Early unsuspected neuron and axon terminal loss in scra pie‐infected mice revealed by morphometry and immunocytochemistry

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Abstract
Neuronal loss is often quoted as an element of the pathology of the transmissible spongiform encephalo‐pathies, but few data are published. To determine whether neuronal loss is a salient feature of murine scrapie, and whether there is a relationship with the other hallmark lesions of scrapie we compared the numbers of neurons, severity of vacuolation, axonal bouton density and distribution of prion protein (PrP) in the dorsal lateral geniculate nucleus (dLGN) following intraocular infection of C57BL/FaBtDk mice with ME 7 scrapie. This route of infection limits the initial spread of infection to the retinal efferents, thus directing infectivity and subsequent pathological changes to the dLGN which is a major projection of the optic nerve. Morphometric assessment of neuron number in the dLGN was made on semi‐serial sections from five infected and five normal brain injected controls at four 50‐day intervals during the incubation period, and on terminally affected mice. The number of neurons decreased from around 20 000 at 50 days to under 1000 in the terminal group. Significant loss was identified in individual mice at 150 days post‐infection, coincident with the onset of vacuolation: neuron number was found to have an inverse relationship to the severity of vacuolation. Axonal boutons in the dLGN (demonstrated by synaptophysin immunolabelling) were reduced at 200 days, and virtually absent in terminal mice. The intensity of PrP immunostaining progressively increased from 150 days, and in a separate experiment PrP was detected from 175 days by polyacrylamide gel electrophoresis of brain extracts. These results show that early neuronal loss is a significant feature of experimental scrapie infection, and the possible mechanisms of this degeneration are discussed.