Spine dynamics and synapse remodeling during LTP and memory processes

Proteomic-Based Studies on Memory Formation in Normal and Neurodegenerative Disease-Affected Brains

A critical aspect of cognition is the ability to acquire, consolidate, and evoke memories, which is considerably impaired by neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. These mnemonic processes are dependent on signaling cascades, which involve protein expression and degradation. Recent mass spectrometry (MS)-based proteomics has opened a range of possibilities for the study of memory formation and impairment, making it possible to research protein systems not studied before. However, in the context of synaptic proteome related to learning processes and memory formation, a deeper understanding of the synaptic proteome temporal dynamics after induction of synaptic plasticity and the molecular changes underlying the cognitive deficits seen in neurodegenerative diseases is needed. This review analyzes the applications of proteomics for understanding memory processes in both normal and neurodegenerative conditions. Moreover, the most critical experimental studies have been summarized using the PANTHER overrepresentation test. Finally, limitations associated with investigations of memory studies in physiological and neurodegenerative disorders have also been discussed.

Stabilizing Immature Dendritic Spines in the Auditory Cortex: A Key Mechanism for mTORC1-Mediated Enhancement of Long-Term Fear Memories

Mammalian target of rapamycin (mTOR) pathway has emerged as a key molecular mechanism underlying memory processes. Although mTOR inhibition is known to block memory processes, it remains elusive whether and how an enhancement of mTOR signaling may improve memory processes. Here we found in male mice that the administration of VO-OHpic, an inhibitor of the phosphatase and tensin homolog (PTEN) that negatively modulates AKT-mTOR pathway, enhanced auditory fear memory for days and weeks, while it left short-term memory unchanged. Memory enhancement was associated with a long-lasting increase in immature-type dendritic spines of pyramidal neurons into the auditory cortex. The persistence of spine remodeling over time arose by the interplay between PTEN inhibition and memory processes, as VO-OHpic induced only a transient immature spine growth in the somatosensory cortex, a region not involved in long-term auditory memory. Both the potentiation of fear memories and increase in immature spines were hampered by rapamycin, a selective inhibitor of mTORC1. These data revealed that memory can be potentiated over time by the administration of a selective PTEN inhibitor. In addition to disclosing new information on the cellular mechanisms underlying long-term memory maintenance, our study provides new insights on the molecular processes that aid enhancing memories over time.SIGNIFICANCE STATEMENT The neuronal mechanisms that may help improve the maintenance of long-term memories are still elusive. The inhibition of mammalian-target of rapamycin (mTOR) signaling shows that this pathway plays a crucial role in synaptic plasticity and memory formation. However, whether its activation may strengthen long-term memory storage is unclear. We assessed the consequences of positive modulation of AKT-mTOR pathway obtained by VO-OHpic administration, a phosphatase and tensin homolog inhibitor, on memory retention and underlying synaptic modifications. We found that mTOR activation greatly enhanced memory maintenance for weeks by producing a long-lasting increase of immature-type dendritic spines in pyramidal neurons of the auditory cortex. These results offer new insights on the cellular and molecular mechanisms that can aid enhancing memories over time.

Cofilin overactivation improves hippocampus-dependent short-term memory

Many living organisms of the animal kingdom have the fundamental ability to form and retrieve memories. Most information is initially stored as short-term memory, which is then converted to a more stable long-term memory through a process called memory consolidation. At the neuronal level, synaptic plasticity is crucial for memory storage. It includes the formation of new spines, as well as the modification of existing spines, thereby tuning and shaping synaptic efficacy. Cofilin critically contributes to memory processes as upon activation, it regulates the shape of dendritic spines by targeting actin filaments. We previously found that prolonged activation of cofilin in hippocampal neurons attenuated the formation of long-term object-location memories. Because the modification of spine shape and structure is also essential for short-term memory formation, we determined whether overactivation of hippocampal cofilin also influences the formation of short-term memories. To this end, mice were either injected with an adeno-associated virus expressing catalytically active cofilin, or an eGFP control, in the hippocampus. We show for the first time that cofilin overactivation improves short-term memory formation in the object-location memory task, without affecting anxiety-like behavior. Surprisingly, we found no effect of cofilin overactivation on AMPA receptor expression levels. Altogether, while cofilin overactivation might negatively impact the formation of long-lasting memories, it may benefit short-term plasticity.

Role of Aβ in Alzheimer’s-related synaptic dysfunction

Synaptic dysfunction is closely related to Alzheimer’s disease (AD) which is also recognized as synaptic disorder. β-amyloid (Aβ) is one of the main pathogenic factors in AD, which disrupts synaptic plasticity and mediates the synaptic toxicity through different mechanisms. Aβ disrupts glutamate receptors, such as NMDA and AMPA receptors, which mediates calcium dyshomeostasis and damages synapse plasticity characterized by long-term potentiation (LTP) suppression and long-term depression (LTD) enhancement. As Aβ stimulates and Ca2+ influx, microglial cells and astrocyte can be activated and release cytokines, which reduces glutamate uptake and further impair synapse function. Besides, extracellular glutamate accumulation induced by Aβ mediates synapse toxicity resulting from reduced glutamate receptors and glutamate spillovers. Aβ also mediates synaptic dysfunction by acting on various signaling pathways and molecular targets, disrupting mitochondria and energy metabolism. In addition, Aβ overdeposition aggravates the toxic damage of hyperphosphorylated tau to synapses. Synaptic dysfunction plays a critical role in cognitive impairment of AD. The review addresses the possible mechanisms by which Aβ mediates AD-related synaptic impairment from distant perspectives.

