[Prions and Trypanosoma as a source of knowledge of the brain].

Recent research on prions and trypanosomes is reviewed as they constitute examples of how infectious agents can cause degenerative diseases or functional disturbances in the nervous system. Prions are novel, transmissible pathogens causing degenerative diseases of the central nervous system both in humans and in animals. Kuru, Creutzfeldt-Jakob's disease and Gerstmann-Sträussler-Scheinker's (GSS) syndrome illustrate the acquired, sporadic and genetic manifestations of the human prion diseases, respectively. Prions are composed largely, if not entirely, of an abnormal isoform of the prion protein, which is a normal cellular protein occurring at high concentrations in the central nervous system. Specific mutations in the prion gene may cause the sporadic and hereditary human disease. S B Prusiner's group at the University of California has used genetic engineering techniques to recreate GSS syndrome by transplanting the mutated prion gene into mice in which the effects of the prion protein genes can then be studied. Trypanosomes are extracellular parasites which cause distinct functional disturbances of the nervous system, although they do not penetrate the blood-brain barrier. Factors are now being isolated in the interactions between the parasites and the immunesystem, and the effects of such factors on the functional properties of neurons are studied.