Injection of the cAMP-responsive element into the nucleus of Aplysia sensory neurons blocks long-term facilitation

Uncovering the Cellular and Molecular Mechanisms of Synapse Formation and Functional Specificity Using Central Neurons of Lymnaea stagnalis

All functions of the nervous system are contingent upon the precise organization of neuronal connections that are initially patterned during development, and then continually modified throughout life. Determining the mechanisms that specify the formation and functional modulation of synaptic circuitry are critical to advancing both our fundamental understanding of the nervous system as well as the various neurodevelopmental, neurological, neuropsychiatric, and neurodegenerative disorders that are met in clinical practice when these processes go awry. Defining the cellular and molecular mechanisms underlying nervous system development, function, and pathology has proven challenging, due mainly to the complexity of the vertebrate brain. Simple model system approaches with invertebrate preparations, on the other hand, have played pivotal roles in elucidating the fundamental mechanisms underlying the formation and plasticity of individual synapses, and the contributions of individual neurons and their synaptic connections that underlie a variety of behaviors, and learning and memory. In this Review, we discuss the experimental utility of the invertebrate mollusc Lymnaea stagnalis, with a particular emphasis on in vitro cell culture, semi-intact and in vivo preparations, which enable molecular and electrophysiological identification of the cellular and molecular mechanisms governing the formation, plasticity, and specificity of individual synapses at a single-neuron or single-synapse resolution.

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

Important insights into the selection pressures and core molecular modules contributing to the evolution of pain-related processes have come from studies of nociceptive systems in several molluscan and arthropod species. These phyla, and the chordates that include humans, last shared a common ancestor approximately 550 million years ago. Since then, animals in these phyla have continued to be subject to traumatic injury, often from predators, which has led to similar adaptive behaviors (e.g., withdrawal, escape, recuperative behavior) and physiological responses to injury in each group. Comparisons across these taxa provide clues about the contributions of convergent evolution and of conservation of ancient adaptive mechanisms to general nociceptive and pain-related functions. Primary nociceptors have been investigated extensively in a few molluscan and arthropod species, with studies of long-lasting nociceptive sensitization in the gastropod, Aplysia, and the insect, Drosophila, being especially fruitful. In Aplysia, nociceptive sensitization has been investigated as a model for aversive memory and for hyperalgesia. Neuromodulator-induced, activity-dependent, and axotomy-induced plasticity mechanisms have been defined in synapses, cell bodies, and axons of Aplysia primary nociceptors. Studies of nociceptive sensitization in Drosophila larvae have revealed numerous molecular contributors in primary nociceptors and interacting cells. Interestingly, molecular contributors examined thus far in Aplysia and Drosophila are largely different, but both sets overlap extensively with those in mammalian pain-related pathways. In contrast to results from Aplysia and Drosophila, nociceptive sensitization examined in moth larvae (Manduca) disclosed central hyperactivity but no obvious peripheral sensitization of nociceptive responses. Squid (Doryteuthis) show injury-induced sensitization manifested as behavioral hypersensitivity to tactile and especially visual stimuli, and as hypersensitivity and spontaneous activity in nociceptor terminals. Temporary blockade of nociceptor activity during injury subsequently increased mortality when injured squid were exposed to fish predators, providing the first demonstration in any animal of the adaptiveness of nociceptive sensitization. Immediate responses to noxious stimulation and nociceptive sensitization have also been examined behaviorally and physiologically in a snail (Helix), octopus (Adopus), crayfish (Astacus), hermit crab (Pagurus), and shore crab (Hemigrapsus). Molluscs and arthropods have systems that suppress nociceptive responses, but whether opioid systems play antinociceptive roles in these phyla is uncertain.

Regulators of Long-Term Memory Revealed by Mushroom Body-Specific Gene Expression Profiling in Drosophila melanogaster

Memory formation is achieved by genetically tightly controlled molecular pathways that result in a change of synaptic strength and synapse organization. While for short-term memory traces, rapidly acting biochemical pathways are in place, the formation of long-lasting memories requires changes in the transcriptional program of a cell. Although many genes involved in learning and memory formation have been identified, little is known about the genetic mechanisms required for changing the transcriptional program during different phases of long-term memory (LTM) formation. With Drosophila melanogaster as a model system, we profiled transcriptomic changes in the mushroom body—a memory center in the fly brain—at distinct time intervals during appetitive olfactory LTM formation using the targeted DamID technique. We describe the gene expression profiles during these phases and tested 33 selected candidate genes for deficits in LTM formation using RNAi knockdown. We identified 10 genes that enhance or decrease memory when knocked-down in the mushroom body. For vajk-1 and hacd1—the two strongest hits—we gained further support for their crucial role in appetitive learning and forgetting. These findings show that profiling gene expression changes in specific cell-types harboring memory traces provides a powerful entry point to identify new genes involved in learning and memory. The presented transcriptomic data may further be used as resource to study genes acting at different memory phases.

NEDD4L limits cAMP signaling through ubiquitination of CREB-regulated transcription coactivator 3

The transcription factor cAMP-responsive element-binding protein (CREB) is involved in a variety of physiologic processes. Although its activity appears to be largely correlated with its phosphorylation status, cAMP-mediated dephosphorylation and the subsequent nuclear migration of the CREB-regulated transcription factors (CRTCs) are required to stimulate CREB transcriptional activity. Among the 3 identified mammalian homologs of CRTCs, CRTC3 has been shown to be expressed predominantly in adipose tissues in response to catecholamine signals that regulate lipid metabolism. Here, we show that prolonged cAMP signaling down-regulates CRTC3 in a proteasome-dependent manner and that neural precursor cell-expressed developmentally down-regulated gene 4-like (NEDD4L), a specific ubiquitin ligase for CRTC3, is responsible for this process. By recognizing the PY motif of CRTC3, NEDD4L interacts with CRTC3 and promotes its polyubiquitination. Interaction between NEDD4L and CRTC3 is further boosted by cAMP signaling, and this enhanced interaction appears to be dependent on the cAMP-mediated phosphorylation of NEDD4L at the Ser448 site. Furthermore, we show that food withdrawal stimulates NEDD4L phosphorylation in mice, which then show a decrease of adipose tissue CRTC3 protein levels. Together, these results suggest that NEDD4L plays a key role in the feedback regulation of cAMP signaling by limiting CRTC3 protein levels.-Kim, Y.-H., Yoo, H., Hong, A.-R., Kwon, M., Kang, S.-W., Kim, K., Song, Y. NEDD4L limits cAMP signaling through ubiquitination of CREB-regulated transcription coactivator 3.

