Cognitive problems occur in asymptomatic gene carriers of Huntington's disease (HD), and mouse models of the disease exhibit impaired learning and substantial deficits in the cytoskeletal changes that stabilize long-term potentiation (LTP). The latter effects may be related to the decreased production of brainderived neurotrophic factor (BDNF) associated with the HD mutation. This study asked whether up-regulating endogenous BDNF levels with an ampakine, a positive modulator of AMPA-type glutamate receptors, rescues plasticity and reduces learning problems in HD (CAG140) mice. Twice-daily injections of a short half-life ampakine normalized BDNF levels, activity-driven actin polymerization in dendritic spines, and LTP stabilization in 8-week-old mutants. Comparable results were obtained in 16-week-old HD mice with more severe LTP deficits. Ampakine treatments had no measurable effect on the decreased locomotor activity observed in the mutants but offset their impairments in long-term memory. Given that ampakines are well tolerated in clinical trials and were effective in this study after brief exposures, these results suggest a novel strategy for chronic treatment of the cognitive difficulties that occur in the early stages of HD.actin polymerization ͉ CAG140 ͉ long-term potentiation ͉ theta burst stimulation ͉ unsupervised learning H untington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation involving the trinucleotide CAG in the huntingtin gene (1, 2). Although severe motor impairments characterize the disease, cognitive and memory deficits are also present and often appear in advance of other symptoms (3-6). Impaired learning that occurs before or concurrent with motor dysfunction or neuron loss has been described for HD mouse models (7-9) as have deficits in hippocampal long-term potentiation (LTP) (10-13), a form of synaptic plasticity widely regarded as a neurobiological substrate for memory. Understanding why LTP is severely impaired in HD mice could explain the cognitive dysfunction seen in patients with the disease.Our recent investigations into HD-associated plasticity deficits established that actin polymerization in dendritic spines, which normally stabilizes LTP (14, 15), is defective in HD knockin mice and most likely explains the rapid decay of potentiation (11). Pertinent to this finding, both HD mice and patients have reduced forebrain levels of brain-derived neurotrophic factor (BDNF) and its TrkB receptor (16,17). BDNF is a releasable neurotrophin that promotes activity-driven actin polymerization in dendritic spines (15, 18) and potently facilitates LTP induction by theta burst stimulation (TBS; refs. 19 and 20). Thus, an HD-related failure in BDNF signaling could remove an essential element of the system that modifies the spine cytoskeleton, thereby disrupting the stable synaptic changes needed to encode memory. Accordingly, applying low concentrations of BDNF fully restored TBS-induced actin polymerization and LTP in hippocampal slices prepared from HD ...

Stabilization of long-term potentiation (LTP) depends on multiple signaling cascades linked to actin polymerization. We used one of these, involving phosphorylation of the regulatory protein cofilin, as a marker to test whether LTP-related changes occur in hippocampal synapses during unsupervised learning. Well handled rats were allowed to explore a compartmentalized environment for 30 min after an injection of vehicle or the NMDA receptor antagonist (Ϯ)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). Another group of rats consisted of vehicle-injected, home-cage controls. Vehicle-treated rats that explored the environment had 30% more spines with dense phosphorylated (p) cofilin immunoreactivity in hippocampal field CA1 than did rats in the home-cage group. The increase in pCofilin-positive spines and behavioral evidence for memory of the explored environment were both eliminated by CPP. Coimmunostaining for pCofilin and the postsynaptic density protein 95 (PSD-95) showed that synapses on pCofilin-positive spines were substantially larger than those on neighboring (pCofilin-negative) spines. These results establish that uncommon cellular events associated with LTP, including changes in synapse size, occur in individual spines during learning, and provide a technique for mapping potential engrams.

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