Genomic analysis of sleep deprivation reveals translational regulation in the hippocampus.

Sleep deprivation is a common problem of considerable health and economic impact in today's society. Sleep loss is associated with deleterious effects on cognitive functions such as memory and has a high comorbidity with many neurodegenerative and neuropsychiatric disorders. Therefore, it is crucial to understand the molecular basis of the effect of sleep deprivation in the brain. In this study, we combined genome-wide and traditional molecular biological approaches to determine the cellular and molecular impacts of sleep deprivation in the mouse hippocampus, a brain area crucial for many forms of memory. Microarray analysis examining the effects of 5 h of sleep deprivation on gene expression in the mouse hippocampus found 533 genes with altered expression. Bioinformatic analysis revealed that a prominent effect of sleep deprivation was to downregulate translation, potentially mediated through components of the insulin signaling pathway such as the mammalian target of rapamycin (mTOR), a key regulator of protein synthesis. Consistent with this analysis, sleep deprivation reduced levels of total and phosphorylated mTOR, and levels returned to baseline after 2.5 h of recovery sleep. Our findings represent the first genome-wide analysis of the effects of sleep deprivation on the mouse hippocampus, and they suggest that the detrimental effects of sleep deprivation may be mediated by reductions in protein synthesis via downregulation of mTOR. Because protein synthesis and mTOR activation are required for long-term memory formation, our study improves our understanding of the molecular mechanisms underlying the memory impairments induced by sleep deprivation.

Aging impairs hippocampus-dependent long-term memory for object location in mice.

The decline in cognitive function that accompanies normal aging has a negative impact on the quality of life of the elderly and their families. Studies in humans and rodents show that spatial navigation and other hippocampus-dependent functions are particularly vulnerable to the deleterious effects of aging. However, reduced motor activity and alterations in the stress response that accompany normal aging can hinder the ability to study certain cognitive behaviors in aged animals. In an attempt to circumvent these potential confounds, we used a hippocampus-dependent object-place recognition task to show that long-term spatial memory is impaired in aged mice. Aged animals performed similarly to young adult mice on an object recognition task that does not rely on hippocampal function.

A molecular basis for interactions between sleep and memory.

The electrophysiological properties of the sleeping brain profoundly influence memory function in various species, yet the molecular nature by which sleep and memory interact remains unclear. We summarize work that has established the cAMP-PKA-CREB intracellular signaling pathway as a major mechanism involved in the wakeful consolidation of memory in many organisms while highlighting newer evidence that this pathway has a role in sleep regulation, sleep deprivation and potentially sleep-memory interactions. We explore the possibility that sleep might influence memory processing by reactivating the same molecular cascades first recruited during learning during a sort of "molecular replay". Lastly, we discuss how new approaches together with established techniques will aid in our understanding of the nature of sleep-memory interactions.

Exchange protein activated by cAMP enhances long-term memory formation independent of protein kinase A.

It is well established that cAMP signaling within neurons plays a major role in the formation of long-term memories--signaling thought to proceed through protein kinase A (PKA). However, here we show that exchange protein activated by cAMP (Epac) is able to enhance the formation of long-term memory in the hippocampus and appears to do so independent of PKA, thus demonstrating the importance of Epac-mediated signaling in memory consolidation.

The role of protein synthesis in memory consolidation: progress amid decades of debate.

A major component of consolidation theory holds that protein synthesis is required to produce the synaptic modification needed for long-term memory storage. Protein synthesis inhibitors have played a pivotal role in the development of this theory. However, these commonly used drugs have unintended effects that have prompted some to reevaluate the role of protein synthesis in memory consolidation. Here we review the role of protein synthesis in memory formation as proposed by consolidation theory calling special attention to the controversy involving the non-specific effects of a group of protein synthesis inhibitors commonly used to study memory formation in vivo. We argue that molecular and genetic approaches that were subsequently applied to the problem of memory formation confirm the results of less selective pharmacological studies. Thus, to a certain extent, the debate over the role of protein synthesis in memory based on interpretational difficulties inherent to the use of protein synthesis inhibitors may be somewhat moot. We conclude by presenting avenues of research we believe will best provide answers to both long-standing and more recent questions facing field of learning and memory.

Dynamic shifts in corticostriatal expression patterns of the immediate early genes Homer 1a and Zif268 during early and late phases of instrumental training.

Adaptive motor actions require prior knowledge of instrumental contingencies. With practice, these actions can become highly automatic in nature. However, the molecular and anatomical substrates mediating these related forms of learning are not understood. In the present study, we used in situ hybridization to measure the mRNA levels of two immediate early genes (IEGs) in an instrumental paradigm where rats learned to lever-press for food. We report that after three training sessions, Homer 1a and Zif268 (an effector and regulatory IEG, respectively) were significantly induced within an extensive corticostriatal network relative to untrained controls. With extended training (23 sessions), however, a shift in the expression patterns of the two genes was evident. Expression of Homer 1a (official symbol Homer1) decreased significantly in frontal and cingulate cortices, whereas striatal expression was generally maintained. Interestingly, Homer 1a expression markedly increased with extensive training in the ventrolateral region of the striatum (VLS) relative to early learners, suggesting that plasticity in the VLS is required for the efficient production of the learned behavior or in habit formation. Zif268 (official symbol Egr1) expression generally decreased with extensive training; however, these changes were not significant. These results demonstrate for the first time, on a molecular level, a dynamic shift in the contribution of corticostriatal systems mediating the early acquisition and consolidation of goal-directed responses to those engaged after extensive training.

