Influence of stereotaxically injected scrapie on neurotransmitter systems of mouse cerebellum

Cerebellar compartmentation of prion pathogenesis

In prion diseases, the brain lesion profile is influenced by the prion "strain" properties, the invasion route to the brain, and still unknown host cell-specific parameters. To gain insight into those endogenous factors, we analyzed the histopathological alterations induced by distinct prion strains in the mouse cerebellum. We show that 22L and ME7 scrapie prion proteins (PrP^22L , PrP^ME7 ), but not bovine spongiform encephalopathy PrP^6PB1 , accumulate in a reproducible parasagittal banding pattern in the cerebellar cortex of infected mice. Such banding pattern of PrP^22L aggregation did not depend on the neuroinvasion route, but coincided with the parasagittal compartmentation of the cerebellum mostly defined by the expression of zebrins, such as aldolase C and the excitatory amino acid transporter 4, in Purkinje cells. We provide evidence that Purkinje cells display a differential, subtype-specific vulnerability to 22L prions with zebrin-expressing Purkinje cells being more resistant to prion toxicity, while in stripes where PrP^22L accumulated most zebrin-deficient Purkinje cells are lost and spongiosis accentuated. In addition, in PrP^22L stripes, enhanced reactive astrocyte processes associated with microglia activation support interdependent events between the topographic pattern of Purkinje cell death, reactive gliosis and PrP^22L accumulation. Finally, we find that in preclinically-ill mice prion infection promotes at the membrane of astrocytes enveloping Purkinje cell excitatory synapses, upregulation of tumor necrosis factor-α receptor type 1 (TNFR1), a key mediator of the neuroinflammation process. These overall data show that Purkinje cell sensitivity to prion insult is locally restricted by the parasagittal compartmentation of the cerebellum, and that perisynaptic astrocytes may contribute to prion pathogenesis through prion-induced TNFR1 upregulation.

Extensive degeneration of catecholaminergic neurons to scrapie agent 87V in the brains of IM mice

Scrapie is a degenerative disease of the central nervous system of sheep and goats. The causative agent has been passaged to a number of laboratory species, including mice and hamster. Amyloid plaque formation and vacuolation, the signs of senile dementia, are found in the brains of mice infected with 87V scrapie agent. Dopamine (DA) and norepinephrine (NE) concentrations in the brains of scrapie-infected mice were measured with high-performance liquid chromatography-electrochemical detector (HPLC-ECD). A significant decrease in NE level was exhibited in all regions tested, whereas the level of DA decreased significantly only in cerebral cortex. Immunohistochemistry was used to examine immunoreactive catecholamine neurons in substantia nigra and locus ceruleus using antisera against tyrosine hydroxylase (TH). The population of TH-immunoreactive neurons in the substantia nigra and locus ceruleus were significantly decreased in scrapie-infected mice compared to controls. These data suggest that both the noradrenergic and dopaminergic system are sensitive to the action of scrapie agent 87V and that changes in the catecholamine levels in the brains of scrapie-infected mice may contribute to some of the clinical symptoms of the diseases, such as ataxia and apraxia.

Scrapie strain infection in vitro induces changes in neuronal cells

PC12 cells, in the presence of nerve growth factor (NGF), support replication of the mouse-derived scrapie strains 139A and ME7, with the former yielding 100-1000-fold higher levels of infectivity. Infectivity remained cell-associated and cells did not show any gross morphological alterations, although changes were observed by electron microscopy in the form of an increased number of lipid droplets in 139A-infected cultures. Analysis of phospholipid metabolism in 139A infected cells indicated that scrapie replication did not change the inositol phosphate levels, but did stimulate phosphoinositide synthesis. Replication was not detected in PC12 cells infected with either the hamster-derived 263K or rat-derived 139R scrapie strains. Since scrapie-infected cultures did not exhibit cell death or any gross changes, any scrapie-induced effects would probably be manifested in nonvital cellular functions. When compared to controls, infection with the 139A scrapie strain resulted in decreased activity of the cholinergic pathway-related enzymes, as well as the GABA synthetic pathway; however, the adrenergic pathway was unaffected by scrapie infection. The effects of the 139A scrapie strain on the cholinergic system appeared to be dose-dependent and were first detected prior to the detection of scrapie agent replication in these cells. No neurotransmitter-related enzymatic changes were detected in 263K- or 139R-infected PC12 cells. The enzymatic changes observed in ME7-infected PC12 cells and in Chandler agent-infected mouse neuroblastoma cells suggest that the significant changes in neurotransmitter levels in cultures exhibiting low infectivity titers must involve factors other than, but not excluding, replication of the agent. The role of additional factors is also suggested in studies of protein kinase C activity in 139A- and 139R-infected PC12 cells. These studies emphasize the value of the PC12 cell model system in examining the scrapie strain-host cell interaction and, in addition, support the concept of variation among scrapie strains.

Taurine allosterically modulates flunitrazepam binding to synaptic membranes

Taurine is hypothesized to exert its inhibitory neuromodulatory effects, in part, by interaction with the GABAA receptor. Although taurine displaces GABA agonist binding to synaptic membranes, its allosteric effects on the benzodiazepine recognition site of the GABAA receptor complex is unsettled. We determined the effects of taurine on [3H]flunitrazepam (Flu) binding to well-washed, frozen-thawed synaptic membranes prepared from rat cortex. Comparative binding studies were conducted at 37 degrees C and on ice (0-4 degrees C). At 37 degrees C taurine increased Flu binding in a concentration dependent way by interaction with a bicuculline sensitive site, similar to GABA. Taurine increased Flu binding by causing a decrease in KD. The maximal effectiveness of taurine on Flu binding could not be increased further by addition of GABA. In contrast, the maximal stimulation of Flu binding by GABA was decreased by addition of taurine to the level attained by taurine alone. These mixed agonist/antagonist effects of taurine are pharmacologically specific and qualify taurine as a partial GABA agonist in this type of allosteric interaction. However, taurine causes opposite effects on Flu binding when measured at 0-4 degrees C: taurine interacts with a bicuculline insensitive site to inhibit Flu binding by increasing the KD. Taurine inhibition of Flu binding is not overcome by increasing concentrations of GABA. Although the mechanism of taurine inhibition of Flu binding at 0-4 degrees C is unclear, it may be an indirect effect of taurine interaction with membrane phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)