Imprinting: When Early Life Memories Make Food Smell Bad

A recent study has found that pathogen exposure early in the life of the nematode Caenorhabditis elegans leads to a long-lasting aversion that requires distinct sets of neurons for the formation and retrieval of the imprinted memory.

The Past, Present, and Future of Phosphodiesterase-4 Modulation for Age-Induced Memory Loss

The purpose of this chapter is to highlight the state of progress for phosphodiesterase-4 (PDE4) modulation as a potential therapeutic for psychiatric illness, and to draw attention to particular hurdles and obstacles that must be overcome in future studies to develop PDE4-mediated therapeutics. Pathological and non-pathological related memory loss will be the focus of the chapter; however, we will at times also touch upon other psychiatric illnesses like anxiety and depression. First, we will provide a brief background of PDE4, and the rationale for its extensive study in cognition. Second, we will explore fundamental differences in individual PDE4 subtypes, and then begin to address differences between pathological and non-pathological aging. Alterations of cAMP/PDE4 signaling that occur within normal vs. pathological aging, and the potential for PDE4 modulation to combat these alterations within each context will be described. Finally, we will finish the chapter with obstacles that have hindered the field, and future studies and alternative viewpoints that need to be addressed. Overall, we hope this chapter will demonstrate the incredible complexity of PDE4 signaling in the brain, and will be useful in forming a strategy to develop future PDE4-mediated therapeutics for psychiatric illnesses.

Molecular correlates of separate components of training that contribute to long-term memory formation after learning that food is inedible in Aplysia

Training Aplysia with inedible food for a period that is too brief to produce long-term memory becomes effective in producing memory when training is paired with a nitric oxide (NO) donor. Lip stimulation for the same period of time paired with an NO donor is ineffective. Using qPCR, we examined molecular correlates of brief training versus lip stimulation, of treatment with an NO donor versus saline, and of the combined stimuli producing long-term memory. Changes were examined in mRNA expression of Aplysia homologs of C/EBP, CREB1, CREB1α, CREB1β, and CREB2, in both the buccal and cerebral ganglia controlling feeding. Both the brief training and the NO donor increased expression of C/EBP, CREB1, CREB1α, and CREB1β, but not CREB2 in the buccal ganglia. For CREB1α, there was a significant interaction between the effects of the brief training and of the NO donor. In addition, the NO donor, but not brief training, increased expression of all of the genes in the cerebral ganglion. These findings show that the components of learning that alone do not produce memory produce molecular changes in different ganglia. Thus, long-term memory is likely to arise by both additive and interactive increases in gene expression.

Memory Reconsolidation

Scientific advances in the last decades uncovered that memory is not a stable, fixed entity. Apparently stable memories may become transiently labile and susceptible to modifications when retrieved due to the process of reconsolidation. Here, we review the initial evidence and the logic on which reconsolidation theory is based, the wide range of conditions in which it has been reported and recent findings further revealing the fascinating nature of this process. Special focus is given to conceptual issues of when and why reconsolidation happen and its possible outcomes. Last, we discuss the potential clinical implications of memory modifications by reconsolidation.

A role for autophagy in long-term spatial memory formation in male rodents

A hallmark of long-term memory formation is the requirement for protein synthesis. Administration of protein synthesis inhibitors impairs long-term memory formation without influencing short-term memory. Rapamycin is a specific inhibitor of target of rapamycin complex 1 (TORC1) that has been shown to block protein synthesis and impair long-term memory. In addition to regulating protein synthesis, TORC1 also phosphorylates Unc-51-like autophagy activating kinase-1 (Ulk-1) to suppress autophagy. As autophagy can be activated by rapamycin (and rapamycin inhibits long-term memory), our aim was to test the hypothesis that autophagy inhibitors would enhance long-term memory. To examine if learning alters autophagosome number, we used male reporter mice carrying the GFP-LC3 transgene. Using these mice, we observed that training in the Morris water maze task increases the number of autophagosomes, a finding contrary to our expectations. For learning and memory studies, male Long Evans rats were used due to their relatively larger size (compared to mice), making it easier to perform intrahippocampal infusions in awake, moving animals. When the autophagy inhibitors 3-methyladenine (3-MA) or Spautin-1 were administered bilaterally into the hippocampii prior to training in the Morris water maze task, the drugs did not alter learning. In contrast, when memory was tested 24 hours later by a probe trial, significant impairments were observed. In addition, intrahippocampal infusion of an autophagy activator peptide (TAT-Beclin-1) improved long-term memory. These results indicate that autophagy is not necessary for learning, but is required for long-term memory formation.

Cross-talk between the epigenome and neural circuits in drug addiction

Drug addiction is a behavioral disorder characterized by dysregulated learning about drugs and associated cues that result in compulsive drug seeking and relapse. Learning about drug rewards and predictive cues is a complex process controlled by a computational network of neural connections interacting with transcriptional and molecular mechanisms within each cell to precisely guide behavior. The interplay between rapid, temporally specific neuronal activation, and longer-term changes in transcription is of critical importance in the expression of appropriate, or in the case of drug addiction, inappropriate behaviors. Thus, these factors and their interactions must be considered together, especially in the context of treatment. Understanding the complex interplay between epigenetic gene regulation and circuit connectivity will allow us to formulate novel therapies to normalize maladaptive reward behaviors, with a goal of modulating addictive behaviors, while leaving natural reward-associated behavior unaffected.

Serotonergic Modulation Enables Pathway-Specific Plasticity in a Developing Sensory Circuit in Drosophila

How experiences during development cause long-lasting changes in sensory circuits and affect behavior in mature animals are poorly understood. Here we establish a novel system for mechanistic analysis of the plasticity of developing neural circuits by showing that sensory experience during development alters nociceptive behavior and circuit physiology in Drosophila larvae. Despite the convergence of nociceptive and mechanosensory inputs on common second-order neurons (SONs), developmental noxious input modifies transmission from nociceptors to their SONs, but not from mechanosensors to the same SONs, which suggests striking sensory pathway specificity. These SONs activate serotonergic neurons to inhibit nociceptor-to-SON transmission; stimulation of nociceptors during development sensitizes nociceptor presynapses to this feedback inhibition. Our results demonstrate that, unlike associative learning, which involves inputs from two sensory pathways, sensory pathway-specific plasticity in the Drosophila nociceptive circuit is in part established through feedback modulation. This study elucidates a novel mechanism that enables pathway-specific plasticity in sensory systems. VIDEO ABSTRACT.