Randomly fluctuating neural connections may implement a consolidation mechanism that explains classic memory laws

How can we reconcile the massive fluctuations in neural connections with a stable long-term memory? Two-photon microscopy studies have revealed that large portions of neural connections (spines, synapses) are unexpectedly active, changing unpredictably over time. This appears to invalidate the main assumption underlying the majority of memory models in cognitive neuroscience, which rely on stable connections that retain information over time. Here, we show that such random fluctuations may in fact implement a type of memory consolidation mechanism with a stable very long-term memory that offers novel explanations for several classic memory 'laws', namely Jost's Law (1897: superiority of spaced learning) and Ribot's Law (1881: loss of recent memories in retrograde amnesia), for which a common neural basis has been postulated but not established, as well as other general 'laws' of learning and forgetting. We show how these phenomena emerge naturally from massively fluctuating neural connections.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

The Science of Learning and Art of Education in Cardiology Fellowship

The science of learning, bolstered by the foundational principles of adult learning, has evolved to allow for a more sophisticated understanding of how humans acquire knowledge. To optimize learning outcomes, cardiology educators should be familiar with these concepts and apply them routinely when teaching trainees. This paper presents an overview of the neurobiology of learning and adult learning principles and offers examples of ways in which this science can be applied in cardiology fellowships. Both fellows and educators benefit from the science of learning and its artistic application to education.

Long-lasting Postnatal Sensory Deprivation Alters Dendritic Morphology of Pyramidal Neurons in the Rat Hippocampus: Behavioral Correlates

The role of normal sensory inputs in the development of sensory cortices is well known, however, their impacts on the hippocampus, an integrator of sensory modalities with important roles in cognitive functions, has received much less attention. Here, we applied a long-term sensory deprivation paradigm by trimming the rats' whiskers bilaterally, from postnatal day 3 to 59. Female sensory-deprived (SD) rats showed more on-wall rearing and visits to the center of the open-field box, shorter periods of grooming, less defecation and less anxiety-like behaviors in the elevated plus-maze compared to controls, who had their intact whiskers brushed. Passive avoidance memory retention was sex-dependently impaired in the female SD rats. In the radial arm maze, however, reference spatial memory was impaired only in the male SD rats. Nonetheless, working memory errors increased in both sexes of SD rats. Besides depletion of CA1 and CA3 pyramidal neurons in SD rats, Sholl analysis of Golgi-Cox stained neurons revealed that prolonged sensory deprivation has retracted the arborization of CA1 basal dendrites in SD group, while solely female SD rats had diminished CA1 apical dendrites. Sholl analysis of CA3 neurons in SD animals also disclosed significantly more branched apical dendrites in males and basal dendrites in females. Sensory deprivation also led to a considerable spine loss and variation of different spine types in a sex-dependent manner. Our findings suggest that experience-dependent structural plasticity is capable of spreading far beyond the manipulated sensory zones and the inevitable functional alterations can be expressed in a multifactorial sex-dependent manner.

Kinesin-3 mediated axonal delivery of presynaptic neurexin stabilizes dendritic spines and postsynaptic components

The functional properties of neural circuits are defined by the patterns of synaptic connections between their partnering neurons, but the mechanisms that stabilize circuit connectivity are poorly understood. We systemically examined this question at synapses onto newly characterized dendritic spines of C. elegans GABAergic motor neurons. We show that the presynaptic adhesion protein neurexin/NRX-1 is required for stabilization of postsynaptic structure. We find that early postsynaptic developmental events proceed without a strict requirement for synaptic activity and are not disrupted by deletion of neurexin/nrx-1. However, in the absence of presynaptic NRX-1, dendritic spines and receptor clusters become destabilized and collapse prior to adulthood. We demonstrate that NRX-1 delivery to presynaptic terminals is dependent on kinesin-3/UNC-104 and show that ongoing UNC-104 function is required for postsynaptic maintenance in mature animals. By defining the dynamics and temporal order of synapse formation and maintenance events in vivo, we describe a mechanism for stabilizing mature circuit connectivity through neurexin-based adhesion.

Carboxypeptidase E Regulates Activity-Dependent TrkB Neuronal Surface Insertion and Hippocampal Memory

Activity-dependent insertion of the tropomyosin-related kinase B (TrkB) receptor into the plasma membrane can explain, in part, the preferential effect of brain-derived neurotrophic factor (BDNF) on active neurons and synapses; however, the underlying molecular mechanisms remain obscure. Here, we report a novel function for carboxypeptidase E (CPE) in controlling chemical long-term potentiation stimuli-induced TrkB surface delivery in hippocampal neurons. Total internal reflection fluorescence assays and line plot assays showed that CPE facilitates TrkB transport from dendritic shafts to the plasma membrane. The Box2 domain in the juxtamembrane region of TrkB and the C terminus of CPE are critical for the activity-dependent plasma membrane insertion of TrkB. Moreover, the transactivator of transcription TAT-CPE452-466, which could block the association between CPE and TrkB, significantly inhibited neuronal activity-enhanced BDNF signaling and dendritic spine morphologic plasticity in cultured hippocampal neurons. Microinfusion of TAT-CPE452-466 into the dorsal hippocampus of male C57BL/6 mice inhibited the endogenous interaction between TrkB and CPE and diminished fear-conditioning-induced TrkB phosphorylation, which might lead to an impairment in hippocampal memory acquisition and consolidation but not retrieval. These results suggest that CPE modulates activity-induced TrkB surface insertion and hippocampal-dependent memory and sheds light on our understanding of the role of CPE in TrkB-dependent synaptic plasticity and memory modulation.SIGNIFICANCE STATEMENT It is well known that BDNF acts preferentially on active neurons; however, the underlying molecular mechanism is not fully understood. In this study, we found that the cytoplasmic tail of CPE could interact with TrkB and facilitate the neuronal activity-dependent movement of TrkB vesicles to the plasma membrane. Blocking the association between CPE and TrkB decreased fear-conditioning-induced TrkB phosphorylation and led to hippocampal memory deficits. These findings provide novel insights into the role of CPE in TrkB intracellular trafficking as well as in mediating BDNF/TrkB function in synaptic plasticity and hippocampal memory.

