Amyloid b-peptide (Ab) 1)42 oligomers have recently been discussed as intermediate toxic species in Alzheimer's disease (AD) pathology. Here we describe a new and highly stable Ab 1)42 oligomer species which can easily be prepared in vitro and is present in the brains of patients with AD and Ab 1)42 -overproducing transgenic mice. Physicochemical characterization reveals a pure, highly water-soluble globular 60-kDa oligomer which we named 'Ab 1)42 globulomer'. Our data indicate that Ab 1)42 globulomer is a persistent structural entity formed independently of the fibrillar aggregation pathway. It is a potent antigen in mice and rabbits eliciting generation of Ab 1)42 globulomer-specific antibodies that do not cross-react with amyloid precursor protein, Ab 1)40 and Ab 1)42 monomers and Ab fibrils. Ab 1)42 globulomer binds specifically to dendritic processes of neurons but not glia in hippocampal cell cultures and completely blocks long-term potentiation in rat hippocampal slices. Our data suggest that Ab 1)42 globulomer represents a basic pathogenic structural principle also present to a minor extent in previously described oligomer preparations and that its formation is an early pathological event in AD. Selective neutralization of the Ab globulomer structure epitope is expected to have a high potential for treatment of AD. Keywords: Alzheimer's disease, amyloid b-peptide, hippocampal neurons, long-term potentiation, oligomers, polymerization. Abbreviations used: Ab, amyloid b-peptide; AD, Alzheimer's disease; ADDL, amyloid b-peptide-derived diffusible ligand; AFM, atomic force microscopy; APP, amyloid precursor protein; CHO, chinese hamster ovary; CL, cross-linked; CSF, cerebrospinal fluid; DAPI, 4¢,6-Diamidino-2-phenylindole; DIV, days in vitro; EPSPs, excitatory postsynaptic potentials; fEPSP, field excitatory postsynaptic potential; GFAP, glial fibrillary acidic protein; HFIP, 1,1,1,3,3,3 hexafluoro-2-propanol; HFS, high-frequency stimulation; LTP, long-term potentiation; MAP2, microtubule associated protein-2; NBT/BCIP, Nitro blue tetrazolium chloride/5-Bromo-4-chloro-3-indolyl phosphate; PAGE, polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline; PSD-95, postsynaptic density protein 95; RT, room temperature; sAPPa, soluble amyloid precursor protein alpha; SDS, sodium dodecyl sulfate; TBS, Tris-buffered saline; TBST, 0.05% Tween 20 in Tris-buffered saline.

The mammalian hippocampus displays a peculiar pattern of fast (≈200 Hz) network oscillations superimposed on slower sharp waves. Such sharp wave–ripple complexes (SPW–R) have been implicated in memory consolidation. We have recently described a novel and unique method for studying SPW–R in naive slices of murine hippocampus. Here, we used this model to analyse network and cellular mechanisms of this type of network activity. SPW–R are usually generated within area CA3 but can also originate within the isolated CA1 region. Cellular synchronisation during SPW–R requires both excitatory and inhibitory synaptic transmission as well as electrical coupling, the latter being particularly important for the high‐frequency component. Extracellular and intracellular recordings revealed a surprisingly strong inhibition of most CA1 pyramidal cells during SPW–R. A minority of active cells, however, increases action potential frequency and fires in strict synchrony with the field ripples. This strong separation between members and non‐members of the network may serve to ensure a high signal‐to‐noise ratio in information processing during sharp wave–ripple complexes.

Abnormal accumulation of soluble oligomers of amyloid ␤ (A␤) is believed to cause malfunctioning of neurons in Alzheimer's disease. It has been shown that A␤ oligomers impair synaptic plasticity, thereby altering the ability of the neuron to store information. We examined the underlying cellular mechanism of A␤ oligomer-induced synaptic modifications by using a recently described stable oligomeric A␤ preparation called "A␤ 1-42 globulomer." Synthetically prepared A␤ 1-42 globulomer has been shown to localize to neurons and impairs long-term potentiation (Barghorn et al., 2005). Here, we demonstrate that A␤ 1-42 globulomer does not affect intrinsic neuronal properties, as assessed by measuring input resistance and discharge characteristics, excluding an unspecific alteration of membrane properties. We provide evidence that A␤ 1-42 globulomer, at concentrations as low as 8 nM, specifically suppresses spontaneous synaptic activity resulting from a reduction of vesicular release at terminals of both GABAergic and glutamatergic synapses. EPSCs and IPSCs were primarily unaffected. A detailed search for the precise molecular target of A␤ 1-42 globulomer revealed a specific inhibition of presynaptic P/Q calcium currents, whereas other voltage-activated calcium currents remained unaltered. Because intact P/Q calcium currents are needed for synaptic plasticity, the disruption of such currents by A␤ 1-42 globulomer may cause deficits in cellular mechanisms of information storage in brains of Alzheimer's disease patients. The inhibitory effect of A␤ 1-42 globulomer on synaptic vesicle release could be reversed by roscovitine, a specific enhancer of P/Q currents. Selective enhancement of the P/Q calcium current may provide a promising strategy in the treatment of Alzheimer's disease.