Exposure to Stressors Facilitates Long-Term Synaptic Potentiation in the Lateral Habenula

The lateral habenula (LHb) is a small part of the epithalamus that projects to monoamine centers in the brain. Previously, neurotransmission onto the LHb was shown to be abnormally potentiated in animal models of depression. However, synaptic plasticity in this brain area and the effect of stressor exposure on synaptic plasticity of the LHb have not been investigated. Thus, we explored whether the LHb undergoes dynamic changes in synaptic efficacy or not. First, we observed that a moderate LTP occurs in a fraction of LHb neurons obtained from naive Sprague Dawley rats. Interestingly, a single exposure to acute stressors, such as inescapable foot shock or restraint plus tail shock (RTS), significantly enhances the magnitude of LTP in the LHb. We also observed an increased number of LHb neurons expressing phosphorylated cAMP response element-binding protein (pCREB) after exposure to stressors, which may contribute to determine the threshold for LTP induction. LTP induction in the LHb resulted in an additional increase in the number of pCREB-expressing neurons in stress-exposed animals but not in naive control animals. Together, we showed that LHb neurons have heterogeneous propensity for synaptic potentiation at rest; however, a single exposure to stressors greatly facilitates LTP induction in the LHb, suggesting that fundamental alterations in synaptic plasticity in the LHb may occur in animal models of depression or post-traumatic stress disorder.SIGNIFICANCE STATEMENT Stress exposure is known to cause depression in human patients and animal models, although explanations at the cellular level remain to be elaborated. Here, we show that the lateral habenula (LHb) exhibits LTP after a pattern of brief strong stimulation. In addition, we show that stress exposure facilitates LTP in the LHb by lowering the threshold for LTP induction. We observed a selective increase in the number of neurons expressing pCREB in the LHb of animal models of depression. LTP induction results in a further increase in the density of pCREB-expressing neurons only after stress exposure. Our study provides the first evidence that animal models of depression exhibit altered synaptic plasticity of the LHb.

Shank is a dose-dependent regulator of Cav1 calcium current and CREB target expression

Shank is a post-synaptic scaffolding protein that has many binding partners. Shank mutations and copy number variations (CNVs) are linked to several psychiatric disorders, and to synaptic and behavioral defects in mice. It is not known which Shank binding partners are responsible for these defects. Here we show that the C. elegans SHN-1/Shank binds L-type calcium channels and that increased and decreased shn-1 gene dosage alter L-channel current and activity-induced expression of a CRH-1/CREB transcriptional target (gem-4 Copine), which parallels the effects of human Shank copy number variations (CNVs) on Autism spectrum disorders and schizophrenia. These results suggest that an important function of Shank proteins is to regulate L-channel current and activity induced gene expression.

DOI:http://dx.doi.org/10.7554/eLife.18931.001

Appetitive Olfactory Learning and Long-Term Associative Memory in Caenorhabditis elegans

Because of the relative simplicity of its nervous system, Caenorhabditis elegans is a useful model organism to study learning and memory at cellular and molecular levels. For appetitive conditioning in C. elegans, food has exclusively been used as an unconditioned stimulus (US). It may be difficult to analyze neuronal circuits for associative memory since food is a multimodal combination of olfactory, gustatory, and mechanical stimuli. Here, we report classical appetitive conditioning and associative memory in C. elegans, using 1-nonanol as a conditioned stimulus (CS), and potassium chloride (KCl) as a US. Before conditioning, C. elegans innately avoided 1-nonanol, an aversive olfactory stimulus, and was attracted by KCl, an appetitive gustatory stimulus, on assay agar plates. Both massed training without an intertrial interval (ITI) and spaced training with a 10-min ITI induced significant levels of memory of association regarding the two chemicals. Memory induced by massed training decayed within 6 h, while that induced by spaced training was retained for more than 6 h. Animals treated with inhibitors of transcription or translation formed the memory induced by spaced training less efficiently than untreated animals, whereas the memory induced by massed training was not significantly affected by such treatments. By definition, therefore, memories induced by massed training and spaced training are classified as short-term memory (STM) and long-term memory (LTM), respectively. When animals conditioned by spaced training were exposed to 1-nonanol alone, their learning index was lower than that of untreated animals, suggesting that extinction learning occurs in C. elegans. In support of these results, C. elegans mutants defective in nmr-1, encoding an NMDA receptor subunit, formed both STM and LTM less efficiently than wild-type animals, while mutations in crh-1, encoding a ubiquitous transcription factor CREB required for memory consolidation, affected LTM, but not STM. The paradigm established in the present study should allow us to elucidate neuronal circuit plasticity for appetitive learning and memory in C. elegans.

Reward Network Immediate Early Gene Expression in Mood Disorders

Over the past three decades, it has become clear that aberrant function of the network of interconnected brain regions responsible for reward processing and motivated behavior underlies a variety of mood disorders, including depression and anxiety. It is also clear that stress-induced changes in reward network activity underlying both normal and pathological behavior also cause changes in gene expression. Here, we attempt to define the reward circuitry and explore the known and potential contributions of activity-dependent changes in gene expression within this circuitry to stress-induced changes in behavior related to mood disorders, and contrast some of these effects with those induced by exposure to drugs of abuse. We focus on a series of immediate early genes regulated by stress within this circuitry and their connections, both well-explored and relatively novel, to circuit function and subsequent reward-related behaviors. We conclude that IEGs play a crucial role in stress-dependent remodeling of reward circuitry, and that they may serve as inroads to the molecular, cellular, and circuit-level mechanisms of mood disorder etiology and treatment.

Role of circadian rhythm and REM sleep for memory consolidation

Although sleep is strongly implicated in memory consolidation, the molecular basis for the role of sleep in memory is not known. It has been established that the consolidation of hippocampus-dependent memory depends on the activation of the Erk1,2 MAP kinase (MAPK) pathway which activates de novo CRE-mediated transcription and translation, two processes required for memory consolidation pathway. The activation of MAPK during memory formation and its nuclear translocation both depend upon cAMP signals generated by the calmodulin-stimulated adenylyl cyclases, type 1 and type 8 (AC1 and AC8). This signaling pathway undergoes a circadian oscillation in the hippocampus with maximal activation during REM sleep. This data supports the hypothesis that the persistence of long-term memory traces may depend upon the reactivation and circadian oscillation of the cAMP/MAP kinase/CRE transcriptional pathway in tagged neurons which reaches a maximum during REM sleep.

