Showing papers in "Learning & Memory in 2006"

Microstimulation reveals opposing influences of prelimbic and infralimbic cortex on the expression of conditioned fear

Abstract: Recent studies using lesion, infusion, and unit-recording techniques suggest that the infralimbic (IL) subregion of medial prefrontal cortex (mPFC) is necessary for the inhibition of conditioned fear following extinction. Brief microstimulation of IL paired with conditioned tones, designed to mimic neuronal tone responses, reduces the expression of conditioned fear to the tone. In the present study we used microstimulation to investigate the role of additional mPFC subregions: the prelimbic (PL), dorsal anterior cingulate (ACd), and medial precentral (PrCm) cortices in the expression and extinction of conditioned fear. These are tone-responsive areas that have been implicated in both acquisition and extinction of conditioned fear. In contrast to IL, microstimulation of PL increased the expression of conditioned fear and prevented extinction. Microstimulation of ACd and PrCm had no effect. Under low-footshock conditions (to avoid ceiling levels of freezing), microstimulation of PL and IL had opposite effects, respectively increasing and decreasing freezing to the conditioned tone. We suggest that PL excites amygdala output and IL inhibits amygdala output, providing a mechanism for bidirectional modulation of fear expression.

Sleep after learning aids memory recall

Abstract: In recent years, the effect of sleep on memory consolidation has received considerable attention. In humans, these studies concentrated mainly on procedural types of memory, which are considered to be hippocampus-independent. Here, we show that sleep also has a persisting effect on hippocampus-dependent declarative memory. In two experiments, we examined high school students' ability to remember vocabulary. We show that declarative memory is enhanced when sleep follows within a few hours of learning, independent of time of day, and with equal amounts of interference during retention intervals. Sleep deprivation has a detrimental effect on memory, which was significant after a night of recovery sleep. Thus, fatigue accumulating during wake intervals could be ruled out as a confound.

ERK/MAPK regulates hippocampal histone phosphorylation following contextual fear conditioning.

Abstract: Long-term memory formation is regulated by many distinct molecular mechanisms that control gene expression. An emerging model for effecting a stable, coordinated pattern of gene transcription involves epigenetic tagging through modifications of histones or DNA. In this study, we investigated the regulation of histone phosphorylation in the hippocampus by the ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) pathway. We found that activation of ERK/MAPK in vitro significantly increased histone H3 phosphorylation in hippocampal area CA1. Furthermore, we found that contextual fear conditioning in vivo leads to a rapid time-dependent increase in histone H3 phosphorylation in area CA1. This increase paralleled the time course of contextual fear-dependent activation of ERK, and was inhibited in vivo by a latent inhibition paradigm as well as by injection of an N-methyl-d-aspartic acid receptor (NMDA-R) antagonist. Finally, injection of an inhibitor of MEK (MAP kinase/ERK kinase), the unique dual-specificity kinase upstream of ERK, blocked the increase in histone H3 phosphorylation seen after contextual fear conditioning. These results demonstrate that changes in histone phosphorylation in the hippocampus are regulated by ERK/MAPK following a behavioral fear conditioning paradigm.

In memory of consolidation

Abstract: How an animal learns, remembers, and uses information to guide adaptive behavior remains one of the most challenging questions in science today. Much progress was made in the twentieth century, and new tools available to neurobiological investigators have accelerated progress in the new century. Nevertheless, the road has been rocky and progress sometimes impeded by periodic polemic debates at a conceptual level. Retrospective examination of the nature of the divisive issues and how they were (or were not) resolved could help steer a new generation of investigators away from similar pitfalls and impasses. The same applies to scientists from other disciplines, recently joining in the “search for the engram,” who might not be aware of the vast literature generated, mainly by psychologists, in the middle decades of the last century. Our purpose here is not to furnish a complete review of this literature, but to provide a historical perspective for some of the unresolved issues that continue to be discussed within the context of the field of neurobiology of memory. For more general reviews, refer to McGaugh (2000) and Dudai (2004). Scientific investigation of memory processes was initiated at the end of the 19th century by psychologists in Germany, Ebbinghaus (1885) and then Mueller and Pilzecker (1900). Their studies of verbal learning and retention in human subjects led them to conclude that a memory trace was formed gradually over time after acquisition and they coined the term consolidation. Contemporary with this were the very influential clinical observations and theoretical elaborations of the French psychiatrist, Ribot (1882). From his studies of amnesic patients, he formulated “La loi de regression,” which simply notes that, as memories age, they become more resistant to trauma-induced amnesia.

Different mechanisms of fear extinction dependent on length of time since fear acquisition.