A focused review on CB2 receptor-selective pharmacological properties and therapeutic potential of β-caryophyllene, a dietary cannabinoid

The endocannabinoid system (ECS), a conserved physiological system emerged as a novel pharmacological target for its significant role and potential therapeutic benefits ranging from neurological diseases to cancer. Among both, CB1 and CB2R types, CB2R have received attention for its pharmacological effects as antioxidant, anti-inflammatory, immunomodulatory and antiapoptotic that can be achieved without causing psychotropic adverse effects through CB1R. The ligands activate CB2R are of endogenous, synthetic and plant origin. In recent years, β-caryophyllene (BCP), a natural bicyclic sesquiterpene in cannabis as well as non-cannabis plants, has received attention due to its selective agonist property on CB2R. BCP has been well studied in a variety of pathological conditions mediating CB2R selective agonist property. The focus of the present manuscript is to represent the CB2R selective agonist mediated pharmacological mechanisms and therapeutic potential of BCP. The present narrative review summarizes insights into the CB2R-selective pharmacological properties and therapeutic potential of BCP such as cardioprotective, hepatoprotective, neuroprotective, nephroprotective, gastroprotective, chemopreventive, antioxidant, anti-inflammatory, and immunomodulator. The available evidences suggest that BCP, can be an important candidate of plant origin endowed with CB2R selective properties that may provide a pharmacological rationale for its pharmacotherapeutic application and pharmaceutical development like a drug. Additionally, given the wide availability in edible plants and dietary use, with safety, and no toxicity, BCP can be promoted as a nutraceutical and functional food for general health and well-being. Further, studies are needed to explore pharmacological and pharmaceutical opportunities for therapeutic and preventive applications of use of BCP in human diseases.

Modulation of Synaptic Plasticity by Vibratory Training in Young and Old Mice

In the past 40 years, scientific research has shown how Whole Body Vibration concept represents a strong stimulus for the whole organism. Low (<30 Hz), medium (30–80 Hz), and high (>80 Hz) frequency vibrations can have both positive and negative effects, depending on the oscillation type and duration of exposure to which the body is subjected. However, very little is known about the effects of vibratory training on the brain. In this regard, we verified whether three vibratory training protocols, differing in terms of vibration frequency and exposure time to vibration, could modulate synaptic plasticity in an experimental mouse model, by extracellular recordings in vitro in hippocampal slices of mice of 4 and 24 months old. Our results showed that vibratory training can modulate synaptic plasticity differently, depending on the protocol used, and that the best effects are related to the training protocol characterized by a low vibration frequency and a longer recovery time. Future studies will aim to understand the brain responses to various types of vibratory training and to explore the underlying mechanisms, also evaluating the presence of any structural and functional changes due to vibratory training.

Antidepressant-like effects of β-caryophyllene on restraint plus stress-induced depression

Chronic stress is depressogenic by altering neurotrophic and neuroinflammatory environments of the organism. The endocannabinoid system controls cognitive and emotional responses related with stress through the interaction with endocannabinoid receptors. β-Caryophyllene (BCP) is a CB2 agonist that exhibited anti-inflammatory, analgesic effects but minimal psychoactive effects. To test if BCP exhibits antidepressant-like action, animals were chronically restrained with additional stressors for 28 days, and BCP (25, 50, 100 mg/kg) was intraperitoneally injected once a day during the stress inflicting period. Then despair related behaviors and hippocampal expression of neurotrophic, inflammatory and cannabinoid receptor levels were measured. To test the effect of BCP on long-term depression, field potentials were measured during the application of lipopolysaccharide and low frequency stimulation. In the tail suspension test and forced swim test, chronic stress-induced despair behaviors were reduced by BCP. Also BCP improved the stress-related changes in the hippocampal expression of COX-2, BDNF, and CB2 receptor expression. In organotypic hippocampal slices, BCP reduced the lipopolysaccharide-induced intensification of the long-term depression. In conclusion, BCP improved chronic stress related behavioral and biochemical changes. These results suggest that BCP may be effective in treating depression and stress related mental illnesses.

Lactate: A Novel Signaling Molecule in Synaptic Plasticity and Drug Addiction

l-Lactate is emerging as a crucial regulatory nexus for energy metabolism in the brain and signaling transduction in synaptic plasticity, memory processes, and drug addiction instead of being merely a waste by-product of anaerobic glycolysis. In this review, the role of lactate in various memory processes, synapse plasticity and drug addiction on the basis of recent studies is summarized and discussed. To this end, three main parts are presented: first, lactate as an energy substrate in energy metabolism of the brain is described; second, lactate as a novel signaling molecule in synaptic plasticity, neural circuits, memory, and drug addiction is described; and third, in light of the above descriptions, it is plausible to speculate that lactate is predominantly a signaling molecule in specific memory processes and partly acts as an energy substrate. The future perspective in lactate signaling involving microglia and associated precise signaling pathways in the brain is highlighted.

