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

Quantification of glial fibrillary acidic protein: comparison of slot-immunobinding assays with a novel sandwich ELISA

Detailed protocols are presented for assaying glial fibrillary acidic protein (GFAP), an astrocyte localized protein which serves as a quantitative marker of toxicant-induced injury to the central nervous system. Two different solid-phase assay procedures are described: 1) a nitrocellulose based slot-immunobinding assay and 2) a novel microtiter plate based sandwich ELISA. The performance of both assays was assessed by measuring the content of GFAP in homogenates of specific regions of the rat brain and in homogenates of brain regions damaged by the prototype neurotoxicants, trimethyltin (TMT) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Both procedures gave similar results that were consistent with previously published observations. By comparing the simplicity, cost effectiveness, safety and speed of the two methods, it appears likely that the sandwich ELISA has several advantages over slot-immunobinding assays.

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.

Trimethyltin increases choline acetyltransferase in rat hippocampus

The environmental neurotoxin trimethyltin (TMT) destroys parts of the hippocampal formation as well as the entorhinal cortex but leaves the septal cholinergic projection to the hippocampus and dentate gyrus intact. In this study we measured choline acetyltransferase (ChAT) activity in micropunch samples of the dentate gyrus, the CA1 region of Ammon's horn, and the caudate-putamen as a measure of density of cholinergic innervation in control rats and rats exposed to 7 mg/kg TMT by means of gastric intubation. Three months after the rats were exposed to a single dose of TMT both the dentate gyrus and CA1 demonstrated significantly higher ChAT activity in TMT-exposed rats than in control rats. No differences were found between groups for the caudate-putamen samples. These results support the hypothesis that exposure to TMT causes reactive synaptogenesis in the cholinergic septohippocampal system.

Trimethyltin effects on auditory function and cochlear morphology

Trimethyltin (TMT) is a neurotoxicant known to alter auditory function. The present study was designed to compare TMT-induced auditory dysfunction using behavioral, electrophysiological, and anatomical techniques. Adult male Long-Evans hooded rats (n = 9-12/group) were acutely exposed to saline, 3, 5, or 7 mg/kg TMT. Auditory thresholds were determined 11 weeks postdosing for 5- and 40-kHz tones using reflex modification of the auditory startle response (ASR). Brainstem auditory evoked response (BAER) thresholds were determined for 5-, 40-, and 80-kHz tonal stimuli 9 weeks postdosing. Cochlear histology was assessed at 13 weeks postdosing. Functional endpoints demonstrated a high-frequency hearing loss. ASR thresholds for 40-kHz tones were elevated 25-35 dB in all dosage groups. BAER thresholds for 40- and 80-kHz tones were elevated 30-50 dB in the 5 and 7 mg/kg groups. Organ of Corti surface preparations revealed a pattern of damage suggesting classical ototoxicity. That is, outer hair cells died preferentially in regions associated with high-frequency hearing, in a dosage-dependent manner from base to apex. These data demonstrate the utility of the ASR and BAER in detecting functional alterations in audition and indicate that TMT-induced high-frequency hearing loss is associated with cochlear damage.

Characterization of the origins of astrocyte response to injury using the dopaminergic neurotoxicant, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We used the dopaminergic neurotoxicant, 1-methyl-1,2,3,6-tetrahydropyridine (MPTP), as a tool to characterize the origins of astroglial response to injury. Radioimmunoassay of the astrocyte protein, glial fibrillary acidic protein (GFAP), was used to quantify the astrocyte reaction to MPTP. Assays of neuron-localized proteins and of dopamine were used to assess neuronal damage caused by MPTP. A single administration of MPTP (12.5 mg/kg, s.c.) to the C57BL/6J mouse resulted in more than a 3-fold increase in striatal GFAP within 48 h, followed by a decline to baseline at 3 weeks. A decrease in the amount of striatal tyrosine hydroxylase (TH), a marker of dopaminergic neurons, preceded the rise in GFAP. The concentration of striatal DARPP-32, a phosphoprotein enriched in neurons receiving dopaminergic input, was not affected by MPTP. Protecting the dopaminergic neurons from the neurotoxic metabolite of MPTP, 1-methyl-4-phenylpyridinium (MPP+), either by blocking its formation or by preventing its uptake into dopaminergic neurons, completely blocked the increase in GFAP. MPTP did not appear to disrupt the blood-brain barrier, therefore, blood-borne elements probably did not mediate the increase in GFAP. In addition, immunoblot data indicated that brain-derived interleukin 1, an astrocyte growth factor, also did not play a role in MPTP-induced gliosis. Together, these findings suggest that diffusible factors derived from damaged dopaminergic neurons initiate the astrocyte response to MPTP and that large increases in GFAP can be induced without the participation of serum-derived growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of avidin-biotin subtractive hybridization to characterize mRNA common to neurons destroyed by the selective neurotoxicant trimethyltin