Abstract: Fear extinction is defined as a decline in conditioned fear responses (CRs) following nonreinforced exposure to a feared conditioned stimulus (CS). Behavioral evidence indicates that extinction is a form of inhibitory learning: Extinguished fear responses reappear with the passage of time (spontaneous recovery), a shift of context (renewal), and unsignaled presentations of the unconditioned stimulus (reinstatement). However, there also is evidence to suggest that extinction is an "unlearning" process corresponding to depotentiation of potentiated synapses within the amygdala. Because depotentiation is induced more readily at short intervals following LTP induction and is not inducible at all at a sufficient delay, it may be that extinction initiated shortly following fear acquisition preferentially engages depotentiation/"unlearning," whereas extinction initiated at longer delays recruits a different mechanism. We investigated this possibility through a series of behavioral experiments examining the recoverability of conditioned fear following extinction. Consistent with an inhibitory learning mechanism of extinction, rats extinguished 24-72 h following acquisition exhibited moderate to strong reinstatement, renewal, and spontaneous recovery. In contrast, and consistent with an erasure mechanism, rats extinguished 10 min to 1 h after acquisition exhibited little or no reinstatement, renewal, or spontaneous recovery. These data support a model in which different neural mechanisms are recruited depending on the temporal delay of fear extinction.

Impaired memory retrieval correlates with individual differences in cortisol response but not autonomic response.

Abstract: Stress can enhance or impair memory performance. Both cortisol release and sympathetic nervous system responses have been implicated in these differential effects. Here we investigated how memory retrieval might be affected by stress-induced cortisol release, independently of sympathetic nervous system stress responses. Thirty-two healthy participants (16 women) learned emotionally arousing and neutral words. One hour later, half of the participants underwent a stressor (cold pressor test) and the other half, a control warm water exposure, both followed by a delayed free recall task. The stressed participants were split into those who did (responders, N = 8) and those who did not (nonresponders, N = 6) show a cortisol response. Both responders and nonresponders showed comparable sympathetic nervous system activity (skin conductance level) during the cold pressor. The cortisol responders recalled significantly fewer words compared to nonresponders, and compared to control participants; this effect was most pronounced for moderately arousing words (compared to highly arousing and neutral words). These results suggest that individual differences in cortisol reactivity affect memory retrieval performance, and help to explain the differential effects of stress on memory.

Stability of recent and remote contextual fear memory.

Abstract: Following initial encoding, memories undergo a prolonged period of reorganization. While such reorganization may occur in many different memory systems, its purpose is not clear. Previously, we have shown that recall of recent contextual fear memories engages the dorsal hippocampus (dHPC). In contrast, recall of remote contextual fear memories engages a number of different cortical regions, including the anterior cingulate cortex (ACC). To examine whether this reorganization leads to greater memory stability, we examined reconsolidation of 1 d-old (recent) and 36 d-old (remote) contextual fear memory in mice. We infused the protein synthesis inhibitor, anisomycin (ANI), into either the dHPC or ACC immediately following retrieval of either a recent or remote contextual fear memory. In the dHPC, ANI infusions disrupted subsequent expression of recent, but not remote, contextual fear memory. Similar infusions into the ACC had no effect on either recent or remote contextual fear memories, whereas systemically applied ANI blocked subsequent remote memory expression when long re-exposure durations were used. Together, these data suggest that as memories mature they become increasingly stable. Furthermore, the dissociation between the effects of systemically and centrally administered ANI on remote memory suggests that stability is due, in part, to the distributed nature of remote contextual fear memories.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory.

Abstract: The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

A Transcription Factor-Binding Domain of the Coactivator CBP Is Essential for Long-Term Memory and the Expression of Specific Target Genes.

Abstract: Transcriptional activation is a key process required for long-term memory formation. Recently, the transcriptional coactivator CREB-binding protein (CBP) was shown to be critical for hippocampus-dependent long-term memory and hippocampal synaptic plasticity. As a coactivator with intrinsic histone acetyltransferase activity, CBP interacts with numerous transcription factors and contains multiple functional domains. Currently, it is not known which transcription factor-binding domain of CBP is essential for memory storage. Using mice that carry inactivating mutations in the CREB-binding (KIX) domain of the coactivator CBP (CBPKIX/KIX mice), we show that the KIX domain is required for long-term memory storage. These results are the first to identify an in vivo function for the KIX domain of CBP in the brain, and they suggest that KIX-interacting transcription factors recruit CBP histone acetyltransferase activity during long-term memory storage. One such KIX-interacting factor is the transcription factor CREB. Using quantitative real-time RT-PCR, we find that the expression of specific CREB target genes is reduced in the hippocampi of CBPKIX/KIX mice during memory consolidation. The recruitment of the transcriptional coactivator CBP via the KIX domain thus imparts target gene-dependent selectivity to CREB-driven transcriptional regulation, thereby activating genes required for the long-term storage of hippocampus-dependent memory.

