Improved Learning and Memory in Aged Mice Deficient in Amyloid β-Degrading Neutral Endopeptidase

BACKGROUND: Neutral endopeptidase, also known as neprilysin and abbreviated NEP, is considered to be one of the key enzymes in initial human amyloid-beta (Abeta) degradation. The aim of our study was to explore the impact of NEP deficiency on the initial development of dementia-like symptoms in mice. METHODOLOGY/PRINCIPAL FINDINGS: We found that while endogenous Abeta concentrations were elevated in the brains of NEP-knockout mice at all investigated age groups, immunohistochemical analysis using monoclonal antibodies did not detect any Abeta deposits even in old NEP knockout mice. Surprisingly, tests of learning and memory revealed that the ability to learn was not reduced in old NEP-deficient mice but instead had significantly improved, and sustained learning and memory in the aged mice was congruent with improved long-term potentiation (LTP) in brain slices of the hippocampus and lateral amygdala. Our data suggests a beneficial effect of pharmacological inhibition of cerebral NEP on learning and memory in mice due to the accumulation of peptides other than Abeta degradable by NEP. By conducting degradation studies and peptide measurements in the brain of both genotypes, we identified two neuropeptide candidates, glucagon-like peptide 1 and galanin, as first potential candidates to be involved in the improved learning in aged NEP-deficient mice. CONCLUSIONS/SIGNIFICANCE: Thus, the existence of peptides targeted by NEP that improve learning and memory in older individuals may represent a promising avenue for the treatment of neurodegenerative diseases

A Fear-Inducing Odor Alters PER2 and c-Fos Expression in Brain Regions Involved in Fear Memory

Evidence demonstrates that rodents learn to associate a foot shock with time of day, indicating the formation of a fear related time-stamp memory, even in the absence of a functioning SCN. In addition, mice acquire and retain fear memory better during the early day compared to the early night. This type of memory may be regulated by circadian pacemakers outside of the SCN. As a first step in testing the hypothesis that clock genes are involved in the formation of a time-stamp fear memory, we exposed one group of mice to fox feces derived odor (TMT) at ZT 0 and one group at ZT 12 for 4 successive days. A separate group with no exposure to TMT was also included as a control. Animals were sacrificed one day after the last exposure to TMT, and PER2 and c-Fos protein were quantified in the SCN, amygdala, hippocampus, and piriform cortex. Exposure to TMT had a strong effect at ZT 0, decreasing PER2 expression at this time point in most regions except the SCN, and reversing the normal rhythm of PER2 expression in the amygdala and piriform cortex. These changes were accompanied by increased c-Fos expression at ZT0. In contrast, exposure to TMT at ZT 12 abolished the rhythm of PER2 expression in the amygdala. In addition, increased c-Fos expression at ZT 12 was only detected in the central nucleus of the amygdala in the TMT12 group. TMT exposure at either time point did not affect PER2 or c-Fos in the SCN, indicating that under a light-dark cycle, the SCN rhythm is stable in the presence of repeated exposure to a fear-inducing stimulus. Taken together, these results indicate that entrainment to a fear-inducing stimulus leads to changes in PER2 and c-Fos expression that are detected 24 hours following the last exposure to TMT, indicating entrainment of endogenous oscillators in these regions. The observed effects on PER2 expression and c-Fos were stronger during the early day than during the early night, possibly to prepare appropriate systems at ZT 0 to respond to a fear-inducing stimulus

Capsaicin-Induced Changes in LTP in the Lateral Amygdala Are Mediated by TRPV1

The transient receptor potential vanilloid type 1 (TRPV1) channel is a well recognized polymodal signal detector that is activated by painful stimuli such as capsaicin. Here, we show that TRPV1 is expressed in the lateral nucleus of the amygdala (LA). Despite the fact that the central amygdala displays the highest neuronal density, the highest density of TRPV1 labeled neurons was found within the nuclei of the basolateral complex of the amygdala. Capsaicin specifically changed the magnitude of long-term potentiation (LTP) in the LA in brain slices of mice depending on the anesthetic (ether, isoflurane) used before euthanasia. After ether anesthesia, capsaicin had a suppressive effect on LA-LTP both in patch clamp and in extracellular recordings. The capsaicin-induced reduction of LTP was completely blocked by the nitric oxide synthase (NOS) inhibitor L-NAME and was absent in neuronal NOS as well as in TRPV1 deficient mice. The specific antagonist of cannabinoid receptor type 1 (CB1), AM 251, was also able to reduce the inhibitory effect of capsaicin on LA-LTP, suggesting that stimulation of TRPV1 provokes the generation of anandamide in the brain which seems to inhibit NO synthesis. After isoflurane anesthesia before euthanasia capsaicin caused a TRPV1-mediated increase in the magnitude of LA-LTP. Therefore, our results also indicate that the appropriate choice of the anesthetics used is an important consideration when brain plasticity and the action of endovanilloids will be evaluated. In summary, our results demonstrate that TRPV1 may be involved in the amygdala control of learning mechanisms

Effects of Inescapable Stress on LTP in the Amygdala versus the Dentate Gyrus of Freely Behaving Rats

Stress impairs hippocampal long‐term potentiation (LTP), a model of synaptic plasticity that is assumed to underlie memory formation. In the amygdala, little is known about the effects of stress on LTP, or about its longevity. Here we assessed the ability of entorhinal cortex (EC) stimulation to induce LTP simultaneously in the basal amygdaloid nucleus (B) and in the dentate gyrus (DG) of freely behaving Wistar rats. We also tested whether LTP persists over days. Once established, we investigated the effects of acute vs. repeated inescapable stressful experiences on LTP in both structures. Results show that B, like DG, sustained LTP for 7 days. Furthermore, a single exposure to moderate stress facilitated LTP in B but did not affect DG LTP. Stress re‐exposure inhibited LTP in DG but only long‐lasting LTP (\u3e3 days) in B. Behaviourally, animals exhibited a higher immobility when re‐exposed to the stressor than with a single/first exposure. These data support a role for B in memory storage. Furthermore, they support a differential involvement of the amygdala and hippocampus in memory formation and storage depending on the emotional characteristics of the experience

Multiple spine boutons are formed after long-lasting LTP in the awake rat

The formation of multiple spine boutons (MSBs) has been associated with cognitive abilities including hippocampal-dependent associative learning and memory. Data obtained from cultured hippocampal slices suggest that the long-term maintenance of synaptic plasticity requires the formation of new synaptic contacts on pre-existing synapses. This postulate however, has never been tested in the awake, freely moving animals. In the current study, we induced long-term potentiation (LTP) in the dentate gyrus (DG) of awake adult rats and performed 3-D reconstructions of electron micrographs from thin sections of both axonal boutons and dendritic spines, 24 h post-induction. The specificity of the observed changes was demonstrated by comparison with animals in which long-term depression (LTD) had been induced, or with animals in which LTP was blocked by an N-methyl-D-aspartate (NMDA) antagonist. Our data demonstrate that whilst the number of boutons remains unchanged, there is a marked increase in the number of synapses per bouton 24 h after the induction of LTP. Further, we demonstrate that this increase is specific to mushroom spines and not attributable to their division. The present investigation thus fills the gap existing between behavioural and in vitro studies on the role of MSB formation in synaptic plasticity and cognitive abilities