Biochemical and histopathological changes in the brains of mice inoculated with scrapie by the intraperitoneal route

Learning ability of mice infected with different strains of scrapie

CD-1 mice were infected intraperitoneally with one of 4 different strains of scrapie and tested simultaneously at or near the end of incubation. There were no differences between any of the scrapie injected groups and controls in spontaneous motor activity, or in the shock thresholds and entry latencies measured during training in a one-trial passive avoidance test. On testing, the avoidance responses were normal for the mice infected with 22C or ME7, but these mice did not show overt clinical signs (e.g., ataxia) at the time. The 139A and 79A infected mice were showing clinical signs when tested and impaired responses were found in the mice trained at a low shock threshold. However the impairment was completely overcome at a higher shock level. These results suggest that only limited reductions in learning ability are associated with some of the shorter incubation models of scrapie. Specific suggestions are made of how a learning deficit might be produced in certain scrapie models which could be useful in studies of some aspects of dementia.

Pathogenesis of mouse scrapie: dynamics of vacuolation in brain and spinal cord after intraperitoneal infection

At the late clinical stage of scrapie in mice, the severity and distribution of vacuolation in the brain (the lesion profile) is largely determined by the strain of agent and the genotype of the mouse: under controlled conditions, lesion profiles can be used to distinguish between scrapie strains. This paper describes the sequential development of lesions in brain at much earlier times and includes a study of spinal cord. Mice (CW) were infected intraperitoneally with 139A scrapie. Grey matter vacuolation first occurred in thoracic cord, developing later in lumbar and cervical cords, and then in various brain regions in a caudal to rostral sequence. This pattern closely matches the sequential spread of infection from mid-thoracic cord to much of the CNS that was previously found in this scrapie model. Further studies of grey matter in spinal cord suggest that agent entered the mid-thoracic region via sympathetic fibres. Vacuolation in white matter mirrored the grey matter pattern within an area but always occurred later. The severity of grey matter vacuolation in the four areas of the CNS where it developed early, reached plateau levels before the clinical stage of scrapie, but the severity was still increasing at the clinical stage in areas where vacuolation had started late. Hence the severity of lesions in a particular area may sometimes be limited by the time available for them to develop before the host dies. It appears that the distribution of vacuolation in this particular scrapie model is initially influenced by that of the infectious agent and only later does it reflect the distribution of vacuolation target areas shown by the characteristic lesion profile.

Scrapie-induced changes in the percentage of polymorphonuclear neutrophils in mouse peripheral blood

A decrease in the percentage of polymorphonuclear neutrophils (PMN) in the peripheral blood of mice appeared 3 days after intracerebral (IC) inoculation with scrapie mouse brain homogenate. Mice inoculated IC with normal mouse brain had PMN percentages similar to those found for uninoculated mice. This difference between normal and scrapie-inoculated mice continued throughout the preclinical phase of the disease. In the clinical phase of the disease, the percentage of PMN was either higher or lower than that found in normals. The factor causing the decrease in PMN percentages was found in the filtrates from 220-, 100-, and 50-nm filters, but not in the filtrates from a 25-nm filter. Sodium periodate treatment of the scrapie brain samples eliminated their ability to cause the decrease in PMN percentages, whereas sodium iodate had no effect. In addition to two genetically different scrapie mouse brain isolates, homogenates of mouse spleen, sheep brain, and sheep spleen from scrapie-affected animals caused a decrease in percent PMN, whereas the corresponding normal tissue homogenates did not.