Spatial and nonspatial escape strategies in the Barnes maze

Abstract: The Barnes maze is a spatial memory task that requires subjects to learn the position of a hole that can be used to escape the brightly lit, open surface of the maze. Two experiments assessed the relative importance of spatial (extra-maze) versus proximal visible cues in solving the maze. In Experiment 1, four groups of mice were trained either with or without a discrete visible cue marking the location of the escape hole, which was either in a fixed or variable location across trials. In Experiment 2, all mice were trained with the discrete visible cue marking the target hole location. Two groups were identical to the cued-target groups from Experiment 1, with either fixed or variable escape locations. For these mice, the discrete cue either was the sole predictor of the target location or was perfectly confounded with the spatial extra-maze cues. The third group also used a cued variable target, but a curtain was drawn around the maze to prevent the use of spatial cues to guide navigation. Probe trials with all escape holes blocked were conducted to dissociate the use of spatial and discrete proximal cues. We conclude that the Barnes maze can be solved efficiently using spatial, visual cue, or serial-search strategies. However, mice showed a strong preference for using the distal room cues, even when a discrete visible cue clearly marked the escape location. Importantly, these data show that the cued-target control version of the Barnes maze as typically conducted does not dissociate spatial from nonspatial abilities.

NT-3 facilitates hippocampal plasticity and learning and memory by regulating neurogenesis.

Abstract: In the adult brain, the expression of NT-3 is largely confined to the hippocampal dentate gyrus (DG), an area exhibiting significant neurogenesis. Using a conditional mutant line in which the NT-3 gene is deleted in the brain, we investigated the role of NT-3 in adult neurogenesis, hippocampal plasticity, and memory. Bromodeoxyuridine (BrdU)-labeling experiments demonstrated that differentiation, rather than proliferation, of the neuronal precursor cells (NPCs) was significantly impaired in DG lacking NT-3. Triple labeling for BrdU, the neuronal marker NeuN, and the glial marker GFAP indicated that NT-3 affects the number of newly differentiated neurons, but not glia, in DG. Field recordings revealed a selective impairment in long-term potentiation (LTP) in the lateral, but not medial perforant path-granule neuron synapses. In parallel, the NT-3 mutant mice exhibited deficits in spatial memory tasks. In addition to identifying a novel role for NT-3 in adult NPC differentiation in vivo, our study provides a potential link between neurogenesis, dentate LTP, and spatial memory.

Anisomycin infused into the hippocampus fails to block “reconsolidation” but impairs extinction: The role of re-exposure duration

Abstract: Recent studies have reported new evidence consistent with the hypothesis that reactivating a memory by re-exposure to a training context destabilizes the memory and induces "reconsolidation." In the present experiments, rats' memory for inhibitory avoidance (IA) training was tested 6 h (Test 1), 2 d (Test 2), and 6 d (Test 3) after training. On Test 1 the rats were either removed from the shock compartment immediately after entry or retained in the shock context for 200 sec, and intrahippocampal infusions of the protein synthesis inhibitor anisomycin (75 microg/side) were administered immediately after the test. Anisomycin infusions administered after Test 1 impaired IA performance on Test 2 in animals given the brief re-exposure, but impaired extinction in animals exposed to the context for 200 sec. Rats with anisomycin-induced retention impairment on Test 2 demonstrated spontaneous recovery of retention performance on Test 3, whereas rats showing extinction on Test 2 showed further extinction on Test 3. The findings indicate that post-retrieval administration of anisomycin impairs subsequent retention performance only in the absence of extinction and that this impairment is temporary.

Foreground Contextual Fear Memory Consolidation Requires Two Independent Phases of Hippocampal ERK/CREB Activation.

Abstract: Fear conditioning is a popular model for investigating physiological and cellular mechanisms of memory formation. In this paradigm, a footshock is either systematically associated to a tone (paired conditioning) or is pseudorandomly distributed (unpaired conditioning). In the former procedure, the tone/shock association is acquired, whereas in the latter procedure, the context/shock association will prevail. Animals with chronically implanted recording electrodes show enhanced amplitude of the extracellularly recorded field EPSP in CA1 pyramidal cells for up to 24 h after unpaired, but not paired, fear conditioning. This is paralleled by a differential activation of the ERK/CREB pathway in CA1, which is monophasic in paired conditioning (0–15 min post-conditioning), but biphasic (0–1 h and 9–12 h post-conditioning) in unpaired conditioning as revealed by immunocytochemistry and Western blotting. Intrahippocampal injection of the MEK inhibitor U0126 prior to each phase prevents the activation of both ERK1/2 and CREB after unpaired conditioning. Block of any activation phase leads to memory impairment. We finally reveal that the biphasic activation of ERK/CREB activity is independently regulated, yet both phases are critically required for the consolidation of long-term memories following unpaired fear conditioning. These data provide compelling evidence that CA1 serves different forms of memory by expressing differential cellular mechanisms that are dependent on the training regime.