Phosphoproteome Profiling Reveals Circadian Clock Regulation of Posttranslational Modifications in the Murine Hippocampus

The circadian clock is an endogenous oscillator that drives daily rhythms in physiology, behavior, and gene expression. The underlying mechanisms of circadian timekeeping are cell-autonomous and involve oscillatory expression of core clock genes that is driven by interconnecting transcription–translation feedback loops (TTFLs). Circadian clock TTFLs are further regulated by posttranslational modifications, in particular, phosphorylation. The hippocampus plays an important role in spatial memory and the conversion of short- to long-term memory. Several studies have reported the presence of a peripheral oscillator in the hippocampus and have highlighted the importance of circadian regulation in memory formation. Given the general importance of phosphorylation in circadian clock regulation, we performed global quantitative proteome and phosphoproteome analyses of the murine hippocampus across the circadian cycle, applying spiked-in labeled reference and high accuracy mass spectrometry (MS). Of the 3,052 proteins and 2,868 phosphosites on 1,368 proteins that were accurately quantified, 1.7% of proteins and 5.2% of phosphorylation events exhibited time-of-day-dependent expression profiles. The majority of circadian phosphopeptides displayed abrupt fluctuations at mid-to-late day without underlying rhythms of protein abundance. Bioinformatic analysis of cyclic phosphorylation events revealed their diverse distribution in different biological pathways, most notably, cytoskeletal organization and neuronal morphogenesis. This study provides the first large-scale, quantitative MS analysis of the circadian phosphoproteome and proteome of the murine hippocampus and highlights the significance of rhythmic regulation at the posttranslational level in this peripheral oscillator. In addition to providing molecular insights into the hippocampal circadian clock, our results will assist in the understanding of genetic factors that underlie rhythms-associated pathological states of the hippocampus.

CREB decreases astrocytic excitability by modifying subcellular calcium fluxes via the sigma-1 receptor

Astrocytic excitability relies on cytosolic calcium increases as a key mechanism, whereby astrocytes contribute to synaptic transmission and hence learning and memory. While it is a cornerstone of neurosciences that experiences are remembered, because transmitters activate gene expression in neurons, long-term adaptive astrocyte plasticity has not been described. Here, we investigated whether the transcription factor CREB mediates adaptive plasticity-like phenomena in astrocytes. We found that activation of CREB-dependent transcription reduced the calcium responses induced by ATP, noradrenaline, or endothelin-1. As to the mechanism, expression of VP16-CREB, a constitutively active CREB mutant, had no effect on basal cytosolic calcium levels, extracellular calcium entry, or calcium mobilization from lysosomal-related acidic stores. Rather, VP16-CREB upregulated sigma-1 receptor expression thereby increasing the release of calcium from the endoplasmic reticulum and its uptake by mitochondria. Sigma-1 receptor was also upregulated in vivo upon VP16-CREB expression in astrocytes. We conclude that CREB decreases astrocyte responsiveness by increasing calcium signalling at the endoplasmic reticulum-mitochondria interface, which might be an astrocyte-based form of long-term depression.

The Kinase Function of MSK1 Regulates BDNF Signaling to CREB and Basal Synaptic Transmission, But Is Not Required for Hippocampal Long-Term Potentiation or Spatial Memory

The later stages of long-term potentiation (LTP) in vitro and spatial memory in vivo are believed to depend upon gene transcription. Accordingly, considerable attempts have been made to identify both the mechanisms by which transcription is regulated and indeed the gene products themselves. Previous studies have shown that deletion of one regulator of transcription, the mitogen- and stress-activated kinase 1 (MSK1), causes an impairment of spatial memory. Given the ability of MSK1 to regulate gene expression via the phosphorylation of cAMP response element binding protein (CREB) at serine 133 (S133), MSK1 is a plausible candidate as a prime regulator of transcription underpinning synaptic plasticity and learning and memory. Indeed, prior work has revealed the necessity for MSK1 in homeostatic and experience-dependent synaptic plasticity. However, using a knock-in kinase-dead mouse mutant of MSK1, the current study demonstrates that, while the kinase function of MSK1 is important in regulating the phosphorylation of CREB at S133 and basal synaptic transmission in hippocampal area CA1, it is not required for metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD), two forms of LTP or several forms of spatial learning in the watermaze. These data indicate that other functions of MSK1, such as a structural role for the whole enzyme, may explain previous observations of a role for MSK1 in learning and memory.

CREB overexpression in dorsal CA1 ameliorates long-term memory deficits in aged rats

The molecular mechanisms underlying age-related cognitive deficits are not yet fully elucidated. In aged animals, a decrease in the intrinsic excitability of CA1 pyramidal neurons is believed to contribute to age-related cognitive impairments. Increasing activity of the transcription factor cAMP response element-binding protein (CREB) in young adult rodents facilitates cognition, and increases intrinsic excitability. However, it has yet to be tested if increasing CREB expression also ameliorates age-related behavioral and biophysical deficits. To test this hypothesis, we virally overexpressed CREB in CA1 of dorsal hippocampus. Rats received CREB or control virus, before undergoing water maze training. CREB overexpression in aged animals ameliorated the long-term memory deficits observed in control animals. Concurrently, cells overexpressing CREB in aged animals had reduced post-burst afterhyperpolarizations, indicative of increased intrinsic excitability. These results identify CREB modulation as a potential therapy to treat age-related cognitive decline.

DOI:http://dx.doi.org/10.7554/eLife.19358.001

CREB at the Crossroads of Activity-Dependent Regulation of Nervous System Development and Function

The central nervous system is a highly plastic network of cells that constantly adjusts its functions to environmental stimuli throughout life. Transcription-dependent mechanisms modify neuronal properties to respond to external stimuli regulating numerous developmental functions, such as cell survival and differentiation, and physiological functions such as learning, memory, and circadian rhythmicity. The discovery and cloning of the cyclic adenosine monophosphate (cAMP) responsive element binding protein (CREB) constituted a big step toward deciphering the molecular mechanisms underlying neuronal plasticity. CREB was first discovered in learning and memory studies as a crucial mediator of activity-dependent changes in target gene expression that in turn impose long-lasting modifications of the structure and function of neurons. In this chapter, we review the molecular and signaling mechanisms of neural activity-dependent recruitment of CREB and its cofactors. We discuss the crosstalk between signaling pathways that imprints diverse spatiotemporal patterns of CREB activation allowing for the integration of a wide variety of stimuli.

