and, eventually, cell death. Many neurotoxic agents with diverse mechanisms have been reported to affect mechanisms associated with calcium. Among these are organotin compounds: they can both raise the cytosolic and synaptosomal [Ca(2+)](i) concentrations and interfere with the [Ca(2+)](i) response evoked by different agonists. Furthermore, some of these compounds cause actin depolymerization. The interference of triethyltin (TET)-a compound inducing myelin vacuolization and brain oedema-with Ca(2+) homoeostasis and actin polymerization results in an adverse effect on neurotransmitter release in different neural cell lines. However, another neurotoxic organotin compound (trimethyltin, TMT) induces apoptosis in neural cells through the activation of a Ca(2+)-dependent pathway. In conclusion, the identification of the key changes in actin and Ca(2+) homoeostasis could give early information on neural cell perturbation resulting in altered functionality or even cell death.

The reaction of astrocytes and neurons in the hippocampus of adult rats during chronic ethanol treatment and correlations to behavioral impairments

Chronic ethanol treatment of Wistar rats to 10% (v/v) ethanol over a period of 4, 12, and 36 weeks produced distinct alterations of the glial fibrillary acidic protein immunoreactivity (GFAP-IR) of dorsal hippocampal astrocytes. Ethanol consumption over a period of 4 weeks caused an increase in the total GFAP-IR of the astrocytes. Down-regulation of the total GFAP-IR was measured in all examined brain regions after 36 weeks of ethanol treatment. Prolonged ethanol treatment induced a significant loss of the total number of hippocampal pyramidal and dentate gyrus granule cells. Regional differences in the vulnerability to the neurotoxic effects of chronic ethanol intake over 36 weeks were found: CA3 > CA1 + CA2 > > CA4 > GD. In agreement with the degree of neuronal cell loss, ethanol-induced behavioral impairments were found. The acquisition of maze performance using a complex elevated labyrinth was deteriorated after 36 weeks of ethanol treatment, suggesting a deficit in learning and memory. These findings illustrate the importance of time-response analysis when determining the structural and functional changes produced by chronic ethanol treatment.

S-100 proteins in trimethyltin-induced neurodegeneration in the rat hippocampus. An immunochemical and immunocytochemical study

After acute trimethyltin (TMT) intoxication (21 d after a single i.p. injection at a dose of 8 mg/kg) the histological, immunohistochemical, and immunochemical investigation of adult rat hippocampus showed a distinct pattern of neuronal loss, and an increase in both glial fibrillary acidic protein- (GFAP) immunoreactive cells and GFAP concentration, as expected. S-100-immunoreactive cells also increased markedly, whereas the concentration of S-100 increased even more than that of GFAP. The data show that S-100 is an index of glial reaction to damage after TMT intoxication and suggest the potential usefulness of exploring the possibility that it may play a role in induced neurodegenerative processes.

Calretinin-containing neurons in trimethyltin-induced neurodegeneration in the rat hippocampus: an immunocytochemical study

The present study uses immunocytochemistry to investigate the behavior of the calretinin (CR)-containing neuronal subpopulation (interneurons) of the rat hippocampus in neurodegenerative processes induced by the neurotoxicant trimethyltin. Cell counts of CR-immunolabeled interneurons indicated that these cells are spared by the neurotoxicant-induced degeneration, characterized by a generalized neuronal loss, as shown by quantitative analysis after cresyl violet staining.

Trimethyltin induces gelatinase B and urokinase in rat brain

The neurotoxicant trimethyltin (TMT) increases mRNA levels for cytokines, tumor necrosis factor-alpha, interleukin-1alpha, and interleukin-6. Cytokines induce matrix metalloproteinases (MMPs) and urokinase-type plasminogen activator (uPA). MMPs and uPA disrupt extracellular matrix. Since matrix damage may play a role in the neuropathological changes seen with TMT toxicity, we determined the effect of TMT on proteolytic enzyme production. Adult rats were injected with 8.0 mg TMT/kg. At different times after TMT injection, tissue samples from frontal lobe and hippocampus were assayed for MMPs and uPA, using gelatin-substrate and casein/plasminogen-substrate zymography. Gelatinase B (92 kDa type IV collagenase) production increased significantly in frontal lobe tissue at 24, 48 and 96 h, and in hippocampus at 48 h compared to saline-injected controls. Gelatinase A (72 kDa type IV collagenase) was significantly decreased at 12 and 24 h in frontal lobe compared to controls. Urokinase-type PA was significantly increased in hippocampus at 12 and 96 h, and in frontal lobe at 96 h compared to controls. Gelatinase B and uPA are up-regulated by TMT in frontal lobe and hippocampus, suggesting that they may contribute to the neuropathology of TMT.

Hippocampal degeneration inducing impairment of learning in rats: model of dementia?