Molecular, Morphological, and Functional Characterization of Corticotropin-Releasing Factor Receptor 1-Expressing Neurons in the Central Nucleus of the Amygdala

The central nucleus of the amygdala (CeA) is a brain region implicated in anxiety, stress-related disorders and the reinforcing effects of drugs of abuse. Corticotropin-releasing factor (CRF, Crh) acting at cognate type 1 receptors (CRF₁, Crhr1) modulates inhibitory and excitatory synaptic transmission in the CeA. Here, we used CRF₁:GFP reporter mice to characterize the morphological, neurochemical and electrophysiological properties of CRF₁-expressing (CRF₁+) and CRF₁-non-expressing (CRF₁-) neurons in the CeA. We assessed these two neuronal populations for distinctions in the expression of GABAergic subpopulation markers and neuropeptides, dendritic spine density and morphology, and excitatory transmission. We observed that CeA CRF₁+ neurons are GABAergic but do not segregate with calbindin (CB), calretinin (CR), parvalbumin (PV), or protein kinase C-δ (PKCδ). Among the neuropeptides analyzed, Penk and Sst had the highest percentage of co-expression with Crhr1 in both the medial and lateral CeA subdivisions. Additionally, CeA CRF₁+ neurons had a lower density of dendritic spines, which was offset by a higher proportion of mature spines compared to neighboring CRF₁- neurons. Accordingly, there was no difference in basal spontaneous glutamatergic transmission between the two populations. Application of CRF increased overall vesicular glutamate release onto both CRF₁+ and CRF₁- neurons and does not affect amplitude or kinetics of EPSCs in either population. These novel data highlight important differences in the neurochemical make-up and morphology of CRF₁+ compared to CRF₁- neurons, which may have important implications for the transduction of CRF signaling in the CeA.

Role of Cdk5 in Kalirin7-Mediated Formation of Dendritic Spines

A majority of excitatory synapses in the brain are localized on the dendritic spines. Alterations of spine density and morphology are associated with many neurological diseases. Understanding the molecular mechanisms underlying spine formation is important for understanding these diseases. Kalirin7 (Kal-7) is localized to the postsynaptic side of excitatory synapses in the neurons. Overexpression of Kal-7 causes an increase in spine density whereas knockdown expression of endogenous Kal-7 results in a decrease in spine density in primary cultured cortical neurons. However, the mechanisms underlying Kal-7-mediated spine formation are not entirely clear. Cyclin-dependent kinase 5 (Cdk5) plays a vital role in the formation of spines and synaptic plasticity. Kal-7 is phosphorylated by CDK5 at Thr¹⁵⁹⁰, the unique Cdk5 phosphorylation site in the Kal-7 protein. This study was to explore the role of CDK5-mediated phosphorylation of Kal-7 in spine formation and the underlying mechanisms. Our results showed expression of Kal-7T/D (mimicked phosphorylation), Kal-7T/A mutants (blocked phosphorylation) or wild-type (Wt) Kal-7 caused in a similar increase in spine density, while spine size of Wt Kal-7-expressing cortical neurons was bigger than that in Kal-7 T\A-expressing neurons, but smaller than that in Kal-7T/D-expressing neurons. The fluorescence intensity of NMDA receptor subunit NR2B (GluN2B) staining was stronger along the MAP2 positive dendrites of Kal-7T/D-expressing neurons than that in Kal-7T/A- or Wt Kal-7-expressing neurons. The fluorescence intensity of AMPA receptor subunit GluR1 (GluA1) staining showed the same trend as GluN2B staining. These findings suggest that Cdk5 affects the function of Kal-7 on spine morphology and function via GluN2B and GluA1 receptors during dendritic spine formation.

Quantitative Systems Pharmacological Analysis of Drugs of Abuse Reveals the Pleiotropy of Their Targets and the Effector Role of mTORC1

Existing treatments against drug addiction are often ineffective due to the complexity of the networks of protein-drug and protein-protein interactions (PPIs) that mediate the development of drug addiction and related neurobiological disorders. There is an urgent need for understanding the molecular mechanisms that underlie drug addiction toward designing novel preventive or therapeutic strategies. The rapidly accumulating data on addictive drugs and their targets as well as advances in machine learning methods and computing technology now present an opportunity to systematically mine existing data and draw inferences on potential new strategies. To this aim, we carried out a comprehensive analysis of cellular pathways implicated in a diverse set of 50 drugs of abuse using quantitative systems pharmacology methods. The analysis of the drug/ligand-target interactions compiled in DrugBank and STITCH databases revealed 142 known and 48 newly predicted targets, which have been further analyzed to identify the KEGG pathways enriched at different stages of drug addiction cycle, as well as those implicated in cell signaling and regulation events associated with drug abuse. Apart from synaptic neurotransmission pathways detected as upstream signaling modules that “sense” the early effects of drugs of abuse, pathways involved in neuroplasticity are distinguished as determinants of neuronal morphological changes. Notably, many signaling pathways converge on important targets such as mTORC1. The latter emerges as a universal effector of the persistent restructuring of neurons in response to continued use of drugs of abuse.