Degenerative dementia is mainly caused by Alzheimer's disease and/or cerebrovascular abnormalities. Disturbance of the intracellular calcium homeostasis is central to the pathophysiology of neurodegeneration. In Alzheimer's disease, enhanced calcium load may be brought about by extracellular accumulation of amyloid-β. Recent studies suggest that soluble forms facilitate influx through calcium-conducting ion channels in the plasma membrane, leading to excitotoxic neurodegeneration. Calcium channel blockade attenuates amyloid-β-induced neuronal decline in vitro and is neuroprotective in animal models. Vascular dementia, on the other hand, is caused by cerebral hypoperfusion and may benefit from calcium channel blockade due to relaxation of the cerebral vasculature. Several calcium channel blockers have been tested in clinical trials of dementia and the outcome is heterogeneous. Nimodipine as well as nilvadipine prevent cognitive decline in some trials, whereas other calcium channel blockers failed. In trials with a positive outcome, BP reduction did not seem to play a role in preventing dementia, indicating a direct protecting effect on neurons. An optimization of calcium channel blockers for the treatment of dementia may involve an increase of selectivity for presynaptic calcium channels and an improvement of the affinity to the inactivated state. Novel low molecular weight compounds suitable for proof-of-concept studies are now available. AbbreviationsAβ, amyloid-β; AD, Alzheimer's disease; APP, amyloid precursor protein; LTP, long-term potentiation; VaD, vascular dementia; VGCC, voltage-gated calcium channel IntroductionDementia affects 7% of the population over the age of 65, and then progressively increases with age Rocca et al., 1991). Alzheimer's disease (AD) is the leading cause of dementia, followed by vascular dementia (VaD). AD is characterized by three neuropathological hallmarks: extracellular aggregates of amyloid-β (Aβ) peptide (amyloid plaques), neurofibrillary tangles and synaptic loss (Bell and Cuello, 2006). According to the amyloid cascade hypothesis, overproduction of the hydrophobic peptide Aβ 1-42 is the basis for AD pathology (Hardy and Higgins, 1992).Aggregation of Aβ 1-42 is thought to occur in several steps via fibrils, which are finally deposited as amyloid plaques. It was suggested that an alternative pathway leads to the generation of stable oligomeric aggregates, Aβ oligomers (Gellermann et al., 2008;Yu et al., 2009b), which are considered to mediate the toxic Aβ effects (Hardy and Selkoe, 2002). Different forms of Aβ oligomers can be generated synthetically (e.g. Stine et al., 2003; Barghorn et al., 2005) or are harvested from cell lines (Walsh et al., 2002). These preparations were tested in vitro and in animal models, and the results lead to the generally accepted view that Aβ oligomers specifically disturb synaptic function . Constant impairment of neurotransmission leads to a retraction of synapses, which is then evident in the autopsy of AD brains (for a review, see Nimmrich a...

The characteristic, behaviour-related network oscillations of the mammalian hippocampus (θ, γ and ripples) are accompanied by strongly phase-coupled action potentials in specific subsets of GABAergic interneurones. It has been suggested that the resulting phasic, repetitive inhibition shapes rhythmic coherent activity of the neuronal network. Here, we examined whether synaptic inhibition entrains ∼200 Hz network ripples by applying the GABA A receptor antagonist gabazine to CA1 minislices of mouse hippocampus. Gabazine blocked spontaneously occurring sharp wave-ripple (SPW-R) activity. However, local application of KCl to the dendritic layer elicited excitatory sharp waves on which ∼200 Hz ripple oscillations were superimposed with equal temporal properties of native SPW-R. The activity also persisted in the additional presence of blockers of glutamatergic synaptic transmission. In contrast, synchrony was largely abolished after addition of gap junction blockers. Thus, GABAergic transmission appears to be involved in the generation of sharp waves but phasic inhibition is no prerequisite for the precise synchronization of hippocampal neurones during high-frequency oscillations at ∼200 Hz. Gap junctions on the other hand seem to be necessary to orchestrate coordinated activity within the ripple frequency domain.