A morphometric analysis of trimethyltin-induced change in rat brain using the Timm technique

Type II glucocorticoid receptors are involved in neuronal death and astrocyte activation induced by trimethyltin in the rat hippocampus

According to our previous study, trimethyltin (TMT), a neurotoxicant, induces the loss of pyramidal neurons in the rat hippocampus, which is preceded by a transient increase in plasma corticosterone concentration. To address whether this transient activation of the hypothalamopituitary-adrenocortical axis is related to neuronal loss in the hippocampus, we evaluated the effects of bilateral adrenalectomy (ADX) and the chronic supplemental treatment of glucocorticoid receptor agonists after ADX on TMT-induced hippocampal damage. Peroral administration of a single dose of TMT (9 mg/kg body wt) induced the extensive loss of CA3 pyramidal neurons and reactive astrocytosis in the hippocampus, as evidenced by results of vimentin and glial fibrillary acidic protein immunohistochemistry, and the effects were profoundly exacerbated by bilateral adrenalectomy. Prolonged administration of corticosterone not only attenuated the exacerbating effects of adrenalectomy but also partially reversed the TMT-induced neuronal loss and reactive astrocytosis. Dexamethasone, but not aldosterone, could be substituted for corticosterone, suggesting a novel neuroprotective action of type II glucocorticoid receptors in the hippocampus.

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.

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.