Daytime Light Intensity Modulates Spatial Learning and Hippocampal Plasticity in Female Nile Grass Rats (Arvicanthis niloticus)

Light has pervasive effects on the physiology and behavior of mammals. Several human studies have shown that light modulates cognitive functions; however, the mechanisms responsible for the effects of light remain unclear. Our previous work using diurnal male Nile grass rats (Arvicanthis niloticus) revealed that reduced illuminance during the day leads to impairments in hippocampal-dependent spatial learning/memory, reduced CA1 dendritic spine density, and attenuated hippocampal brain-derived neurotrophic factor (BDNF) expression in males. The present study examined the impact of ambient light intensity on hippocampal functions in female grass rats and explored sex differences in behavioral and hippocampal responses. Female grass rats were housed in either a 12:12-hr bright light-dark (brLD, 1000 lx) or dim light-dark (dimLD, 50 lx) cycle for four weeks. The dimLD group showed impaired spatial memory in the Morris water maze task and reduced CA1 apical dendritic spine density, similar to prior observations in males. However, the behavioral deficits seen in females were more severe than those seen in males, with dimLD females showing no evidence of long-term retention over the 24-hour periods between training sessions. In contrast to the attenuated hippocampal BDNF expression found in dimLD males, there was no significant difference in the expression of BDNF and of its receptor TrkB between females in brLD and dimLD. The results suggest that, as seen in male grass rats, reduced illuminance during the day impairs hippocampal-dependent spatial memory and hippocampal plasticity in female diurnal grass rats, but the underlying signaling pathways responsible for the effects of light restriction may differ between the sexes.

Chronic treatment with N-acetylcysteine decreases extinction responding and reduces cue-induced nicotine-seeking

N-acetylcysteine (NAC), a promising glutamatergic therapeutic agent, has shown some clinical efficacy in reducing nicotine use in humans and has been shown to reverse drug-induced changes in glutamatergic neurophysiology. In rats, nicotine-seeking behavior is associated with alterations in glutamatergic plasticity within the nucleus accumbens core (NAcore). Specifically, cue-induced nicotine-seeking is associated with rapid, transient synaptic plasticity (t-SP) in glutamatergic synapses on NAcore medium spiny neurons. The goal of the present study was to determine if NAC reduces nicotine-seeking behavior and reverses reinstatement-associated NAcore glutamatergic alterations. Rats were extinguished from nicotine self-administration, followed by subchronic NAC administration (0 or 100 mg/kg/d) for 4 days prior to cue-induced reinstatement. NAcore synaptic potentiation was measured via dendritic spine morphology and mRNA and protein of relevant glutamatergic genes were quantified. Nicotine-seeking behavior was not reduced by subchronic NAC treatment. Also, NAcore transcript and protein expression of multiple glutamatergic genes, as well as spine morphological measures, were unaffected by subchronic NAC. Finally, chronic NAC treatment (15 days total) during extinction and prior to reinstatement significantly decreased extinction responding and reduced reinstatement of nicotine-seeking compared to vehicle. Together, these results suggest that chronic NAC treatment is necessary for its therapeutic efficacy as a treatment strategy for nicotine addiction and relapse.

The Role of Glia in Sleep Regulation and Function

The cellular mechanisms governing the expression, regulation, and function of sleep are not entirely understood. The traditional view is that these mechanisms are neuronal. An alternative view is that glial brain cells may play important roles in these processes. Their ubiquity in the central nervous system makes them well positioned to modulate neuronal circuits that gate sleep and wake. Their ability to respond to chemical neuronal signals suggests that they form feedback loops with neurons that may globally regulate neuronal activity. Their potential role in detoxifying the brain, regulating neuronal metabolism, and promoting synaptic plasticity raises the intriguing possibility that glia mediate important functions ascribed to sleep.

The Synaptic Theory of Memory: A Historical Survey and Reconciliation of Recent Opposition

Trettenbrein (2016) has argued that the concept of the synapse as the locus of memory is outdated and has made six critiques of this concept. In this article, we examine these six critiques and suggest that the current theories of the neurobiology of memory and the empirical data indicate that synaptic activation is the first step in a chain of cellular and biochemical events that lead to memories formed in cell assemblies and neural networks that rely on synaptic modification for their formation. These neural networks and their modified synaptic connections can account for the cognitive basis of learning and memory and for memory deterioration in neurological disorders. We first discuss Hebb's (1949) theory that synaptic change and the formation of cell assemblies and phase sequences can link neurophysiology to cognitive processes. We then examine each of Trettenbrein's (2016) critiques of the synaptic theory in light of Hebb's theories and recent empirical data. We examine the biochemical basis of memory formation and the necessity of synaptic modification to form the neural networks underlying learning and memory. We then examine the use of Hebb's theories of synaptic change and cell assemblies for integrating neurophysiological and cognitive conceptions of learning and memory. We conclude with an examination of the applications of the Hebb synapse and cell assembly theories to the study of the neuroscience of learning and memory, the development of computational models of memory and the construction of "intelligent" robots. We conclude that the synaptic theory of memory has not met its demise, but is essential to our understanding of the neural basis of memory, which has two components: synaptic plasticity and intrinsic plasticity.

Morphological and functional evidence of increased excitatory signaling in the prelimbic cortex during ethanol withdrawal

Excessive alcohol consumption in humans induces deficits in decision making and emotional processing, which indicates a dysfunction of the prefrontal cortex (PFC). The present study aimed to determine the impact of chronic intermittent ethanol (CIE) inhalation on mouse medial PFC pyramidal neurons. Data were collected 6-8 days into withdrawal from 7 weeks of CIE exposure, a time point when mice exhibit behavioral symptoms of withdrawal. We found that spine maturity in prelimbic (PL) layer 2/3 neurons was increased, while dendritic spines in PL layer 5 neurons or infralimbic (IL) neurons were not affected. Corroborating these morphological observations, CIE enhanced glutamatergic transmission in PL layer 2/3 pyramidal neurons, but not IL layer 2/3 neurons. Contrary to our predictions, these cellular alterations were associated with improved, rather than impaired, performance in reversal learning and strategy switching tasks in the Barnes maze at an earlier stage of chronic ethanol exposure (5-7 days withdrawal from 3 to 4 weeks of CIE), which could result from the anxiety-like behavior associated with ethanol withdrawal. Altogether, this study adds to a growing body of literature indicating that glutamatergic activity in the PFC is upregulated following chronic ethanol exposure, and identifies PL layer 2/3 pyramidal neurons as a sensitive target of synaptic remodeling. It also indicates that the Barnes maze is not suitable to detect deficits in cognitive flexibility in CIE-withdrawn mice.