In the pharmacological field, the development of drugs effective for dementia is now widely anticipated because of the increase in the elderly population. Dementia has some histological degeneration in the brain, including the hippocampus. Preclinical evaluations of such drugs use animal models with memory impairment, since memory impairment is a major criterion of dementia. We therefore investigated two animal models with hippocampal degeneration. Neonatal administration of monosodium glutamate (MSG) induced specific degeneration of hippocampal pyramidal cells in the CA1 region of Wistar rats in adulthood. In these animals, the correct response rate during the acquisition period of light-dark discrimination learning was significantly lower than that in the control group. No significant changes were noted in the hippocampal concentrations of neurotransmitter substances, including acetylcholine and glutamate. In the second model, similar histological changes were observed at 3 weeks after oral administration of trimethyltin (TMT). These histological changes were accompanied by a reduction in the intrahippocampal concentrations of acetylcholine and glutamate. In the case of light-dark discrimination learning, neither pre- nor post-training administration of TMT affected the correct response rate during both the acquisition and retention test periods. In the case of 8-arm radial maze learning, the increase in correct response rate was significantly suppressed in comparison with that of the control group when TMT was administered at 4 weeks before starting the acquisition trial. This suppression was followed by a lower response rate in the retention test. On the other hand, the correct response rates in retention tests were not affected when TMT was administered after completion of the acquisition trial. These findings indicate that sole degeneration of the hippocampus was able to induce different types of memory impairment, and single evaluation of a drug with one learning paradigm was difficult to justify that a drug is effective for dementia.

Alterations in hippocampal expression of SNAP-25, GAP-43, stannin and glial fibrillary acidic protein following mechanical and trimethyltin-induced injury in the rat

A set of well-defined antisera against neuronal and glial proteins were used to characterize patterns of protein expression in rat hippocampus following transection of the fimbira-fornix and perforant pathways or after administration of the selective neurotoxicant trimethyltin (8 mg/kg, i.p.). SNAP-25 (synaptosomal protein, mol. wt 25,000) is a neuron-specific, developmentally regulated presynaptic protein, stannin is a protein enriched in cells sensitive to trimethyltin, and GAP-43 (growth-associated protein, mol. wt 43,000) is associated with axonal growth and regeneration. Glial fibrillary acidic protein is an astrocyte-specific intermediate filament protein and a marker for reactive gliosis. SNAP-25 immunoreactivity was altered following both neurotoxicant and mechanical injury. Three days after fimbria-fornix/perforant path lesions, there was a loss of SNAP-25 immunoreactivity in hippocampal efferent pathways and in the lesioned entorhinal cortex. By day 12, there was evidence of reinnervation of hippocampal subfields by SNAP-25-immunopositive commissural afferent fibers. On day 3, immunoblots showed the appearance of SNAP-25a, a developmental isoform produced by alternative splicing of nine amino acids in exon 5, in lesioned tissues. This isoform declined by day 12 and was not found in contralateral control hippocampus or non-lesioned brain regions. Stannin immunoreactivity was unchanged, while GAP-43 was prominent on day 12 post-lesion. Glial fibrillary acidic protein immunoreactivity indicated gliosis near the site of pathway transection. In contrast, trimethyltin induced a marked loss of stannin immunoreactivity in hippocampal neurons seven days after injection. Trimethyltin increased glial fibrillary acidic protein staining in the hippocampus and other damaged regions. SNAP-25 immunoreactivity was markedly increased in mossy fibers and other hippocampal fields seven days following trimethyltin. Immunoblot analysis showed that only the adult SNAP-25b isoform was expressed after trimethyltin intoxication. These data suggest that SNAP-25 is a useful marker for presynaptic damage. Furthermore, reexpression of developmental isoforms of SNAP-25a may precede functional reinnervation when the postsynaptic target remains intact.

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Glial fibrillary acidic protein and RNA expression in adult rat hippocampus following low-level lead exposure during development

The astroglial cytoskeletal element, glial fibrillary acidic protein (GFAP), is a generally accepted sensitive indicator for neurotoxic effects in the mature brain. We used GFAP as a marker for structural changes in rat hippocampus related to chronic low level lead exposure during different developmental periods. Four groups of rats were investigated: a control group, a perinatal group, which was exposed during brain development (E0-P16), a permanent group, exposed during and after brain development (E0-P100), and a postweaning group, exposed after brain development (P16-P100). Sections were processed for light microscopy (hematoxylin-eosin, Nissl, periodic acid Schiff (PAS) and GFAP-specific immunohistology), for electron microscopy, and for in-situ hybridization (GFAP). Sections were prepared from animals tested for active avoidance learning (AAL) and long-term potentiation (LTP). Chronic lead exposure did not affect glial and neuronal functions, as assessed by LTP and AAL, when lead exposure started after brain development (postweaning group). In this group, astrocytes displayed increased GFAP and GFAP gene transcript levels. However, lead exposure affected neuronal and glial function when the intoxication fell into the developmental period of the brain (perinatal and permanent groups). In these groups, LTP and AAL were impaired, and astrocytes failed to react to the toxic exposure with an adequate increase of GFAP and GFAP gene transcripts. Although GFAP is an accepted marker for neurotoxicity, our data suggest the marker function of GFAP to be restricted to postnatal toxic insult.

