Neurochemical and behavioural modifications induced by scrapie infection in golden hamsters

After intracerebral (i.c.) infection of hamsters, the 263K strain of scrapie replicated at a nearly constant exponential rate until clinical disease developed when titres in brain averaged 9.8 log10 LD50 i.c. units/g. After intraperitoneal infection, scrapie replication was first detected in spleen, then in thoracic spinal cord and finally in lumbar cord and brain. This pattern suggests that invasion of the central nervous system occurs by spread of infection along certain visceral autonomic nerves. Infectivity was detected in the thoracic cord only 3 to 4 weeks after infection (incubation period 16 weeks) indicating the exceptional neuroinvasiveness of this scrapie model. This observation and the failure of splenectomy to lengthen incubation period raises the possibility of direct infection of nerve tissue in the peritoneum and transport to the thoracic cord with minimal prior replication of scrapie agent extraneurally. After intraocular infection of the right eye, replication (or accumulation) of scrapie was detected in the right optic nerve and left superior colliculus, then in the right superior colliculus and finally in the left optic nerve and medulla. This pattern shows that scrapie infection can spread along nerves, possibly by intra-axonal transport. The duration of agent replication in brain (between detectable onset of replication and clinical disease) was shortest after intraperitoneal infection (51 to 58 days), longer after intracerebral infection (81 to 88 days) and longest after intraocular infection (greater than 121 days). These differences may reflect the relative efficiency of the neural pathways by which infectivity spreads from different sites of entry in the brain to the postulated 'clinical target areas'.

Behavioural testing can reveal effects in scrapie-infected mice long before overt clinical signs appear (Betmouni et al., 1999, Psychobiology, 27, 63-71). These effects may be partly attributable to an early, atypical inflammatory response in the brain (Betmouni et al., 1996, Neuroscience, 74, 1-5). The present study replicated and extended these findings, and examined the effect of chronic treatment with dapsone. This anti-inflammatory compound has been reported to delay disease onset in a rat model of Creutzfeldt-Jakob disease (Manuelidis et al., 1998, Lancet, 352, 456). Although the doses used in the present study were higher than those of Manuelidis et al. (1998), no attenuation of the disease was seen in either behavioural or subsequent histological tests. Burrowing, i.e. displacing food pellets from a tube in the home cage, decreased from around week 12 in scrapie-infected mice, as did consumption of palatable glucose solution. Concurrently, ambulation in an open field increased, as did rearing at around week 17. Spontaneous alternation was impaired around this time. Around 18 weeks, motor performance on an inverted screen, horizontal bar, rotating rod and static rods decreased. Nest construction was impaired at 20 weeks. Overt clinical signs (reduction in mobility, hunched posture, poor coat condition, bladder enlargement) only occurred after week 20, when the mice were prepared for histology. The ME7 scrapie-infected mice thus showed a characteristic complex of neurological and behavioural changes during the course of the disease that were not ameliorated by dapsone. These changes appeared well before clinical signs were prominent.

Enriching fractions from Syrian hamster (SHa) brain for scrapie prion infectivity led to the discovery of the prion protein (PrP). Prion diseases include scrapie of sheep and bovine spongiform encephalopathy of cattle as well as Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome (GSS) of humans. Transgenic (Tg) mice expressing both SHa and mouse (Mo) PrP genes were used to probe the molecular basis of the species barrier and the mechanism of scrapie prion replication. Bioassays of brain extracts from two scrapie-infected Tg lines showed that the prion inoculum determines that prions are synthesized de novo, even though the cells express both PrP genes. Studies with artificial prions produced from chimeric Mo/SHaPrP transgenes underscore the concept that inoculated prion dictates which prion will be replicated. Discovery of mutations in the PrP genes of humans with GSS and familial CJD established that prion diseases are both genetic and infectious. Transgenic mice expressing high levels of MoPrP-P101L, corresponding to the GSS point mutation (P102L) in human PrP, spontaneously develop neurologic dysfunction, spongiform degeneration, and astrocytic gliosis. Inoculation of brain extracts prepared from these Tg (MoPrP-P101L) mice produced neurodegeneration in recipient animals after prolonged incubation times. These results are in accord with those of other studies and argue that prions are devoid of foreign nucleic acid. Structural investigations of cellular prion protein (PrPC) and prion protein scrapie (PrPSc) suggest that the difference may be conformational. Conditions that diminished the beta-sheet content of PrPSc were the same as those identified previously that inactivate prion infectivity. Whether prion diversity as reflected by distinct "strains" producing different patterns of PrPSc accumulation is due to different conformers of PrPSc remains to be established. Advances in the purification and characterization of both PrPC and PrPSc seem to have identified the central event in PrPSc synthesis and prion propagation, ie, the unfolding of PrPC followed by its refolding into PrPSc. These findings underscore the fundamental features of prion structure and propagation that differentiate prions from other transmissible pathogens.