Timing is Essential for Rapid Effects of Corticosterone on Synaptic Potentiation in the Mouse Hippocampus.

Abstract: Stress facilitates memory formation, but only when the stressor is closely linked to the learning context. These effects are, at least in part, mediated by corticosteroid hormones. Here we demonstrate that corticosterone rapidly facilitates synaptic potentiation in the mouse hippocampal CA1 area when high levels of the hormone and high-frequency stimulation coincide in time, but not when corticosterone is given either before or after repetitive stimulation. This effect could not be blocked by antagonists of the mineralocorticoid receptor and glucocorticoid receptor (spironolactone and RU 38486, respectively). These data provide a biological substrate for the important behavioral observation that stress and corticosteroid hormones can facilitate learning and memory processes.

Conditioned fear extinction and reinstatement in a human fear-potentiated startle paradigm.

Abstract: The purpose of this study was to analyze fear extinction and reinstatement in humans using fear-potentiated startle. Participants were fear conditioned using a simple discrimination procedure with colored lights as the conditioned stimuli (CSs) and an airblast to the throat as the unconditioned stimulus (US). Participants were extinguished 24 h after fear conditioning. Upon presentation of unsignaled USs after extinction, participants displayed significant fear reinstatement. In summary, these procedures produced robust fear-potentiated startle, significant CS+/CS- discrimination, within-session extinction, and significant reinstatement. This is the first demonstration of fear extinction and reinstatement in humans using startle measures.

Dopamine D1 Receptors in the Anterior Cingulate Cortex Regulate Effort-Based Decision Making.

Abstract: The anterior cingulate cortex (ACC) has been implicated in encoding whether or not an action is worth performing in view of the expected benefit and the cost of performing the action. Dopamine input to the ACC may be critical for this form of effort-based decision making; however, the role of distinct ACC dopamine receptors is yet unknown. Therefore, we examined in rats the effects of an intra-ACC D1 and D2 receptor blockade on effort-based decision making tested in a T-maze cost-benefit task. In this task, subjects could either choose to climb a barrier to obtain a high reward in one arm or a low reward in the other arm without a barrier. Unlike vehicle-treated rats, rats with intra-ACC infusion of the D1 receptor antagonist SCH23390 exhibited a reduced preference for the high-cost- high-reward response option when having the choice to obtain a low reward with little effort. In contrast, in rats with intra-ACC infusion of the D2 receptor antagonist eticlopride, the preference for the high-cost-high-reward response option was not altered relative to vehicle-treated rats. These data provide the first evidence that D1 receptors in the ACC regulate effort-based decision making.

Reversible hippocampal lesions disrupt water maze performance during both recent and remote memory tests

Abstract: Conventional lesion methods have shown that damage to the rodent hippocampus can impair previously acquired spatial memory in tasks such as the water maze. In contrast, work with reversible lesion methods using a different spatial task has found remote memory to be spared. To determine whether the finding of spared remote spatial memory depends on the lesion method, we reversibly inactivated the hippocampus with lidocaine either immediately (0-DAY) or 1 mo (30-DAY) after training in a water maze. For both the 0-DAY and 30-DAY retention tests, rats that received lidocaine infusions exhibited impaired performance. In addition, when the 0-DAY group was retested 2 d later, (when the drug was no longer active), the effect was reversed. That is, rats that had previously received lidocaine performed as well as control rats did. These findings indicate that the rodent hippocampus is important for both recent and remote spatial memory, as assessed in the water maze. What determines whether remote spatial memory is preserved or impaired following disruption of hippocampal function appears to be the type of task used to assess spatial memory, not the method used to disrupt the hippocampus.

Distinct roles for medial temporal lobe structures in memory for objects and their locations.

Abstract: The ability to learn and retain novel information depends on a system of structures in the medial temporal lobe (MTL) including the hippocampus and the surrounding entorhinal, perirhinal, and parahippocampal cortices. Damage to these structures produces profound memory deficits; however, the unique contribution to memory of each of these structures remains unclear. Here we have used functional magnetic resonance imaging (fMRI) to determine whether the perirhinal and parahippocampal cortices show differential memory-related activity. Based on the distinct patterns of cortical input to these two areas, we reasoned that these structures might show differential activity for spatial and object recognition memory. In each of 11 subjects, we found that the perirhinal cortex was active during both spatial and object memory encoding, while the anterior parahippocampal cortex was active only during spatial encoding. These data support the idea that MTL structures make distinct contributions to recognition memory performance.

Roles for Drosophila mushroom body neurons in olfactory learning and memory.