Wnt Signaling Mediates LTP-Dependent Spine Plasticity and AMPAR Localization through Frizzled-7 Receptors

The structural and functional plasticity of synapses is critical for learning and memory. Long-term potentiation (LTP) induction promotes spine growth and AMPAR accumulation at excitatory synapses, leading to increased synaptic strength. Glutamate initiates these processes, but the contribution from extracellular modulators is not fully established. Wnts are required for spine formation; however, their impact on activity-mediated spine plasticity and AMPAR localization is unknown. We found that LTP induction rapidly increased synaptic Wnt7a/b protein levels. Acute blockade of endogenous Wnts or loss of postsynaptic Frizzled-7 (Fz7) receptors impaired LTP-mediated synaptic strength, spine growth, and AMPAR localization at synapses. Live imaging of SEP-GluA1 and single-particle tracking revealed that Wnt7a rapidly promoted synaptic AMPAR recruitment and trapping. Wnt7a, through Fz7, induced CaMKII-dependent loss of SynGAP from spines and increased extrasynaptic AMPARs by PKA phosphorylation. We identify a critical role for Wnt-Fz7 signaling in LTP-mediated synaptic accumulation of AMPARs and spine plasticity.

A brief period of sleep deprivation causes spine loss in the dentate gyrus of mice

Sleep and sleep loss have a profound impact on hippocampal function, leading to memory impairments. Modifications in the strength of synaptic connections directly influences neuronal communication, which is vital for normal brain function, as well as the processing and storage of information. In a recently published study, we found that as little as five hours of sleep deprivation impaired hippocampus-dependent memory consolidation, which was accompanied by a reduction in dendritic spine numbers in hippocampal area CA1. Surprisingly, loss of sleep did not alter the spine density of CA3 neurons. Although sleep deprivation has been reported to affect the function of the dentate gyrus, it is unclear whether a brief period of sleep deprivation impacts spine density in this region. Here, we investigated the impact of a brief period of sleep deprivation on dendritic structure in the dentate gyrus of the dorsal hippocampus. We found that five hours of sleep loss reduces spine density in the dentate gyrus with a prominent effect on branched spines. Interestingly, the inferior blade of the dentate gyrus seems to be more vulnerable in terms of spine loss than the superior blade. This decrease in spine density predominantly in the inferior blade of the dentate gyrus may contribute to the memory deficits observed after sleep loss, as structural reorganization of synaptic networks in this subregion is fundamental for cognitive processes.

Piriform cortical glutamatergic and GABAergic neurons express coordinated plasticity for whisker-induced odor recall

Neural plasticity occurs in learning and memory. Coordinated plasticity at glutamatergic and GABAergic neurons during memory formation remains elusive, which we investigate in a mouse model of associative learning by cellular imaging and electrophysiology. Paired odor and whisker stimulations lead to whisker-induced olfaction response. In mice that express this cross-modal memory, the neurons in the piriform cortex are recruited to encode newly acquired whisker signal alongside innate odor signal, and their response patterns to these associated signals are different. There are emerged synaptic innervations from barrel cortical neurons to piriform cortical neurons from these mice. These results indicate the recruitment of associative memory cells in the piriform cortex after associative memory. In terms of the structural and functional plasticity at these associative memory cells in the piriform cortex, glutamatergic neurons and synapses are upregulated, GABAergic neurons and synapses are downregulated as well as their mutual innervations are refined in the coordinated manner. Therefore, the associated activations of sensory cortices triggered by their input signals induce the formation of their mutual synapse innervations, the recruitment of associative memory cells and the coordinated plasticity between the GABAergic and glutamatergic neurons, which work for associative memory cells to encode cross-modal associated signals in their integration, associative storage and distinguishable retrieval.

Filopodia: A Rapid Structural Plasticity Substrate for Fast Learning

Formation of new synapses between neurons is an essential mechanism for learning and encoding memories. The vast majority of excitatory synapses occur on dendritic spines, therefore, the growth dynamics of spines is strongly related to the plasticity timescales. Especially in the early stages of the developing brain, there is an abundant number of long, thin and motile protrusions (i.e., filopodia), which develop in timescales of seconds and minutes. Because of their unique morphology and motility, it has been suggested that filopodia can have a dual role in both spinogenesis and environmental sampling of potential axonal partners. I propose that filopodia can lower the threshold and reduce the time to form new dendritic spines and synapses, providing a substrate for fast learning. Based on this proposition, the functional role of filopodia during brain development is discussed in relation to learning and memory. Specifically, it is hypothesized that the postnatal brain starts with a single-stage memory system with filopodia playing a significant role in rapid structural plasticity along with the stability provided by the mushroom-shaped spines. Following the maturation of the hippocampus, this highly-plastic unitary system transitions to a two-stage memory system, which consists of a plastic temporary store and a long-term stable store. In alignment with these architectural changes, it is posited that after brain maturation, filopodia-based structural plasticity will be preserved in specific areas, which are involved in fast learning (e.g., hippocampus in relation to episodic memory). These propositions aim to introduce a unifying framework for a diversity of phenomena in the brain such as synaptogenesis, pruning and memory consolidation.