Repeated acquisition and the assessment of centrally acting compounds

Repeated acquisition (RA) procedures are behavioral preparations in which subjects learn new sequences of responses during each experimental session. They have been used with great success to assess the effects of drugs and other compounds on learning processes. As learning can be measured over many sessions in individual subjects, RA procedures can prove invaluable when conducting studies of the effects of chronic drug administration, aging and the long-term effects of exposure to toxic compounds. Analyzing the patterns of responding during acquisition can provide insights into the behavioral mechanisms underlying the effects of drugs and other centrally acting compounds on learning. Systematic comparisons are needed on the influence of many procedural variables on RA and the extent to which they may modulate the effects of chemicals.

Integrated evaluation of central nervous system lesions: stains for neurons, astrocytes, and microglia reveal the spatial and temporal features of MK-801-induced neuronal necrosis in the rat cerebral cortex

Routinely processed, hematoxylin and eosin (H&E)-stained slides are typically used to assess the morphologic integrity of the central nervous system in neurotoxicity safety studies. However, the value of special stains for improving neuropathologic evaluations during the assessment of neurotoxicity has been emphasized in the neuroscience literature and by regulatory agencies. The primary objective of the present study was to characterize the spatial and temporal changes in neurons, astrocytes, and microglia after dizocilpine maleate (MK-801)-induced focal neuronal necrosis in the posterior cingulate/retrosplenial (PC/RS) cortex of the rat. A secondary objective was to evaluate the application of special stains and a novel sectioning procedure for detecting neurotoxicity. Sixty adult male Sprague-Dawley rats were treated with sterile water vehicle or 10 mg/kg MK-801 and perfused through the left ventricle (pumped at 65 mm Hg pressure) with 10% neutral buffered formalin or 4% paraformaldehyde at 4 hr and on days 1, 3, 7, 14, and 28 after treatment. For light microscopic evaluation, brain sections were stained with H&E, a special cupric-silver (CS) stain that selectively impregnates degenerating neurons and makes them readily evident, glial fibrillary acidic protein (GFAP) immunohistochemistry for astrocytes, and Griffonia simplicifolia isolectin B4(GSA) histochemistry for microglia. Brains perfusion-fixed with 4% paraformaldehyde were prepared for CS staining with a novel frozen-sectioning procedure for multiple embedding in a composite gelatin block. In H&E sections from treated rats, necrotic nerve cell bodies were observed in PC/RS cortical layers 3 and 4 on days 1, 3, 7, and 14, but not on day 28. These necrotic neurons required high magnification for detection (x20 objective, x10 ocular). In contrast, degenerating neurons selectively stained with CS were observed in the same location as necrotic neurons seen with H&E but at low magnification (x2 objective, x10 ocular). Cupric-silver staining showed details not seen with H&E, including dendritic and axonal degeneration with progressive fragmentation. Beginning on day 3, GFAP immunohistochemistry revealed hypertrophic astrocytes in a diffuse pattern throughout the region of cell body necrosis, a change that persisted throughout the study. However, GSA lectin histochemistry identified a few reactive microglia on day 1 in a multifocal pattern throughout the region of cell body necrosis. Reactive microglia were observed on days 3, 7, and 14, but not on day 28. Glial changes observed with H&E staining were limited to an increase in the cellularity of glial cell nuclei in the area of neuronal necrosis. This study provides a comprehensive and integrated view of the temporal changes occurring in neurons, astrocytes, and microglia during acute neurotoxic injury. Moreover, advantages for using new staining and sectioning methodologies to enhance the toxicologic evaluation of the central nervous system are demonstrated.

Differential activation of microglia and astrocytes following trimethyl tin-induced neurodegeneration

We have investigated the response of astrocytes and microglia to trimethyl tin intoxication in the septum, hippocampus, olfactory bulb, and pyriform cortex of the rat. Microglia were studied qualitatively using lectin histochemistry, and astrocytes were examined both qualitatively with immunohistochemistry, and quantitatively using an immunoassay for glial fibrillary acidic protein. Our results show that activated microglia first appeared 2 days after trimethyl tin intoxication in the lateral septum and hippocampus. Four days after trimethyl tin intoxication, the same regions revealed a most intense microglial reaction characterized by microglial hypertrophy and the formation of phagocytic clusters. By day 7, microglial activation in the septum and hippocampus had lessened, suggesting that the cells were reverting to the resting phenotype. The microglial response in the pyriform cortex and olfactory bulb, while being later in onset than in the septum and hippocampus, showed a similar progression of microglial changes reaching maximal intensity 7 days after trimethyl tin intoxication. Significant increases in the expression of glial fibrillary acidic protein were observed in all regions examined and typically occurred after microglial activation was already underway. We conclude that microglial and astroglial reactions which occur in response to trimethyl tin-induced neuronal necrosis are separated in time, with microglial activation preceding astrogliosis. In addition, our study stresses the importance of microglia as an endogenous source of CNS macrophages, and illustrates the merit of histochemical analysis with microglial markers for the early delineation of neurotoxicant-induced brain damage.