Abstract: Olfactory learning assays in Drosophila have revealed that distinct brain structures known as mushroom bodies (MBs) are critical for the associative learning and memory of olfactory stimuli. However, the precise roles of the different neurons comprising the MBs are still under debate. The confusion surrounding the roles of the different neurons may be due, in part, to the use of different odors as conditioned stimuli in previous studies. We investigated the requirements for the different MB neurons, specifically the alpha/beta versus the gamma neurons, and whether olfactory learning is supported by different subsets of MB neurons irrespective of the odors used as conditioned stimuli. We expressed the rutabaga (rut)-encoded adenylyl cyclase in either the gamma or alpha/beta neurons and examined the effects on restoring olfactory associative learning and memory of rut mutant flies. We also expressed a temperature-sensitive shibire (shi) transgene in these neuron sets and examined the effects of disrupting synaptic vesicle recycling on Drosophila olfactory learning. Our results indicate that although we did not detect odor-pair-specific learning using GAL4 drivers that primarily express in gamma neurons, expression of the transgenes in a subset of alpha/beta neurons resulted in both odor-pair-specific rescue of the rut defect as well as odor-pair-specific disruption of learning using shi(ts1).

Retrieval induces hippocampal-dependent reconsolidation of spatial memory.

Abstract: Nonreinforced retrieval can cause extinction and/or reconsolidation, two processes that affect subsequent retrieval in opposite ways. Using the Morris water maze task we show that, in the rat, repeated nonreinforced expression of spatial memory causes extinction, which is unaffected by inhibition of protein synthesis within the CA1 region of the dorsal hippocampus. However, if the number of nonreinforced retrieval trials is insufficient to induce long-lasting extinction, then a hippocampal protein synthesis-dependent reconsolidation process recovers the original memory. Inhibition of hippocampal protein synthesis after reversal learning sessions impairs retention of the reversed preference and blocks persistence of the original one, suggesting that reversal learning involves reconsolidation rather than extinction of the original memory. Our results suggest the existence of a hippocampal protein synthesis-dependent reconsolidation process that operates to recover or update retrieval-weakened memories from incomplete extinction.

Emotional memories are not all created equal: evidence for selective memory enhancement.

Abstract: Human brain imaging studies have shown that greater amygdala activation to emotional relative to neutral events leads to enhanced episodic memory. Other studies have shown that fearful faces also elicit greater amygdala activation relative to neutral faces. To the extent that amygdala recruitment is sufficient to enhance recollection, these separate lines of evidence predict that recognition memory should be greater for fearful relative to neutral faces. Experiment 1 demonstrated enhanced memory for emotionally negative relative to neutral scenes; however, fearful faces were not subject to enhanced recognition across a variety of delays (15 min to 2 wk). Experiment 2 demonstrated that enhanced delayed recognition for emotional scenes was associated with increased sympathetic autonomic arousal, indexed by the galvanic skin response, relative to fearful faces. These results suggest that while amygdala activation may be necessary, it alone is insufficient to enhance episodic memory formation. It is proposed that a sufficient level of systemic arousal is required to alter memory consolidation resulting in enhanced recollection of emotional events.

Differential involvement of the central amygdala in appetitive versus aversive learning.

Abstract: Understanding the function of the distinct amygdaloid nuclei in learning comprises a major challenge. In the two studies described herein, we used c-Fos immunolabeling to compare the engagement of various nuclei of the amygdala in appetitive and aversive instrumental training procedures. In the first experiment, rats that had already acquired a bar-pressing response to a partial food reinforcement were further trained to learn that an acoustic stimulus signaled either continuous food reinforcement (appetitive training) or a footshock (aversive training). The first training session of the presentation of the acoustic stimulus resulted in significant increases of c-Fos immunolabeling throughout the amygdala; however, the pattern of activation of the nuclei of the amygdala differed according to the valence of motivation. The medial part of the central amygdala (CE) responded, surprisingly, to the appetitive conditioning selectively. The second experiment was designed to extend the aversive versus appetitive conditioning to mice, trained either for place preference or place avoidance in an automated learning system (INTELLICAGE). Again, much more intense c-Fos expression was observed in the medial part of the CE after the appetitive training as compared to the aversive training. These data, obtained in two species and by means of novel experimental approaches balancing appetitive versus aversive conditioning, support the hypothesis that the central nucleus of the amygdala is particularly involved in appetitively motivated learning processes.

Early boost and slow consolidation in motor skill learning

Abstract: Motorskill learning is a dynamic process that continues covertly after training has ended and eventually leads to delayed increments in performance. Current theories suggest that this off-line improvement takes time and appears only after several hours. Here we show an early transient and short-lived boost in performance, emerging as early as 5-30 min after training but no longer observed 4 h later. This early boost is predictive of the performance achieved 48 h later, suggesting its functional relevance for memory processes.

