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.

Routine histopathologic evaluation of the brain (paraffin embedding, hematoxylin and eosin staining) makes it difficult for an investigator to identify the overall location and relative extent of lesions as they relate to neural substructures. Moreover, it is very difficult to convey this information to others who are less familiar with neuroanatomy. This study combined a 3-dimensional imaging program with a cupric silver stain for neuronal degeneration in order to determine the location and extent of a focal lesion produced by (dizocilpine maleate), a glutamate receptor antagonist that induces necrosis in a small population of neurons in the cortex of rats. A male Sprague-Dawley rat was treated with a subcutaneous dose of MK-801 (10 mg/kg) and was perfused with fixative through the left ventricle 3 days after treatment, a time point known to reveal maximal neurotoxic effects. The brain was embedded in a gelatin matrix, frozen, and serially sectioned at a thickness of 40 μm. The cupric silver method of de Olmos was used to stain frozen sections at 320-μm intervals. Using a color charged-couple device (CCD) camera and a macro lens, a series of 2-dimensional images, which encompassed the entire rostral to caudal extent of the brain, was captured. A computer program was written to define internal and external boundaries in these 2-dimensional images. Then, 3-dimensional reconstructions were generated on a Silicon Graphics workstation using IRIS "Explorer." The quality of the 3-dimensional reconstructions allowed for easy identification of various neural substructures while clearly revealing the exact location and extent of the resulting necrotic neurons that were positively identified by the cupric silver stain. This 3-dimensional lesion reconstruction method provides a powerful tool for conveying spatial information about the nature of neurotoxic lesions in the brain. In addition, it may be used to investigate further dose-response relationships and the effects of other neurotoxicants.

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