Acute trimethyltin exposure produces nonspecific effects on learning in rats working under a multiple repeated acquisition and performance schedule

Previous research has explored the adverse effects of trimethyltin (TMT) on learning and memory in laboratory animals. Virtually all studies of TMT effects on learning have not, however, included appropriate controls to establish a selective effect on learning. This experiment investigated the effects of TMT on the repeated acquisition (learning) and performance of response sequences. Adult male Long-Evans rats, maintained at 300 g b.wt., were trained with food reinforcement under a multiple repeated acquisition (RA) and performance (P) schedule. The RA component required rats to learn a different three-member response sequence during each session (Center Right Left, RLC, RCL, LCR, or LRC); the correct response sequence remained constant in the P component (CLR). RA and P components alternated twice during a session. Rats were given 0, 4, or 8 mg/kg TMT IV after 30 sessions of stable baseline performance, and an additional 40 sessions were conducted following TMT. Prior to TMT, all groups maintained comparable accuracy levels in both RA and P components. Following TMT, significant decreases in both accuracy and response rate were obtained in the 8 mg/kg group. Thereafter, response rate and accuracy both recovered to near baseline levels, although large individual differences were observed. No selective effects of TMT were obtained on RA when compared to P. These data suggest that TMT-induced impairments on learning may be due to a generalized performance decrement rather than a specific effect on learning.

Quantitative histological evaluation of neuroprotective compounds

The application of quantitative morphometric methods to neurotoxicology is a relatively recent endeavor, and appropriate techniques are still evolving. However, such methods are essential for subsequent use of neurohistological data in mathematical representations of the risk of exposure to neurotoxicants. It can be predicted that the same methods will also be of great utility in studies of the efficacy of neuroprotective drugs. When the neuropathological conditions to be prevented or reversed are best monitored by neurohistology, quantitative morphometry should be considered as the most direct means to demonstrate the efficacy of a neuroprotective agent. Initially, a decision to choose the most appropriate histological procedure must be made. The rationale for such decisions with regard to several common histochemical techniques was discussed. The appropriate stereological and statistical considerations to be addressed by the sampling strategy were also presented. It is anticipated that quantitative morphometric methods will play an increasingly important role in the evaluation of the efficacy and toxicity of neuroactive compounds.

Endogenous excitatory amino acid release from brain slices and astrocyte cultures evoked by trimethyltin and other neurotoxic agents

Trimethyltin (TMT) is a toxic alkyltin compound that is known to produce neuronal necrosis in the CNS. The present study examined the effects of TMT on the release of excitatory amino acids (EAA) from cortical slices prepared from adult and aged (24 months old) rats. The calcium dependence of TMT-induced EAA efflux was evaluated and compared to other neurotoxic agents. The actions of TMT were also evaluated in an astrocyte culture model to assess glial contributions to TMT-induced EAA efflux. TMT (10-1000 microM) evoked a dose-related increase in GLU and ASP efflux during a 30 min incubation period and this efflux was sustained or slightly higher during a 15 min recovery period. TMT-stimulated GLU efflux was not altered in aged rats. TMT-induced GLU efflux was significantly reduced by removing extracellular calcium and including 10 microM EGTA in the incubation media. Calcium channel blockers (nifedipine, verapamil, flunarizine, amiloride, neomycin) and MK-801 did not significantly attenuate TMT-induced GLU efflux. Diltiazem (25 microM) produced modest but inconsistent reductions in TMT-induced GLU efflux from brain slices, and significantly inhibited the leakage of lactate dehydrogenase (LDH) from TMT-treated astrocyte cultures. TMT did not increase GLU efflux from glial cultures during a 30 min incubation period, but did significantly elevate GLU efflux during the 15 min recovery period. TMT evoked the release of EAA by both calcium dependent and independent mechanisms in brain slices. TMT at high concentrations also produced a delayed increase in glial GLU efflux. These studies suggest that excitotoxic mechanisms may contribute to TMT-induced neurotoxicity.