Role of the adhesion molecule F3/Contactin in synaptic plasticity and memory

Cell adhesion molecules (CAMs) have a pivotal role in building and maintaining synaptic structures during brain development participating in axonal elongation and pathfinding, glial guidance of neuronal migration, as well as myelination. CAMs expression persists in the adult brain particularly in structures undergoing postnatal neurogenesis and involved in synaptic plasticity and memory as the hippocampus. Among the neural CAMs, we have recently focused on F3/Contactin, a glycosylphosphatidyl inositol-anchored glycoprotein belonging to the immunoglobulin superfamily, involved in neuronal development, synaptic maintenance and organization of neuronal networks. Here, we discuss our recent data suggesting that F3/Contactin exerts a role in hippocampal synaptic plasticity and memory in adult and aged mice. In particular, we have studied long-term potentiation (LTP), spatial and object recognition memory, and phosphorylation of the transcription factor cAMP-Responsive-Element Binding protein (CREB) in a transgenic mouse model of F3/Contactin overexpression. We also investigated whether F3/Contactin might influence neuronal apoptosis and the production of amyloid-beta peptide (Aβ), known to be one of the main pathogenetic hallmarks of Alzheimer's disease (AD). In conclusion, a further understanding of F3/Contactin role in synaptic plasticity and memory might have interesting clinical outcomes in cognitive disorders, such as aging and AD, offering innovative therapeutic opportunities.

CCR5 is a suppressor for cortical plasticity and hippocampal learning and memory

Although the role of CCR5 in immunity and in HIV infection has been studied widely, its role in neuronal plasticity, learning and memory is not understood. Here, we report that decreasing the function of CCR5 increases MAPK/CREB signaling, long-term potentiation (LTP), and hippocampus-dependent memory in mice, while neuronal CCR5 overexpression caused memory deficits. Decreasing CCR5 function in mouse barrel cortex also resulted in enhanced spike timing dependent plasticity and consequently, dramatically accelerated experience-dependent plasticity. These results suggest that CCR5 is a powerful suppressor for plasticity and memory, and CCR5 over-activation by viral proteins may contribute to HIV-associated cognitive deficits. Consistent with this hypothesis, the HIV V3 peptide caused LTP, signaling and memory deficits that were prevented by Ccr5 knockout or knockdown. Overall, our results demonstrate that CCR5 plays an important role in neuroplasticity, learning and memory, and indicate that CCR5 has a role in the cognitive deficits caused by HIV.

Different contribution of glucocorticoids in the basolateral amygdala to the formation and expression of opiate withdrawal-associated memories

Drug-withdrawal aversive memories generate a motivational state leading to compulsive drug taking, with plasticity changes in the basolateral amygdala (BLA) being essential in aversive motivational learning. The conditioned-place aversion (CPA) paradigm allows for measuring the negative affective component of drug withdrawal. First, CPA triggers association between negative affective consequences of withdrawal with context (memory consolidation). Afterwards, when the animals are re-exposed to the paired environment, they avoid it due to the association between the context and aversive memories (memory retrieval). We examined the influence of glucocorticoids (GCs) for a morphine-withdrawal CPA paradigm, along with plasticity changes in the BLA, in sham-operated and adrenalectomized (ADX) animals. We demonstrated that sham+morphine animals robustly displayed CPA, whereas ADX-dependent animals lacked the affective-like signs of opiate withdrawal but displayed increased somatic signs of withdrawal. Glucocorticoid receptor (GR) actions promote memory consolidation but highly depend on increases in GC levels. Interestingly, we observed that GCs were only increased in sham-dependent rodents during aversive-withdrawal memory consolidation, and that GR expression correlated with phosphorylated cAMP response element binding (pCREB) protein, early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated (Arc) mRNA induction in this experimental group. In contrast, ADX-animals displayed reduced (pCREB). GCs are also known to impair memory retrieval. Accordingly, we showed that GCs levels remained at basal levels in all experimental groups following memory retrieval, and consequently GRs no longer acted as transcriptional regulators. Importantly, memory retrieval elicited increased pCREB levels in sham+morphine animals (not in ADX+morphine group), which were directly correlated with enhanced Arc mRNA/protein expression mainly in glutamatergic neurons. In conclusion, context-withdrawal associations are accompanied plasticity changes in the BLA, which are, in part, regulated by GR signaling. Moreover, dysregulation of CREB signaling, in part through Arc expression, may enhance reconsolidation, resulting in the maintenance of excessive aversive states. These findings might have important implications for drug-seeking behavior.

The Contribution of Spatial and Temporal Molecular Networks in the Induction of Long-term Memory and Its Underlying Synaptic Plasticity

The ability to form long-lasting memories is critical to survival and thus is highly conserved across the animal kingdom. By virtue of its complexity, this same ability is vulnerable to disruption by a wide variety of neuronal traumas and pathologies. To identify effective therapies with which to treat memory disorders, it is critical to have a clear understanding of the cellular and molecular mechanisms which subserve normal learning and memory. A significant challenge to achieving this level of understanding is posed by the wide range of distinct temporal and spatial profiles of molecular signaling induced by learning-related stimuli. In this review we propose that a useful framework within which to address this challenge is to view the molecular foundation of long-lasting plasticity as composed of unique spatial and temporal molecular networks that mediate signaling both within neurons (such as via kinase signaling) as well as between neurons (such as via growth factor signaling). We propose that evaluating how cells integrate and interpret these concurrent and interacting molecular networks has the potential to significantly advance our understanding of the mechanisms underlying learning and memory formation.

Mechanisms underlying long-term fear memory formation from a metaplastic neuronal state

We previously showed that a single weak fear conditioning trial, that does not produce a long-term fear memory (LTM), appeared to prime memory formation such that when a second trial followed within a circumscribed time window a robust and long-lasting fear memory was formed. We also showed that this priming effect could be blocked if we interfered with protein kinase A (PKA) signaling in the amygdala during the first conditioning trial. The goals of the current study were to determine if LTM formation after the second trial depends on PKA signaling in the amygdala and to characterize the underlying memory processes engaged during the second trial that allows for LTM formation. Our interpretation of the original findings is that the second conditioning trial triggers LTM from a metaplastic state that is engaged by the first conditioning trial. However, it is also possible that the second conditioning trial acts as a reminder of the first and engages a reconsolidation-like process. Several experiments were conducted to distinguish between these two possibilities. We show that interfering with PKA signaling during the second conditioning trial disrupts memory formation. However, if a third trial follows the second or if the second trial was presented without shock, the PKA inhibitor was no longer effective. Our findings demonstrate that the induction of fear memory from a metaplastic state involves new learning that is distinct from retrieval-dependent updating of memories.