α2A-adrenoceptor stimulation improves prefrontal cortical regulation of behavior through inhibition of cAMP signaling in aging animals

Abstract: The working-memory functions of the prefrontal cortex (PFC) are improved by stimulation of postsynaptic, alpha2A-adrenoceptors, especially in aged animals with PFC cognitive deficits. Thus, the alpha2A-adrenoceptor agonist, guanfacine, greatly improves working-memory performance in monkeys and rats following systemic administration or intra-PFC infusion. Alpha2A-adrenoceptors are generally coupled to Gi, which can inhibit adenylyl cyclases and reduce the production of cAMP. However, no study has directly examined whether the working-memory enhancement observed with guanfacine or other alpha2A-adrenoceptor agonists results from cAMP inhibition. The current study confirmed this hypothesis in both rats and monkeys, showing that treatments that increase cAMP-mediated signaling block guanfacine's beneficial effects. In aged rats, guanfacine was infused directly into the prelimbic PFC and was challenged with co-infusions of the cAMP analog, Sp-cAMPS. In aging monkeys, systemically administered guanfacine was challenged with the phosphodiesterase 4 inhibitor, rolipram, using intramuscular doses known to have no effect on their own. In both studies, agents that mimicked the actions of cAMP (rats) or increased endogenous cAMP (monkeys) completely blocked the enhancing effects of guanfacine on working-memory performance. These results are consistent with alpha2A-adrenoceptor stimulation enhancing PFC working-memory function via inhibition of cAMP-mediated signaling.

Critical role of nitric oxide-cGMP cascade in the formation of cAMP-dependent long-term memory

Abstract: In both vertebrates and invertebrates, nervous systems store information for short-term memory (STM) and long-term memory (LTM) by changing the strength of their synaptic connections (Kandel 2001). Studies in many species, including mollusca Aplysia, fruitflies Drosophila, and mice, suggest that STM storage is accompanied by transient changes in the strength of synaptic connections by covalent modifications of pre-existing proteins and that LTM storage, in contrast, is accompanied by enduring changes in synaptic strength that require transcription and translation of genes (Montarolo et al. 1986; DeZazzo and Tully 1995). In all of these species, formation of LTM requires an increase in intracellular cAMP and recruitment of the cAMP-dependent protein kinase (PKA) that phosphorylates the transcription factor, cAMP-responsive element-binding protein (CREB) (Bartsch et al. 1995; Yin et al. 1995; Abel et al. 1997). The roles of the cAMP pathway in the formation of LTM are often supplemented by other signaling pathways, most notably by the nitric oxide (NO)-cGMP signaling pathway (Lewin and Walters 1999; Lu et al. 1999). NO is a membrane-permeable molecule that functions in intercellular signaling in the brain (Garthwaite et al. 1988). In mice, NO contributes to late-phase long-term potentiation of synaptic transmission by stimulating soluble guanylate cyclase in target cells, and the resulting increase in cGMP concentration stimulates cGMP-dependent protein kinase (PKG), which acts in parallel with PKA to increase the phosphorylation of CREB (Lu et al. 1999). In nociceptive sensory neurons of Aplysia, the induction of long-term hyper-excitability after a noxious stimulus depends on the NO-cGMP-PKG pathway, which acts in parallel with the cAMP-PKA pathway (Lewin and Walters 1999). In associative olfactory learning in honey bees, it has been shown that prolonged PKA activation in the antennal lobe (primary olfactory center) mediated by the NO-cGMP pathway is critical for LTM induction (Muller 1996, 2000) and it has been speculated that PKA is activated by either cAMP or cGMP and thus, the cAMP pathway and NO-cGMP pathway act in parallel to activate PKA, although this hypothesis remains to be tested. Histochemical studies of the brains of various insect species (Muller and Bicker 1994; Bicker 2001) have suggested that NO synthase is distributed in the antennal lobe and the mushroom body, both centers implicated in olfactory learning and memory (Erber et al. 1980; Hammer and Menzel 1998). In crickets, we examined the biochemical pathway involved in the formation of LTM in associative olfactory learning. These animals are newly emerging subjects for the study of learning and memory. Crickets are capable of quickly learning olfactory signals and memorizing them for practically a lifetime (Matsumoto and Mizunami 2000, 2002b, 2004), and they can be easily used for detailed pharmacological (Matsumoto et al. 2003; Unoki et al. 2005) and electrophysiological (Paydar et al. 1999) studies as well as for RNAi analysis (Miyawaki et al. 2004). Our results indicate that sequential activation of the NO-cGMP signaling pathway, cyclic nucleotide-gated anion channel, calcium-calmodulin, and then the cAMP-PKA pathway, induces protein synthesis-dependent LTM, thereby demonstrating a new role of the NO-cGMP pathway, namely, induction of LTM by activating the cAMP pathway.