A direct comparison of GFAP immunocytochemistry and GFAP concentration in various regions of ethanol-fixed rat and mouse brain

Glial fibrillary acidic protein (GFAP) immunostaining is the most commonly used method to examine the distribution of astrocytes and the hypertrophy of astrocytes in response to neural degeneration or injury. More recently, a variety of biochemical assays for GFAP have been developed. Both qualitative immunocytochemical evaluations of GFAP and quantitative biochemical measurements of GFAP have been used to examine the regional distribution of GFAP within the central nervous system (CNS). The former method has largely been based on aldehyde-fixed tissue, while the latter approach has been based on the use of fresh tissue extracts or homogenates. In the present study, we used ethanol as a fixative to permit both immunocytochemical and biochemical procedure to be carried out on brain tissue from a single animal. Normal adult rats and mice were perfused with a 70% ethanol/saline solution, and each brain was hemisectioned. The concentration of GFAP was measured in regions of 1 hemisection, using an enzyme-linked immunosorbent assay (ELISA), while the other hemisection was used for GFAP immunostaining. Regional differences occurred in the brains of both species, with the highest concentration of GFAP found in the brainstem, and the lowest concentrations found in the striatum and cortex. The specific patterns of GFAP immunoreactivity corresponded to regional concentrations in most brain area of both species. These data show that it is possible to assay GFAP concentrations in tissue prepared for immunocytochemical analysis, providing both qualitative and quantitative information from one set of tissue.

Quantitative aspects of drug and toxicant-induced astrogliosis

A universal cellular reaction to damage of the CNS is hypertrophy of astrocytes. The hallmark of this response, often termed 'reactive gliosis', is the enhanced expression of the major intermediate filament protein of astrocytes, glial fibrillary acidic protein (GFAP). This latter observation suggests that increased synthesis of GFAP would occur in response to diverse neurotoxic insults. To investigate this possibility, prototype neurotoxicants were administered to experimental animals and the effects of these agents on the tissue content of GFAP was determined by immunoassay. Assays of GFAP were found to reveal dose-, time- and region-dependent patterns of neurotoxicity at toxicant dosages below those that cause light microscopic evidence of cell loss or damage. Moreover, the temporal and regional increments in GFAP correspond to the temporal and regional patterns of argyrophilia, as revealed by the cupric silver degeneration stain of de Olmos. Our findings indicate that assays of GFAP represent a sensitive, simple and quantitative approach for evaluation of nervous system damage. Combining this indirect yet quantitative indicator of neurotoxicity with more traditional neuroanatomical endpoints, should augment the armamentarium of techniques useful for detection and characterization of neurotoxicity.

Use of an amino-cupric-silver technique for the detection of early and semiacute neuronal degeneration caused by neurotoxicants, hypoxia, and physical trauma

A new amino-cupric silver protocol is described for detection of neuronal degeneration. We describe its selectivity in visualizing both early and semiacute degeneration after intracerebral or systemic administration of a variety of neurotoxicants in rats, and after transient ischemic episodes in gerbils. As early as 5 min after physical trauma, or 15 min following either intrastriatal injections of glutamate analogs or exposure to ischemic episodes, neuronal silver staining was evident at primary sites of trauma (i.g. injection sites) and at hodologically related secondary sites. With intoxication by peripheral injections of trimethyltin (IP) or intracerebral injections of Doxorubicin, reproducible patterns of degeneration are demonstrable after 24 h or after 9-13 days, respectively. The amino-cupric silver method permits simultaneous detection of all neuronal compartments against a clear background. Degeneration in the neuronal cell bodies, dendrites, axons and terminals, as well as the recruitment of new structures in a progressive pathologic process, could be accurately followed. The inclusion of new reagents increased the sensitivity vis-à-vis previous versions of the cupric-silver method. The advantages and disadvantages of the current method in comparison with other means of neurotoxic assessment are discussed in detail, with special emphasis on its unique ability to discriminate irreversible degenerative phenomena and degeneration of axonal components in cases where the cell body remains apparently intact. The amino-cupric silver method is an especially useful tool for surveying neuronal damage in basic neuroscience investigations and in neuropathologic and neurotoxic assessment.

The effect of time following exposure to trimethyltin (TMT) on cholinergic muscarinic receptor binding in rat hippocampus

Adult male Long-Evans rats were given 6 mg/kg trimethyltin (TMT). Rats were killed 1, 3, 7, 14, 21, 35, or 60 d later. An untreated control group was included. Brain sections were processed using film autoradiography to visualize in the hippocampus either total muscarinic receptor binding ([3H]quinuclidinyl benzilate; [3H]QNB), or M1 receptors ([3H]pirenzepine; [3H]PZ), or M2 receptors ([3H]oxotremorine-M; [3H]OXO-M). A reduction in [3H]QNB binding was found in CA1 and CA3c 7 d after TMT, but not in CA3a, b, or the dentate gyrus. [3H]PZ binding was decreased throughout Ammon's horn by 14 d after treatment. [3H]OXO-M binding decreased 1 d after exposure in CA1 and in all subfields of Ammon's horn by d 3. Neither [3H]PZ or [3H]OXO-M binding decreased in the dentate gyrus of TMT-treated rat at any time point. The temporal patterns of receptor loss may be explicable by reference to timing of fiber and cell body degeneration reported in previous studies and the regional differences may account for discrepancies between reports of either substantial decreases or no loss in hippocampal muscarinic receptors after TMT exposure.