The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function

Dendritic spines are the major sites of synaptic transmission in the central nervous system. Alterations in the strength of synaptic connections directly affect the neuronal communication, which is crucial for brain function as well as the processing and storage of information. Sleep and sleep loss bidirectionally alter structural plasticity, by affecting spine numbers and morphology, which ultimately can affect the functional output of the brain in terms of alertness, cognition, and mood. Experimental data from studies in rodents suggest that sleep deprivation may impact structural plasticity in different ways. One of the current views, referred to as the synaptic homeostasis hypothesis, suggests that wake promotes synaptic potentiation whereas sleep facilitates synaptic downscaling. On the other hand, several studies have now shown that sleep deprivation can reduce spine density and attenuate synaptic efficacy in the hippocampus. These data are the basis for the view that sleep promotes hippocampal structural plasticity critical for memory formation. Altogether, the impact of sleep and sleep loss may vary between regions of the brain. A better understanding of the role that sleep plays in regulating structural plasticity may ultimately lead to novel therapeutic approaches for brain disorders that are accompanied by sleep disturbances and sleep loss.

Cognitive deficits associated with a high-fat diet and insulin resistance are potentiated by overexpression of ecto-nucleotide pyrophosphatase phosphodiesterase-1

There is growing evidence that over consumption of high-fat foods and insulin resistance may alter hippocampal-dependent cognitive function. To study the individual contributions of diet and peripheral insulin resistance to learning and memory, we used a transgenic mouse line that overexpresses ecto-nucleotide pyrophosphatase phosphodiesterase-1 in adipocytes, which inhibits the insulin receptor. Here, we demonstrate that a model of peripheral insulin resistance exacerbates high-fat diet induced deficits in performance on the Morris Water Maze task. This finding was then reviewed in the context of the greater literature to explore potential mechanisms including triglyceride storage, adiponectin, lipid composition, insulin signaling, oxidative stress, and hippocampal signaling. Together, these findings further our understanding of the complex relationship among peripheral insulin resistance, diet and memory.

A Mercaptoacetamide-Based Class II Histone Deacetylase Inhibitor Increases Dendritic Spine Density via RasGRF1/ERK Pathway

BACKGROUND

The accumulation of amyloid-β (Aβ) leads to the loss of dendritic spines and synapses, which is hypothesized to cause cognitive impairments in Alzheimer's disease (AD) patients. In our previous study, we demonstrated that a novel mercaptoacetamide-based class II histone deacetylase inhibitor (HDACI), known as W2, decreased Aβ levels and improved learning and memory in mice. However, the underlying mechanism of this effect is unknown.

OBJECTIVE

Because dendritic spine formation is associated with cognitive performance, here we investigated whether HDACI W2 regulates dendritic spine density and its molecular mechanism of action.

METHODS

To examine the effect of HDACI W2 on dendritic spine density, we conducted morphological analysis of dendritic spines using GFP transfection and Golgi staining. In addition, to determine the molecular mechanism of W2 effects on spines, we measured the levels of mRNAs and proteins involved in the Ras signaling pathway using quantitative real-time PCR, immunocytochemistry, and western analysis.

RESULTS

We found that HDACI W2 altered dendritic spine density and morphology in vitro and in vivo. Additionally, W2 increased the mRNA or protein levels of Ras GRF1 and phospho-ERK. Moreover, knockdown of RasGRF1 and inhibition of ERK activity prevented the W2-mediated spinogenesis in primary hippocampal neurons.

CONCLUSION

Our Class II-selective HDACI W2 promotes the formation and growth of dendritic spines in a RasGRF1 and ERK dependent manner in primary hippocampal neurons.

Structural and functional plasticity of dendritic spines - root or result of behavior?

Dendritic spines are multifunctional integrative units of the nervous system and are highly diverse and dynamic in nature. Both internal and external stimuli influence dendritic spine density and morphology on the order of minutes. It is clear that the structural plasticity of dendritic spines is related to changes in synaptic efficacy, learning and memory and other cognitive processes. However, it is currently unclear whether structural changes in dendritic spines are primary instigators of changes in specific behaviors, a consequence of behavioral changes, or both. In this review, we first examine the basic structure and function of dendritic spines in the brain, as well as laboratory methods to characterize and quantify morphological changes in dendritic spines. We then discuss the existing literature on the temporal and functional relationship between changes in dendritic spines in specific brain regions and changes in specific behaviors mediated by those regions. Although technological advancements have allowed us to better understand the functional relevance of structural changes in dendritic spines that are influenced by environmental stimuli, the role of spine dynamics as an underlying driver or consequence of behavior still remains elusive. We conclude that while it is likely that structural changes in dendritic spines are both instigators and results of behavioral changes, improved research tools and methods are needed to experimentally and directly manipulate spine dynamics in order to more empirically delineate the relationship between spine structure and behavior.

The Brain on Drugs: From Reward to Addiction

Advances in neuroscience identified addiction as a chronic brain disease with strong genetic, neurodevelopmental, and sociocultural components. We here discuss the circuit- and cell-level mechanisms of this condition and its co-option of pathways regulating reward, self-control, and affect. Drugs of abuse exert their initial reinforcing effects by triggering supraphysiologic surges of dopamine in the nucleus accumbens that activate the direct striatal pathway via D1 receptors and inhibit the indirect striato-cortical pathway via D2 receptors. Repeated drug administration triggers neuroplastic changes in glutamatergic inputs to the striatum and midbrain dopamine neurons, enhancing the brain's reactivity to drug cues, reducing the sensitivity to non-drug rewards, weakening self-regulation, and increasing the sensitivity to stressful stimuli and dysphoria. Drug-induced impairments are long lasting; thus, interventions designed to mitigate or even reverse them would be beneficial for the treatment of addiction.

mGluR5 Positive and Negative Allosteric Modulators Differentially Affect Dendritic Spine Density and Morphology in the Prefrontal Cortex

