Alzheimer's disease is associated with an increased risk of unprovoked seizures. However, the underlying mechanisms of seizure induction remain elusive. Here, we performed video-EEG recordings in mice carrying mutant human APPswe and PS1dE9 genes (APdE9 mice) and their wild-type littermates to determine the prevalence of unprovoked seizures. In two recording episodes at the onset of amyloid ␤ (A␤) pathogenesis (3 and 4.5 months of age), at least one unprovoked seizure was detected in 65% of APdE9 mice, of which 46% had multiple seizures and 38% had a generalized seizure. None of the wild-type mice had seizures. In a subset of APdE9 mice, seizure phenotype was associated with a loss of calbindin-D28k immunoreactivity in dentate granular cells and ectopic expression of neuropeptide Y in mossy fibers. In APdE9 mice, persistently decreased resting membrane potential in neocortical layer 2/3 pyramidal cells and dentate granule cells underpinned increased network excitability as identified by patch-clamp electrophysiology. At stimulus strengths evoking single-component EPSPs in wild-type littermates, APdE9 mice exhibited decreased action potential threshold and burst firing of pyramidal cells. Bath application (1 h) of A␤1-42 or A␤25-35 (proto-)fibrils but not oligomers induced significant membrane depolarization of pyramidal cells and increased the activity of excitatory cell populations as measured by extracellular field recordings in the juvenile rodent brain, confirming the pathogenic significance of bath-applied A␤ (proto-)fibrils. Overall, these data identify fibrillar A␤ as a pathogenic entity powerfully altering neuronal membrane properties such that hyperexcitability of pyramidal cells culminates in epileptiform activity.

We assessed baseline and KCl‐stimulated glutamate release by using microdialysis in freely moving young adult (7 months) and middle‐aged (17 months) transgenic mice carrying mutated human amyloid precursor protein and presenilin genes (APdE9 mice) and their wild‐type littermates. In addition, we assessed the age‐related development of amyloid pathology and spatial memory impaired in the water maze and changes in glutamate transporters. APdE9 mice showed gradual spatial memory impairment between 6 and 15 months of age. The stimulated glutamate release declined very robustly in 17‐month‐old APdE9 mice as compared to 7‐month‐old APdE9 mice. This age‐dependent decrease in stimulated glutamate release was also evident in wild‐type mice, although it was not as robust as in APdE9 mice. When compared to individual baselines, all aged wild‐type mice showed 25% or greater increase in glutamate release upon KCl stimulation, but none of the aged APdE9 mice. There was an age‐dependent decline in VGLUT1 levels, but not in the levels of VGLUT2, GLT‐1 or synaptophysin. Astrocyte activation as measured by glial acidic fibrillary protein was increased in middle‐aged APdE9 mice. Blunted pre‐synaptic glutamate response may contribute to memory deficit in middle‐aged APdE9 mice.

Memantine, a low-to moderate-affinity uncompetitive N-methyl-D-aspartate receptor antagonist, has been shown to improve learning and memory in several pharmacological models of Alzheimer's disease (AD). In the present study, the effect of memantine on locomotor activity, social behavior, and spatial learning was assessed in a transgenic mouse model of AD. Eight-month-old male C57BL/6J mice carrying mutated human APP and PS1 genes (APP/PS1) and their nontransgenic (NT) litter mates were administered a therapeutic dose of memantine (30 mg/kg/day p.o.) for 2 to 3 weeks. At this age, APP/PS1 mice show elevated levels of ␤-amyloid peptides in several brain regions. APP/PS1 mice exhibited less exploratory rearing and increased aggressive behavior compared with NT mice. In the water maze test for spatial learning, APP/PS1 mice had longer escape latencies to both hidden and visible platforms, but they did not differ from NT mice in their swimming speed. Memantine significantly improved the acquisition of the water maze in APP/PS1 mice without affecting swimming speed. Memantine did not affect either locomotor activity or aggressive behavior in either genotype. These data indicate that memantine improves hippocampus-based spatial learning in a transgenic mouse model of AD without producing nonspecific effects on locomotion/exploratory activity.In the mammalian brain, NMDA receptors are involved in important physiological functions such as synaptic plasticity and synapse formation, which play important roles in memory, learning, and the formation of neural networks during development (Mayer and Westbrook, 1987). NMDA receptors are also thought to be involved in a variety of neuropathological states caused by excitotoxic neuronal injury such as ischemia, epilepsy, and several neurodegenerative diseases (e.g., Alzheimer's disease, Parkinson's disease, and Huntington's disease). In fact, any central nervous system disorder in which neuronal loss is caused by glutamate-induced excitotoxicity has the potential to be treated by NMDA receptor antagonists. However, given the critical role of NMDA receptors in learning and memory (Morris, 1989;Tsien et al., 1996), it may appear counterintuitive that an NMDA receptor antagonist could improve the symptomatology of Alzheimer's disease (AD).Several NMDA receptor antagonists possessing high affinity for NMDA receptors [e.g., (ϩ)MK-801] have been found to cause neurobehavioral adverse effects such as hallucination and cognitive impairment (Benvenga and Spaulding, 1988;Abi-Saab et al., 1998). These adverse events have largely limited the clinical development of high-affinity NMDA receptor antagonists. An alternative approach to avoid such side effects is to produce a partial rather than complete blockade of the NMDA receptor. Partial receptor blockade can be achieved, for example, by low-affinity NMDA receptor antagonists, which typically possess a better therapeutic window than high-affinity NMDA receptor antagonists (Rogawski, 2000). Memantine, a low-to moderate-affinity NMDA r...

