A comparison of immunohistochemical and silver staining methods for the detection of diffuse plaques in the aged canine brain.

Plaques and amyloid angiopathy represent frequent findings in the aging canine brain, whereas neuritic (senile) plaques and neurofibrillary tangles, the hallmark of Alzheimer's disease in humans, have only rarely been described in this species. Most studies on canine age-related CNS changes have used the modified (mod.) Bielschowsky method. Therefore, it remains to be determined, whether results obtained represent species-specific age-related changes or are due to the use of inappropriate methods. Consequently, the aim of this study was to compare the usefulness of various methods for the detection of age-related changes in the canine CNS. In a preliminary study, 130 brains from dogs between 1 month and 18 years of age were screened for the presence of A-beta protein using a monoclonal antibody against the beta-amyloid peptide. Thereafter, 30 brains of aged dogs, 10-18 years of age, out of the 130, were comparatively investigated by immunohistochemistry and silver stains such as the method of Campbell-Switzer, Reusche, Gallyas, modified (mod.) Bielschowsky and mod. Bodian as well as the Congo red-stain. Vascular amyloid angiopathy, plaques and A-beta protein deposits in the hippocampus/dentate gyrus were an age-dependent process starting at the age of 8 years and increased in a linear fashion with age. In contrast, plaque density did not rise in a similar age-related fashion in aged dogs. Moreover, great individual variations were observed in aged animals. Immunohistology was by far the most sensitive method for detection of diffuse plaques and amyloid angiopathy, followed by the Campbell-Switzer, Reusche, mod. Bielschowsky, and mod. Bodian technique. Neurofibrillary tangles and Congo red positive neuritic plaques were not found with any method. Immunohistochemically, two plaque types, both resembling diffuse human plaques, occurring in the superficial or deep cortical layers, were detected. Cluster analysis suggested the presence of three subgroups such as aged dogs with a low, medium and high number of plaques. Summarized, detection of plaques depends strongly upon the method used and the aging canine brain displays diffuse plaques and amyloid angiopathy but lacks neurofibrillary tangles and classic/neuritic and Congo red positive plaques. Cluster analysis indicated, that genetic factors might be important as predisposition for the development of diffuse plaques in dogs. The observed findings substantiate previous observations, that the canine brain represents an important model to study the pathogenesis of diffuse plaques and amyloid angiopathy in the absence of other Alzheimer disease characteristic lesions.