Long-term habituation of the C-start escape response in zebrafish larvae

The cellular and molecular basis of long-term memory in vertebrates remains poorly understood. Knowledge regarding long-term memory has been impeded by the enormous complexity of the vertebrate brain, particularly the mammalian brain, as well as by the relative complexity of the behavioral alterations examined in most studies of long-term memory in vertebrates. Here, we demonstrate a long-term form of nonassociative learning-specifically, long-term habituation (LTH)-of a simple reflexive escape response, the C-start, in zebrafish larvae. The C-start is triggered by the activation of one of a pair of giant neurons in the zebrafish's hindbrain, the Mauthner cells. We show that LTH of the C-start requires the activity of NMDA receptors and involves macromolecular synthesis. We further show that the long-term habituated reflex can by rapidly dishabituated by a brief tactile stimulus. Our results set the stage for rigorous, mechanistic investigations of the long-term memory for habituation of a reflexive behavioral response, one that is mediated by a relatively simple, neurobiologically tractable, neural circuit. Moreover, the demonstration of NMDAR and transcriptionally dependent LTH in a translucent vertebrate organism should facilitate the use of optical recording, and optogenetic manipulation, of neuronal activity to elucidate the cellular basis of a long-term vertebrate memory.

Two proteolytic fragments of menin coordinate the nuclear transcription and postsynaptic clustering of neurotransmitter receptors during synaptogenesis between Lymnaea neurons

Synapse formation and plasticity depend on nuclear transcription and site-specific protein targeting, but the molecular mechanisms that coordinate these steps have not been well defined. The MEN1 tumor suppressor gene, which encodes the protein menin, is known to induce synapse formation and plasticity in the CNS. This synaptogenic function has been conserved across evolution, however the underlying molecular mechanisms remain unidentified. Here, using central neurons from the invertebrate Lymnaea stagnalis, we demonstrate that menin coordinates subunit-specific transcriptional regulation and synaptic clustering of nicotinic acetylcholine receptors (nAChR) during neurotrophic factor (NTF)-dependent excitatory synaptogenesis, via two proteolytic fragments generated by calpain cleavage. Whereas menin is largely regarded as a nuclear protein, our data demonstrate a novel cytoplasmic function at central synapses. Furthermore, this study identifies a novel synaptogenic mechanism in which a single gene product coordinates the nuclear transcription and postsynaptic targeting of neurotransmitter receptors through distinct molecular functions of differentially localized proteolytic fragments.

CREB, cellular excitability, and cognition: Implications for aging

Humans and laboratory animals display cognitive deficits as they age. However, there are currently no effective therapies available to treat these deficits, as the underlying mechanisms are poorly understood. Studies using pharmacological compounds have found a link between cognitive performance and the intrinsic cellular excitability of CA1 hippocampal neurons. Therefore, it is of great interest to identify molecular regulators that may be influencing both cognition and neuronal excitability, which could be changed with age. One possible regulator is the transcription factor cAMP response element binding-protein (CREB). In young adult animals, manipulation of CREB activity has resulted in modulation of both cognitive performance on behavioral tasks, and neuronal excitability. While evidence is sparse, studies also point to a dysfunction in CREB signaling with aging. We propose that CREB may be a viable therapeutic target for the treatment of age-related cognitive deficits, along with potential experiments to test this hypothesis.

REVIEW ■ : Regulation of Gene Expression by Neural Signals

Regulation of gene expression by extracellular signals is a ubiquitous biological mechanism controlling cell proliferation, differentiation, homeostasis, and adaptation to the environment. This article will focus on one set of issues within the broad topic of regulated gene expression: mechanisms by which neurotransmitters and neural activity regulate genes within the mature nervous system. The regulatory actions of growth factors, cytokines, and other types of extracellular signals are complex matters deserving separate review. This article proceeds from a basic overview of transcriptional regulation to a more specific discussion of the actions of two families of transcriptional regulators, the CREB family and the AP-1 family. These families of proteins are discussed because they play a central role in the regulation of gene expression by neurotransmitters and also because they exemplify many general principles of extracellular signal-regulated gene expression. NEUROSCIENTIST 2:217-224, 1996

Gene Expression in Hippocampal Long-Term Potentiation

Recent work on hippocampal LTP has focused on gene expression induced with high-frequency stimulation, as well as the signal transduction cascades responsible for the induction of these genes. Many scenarios for LTP lasting for greater than 5 hours include some or all of the following processes: 1) tagging of potentiated synapses, possibly by phosphorylation; 2) signaling to the nucleus; 3) kinase cascades and transcription factors in the nucleus;, 4) expression of immediate-early genes and/or synaptic proteins; and, finally, 5) targeting of newly synthesized proteins (or RNAs) to the potentiated synapses (and not to the unpotentiated synapses). Unfortunately, most scenarios proposed for the late-phase expression of LTP are still highly speculative at this time. A critical review of the literature relating to the role of gene expression in hippocampal LTP and a discussion of recent work on the subject will be presented.

Transcriptional regulation of long-term potentiation

Long-term potentiation (LTP), the persistent strengthening of synapses following high levels of stimulation, is a form of synaptic plasticity that has been studied extensively as a possible mechanism for learning and memory formation. The strengthening of the synapse that occurs during LTP requires cascades of complex molecular processes and the coordinated remodeling of pre-synaptic and post-synaptic neurons. Despite over four decades of research, our understanding of the transcriptional mechanisms and molecular processes underlying LTP remains incomplete. Identification of all the proteins and non-coding RNA transcripts expressed during LTP may provide greater insight into the molecular mechanisms involved in learning and memory formation.

