Serotoninergic system in scrapie-infected hamsters

Unaltered susceptibility to scrapie in serotonin transporter deficient mice

The serotonergic system has been hypothesized to play an important role in prion diseases. Specifically, hyperactivity of the serotonergic system in prion diseases is suggested by an increase in the turnover rate of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) in human and experimental prion diseases. The 5-HT transporter (5-HTT) determines the duration of serotonergic neurotransmission by way of reuptake of 5-HT from the extracellular space. 5-HTT availability is reduced in brains of patients with the human prion disease familial fatal insomnia. To further clarify a possible role of the 5-HTT in prion diseases we investigated whether mice lacking the 5-HTT display an altered susceptibility to experimental scrapie infection. Surprisingly, 5-HTT knockout mice developed mouse scrapie in a time course similar to wildtype control mice with accumulation of the pathological prion protein, PrP(Sc) and with typical pathological hallmarks of the disease. These findings argue against a major role of the 5-HTT in the pathogenesis of prion diseases in mice.

Effects on the serotoninergic system in sub-acute transmissible spongiform encephalopathies: current data, hypotheses, suggestions for experimentation

Sub-acute transmissible spongiform encephalopathies (TSEs), or prion diseases, are affections in which little is known of their etiology. The predominant theory stipulates that an abnormal protease-resistant prion protein (PrPres) would be infectious by directly inducing its defective conformation to the normal native protein (PrPC). The function of PrPC remains unknown. The preferred localization of PrPC at the level of the synapses supposes a function in neuronal transmission. Several neurotransmitter systems (acetylcholine, GABA, dopamine, etc.) are damaged in TSEs, mainly the serotonin (5-HT) system. At a hypothetical level, PrPC would play a trophic and functional role by regulating the capture of amino acid precursors of neurotransmitters and the functions of neuroreceptors, in particular regarding tryptophan and 5-HT receptors. By comparison with the modes of action of Ras proteins and of the envelope glycoprotein of jaagsiekte sheep retrovirus, the adaptation of an oncogenic model is suggested for the mode of action of PrPres. The sequence of events could be the following: capture of PrPres and forming of an abnormal receptor, chronic disturbance of transduction pathways, more particularly of the phosphatidylinositol-3 kinase (PI-3K)/protein kinase B (Akt)/glycogen synthetase kinase 3 (GSK 3)/Wnt-beta catenin pathway, deregulation of the PrP gene and infrequent and transitory forming of abnormal RNA messengers and, finally, the forming of abnormal proteins and the deterioration of the serotoninergic system. The involvement of endogenous nucleic acids is supposed. The infectious agent of TSEs could be an ancestral form of retrovirus, such as a retrotransposon using the prion protein as an envelope glycoprotein. Pharmacological tests, by comparison with a rare disease of unknown etiology in cattle, bovine spastic paresis, are suggested with the amino acid precursors of neuromediators (tryptophan, tyrosine, glutamic acid, etc.) and with lithium, neuroprotector and regulator of the serotonergic system.

Familial and sporadic fatal insomnia

Familial fatal insomnia (FFI)--a hereditary prion disease caused by a mutation at codon 178 of the prion-protein (PrP) gene (PRNP) that leads to a D178N substitution in the protein--and its sporadic form, sporadic fatal insomnia (SFI), have similar disease phenotypes. Both disorders have clinical features of disrupted sleep (loss of sleep spindles and slow-wave sleep and enacted dreams during rapid-eye-movement sleep), autonomic hyperactivation, and motor abnormalities (myoclonus, ataxia, dysarthria, dysphagia, and pyramidal signs). PET shows pronounced thalamic and limbic hypometabolism that becomes more widespread in later stages. Neuropathological assessment reveals severe neuronal loss and astrogliosis of the anterior medial thalamus and inferior olives, with later cerebral cortical and cerebellar involvement. Accumulation of an isoform of protease-resistant PrP fragment in FFI distinct from that found in a familial form of Creutzfeldt-Jakob disease with the same D178N mutation, shows the effect of the polymorphism at codon 129 of PRNP on phenotypic expression and the possibility of distinct prion "strains" with diverse pathological potential. Intriguing clinicopathological correlations in FFI and SFI suggest a role for the thalamolimbic system in the regulation of sleep and other circadian functions.

Neurotransmitter metabolites, enzymes and receptors in experimental scrapie

A comprehensive study has been made of metabolites, enzymes and receptors for a variety of neurotransmitter systems in different parts of the CNS from mice affected with 139A scrapie. Studies were made at the early clinical stage so as to minimise secondary alterations in terminally sick mice. Even though histological lesions (vacuolation) are widespread in the CNS in this model of scrapie, no consistent neurochemical changes were found in mid- or anterior brain. In the cerebellum, the activity of glutamic acid decarboxylase was reduced and the binding of [3H]muscimol to gamma-aminobutyric acid receptors was increased. In the brain stem, the concentration of 5-hydroxyindoleacetic acid (5-HIAA) was increased and there was a reduced binding of appropriate ligands to serotonin and to muscarinic cholinergic receptors. In the spinal cord, the activity of choline acetyl transferase was reduced and the concentration of 5-HIAA was increased. The extent of these alterations was about 20-40% of control values. The specificity of these changes for a few neurotransmitter systems in hind brain and spinal cord raised the possibility of their being a primary cause of clinical disease and led us to study a different scrapie model. Present and published findings show that an increased concentration of 5-HIAA, some alterations in the binding properties of serotonin receptors and a decreased number of muscarinic receptor sites are common to both 139A scrapie in CW mice and 263K scrapie in hamsters.

Isonicotinic hydrazide causes seizures in scrapie-infected hamsters with shorter latency than in control animals: a possible GABAergic defect

Isonicotinic hydrazide, a drug that decreases the level of GABA, when injected subcutaneously in control and scrapie-infected hamsters induced tonic-clonic seizures in scrapie hamsters significantly earlier (P less than 0.0001) than in control animals. This suggests depression of the GABAergic system in scrapie-infected hamsters. To determine whether this lesion is pre or postsynaptic we measured the level of GABA, glutamate, cGMP and cAMP and the GABA-benzodiazepine receptor complex.

Choline acetyltransferase activity and [3H]quinuclidinylbenzilate binding in brains of scrapie-infected hamsters

Choline acetyltransferase (ChAT) activity and [3H]quinuclidinylbenzilate binding were studied in the brain of scrapie-infected hamsters and sham inoculated controls. Although scrapie-infected hamsters showed no reduction of ChAT activity compared to the controls, they showed a decrease in the affinity and maximum number of post-synaptic muscarinic receptors. Scrapie virus thus alters the cholinergic system at the post-synaptic rather than at the pre-synaptic level.