Effects of trimethyltin (TMT) on choline acetyltransferase activity in the rat hippocampus. Influence of dose and time following exposure

Trimethyltin (TMT) destroys specific subfields of the hippocampus in the rat. TMT also increases choline acetyltransferase (ChAT) activity in CA1 of Ammon's horn and the outer molecular layer of the dentate gyrus. This observation suggests that axonal sprouting occurs in the cholinergic septohippocampal system in response to TMT. However, neither does-response nor time course data are available for the effects of TMT on this enzyme. The effects of three dose levels of TMT on ChAT activity in CA1 and the dentate gyrus were determined in Experiment 1 and ChAT activity in these two areas was measured at six time points following exposure to TMT in Experiment 2. Only the highest dose of TMT (6 mg/kg) significantly increased ChAT activity. ChAT activity in the dentate gyrus increased significantly by 3 d after administration and continued to increase until 21 d after exposure. A significant increase was not observed in CA1 until 7 d after exposure to TMT. Asymptotic levels were still reached at d 21. These results indicate a steep dose-response curve for TMT-induced changes in ChAT activity in the hippocampal formation and that this marker of cholinergic activity is more sensitive to perturbation by TMT in the dentate gyrus than Ammon's horn.

Trimethyltin-induced expression of GABA and vimentin immunoreactivities in astrocytes of the rat brain

Adult Sprague-Dawley rats were given a single dose of trimethyltin chloride (TMT). Three days following treatment, a neuronal alteration was observed in the CA3c pyramidal cell layer of hippocampus which was not accompanied by any apparent astrocyte reaction. At 1 as well as 2 weeks after treatment, a gliosis in hippocampus, piriform, and entorhinal cortices was detected by glial fibrillary acidic protein (GFAP) immunohistochemistry. Concomitant with the enhanced astrocytic GFAP, astrocytes were swollen and expressed immunoreactivity to vimentin and gamma-aminobutyric acid (GABA). The astrocytic GABA immunoreactivity may reflect a trimethyltin-induced alteration in astrocyte phenotype, or alterations in compartmentalization and/or metabolism of GABA.

Developmental differences in neural damage following trimethyl-tin as demonstrated with GFAP immunohistochemistry

Long-Evans rat pups received intraperitoneal (i.p.) injections of trimethyl-tin (TMT) 6 mg/kg hydroxide or saline on postnatal day (PND) 10 or PND 18 and were sacrificed for immunohistochemical staining for glial fibrillary acidic protein (GFAP) on PND 12, 18, 20, or 25. After dosing with TMT on PND 10 there was a transient increase in GFAP immunoreactivity (IR) in the amygdala, piriform, and entorhinal cortex 2 days post-dosing (PND 12) and a persistent increase in GFAP IR in the hippocampus and cingulate cortex up to two weeks post-dosing. Following dosing with TMT on PND 18 there was a delayed (PND 25) increase in GFAP IR in the amygdala, hippocampus, cingulate, piriform, and entorhinal cortex. In addition, increases in GFAP IR were observed in the neocortex 7 days post-dosing, which was not observed following earlier postnatal dosing. The regions in which gliosis and loss of Nissl-staining were consistent for the different time points of TMT treatment were the amygdala, hippocampus, cingulate, piriform, and entorhinal cortex. The present findings indicate the GFAP immunohistochemistry can be used to reveal regional effects of developmental neurotoxicant exposure during early stages of development.

Limbic forebrain toxin trimethyltin reduces behavioral suppression by clonidine

Trimethyltin (TMT) at moderate doses selectively damages hippocampus and related olfactory cortex and produces learning and memory impairments. TMT also increases forebrain beta-adrenergic ligand binding; this could be ancillary to reduced noradrenergic neurotransmission, which in turn could be involved in the cognitive deficit caused by TMT. If this hypothesis is correct, then the alpha 1-adrenergic agonist clonidine, which inhibits noradrenergic neurotransmission in normal subjects, should be less behaviourally effective after TMT poisoning. Thus, rats treated with water vehicle or TMT (6 mg/kg, PO) were given saline or clonidine IP (5, 10, or 20 micrograms/kg) 30 min before placement in a hole-board apparatus. Exploratory activity was reduced in controls by 10 or 20 micrograms/kg. Clonidine at 10 micrograms/kg was ineffective in rats given TMT. At 20 micrograms/kg, an apparent reduction in exploratory activity was not significant because variability of responding was higher after TMT treatment. The results suggest an impairment in noradrenergic neurotransmission following TMT poisoning.

Hearing: the effects of chemicals

Recent studies of human beings exposed to environmental chemicals, as well as experimental animal studies, have identified a number of chemical agents that are commercial products, chemical intermediaries, waste products, or contaminants that are potentially ototoxic. The classes of compounds discussed in this review include organic solvents, asphyxiant gases, and heavy metals that are present in the environment as industrial pollutants or byproducts. Both human and animal investigations are summarized in discussing the actions of these ototoxic compounds. The suggested gaps in our knowledge are highlighted to help direct future research.