Morris Water Maze Training in Mice Elevates Hippocampal Levels of Transcription Factors Nuclear Factor (Erythroid-derived 2)-like 2 and Nuclear Factor Kappa B p65

Research has identified several transcription factors that regulate activity-dependent plasticity and memory, with cAMP-response element binding protein (CREB) being the most well-studied. In neurons, CREB activation is influenced by the transcription factor nuclear factor kappa B (NF-κB), considered central to immunity but more recently implicated in memory. The transcription factor early growth response-2 (Egr-2), an NF-κB gene target, is also associated with learning and memory. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2), an antioxidant transcription factor linked to NF-κB in pathological conditions, has not been studied in normal memory. Given that numerous transcription factors implicated in activity-dependent plasticity demonstrate connections to NF-κB, this study simultaneously evaluated protein levels of NF-κB, CREB, Egr-2, Nrf2, and actin in hippocampi from young (1 month-old) weanling CD1 mice after training in the Morris water maze, a hippocampal-dependent spatial memory task. After a 6-day acquisition period, time to locate the hidden platform decreased in the Morris water maze. Mice spent more time in the target vs. non-target quadrants of the maze, suggestive of recall of the platform location. Western blot data revealed a decrease in NF-κB p50 protein after training relative to controls, whereas NF-κB p65, Nrf2 and actin increased. Nrf2 levels were correlated with platform crosses in nearly all tested animals. These data demonstrate that training in a spatial memory task results in alterations in and associations with particular transcription factors in the hippocampus, including upregulation of NF-κB p65 and Nrf2. Training-induced increases in actin protein levels caution against its use as a loading control in immunoblot studies examining activity-dependent plasticity, learning, and memory.

Synaptic generation of an intracellular retrograde signal requires activation of the tyrosine kinase and mitogen-activated protein kinase signaling cascades in Aplysia

Cellular changes underlying memory formation can be generated in an activity-dependent manner at specific synapses. Thus an important question concerns the mechanisms by which synaptic signals communicate with the cell body to mediate these cellular changes. A monosynaptic circuit that is enhanced by sensitization in Aplysia is well-suited to study this question because three different subcellular compartments: (i) the sensorimotor SN-MN synapses, (ii) the SN projections to MNs via axonal connections, (iii) the SN cell bodies, can all be manipulated and studied independently. Here, we report that activity-dependent (AD) training in either the entire SN-MN circuit or in only the synaptic compartment, activates MAPK in a temporally and spatially specific pattern. Specifically, we find (i) MAPK activation is first transiently generated at SN-MN synapses during training, (ii) immediately after training MAPK is transiently activated in SN-MN axonal connections and persistently activated in SN cell bodies, and finally, (iii) MAPK is activated in SN cell bodies and SN-MN synapses 1h after training. These data suggest that there is an intracellularly transported retrograde signal generated at the synapse which is later responsible for delayed MAPK activation at SN somata. Finally, we find that this retrograde signal requires activation of tyrosine kinase (TK) and MEK signaling cascades at the synapses.

Plasticity-related genes in brain development and amygdala-dependent learning

Learning about motivationally important stimuli involves plasticity in the amygdala, a temporal lobe structure. Amygdala-dependent learning involves a growing number of plasticity-related signaling pathways also implicated in brain development, suggesting that learning-related signaling in juveniles may simultaneously influence development. Here, we review the pleiotropic functions in nervous system development and amygdala-dependent learning of a signaling pathway that includes brain-derived neurotrophic factor (BDNF), extracellular signaling-related kinases (ERKs) and cyclic AMP-response element binding protein (CREB). Using these canonical, plasticity-related genes as an example, we discuss the intersection of learning-related and developmental plasticity in the immature amygdala, when aversive and appetitive learning may influence the developmental trajectory of amygdala function. We propose that learning-dependent activation of BDNF, ERK and CREB signaling in the immature amygdala exaggerates and accelerates neural development, promoting amygdala excitability and environmental sensitivity later in life.

Tet1 Oxidase Regulates Neuronal Gene Transcription, Active DNA Hydroxy-methylation, Object Location Memory, and Threat Recognition Memory

A dynamic equilibrium between DNA methylation and demethylation of neuronal activity-regulated genes is crucial for memory processes. However, the mechanisms underlying this equilibrium remain elusive. Tet1 oxidase has been shown to play a key role in the active DNA demethylation in the CNS. In this study, we used Tet1 gene knockout (Tet1KO) mice to examine the involvement of Tet1 in memory consolidation and storage in the adult brain. We found that Tet1 ablation leads to: altered expression of numerous neuronal activity-regulated genes, compensatory upregulation of active demethylation pathway genes, and upregulation of various epigenetic modifiers. Moreover, Tet1KO mice showed an enhancement in the consolidation and storage of threat recognition (cued and contextual fear conditioning) and object location memories. We conclude that Tet1 plays a critical role in regulating neuronal transcription and in maintaining the epigenetic state of the brain associated with memory consolidation and storage.

NF-κB transcription factor role in consolidation and reconsolidation of persistent memories

Transcriptional regulation is an important molecular process required for long-term neural plasticity and long-term memory (LTM) formation. Thus, one main interest in molecular neuroscience in the last decades has been the identification of transcription factors that are involved in memory processes. Among them, the nuclear factor κB (NF-κB) family of transcription factors has gained interest due to a significant body of evidence that supports a key role of these proteins in synaptic plasticity and memory. In recent years, the interest was particularly reinforced because NF-κB was characterized as an important regulator of synaptogenesis. This function may be explained by its participation in synapse to nucleus communication, as well as a possible local role at the synapse. This review provides an overview of experimental work obtained in the last years, showing the essential role of this transcription factor in memory processes in different learning tasks in mammals. We focus the review on the consolidation and reconsolidation memory phases as well as on the regulation of immediate-early and late genes by epigenetic mechanisms that determine enduring forms of memories.

A new perspective on the role of the CREB family of transcription factors in memory consolidation via adult hippocampal neurogenesis

Adult neurogenesis is the process by which new neurons are generated in the brains of adults. Since its discovery 50 years ago, adult neurogenesis has been widely studied in the mammalian brain and has provided a new perspective on the pathophysiology of many psychiatric and neurodegenerative disorders, some of which affect memory. In this regard, adult hippocampal neurogenesis (AHN), which occurs in the subgranular zone (SGZ) of the dentate gyrus (DG), has been suggested to play a role in the formation and consolidation of new memories. This process involves many transcription factors, of which cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a well-documented one. In the developing brain, CREB regulates crucial cell stages (e.g., proliferation, differentiation, and survival), and in the adult brain, it participates in neuronal plasticity, learning, and memory. In addition, new evidence supports the hypothesis that CREB may also participate in learning and memory through its involvement in AHN. This review examines the CREB family of transcription factors, including the different members and known signaling pathways. It highlights the role of CREB as a modulator of AHN, which could underlie its function in memory consolidation mechanisms.