Reduced extinction of hippocampal-dependent memories in CPEB knockout mice.

Abstract: CPEB is a sequence-specific RNA binding protein that regulates translation at synapses. In neurons of CPEB knockout mice, synaptic efficacy is reduced. Here, we have performed a battery of behavioral tests and find that relative to wild-type animals, CPEB knockout mice, although similar on many baseline behaviors, have reduced extinction of memories on two hippocampal-dependent tasks. A corresponding microarray analysis reveals that about 0.14% of hippocampal genes have an altered expression in the CPEB knockout mouse. These data suggest that CPEB-dependent local protein synthesis may be an important cellular mechanism underlying extinction of hippocampal-dependent memories.

Reconsolidation: a brief history, a retrieval view, and some recent issues.

Abstract: This review briefly traces some of the history of the phenomenon of what has come to be called "reconsolidation." The early findings of retrograde amnesia for an old but reactivated memory led to several interesting but largely behaviorally oriented studies. With only a few sporadic exceptions, research in the area languished until about 2000, when several articles caught the attention of the neuroscience community and led to a number of studies examining the phenomenon at several different levels of analysis. We consider several of the current issues generated by those studies, present a retrieval based model that may account for some findings, and indicate some possible new directions on this topic.

Effects of stress and sex on acquisition and consolidation of human fear conditioning.

Abstract: We examined the relationship between stress hormone (cortisol) release and acquisition and consolidation of conditioned fear learning in healthy adults. Participants underwent acquisition of differential fear conditioning, and consolidation was assessed in a 24-h delayed extinction test. The acquisition phase was immediately followed by an 11-min psychosocial stress period (arithmetic test combined with a public speech). Salivary cortisol was sampled at various time points before and after acquisition and retention of fear conditioning. Results showed two effects of endogenous cortisol. Post-acquisition cortisol correlated with fear acquisition in male but not female participants. In addition, post-acquisition cortisol correlated with consolidation of fear but only in those participants with high cortisol levels. We conclude that in the short term, a robust and sexually dimorphic relationship exists between fear learning and stress hormone levels. For those participants whose fear learning is accompanied by high stress hormone levels, a long-term relationship exists between cortisol release and memory consolidation. These short-term and long-term effects may relate to the differential involvement of mineralocorticoid and glucocorticoid receptor subtypes, respectively. The findings have implications for understanding the role of stress, sex, and hormones in different stages of fear learning and memory.

Effects of Intra-Amygdala Infusion of CB1 Receptor Agonists on the Reconsolidation of Fear-Potentiated Startle.

Abstract: The cannabinoid CB1 receptor has been shown to be critically involved in the extinction of fear memory. Systemic injection of a CB1 receptor antagonist prior to extinction training blocked extinction. Conversely, administration of the cannabinoid uptake inhibitor AM404 facilitated extinction in a dose-dependent manner. Here we show that bilateral infusion of CB1 receptor agonists into the amygdala after memory reactivation blocked reconsolidation of fear memory measured with fear-potentiated startle. The effect was dose-dependent and could be blocked by AM251, a specific CB1 receptor antagonist. In contrast, the effect of CB1 agonists on reconsolidation was no longer seen if memory reactivation was omitted. Concomitant with block of reconsolidation, CB1 agonist-treated animals did not exhibit shock-induced reinstatement or spontaneous recovery of fear. The absence of recovery was not attributable to permanent damage to the amygdala in WIN-treated rats, nor did the effect result from alteration of baseline startle or shock reactivity. These results suggest that CB1 agonists could impair fear memory via blocking reconsolidation.

Combinatorial chromatin modifications and memory storage: a code for memory?