Toxic effects of somatostatin in the cerebellum and vestibular nuclei: multiple sites of action

This study demonstrates that somatostatin (SRIF), an endogenous peptide in vestibular nuclei and cerebellum, can produce both a dose-dependent death of Purkinje cells in distinct sagittal regions of cerebellar cortex and vascular infarcts centered selectively in the inferior vestibular nucleus. Alert, adult male rats were given a 5 microliters intracerebroventricular (i.c.v.) bolus of either SRIF alone (20 or 40 micrograms) or a combined dose of SRIF plus either arginine-vasopressin (AVP, 1 micrograms) or an AVP V1 antagonist, (1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid), 2-(O-methyl)-tyrosine)-arginine 8-vasopressin (mcAVP, 1 micrograms), through an implanted cannula. After a 4-5 day survival, the brains were stained with the cupric-silver selective degeneration method. Two types of dose-dependent lesions were observed in the cerebellar and vestibular nuclei of these animals: degeneration of Purkinje cell responses in the cerebellar cortex and vascular infarcts in vestibular nuclei. These toxic responses were unaffected by application of AVP or mcAVP; hence, they can be attributed to actions of SRIF. The distribution of Purkinje cell degeneration varied with the SRIF dose in different cerebellar regions. Purkinje cell responses in lobules I-III were equivalent at both SRIF doses, and degeneration in the copula pyramis, paraflocculus and paramedian lobule emerged at the higher SRIF dose. Purkinje cells in the medial aspect of lobules IX-X had an intermediate sensitivity to SRIF intoxication. Degenerating Purkinje cells tended to be arranged in parasagittal bands in each region, suggesting parasagittal zonal variations in susceptibility to SRIF intoxication. By contrast, infarctions in the vestibular nuclei only appeared at the higher SRIF dose. These infarcts could be unilateral or bilateral and always involved the inferior vestibular nucleus at the level of the caudal margin of the acoustic tubercle; they often extended into the medial and lateral vestibular nuclei. The infarcts had a necrotic core that was infiltrated by non-neuronal elements. Thus, they appear to reflect a direct or neurally-mediated vascular response to the peptide.

Neonatal exposure to triethyltin disrupts olfactory discrimination learning in preweanling rats

Triethyltin is an organotin compound that is known to produce neurotoxicity in both adult and developing organisms. Although this neurotoxicity has been documented with a variety of behavioral and biological measures, the effects of this compound on learning during early development have been less extensively studied. The present study reports four experiments that examined this question with an odor aversion learning paradigm in which pups received presentations of one odor paired with footshock and an alternate odor without shock. In Experiment 1, Long-Evans rat pups were injected IP on postnatal day 5 (PND 5) with either 0, 3 or 5 mg/kg TET and then tested for olfactory discrimination learning on PND 18. Only the 5-mg/kg dose impaired discrimination learning. In Experiment 2, PND 5 exposure to TET (5 mg/kg) disrupted olfactory learning on PND 18 but not on PND 12, whereas exposure on PND 10 disrupted learning at both ages of testing. In Experiment 3, PND 16 exposure to TET (5 mg/kg) also disrupted acquisition of olfactory learning on PND 18 but had no effect on retention of an olfactory discrimination that was acquired prior to TET exposure (i.e., on PND 14 and PND 15). Unconditioned responses to footshock were also unaffected by TET (Experiment 4). These findings indicate that neonatal exposure to TET impairs associative learning in developing rats and are discussed in relation to other studies of the developmental neurotoxicity of this compound.

Glucocorticoids regulate the synthesis of glial fibrillary acidic protein in intact and adrenalectomized rats but do not affect its expression following brain injury

Short (5 days)- to long-term (4 months) corticosterone (CORT) administration by injection, pellet implantation, or in the drinking water decreased glial fibrillary acidic protein (GFAP) by 20-40% in hippocampus and cortex of intact rats. In contrast to CORT, adrenalectomy (ADX) caused elevations (50-125%) in hippocampus and cortex GFAP within 12 days of surgery that persisted for at least 4 months. CORT replacement of ADX rats decreased GFAP amount in hippocampus and cortex. The effects of long-term CORT and ADX on GFAP in hippocampus and cortex were also seen in striatum, midbrain, and cerebellum, findings suggestive of brain-wide adrenal steroid regulation of this astrocyte protein. The changes in GFAP amount due to CORT and ADX were paralleled by changes in GFAP mRNA, indicating a possible transcriptional or at least genomic effect of adrenal steroids. Glucocorticoid regulation of GFAP was relatively specific; it could not be generalized to other astrocyte proteins or other major structural proteins of neurons. The negative regulation of GFAP and GFAP mRNA by adrenal steroids suggested that increases in GFAP that result from brain injury may be attenuated by glucocorticoids. However, chronic CORT treatment of intact rats did not reverse or reduce the large increases in GFAP caused by trauma- or toxicant-induced brain damage. Thus, glucocorticoids and injury appear to regulate the expression of GFAP through different mechanisms. In contrast to the lack of effects of CORT on brain damage-induced increases in GFAP, CORT treatment begun in 2-week ADX rats, after an increase in GFAP had time to occur, did reverse the ADX-induced increase in GFAP. These results suggest that the increase in GFAP resulting from ADX is not mediated through an injury-linked mechanism.