Trimethyltin (TMT), a selective neurotoxicant, destroys a distinct subpopulation of neurons which possess no known biochemical or anatomic linkage. However, TMT-sensitive neurons may share common gene products related to susceptibility. In an effort to isolate mRNAs common to TMT-sensitive neurons, avidin/biotin based-subtractive hybridization was used to generate a cDNA library specifically related to TMT-toxicity. Out of 50 cDNAs, two clones hybridized only to poly(A+) mRNA isolated from the brains of saline-treated rats. Two of these cDNAs, p9T10 and p9T19, were used for in situ hybridization; both hybridized to hippocampus, limbic cortex, amygdala and other regions destroyed by TMT, suggesting that these probes identified mRNA enriched in TMT-sensitive neurons. The patterns of in situ hybridization coupled with the loss of p9T10 and p9T19 hybridization to mRNA isolated from the brains of TMT-treated rats suggests that one or both of these two clones may represent mRNA found in neurons damaged by TMT. The combination of selective neurotoxic lesions followed by cDNA subtractive hybridization should prove to be a useful strategy for the isolation of gene products from specific neuronal populations.

Exposure to trimethyltin significantly enhances acetylcholinesterase staining in the rat dentate gyrus

Trimethyltin (TMT) is known to produce substantial damage to the hippocampal formation. It also destroys neurons within the entorhinal cortex, thereby causing degeneration of perforant path afferents that terminate in the outer molecular layer (OML) of the dentate gyrus. Surgical destruction of the entorhinal cortex also causes the perforant path to degenerate. This leads to reactive synaptogenesis (axonal sprouting) of septal afferents to the dentate gyrus. The purpose of the present study was to determine whether administration of 6 mg/kg of TMT by gavage to rats would cause axonal sprouting within the septo-dentate projection. A histochemical stain for acetylcholinesterase (AChE) was used. Compared to control subjects rats given TMT exhibited significantly denser AChE staining in the dentate OML. This is putative indication of reactive synaptogenesis within the cholinergic projection to this layer of the dentate and is somewhat surprising because other neurotoxins, such as lead and ethanol, that affect neurons within the hippocampal formation reduce the capacity for reactive synaptogenesis in response to lesions of the entorhinal cortex.

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Variations in the neurotoxic potency of trimethyltin

The organometallic compound, trimethyltin (TMT), is used as a selective denervation tool to validate morphological, biochemical and functional approaches to the detection and characterization of neurotoxicity. Variations in nervous system response TMT have been reported and may complicate the use of this compound as a research tool. We examined the contribution of sample-to-sample differences to variations in TMT-induced neurotoxicity. Seven samples of TMT obtained from three commercial sources were evaluated for neurotoxic potency in the rat. Hippocampus weight, histology and assays of the astrocyte protein, glial fibrillary acidic protein (GFAP), were used as indices of neurotoxicity. A single administration (8.0 mg/kg, IV) of different samples of TMT resulted in markedly different degrees of neurotoxicity as assessed by hippocampus weight and GFAP assays. Subsequent analysis of each sample for trace metal and speciated organotin content revealed that sample-to-sample differences in neurotoxic potency could be attributed to the presence of several impurities. Indeed, in several samples, sodium was present at levels high enough to affect neurotoxic potency simply by diluting the TMT content. A number of samples also showed contamination with the nonneurotoxic organotin, dimethyltin. The data indicate that different sources of TMT produce quantitatively different degrees of neurotoxicity, differences that may be attributed to sample-to-sample variations in TMT content.