Proper morphogenesis of neuronal dendritic spines is essential for the formation of functional synaptic networks. However, it is not known how spines are initiated. Here, we identify the inverse-BAR (I-BAR) protein MIM/MTSS1 as a nucleator of dendritic spines. MIM accumulated to future spine initiation sites in a PIP2-dependent manner and deformed the plasma membrane outward into a proto-protrusion via its I-BAR domain. Unexpectedly, the initial protrusion formation did not involve actin polymerization. However, PIP2-dependent activation of Arp2/3-mediated actin assembly was required for protrusion elongation. Overexpression of MIM increased the density of dendritic protrusions and suppressed spine maturation. In contrast, MIM deficiency led to decreased density of dendritic protrusions and larger spine heads. Moreover, MIM-deficient mice displayed altered glutamatergic synaptic transmission and compatible behavioral defects. Collectively, our data identify an important morphogenetic pathway, which initiates spine protrusions by coupling phosphoinositide signaling, direct membrane bending, and actin assembly to ensure proper synaptogenesis.

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The axon initial segment (AIS) is the site of action potential initiation and serves as a cargo transport filter and diffusion barrier that helps maintain neuronal polarity. The AIS actin cytoskeleton comprises actin patches and periodic sub-membranous actin rings. We demonstrate that tropomyosin isoform Tpm3.1 co-localizes with actin patches and that the inhibition of Tpm3.1 led to a reduction in the density of actin patches. Furthermore, Tpm3.1 showed a periodic distribution similar to sub-membranous actin rings but Tpm3.1 was only partially congruent with submembranous actin rings. Nevertheless, the inhibition of Tpm3.1 affected the uniformity of the periodicity of actin rings. Furthermore, Tpm3.1 inhibition led to reduced accumulation of AIS structural and functional proteins, disruption in sorting somatodendritic and axonal proteins, and a reduction in firing frequency. These results show that Tpm3.1 is necessary for the structural and functional maintenance of the AIS.

Rapid reorganization and stabilization of the actin cytoskeleton in dendritic spines enables cellular processes underlying learning, such as long-term potentiation (LTP). Dendritic spines are enriched in exceptionally short and dynamic actin filaments, but the studies so far have not revealed the molecular mechanisms underlying the high actin dynamics in dendritic spines. Here, we show that actin in dendritic spines is dynamically phosphorylated at tyrosine-53 (Y53) in rat hippocampal and cortical neurons. Our findings show that actin phosphorylation increases the turnover rate of actin filaments and promotes the short-term dynamics of dendritic spines. During neuronal maturation, actin phosphorylation peaks at the first weeks of morphogenesis, when dendritic spines form, and the amount of Y53-phosphorylated actin decreases when spines mature and stabilize. Induction of LTP transiently increases the amount of phosphorylated actin and LTP induction is deficient in neurons expressing mutant actin that mimics phosphorylation. Actin phosphorylation provides a molecular mechanism to maintain the high actin dynamics in dendritic spines during neuronal development and to induce fast reorganization of the actin cytoskeleton in synaptic plasticity. In turn, dephosphorylation of actin is required for the stabilization of actin filaments that is necessary for proper dendritic spine maturation and LTP maintenance.