Neonatal exposure to trimethyltin disrupts spatial delayed alternation learning in preweanling rats

Trimethyltin is an organotin compound that produces marked neurotoxicity in both adult and developing animals. The limbic system is a primary CNS target site for this toxicity, and a prominent behavioral effect of TMT is disruption of learning and memory. Impairment of cognitive development has also been suggested by studies showing that rats neonatally exposed to TMT cannot perform spatial working memory tasks during adulthood. However, the question of how early in ontogeny such deficits can be detected has not been addressed. The present study examined this question with a T-maze delayed alternation learning paradigm. Long-Evans rat pups, injected IP on Postnatal Day 10 (PND 10) with 6 mg/kg TMT and tested on PND 18, were unable to learn delayed alternation in the manner shown by vehicle control pups. However, TMT- and vehicle-treated groups were both able to learn a simple position discrimination. These findings indicate a selective impairment of spatial working memory by neonatal TMT exposure and show that this impairment can be demonstrated during the preweanling period in the rat.

Acute ototoxicity of trialkyltins in the guinea pig

Two trialkyltin compounds, trimethyltin chloride (TMT) and triethyltin bromide (TET) were evaluated for their acute effects on cochlear function in pigmented guinea pigs. Compound action potential (CAP) thresholds and 1 microV RMS cochlear microphonic (CM) isopotential curves were generated for 25 subjects following ip injection of TMT (2 mg/kg), TET (12 or 24 mg/kg) or inert vehicle (0.9% saline or 15% ethanol). The CAP is generated by the release of neurotransmitters from the inner hair cells and the subsequent depolarization of spiral ganglion cells. However, the sensitivity of the CAP is influenced by other cochlear structures including the outer hair cells which are thought to influence sensitivity of the inner hair cells. By contrast, CM reflects electromechanical function of the outer hair cells. CAP function was severely disrupted by organotin exposure while CM was unaffected by these agents. TMT administration impaired CAP thresholds at all frequencies within 30 min of administration. Thresholds deteriorated slightly more between 30 and 60 min. TET also reduced the sensitivity of the CAP to all frequencies. At the lower dose moderate impairments of function were observed at 30 min which became more noticeable at 60 min. Following 24 mg/kg TET injection, CAP sensitivity was markedly impaired even at 30 min. The CM isopotential values were not significantly altered 30 min or 60 min after either TMT or TET treatment at any of the 11 frequencies tested. These data document far more rapid toxic effects of TMT and TET than have been seen in most intact neuronal systems. They indicate that both organotins initially disrupt the functional integrity of either inner hair cells or spiral ganglion cells within the cochlea such that depolarization occurs only following a significant increase in stimulus intensity.

Myelin basic protein-mRNA used to monitor trimethyltin neurotoxicity in rats

Trimethyltin (TMT) is an alkyltin that targets neurons of the limbic system. A gene probe (i.e., mRNA) for myelin basic protein (MBP), a major component of central nervous system myelin, was used to monitor this toxic neuropathy in Sprague-Dawley rats. Animals were administered a single intraperitoneal injection of TMT-hydroxide at a neuropathic (8.0 mg/kg/body wt) or nonneuropathic (0.8 mg/kg/body wt) dose and sampled at 1, 3, or 7 days postexposure to correlate the progression of hippocampal neuropathology with probe (i.e., MBP-mRNA) levels. Microscopic examination of the brain showed only moderate but progressive damage over the 7-day postexposure period in animals treated with the neuropathic dose. Neuronal loss was first observed in the dendate gyrus and CA4 at 1 day postexposure, and progressed to the CA3c sector at 3 and 7 days postexposure. Elsewhere in the brain, minimal involvement of the entorhinal cortex neurons occurred 3 days postexposure and intensified by 7 days. No histological damage was seen at the nonneuropathic (0.8 mg/kg) dose. For gene probe analysis, the brain was divided into anterior and posterior halves. In rats treated with the neuropathic dose of TMT, the anterior brain showed progressive depressions of MBP-mRNA levels over the 1-, 3-, and 7-day postexposure period that correlated with increasing hippocampal neuropathology. The posterior brain showed no significant changes in MBP-mRNA levels with respect to that of controls over the same time period. At the nonneuropathic dose (0.8 mg/kg) a significant depression of MBP-mRNA levels occurred in the anterior brain at 7 days postexposure in the absence of overt histological damage.