CBP-Dependent memory consolidation in the prefrontal cortex supports object-location learning

Recognition of an object's location in space is supported by hippocampus-dependent recollection. Converging evidence strongly suggests that the interplay between the prefrontal cortex and hippocampus is critical for spatial memory. Lesion, pharmacological, and genetic studies have been successful in dissecting the role of plasticity in the hippocampal circuit in a variety of neural processes relevant to spatial memory, including memory for the location of objects. However, prefrontal mechanisms underlying spatial memory are less well understood. Here, we show that an acute hypofunction of the cyclic-AMP regulatory element binding protein (CREB) Binding Protein (CBP) histone acetyltransferase (HAT) in the medial prefrontal cortex (mPFC) results in delay-dependent disruption of object-location memory. These data suggest that mechanisms involving CBP HAT-mediated lysine acetylation of nuclear proteins support selectively long-term encoding in the mPFC circuits. Evidence from the object-location task suggests that long-term memory encoding within the mPFC complements hippocampus-dependent spatial memory mechanisms and may be critical for broader network integration of information necessary for an assessment of subtle spatial differences to guide appropriate behavioral response during retrieval of spatial memories.

Rescue of impaired long-term facilitation at sensorimotor synapses of Aplysia following siRNA knockdown of CREB1

Memory impairment is often associated with disrupted regulation of gene induction. For example, deficits in cAMP response element-binding protein (CREB) binding protein (CBP; an essential cofactor for activation of transcription by CREB) impair long-term synaptic plasticity and memory. Previously, we showed that small interfering RNA (siRNA)-induced knockdown of CBP in individual sensory neurons significantly reduced levels of CBP and impaired 5-HT-induced long-term facilitation (LTF) in sensorimotor cocultures from Aplysia. Moreover, computational simulations of the biochemical cascades underlying LTF successfully predicted training protocols that restored LTF following CBP knockdown. We examined whether simulations could also predict a training protocol that restores LTF impaired by siRNA-induced knockdown of the transcription factor CREB1. Simulations based on a previously described model predicted rescue protocols that were specific to CREB1 knockdown. Empirical studies demonstrated that one of these rescue protocols partially restored impaired LTF. In addition, the effectiveness of the rescue protocol was enhanced by pretreatment with rolipram, a selective cAMP phosphodiesterase inhibitor. These results provide further evidence that computational methods can help rescue disruptions in signaling cascades underlying memory formation. Moreover, the study demonstrates that the effectiveness of computationally designed training protocols can be enhanced with complementary pharmacological approaches.

cJun and CREB2 in the postsynaptic neuron contribute to persistent long-term facilitation at a behaviorally relevant synapse

Basic region leucine zipper (bZIP) transcription factors regulate gene expression critical for long-term synaptic plasticity or neuronal excitability contributing to learning and memory. At sensorimotor synapses of Aplysia, changes in activation or expression of CREB1 and CREB2 in sensory neurons are required for long-term synaptic plasticity. However, it is unknown whether concomitant stimulus-induced changes in expression and activation of bZIP transcription factors in the postsynaptic motor neuron also contribute to persistent long-term facilitation (P-LTF). We overexpressed various forms of CREB1, CREB2, or cJun in the postsynaptic motor neuron L7 in cell culture to examine whether these factors contribute to P-LTF. P-LTF is evoked by 2 consecutive days of 5-HT applications (2 5-HT), while a transient form of LTF is produced by 1 day of 5-HT applications (1 5-HT). Significant increases in the expression of both cJun and CREB2 mRNA in L7 accompany P-LTF. Overexpressing each bZIP factor in L7 did not alter basal synapse strength, while coexpressing cJun and CREB2 in L7 evoked persistent increases in basal synapse strength. In contrast, overexpressing cJun and CREB2 in sensory neurons evoked persistent decreases in basal synapse strength. Overexpressing wild-type cJun or CREB2, but not CREB1, in L7 can replace the second day of 5-HT applications in producing P-LTF. Reducing cJun activity in L7 blocked P-LTF evoked by 2 5-HT. These results suggest that expression and activation of different bZIP factors in both presynaptic and postsynaptic neurons contribute to persistent change in synapse strength including stimulus-dependent long-term synaptic plasticity.

Distinct roles of multiple isoforms of CaMKII in signaling to the nucleus

Long-lasting synaptic changes following information acquisition are critical steps for memory. In this process, long-term potentiation (LTP) is widely considered as one of the major cellular mechanisms modifying synaptic strength. It can be classified into early phase LTP (E-LTP) and late phase LTP (L-LTP) based on its duration. Using genetically modified mice, investigators have recognized the critical role of CaMKII in E-LTP and memory. However, its function in L-LTP, which is strongly dependent on gene transcription and protein synthesis, is still unclear. In this review, we discuss how different isoforms of CaMKII are coordinated to regulate gene expression in an activity-dependent manner, and thus contribute to L-LTP and memory. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Commonalities between pain and memory mechanisms and their meaning for understanding chronic pain

Pain sensing neurons in the periphery (called nociceptors) and the central neurons that receive their projections show remarkable plasticity following injury. This plasticity results in amplification of pain signaling that is now understood to be crucial for the recovery and survival of organisms following injury. These same plasticity mechanisms may drive a transition to a nonadaptive chronic pain state if they fail to resolve following the termination of the healing process. Remarkable advances have been achieved in the past two decades in understanding the molecular mechanisms that underlie pain plasticity following injury. The mechanisms bear a striking resemblance to molecular mechanisms involved in learning and memory processes in other brain regions, including the hippocampus and cerebral cortex. Here those mechanisms, their commonalities and subtle differences, will be highlighted and their role in causing chronic pain will be discussed. Arising from these data is the striking argument that chronic pain is a disease of the nervous system, which distinguishes this phenomena from acute pain that is frequently a symptom alerting the organism to injury. This argument has important implications for the development of disease modifying therapeutics.

Phosphodiesterase-5 Inhibitors: Action on the Signaling Pathways of Neuroinflammation, Neurodegeneration, and Cognition

Phosphodiesterase type 5 inhibitors (PDE5-Is) have recently emerged as a potential therapeutic strategy for neuroinflammatory, neurodegenerative, and memory loss diseases. Mechanistically, PDE5-Is produce an anti-inflammatory and neuroprotection effect by increasing expression of nitric oxide synthases and accumulation of cGMP and activating protein kinase G (PKG), the signaling pathway of which is thought to play an important role in the development of several neurodiseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). The aim of this paper was to review present knowledge of the signaling pathways that underlie the use of PDE5-Is in neuroinflammation, neurogenesis, learning, and memory.