Abstract: Gene transcription and long-term memory storage have been linked in experiments going back for more than 30 years, but the molecular mechanisms responsible for the regulation of gene expression during memory consolidation remain the subject of intense investigation. Much work has focused on the role of individual transcription factors, such as cAMP-response elementbinding protein (CREB) or nuclear factor of B (NFB), in memory storage (Kaplan and Abel 2003; Yeh et al. 2004), but it is now clear that transcriptional regulation involves the concerted action of multiple transcription factors that interact with chromatin, a protein complex that packages DNA (Olins and Olins 2003). Originally thought to be static and structural in purpose, chromatin is now known to be very dynamic, exerting precise control over gene expression (Felsenfeld and Groudine 2003). In particular, the idea that chromatin remodeling may regulate gene expression for memory processes has gained considerable attention recently (Levenson and Sweatt 2005). It is this very concept that Chwang et al. (2006) investigate in their studies of transcriptional regulation during memory storage, which are described in this issue of Learning & Memory. Chwang et al. (2006) reveal that a critical signaling pathway in the hippocampus for memory storage, the extracellular signal-regulated kinase (ERK)/ mitogen-activated protein kinase (MAPK) pathway, functions to modify chromatin during memory consolidation (Fig. 1). To understand how chromatin impacts gene expression, it is necessary to understand the nucleosome, which is the building block of chromatin. A nucleosome consists of a histone protein core (comprised of histones H2A, H2B, H3, and H4) and is the first level of packaging of genomic DNA. The amino-terminal “tails” of these histone proteins extend beyond the globular core and are sites for post-translational modifications, including acetylation, phosphorylation, methylation, ubiquitination, and sumoylation (Peterson and Laniel 2004). These histone modifications orchestrate the recruitment of specific chromatin remodeling protein complexes to mediate celland promoter-specific gene expression. Further, there is a dynamic interplay between histone modifications and DNA modifications (such as DNA methylation), thus creating staggering combinatorial possibilities for gene regulation. Chromatin structure can be modified in three different but related ways: First, nucleosomes may be repositioned by ATP-dependent protein complexes; second, histone variants may replace core histones; and third, histone tails may be covalently modified (Felsenfeld and Groudine 2003). Sitespecific covalent modifications of histone tails can yield distinct transcriptional states. For example, the combination of histone H4 Lys8 acetylation, histone H3 Lys14 acetylation, and histone H3 Ser10 phosphorylation is often associated with transcriptional activation (Fig. 1). In contrast, tri-methylation of histone H3 Lys9 and the lack of histone H3 and H4 acetylation is associated with transcriptional repression (Peterson and Laniel 2004). Recent studies have revealed that histone modifications are especially relevant to mechanisms of transcriptional regulation during memory consolidation (Levenson and Sweatt 2005). Increasing histone acetylation at sites that correspond with transcriptional activation enhances memory and synaptic plasticity (Levenson et al. 2004), and the transcriptional coactivator and histone acetyltransferase CREB-binding protein (CBP) is critical for long-term memory and synaptic plasticity (Oike et al. 1999; Bourtchouladze et al. 2003; Alarcon et al. 2004; Korzus et al. 2004; Wood et al. 2005). Now, Chwang et al. (2006) implicate histone H3 Ser10 phosphorylation and histone H3 Lys14 acetylation, modifications that correlate with transcriptional activation, in transcriptional regulation during memory consolidation. As an initial approach, the investigators took advantage of the well-established roles of protein kinase A (PKA) and protein kinase C (PKC) in hippocampus-dependent long-term memory formation. By pharmacological activation of these kinases in tissue slices, they found that both PKA and PKC induced transient increases in histone H3 Ser10 phosphorylation and histone H3 Lys14 acetylation in area CA1 of the hippocampus. Activation of either PKA or PKC also stimulated phosphorylation of ERK, which has been shown to be involved in H3 phosphorylation in cell culture (Soloaga et al. 2003). Furthermore, an inhibitor of MAPK/ERK kinase (MEK), the kinase primarily responsible for ERK phosphorylation, inhibited both phosphorylation and acetylation of histone H3. To examine the effect of these histone modifications on memory storage, the investigators turned to contextual fear conditioning (Fig. 1). The investigators observed a transient increase in both histone H3 Ser10 phosphorylation and histone H3 Lys14 acetylation that was paralleled by an increase in ERK phosphorylation. The peak level of these modifications was observed at 1 h postconditioning, when each modification was >50% above its baseline level. In support of the hypothesis that these changes are associated with memory consolidation, both ERK phosphorylation and H3 modification were blocked by impairment of memory formation with repeated pre-exposure to the context in a latent inhibition paradigm or blockade of N-methyl-D-aspartate (NMDA) receptors. Finally, the investigators showed that the process that leads to modification of histone H3 requires MEK activity, suggesting a central role for MEK/ERK signaling in H3 modifications. Together, these experiments suggest that changes in histone H3 phosphorylation and acetylation in area CA1 of the hippocampus are regulated by ERK/MAPK during memory consolidation (Fig. 1). Currently, almost a dozen kinases are implicated in phosphorylating histone H3 at Ser 10 in either a stimulation-dependent manner or a mitosis-dependent manner (Bode and Dong 2005). During transcription, histone H3 phosphorylation is thought to neutralize the positive charge on histone tails by introducing a negatively charged phosphate group, thus relaxing chromatin and facilitating transcription (Grant 3Corresponding authors. E-mail mwood@uci.edu; fax (949) 824-8439. E-mail abele@sas.upenn.edu; fax (215) 898-8780. Article and publication are at http://www.learnmem.org/cgi/doi/10.1101/ lm.278206. Commentary