Cracking addiction the second time around: reconsolidation of drug-related memories.

One of the greatest challenges in the understanding and treatment of addiction is cue-elicited relapse to drug use. The present findings of Miller and Marshall and Lee et al. reported in this issue of Neuron demonstrate that retrieved drug-related memories undergo reconsolidation and thus suggest that these maladaptive associations may be more labile than previously thought.

AMPA/kainate, NMDA, and dopamine D1 receptor function in the nucleus accumbens core: a context-limited role in the encoding and consolidation of instrumental memory.

Neural integration of glutamate- and dopamine-coded signals within the nucleus accumbens (NAc) is a fundamental process governing cellular plasticity underlying reward-related learning. Intra-NAc core blockade of NMDA or D1 receptors in rats impairs instrumental learning (lever-pressing for sugar pellets), but it is not known during which phase of learning (acquisition or consolidation) these receptors are recruited, nor is it known what role AMPA/kainate receptors have in these processes. Here we show that pre-trial intra-NAc core administration of the NMDA, AMPA/KA, and D1 receptor antagonists AP-5 (1 microg/0.5 microL), LY293558 (0.01 or 0.1 microg/0.5 microL), and SCH23390 (1 microg/0.5 microL), respectively, impaired acquisition of a lever-pressing response, whereas post-trial administration left memory consolidation unaffected. An analysis of the microstructure of behavior while rats were under the influence of these drugs revealed that glutamatergic and dopaminergic signals contribute differentially to critical aspects of the initial, randomly emitted behaviors that enable reinforcement learning. Thus, glutamate and dopamine receptors are activated in a time-limited fashion-only being required while the animals are actively engaged in the learning context.

Long-term memory for instrumental responses does not undergo protein synthesis-dependent reconsolidation upon retrieval.

Recent evidence indicates that certain forms of memory, upon recall, may return to a labile state requiring the synthesis of new proteins in order to preserve or reconsolidate the original memory trace. While the initial consolidation of "instrumental memories" has been shown to require de novo protein synthesis in the nucleus accumbens, it is not known whether memories of this type undergo protein synthesis-dependent reconsolidation. Here we show that low doses of the protein synthesis inhibitor anisomycin (ANI; 5 or 20 mg/kg) administered systemically in rats immediately after recall of a lever-pressing task potently impaired performance on the following daily test sessions. We determined that the nature of this impairment was attributable to conditioned taste aversion (CTA) to the sugar reinforcer used in the task rather than to mnemonic or motoric impairments. However, by substituting a novel flavored reinforcer (chocolate pellets) prior to the administration of doses of ANI (150 or 210 mg/kg) previously shown to cause amnesia, a strong CTA to chocolate was induced sparing any aversion to sugar. Importantly, when sugar was reintroduced on the following session, we found that memory for the task was not significantly affected by ANI. Thus, these data suggest that memory for a well-learned instrumental response does not require protein synthesis-dependent reconsolidation as a means of long-term maintenance.

Glutamate-mediated plasticity in corticostriatal networks: role in adaptive motor learning.

Little is known about how memories of new voluntary motor actions, also known as procedural memory, are formed at the molecular level. Our work examining acquisition of lever-pressing for food in rats has shown that activation of glutamate NMDA receptors, within broadly distributed but interconnected regions (e.g., nucleus accumbens core, prefrontal cortex, basolateral amygdala), is critical for such learning to occur. This receptor stimulation triggers intracellular cascades that involve protein phosphorylation and new protein synthesis. In support of this idea, we have found that posttrial inhibition of protein synthesis in the ventral striatum impairs learning, whereas posttrial NMDA receptor blockade does not. More recent data show extension of this network to the central amygdala, where infusions of NMDA antagonists also impair learning. We hypothesize that activity in this distributed network (including dopaminergic activity and perhaps muscarinic cholinergic activity) computes coincident events and thus enhances the probability that temporally related actions and events (e.g., lever pressing and delivery of reward) become associated. Such basic mechanisms of plasticity within this reinforcement learning network also appear to be profoundly affected in addiction.

Early consolidation of instrumental learning requires protein synthesis in the nucleus accumbens.

It is widely held that long-term memories are established by consolidation of newly acquired information into stable neural representations, a process that requires protein synthesis and synaptic plasticity. Plasticity within the nucleus accumbens (NAc), a major component of the ventral striatum, is thought to mediate instrumental learning processes and many aspects of drug addiction. Here we show that the inhibition of protein synthesis within the NAc disrupts consolidation of an appetitive instrumental learning task (lever-pressing for food) in rats. Post-trial infusions of anisomycin immediately after the first several training sessions prevented consolidation, whereas infusions delayed by 2 or 4 hours had no effect. However, if the rats were allowed to learn the task, the behavior was not sensitive to disruption by intra-accumbens anisomycin. Control infusions into the medial NAc shell or the dorsolateral striatum did not impair learning; in fact, an enhancement was observed in the latter case. These results show that de novo protein synthesis within the NAc is necessary for the consolidation, but not reconsolidation, of appetitive instrumental memories.