The main conclusion from this study is that natural Abeta oligomers can acutely impair the formation of a contextual fear memory. This conclusion is supported by three experiments that used different time points of Abeta injection. The first experiment used repeated injection before and after fear conditioning and before retrieval. This resulted in impaired retrieval of contextual fear memory. In order to determine which time point caused this effect, we performed a second experiment with a single injection 1 hour before fear conditioning. This again resulted in impaired retrieval of contextual fear memory. In order to determine if this was caused by an acute effect on fear conditioning or a delayed effect on context fear retrieval, we performed a third experiment with a single Abeta injection 2 hours before fear conditioning. This had no effect on the retrieval of contextual fear memory, indicating that the single Abeta injection did not have a lasting effect causing impaired retrieval one day later. Therefore, we conclude that injecting Abeta 1 hour before fear conditioning resulted in an Abeta concentration around the time of fear conditioning that acutely impaired the formation of a contextual fear memory. The lack of effect of Abeta injected 2 hours before fear conditioning might have been caused by the reported 2 hour half-life of Abeta in the brain 
. The impaired retrieval of the contextual fear memory found in experiment 1 and 2 was not caused by a state-dependent effect, meaning the internal state of the mice during retrieval differed from their internal state during fear conditioning. In experiment 1 the mice were injected both before fear conditioning and before retrieval and therefore presumably were in the same state, which nevertheless still resulted in an impaired contextual fear memory. In summary, the data indicate that natural Abeta oligomers have an acute effect during or shortly after fear conditioning, which results in the impaired formation of a contextual fear memory.
An acute effect of Abeta oligomers during or shortly after fear conditioning would impact learning or consolidation respectively. Effects on learning have to be interpreted with caution, since they might be caused by impaired detection of the conditioned stimulus (CS: context) or the unconditioned stimulus (US: footshock). In none of the three experiments we observed an effect of Abeta on the formation and retrieval of a tone fear memory. This shows that Abeta injection did not impair hearing function or shock sensation. In addition, Abeta injection did not impair freezing at the end of the fear conditioning trial when the context was the only available CS. This indicates that Abeta injected mice were able to detect both the CS (context) and the US (footshock). Since Abeta did not impair freezing during fear conditioning, it is likely that learning was unaffected and that the impaired retrieval of the contextual fear memory was caused by impaired consolidation shortly after fear conditioning. However, it can not be excluded that learning was impaired by Abeta in a way that did not affect the immediate expression of fear during fear conditioning. Future studies that inject Abeta right after fear conditioning might be able to distinguish between Abeta effects on learning and consolidation. Either way, our data are in line with previous studies that have reported effects of Abeta oligomers on learning and consolidation 
Contextual fear memory is dependent on the hippocampus and tone fear memory is independent from the hippocampus 
, which suggests that the specific impairment of contextual fear memory in our study was caused by Abeta oligomers that diffused from the ventricle into the hippocampus. Accordingly, previous studies found that injection of natural Abeta oligomers into the lateral ventricle impaired synapse remodeling in the hippocampus 
, and increased glutamate levels in the hippocampus 
. Our inability to detect an effect on tone fear memory might indicate that an insufficient fraction of the injected Abeta reached the amygdala, which is a brain region critical for formation of tone fear memories 
. Since together with the hippocampus, the amygdala is one of the first brain regions affected in AD patients 
, future studies could inject Abeta oligomers directly into the amygdala to explore a possible role for the amygdala in AD associated memory loss.
Our finding that natural Abeta oligomers acutely impair the formation of a contextual fear memory is in agreement with previous studies that injected synthetic Abeta oligomers into the ventricle or hippocampus 
. In contrast, another study that injected synthetic Abeta oligomers into the hippocampus reported an improvement of contextual fear memory, suggesting that low levels of synthetic Abeta oligomers might have beneficial effects on memory 
. Since synthetic and natural Abeta oligomers have different efficacy profiles 
, and no positive effect of natural Abeta oligomers on memory has been reported yet, it remains to be shown if low levels of natural Abeta oligomers can also have positive effects on memory.
The results from our study are in agreement with other data that support a causal role of soluble Abeta oligomers in AD associated memory loss 
. We used 7PA2 cells to produce a natural Abeta oligomer solution that contained Abeta monomers, dimers, trimers, and tetramers. These same Abeta oligomers were found in 7PA2 preparations used in earlier studies 
Earlier studies using 7PA2 Abeta oligomers found binding of Abeta oligomers to EphB2 protein, impaired in-vivo long-term potentiation, and impaired memory as tested in the alternating lever cyclic ratio test, radial arm maze, and passive avoidance test 
. Importantly, the natural Abeta oligomer solution derived from 7PA2 cells used in this and other studies contains Abeta oligomer species similar to those found in AD brain tissue, which when injected into rat brains also impair memory 
. It is therefore conceivable that injection of 7PA2 derived natural Abeta oligomers into the mouse brain recapitulates a causal factor responsible for memory loss in AD patients, and that the mechanism responsible for Abeta oligomer impaired contextual fear conditioning also contributes to AD associated memory loss.
In summary, we found an acute impairing effect of natural Abeta oligomers on contextual fear memory in mice. This finding is in agreement with natural Abeta oligomer induced memory impairments found in previous studies that used rats 
. Our data show, to our knowledge, for the first time that natural Abeta oligomers can also impair memory in mice. Our study thereby paves the way for using genetic mouse models to study the underlying mechanisms by which natural Abeta oligomers impair memory. This is also, to our knowledge, the first time that an effect of natural Abeta oligomers on fear conditioning is reported. Since fear conditioning requires only a single learning trial, it is highly suitable for dissecting the different stages of memory (acquisition, consolidation, storage, and retrieval), allowing to test the contribution of each memory stage to AD associated memory loss. We propose that natural Abeta oligomer impaired fear conditioning can be used to test potential mechanisms and treatments of AD associated memory loss. A better understanding of the acute effects of natural Abeta oligomers might lead to treatments that can reverse cognitive impairments during early stages of AD, and possibly prevent or delay the progress of AD.