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of type 5 metabotropic glutamate receptors (mGluR5) are currently being investigated as novel treatments for neuropsychiatric diseases including drug addiction, schizophrenia, and Fragile X syndrome. However, only a handful of studies have examined the effects of mGluR5 PAMs or NAMs on the structural plasticity of dendritic spines in otherwise naïve animals, particularly in brain regions mediating executive function. In the present study, we assessed dendritic spine density and morphology in pyramidal cells of the medial prefrontal cortex (mPFC) after repeated administration of either the prototypical mGluR5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5- yl)benzamide (CDPPB, 20 mg/kg), the clinically utilized mGluR5 NAM 1-(3-chlorophenyl)-3-(3-methyl-5-oxo-4Himidazol- 2-yl)urea (fenobam, 20 mg/kg), or vehicle in male Sprague-Dawley rats. Following once daily treatment for 10 consecutive days, coronal brain sections containing the mPFC underwent diolistic labeling and 3D image analysis of dendritic spines. Compared to vehicle treated animals, rats administered fenobam exhibited significant increases in dendritic spine density and the overall frequency of spines with small (<0.2 μm) head diameters, decreases in frequency of spines with medium (0.2-0.4 μm) head diameters, and had no changes in frequency of spines with large head diameters (>0.4 μm). Administration of CDPPB had no discernable effects on dendritic spine density or morphology, and neither CDPPB nor fenobam had any effect on spine length or volume. We conclude that mGluR5 PAMs and NAMs differentially affect mPFC dendritic spine structural plasticity in otherwise naïve animals, and additional studies assessing their effects in combination with cognitive or behavioral tasks are needed.

Molecular regulation of dendritic spine dynamics and their potential impact on synaptic plasticity and neurological diseases

The structure and dynamics of dendritic spines reflect the strength of synapses, which are severely affected in different brain diseases. Therefore, understanding the ultra-structure, molecular signaling mechanism(s) regulating dendritic spine dynamics is crucial. Although, since last century, dynamics of spine have been explored by several investigators in different neurological diseases, but despite countless efforts, a comprehensive understanding of the fundamental etiology and molecular signaling pathways involved in spine pathology is lacking. The purpose of this review is to provide a contextual framework of our current understanding of the molecular mechanisms of dendritic spine signaling, as well as their potential impact on different neurodegenerative and psychiatric diseases, as a format for highlighting some commonalities in function, as well as providing a format for new insights and perspectives into this critical area of research. Additionally, the potential strategies to restore spine structure-function in different diseases are also pointed out. Overall, these informations should help researchers to design new drugs to restore the structure-function of dendritic spine, a "hot site" of synaptic plasticity.

Aβ42-oligomer Interacting Peptide (AIP) neutralizes toxic amyloid-β42 species and protects synaptic structure and function

The amyloid-β42 (Aβ42) peptide is believed to be the main culprit in the pathogenesis of Alzheimer disease (AD), impairing synaptic function and initiating neuronal degeneration. Soluble Aβ42 oligomers are highly toxic and contribute to progressive neuronal dysfunction, loss of synaptic spine density, and affect long-term potentiation (LTP). We have characterized a short, L-amino acid Aβ-oligomer Interacting Peptide (AIP) that targets a relatively well-defined population of low-n Aβ42 oligomers, rather than simply inhibiting the aggregation of Aβ monomers into oligomers. Our data show that AIP diminishes the loss of Aβ42-induced synaptic spine density and rescues LTP in organotypic hippocampal slice cultures. Notably, the AIP enantiomer (comprised of D-amino acids) attenuated the rough-eye phenotype in a transgenic Aβ42 fly model and significantly improved the function of photoreceptors of these flies in electroretinography tests. Overall, our results indicate that specifically “trapping” low-n oligomers provides a novel strategy for toxic Aβ42-oligomer recognition and removal.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Neural Plasticity in Multiple Sclerosis: The Functional and Molecular Background

Multiple sclerosis is an autoimmune neurodegenerative disorder resulting in motor dysfunction and cognitive decline. The inflammatory and neurodegenerative changes seen in the brains of MS patients lead to progressive disability and increasing brain atrophy. The most common type of MS is characterized by episodes of clinical exacerbations and remissions. This suggests the presence of compensating mechanisms for accumulating damage. Apart from the widely known repair mechanisms like remyelination, another important phenomenon is neuronal plasticity. Initially, neuroplasticity was connected with the developmental stages of life; however, there is now growing evidence confirming that structural and functional reorganization occurs throughout our lifetime. Several functional studies, utilizing such techniques as fMRI, TBS, or MRS, have provided valuable data about the presence of neuronal plasticity in MS patients. CNS ability to compensate for neuronal damage is most evident in RR-MS; however it has been shown that brain plasticity is also preserved in patients with substantial brain damage. Regardless of the numerous studies, the molecular background of neuronal plasticity in MS is still not well understood. Several factors, like IL-1β, BDNF, PDGF, or CB1Rs, have been implicated in functional recovery from the acute phase of MS and are thus considered as potential therapeutic targets.

Long-term stability of axonal boutons in the mouse barrel cortex

Many lines of evidence indicate that postsynaptic dendritic spines are plastic during development and largely stable in adulthood. It remains unclear to what degree presynaptic axonal terminals undergo changes in the developing and mature cortex. In this study, we examined the formation and elimination of fluorescently-labeled axonal boutons in the living mouse barrel cortex with transcranial two-photon microscopy. We found that the turnover of axonal boutons was significantly higher in 3-week-old young mice than in adult mice (older than 3 months). There was a slight but significant net loss of axonal boutons in mice from 1 to 2 months of age. In both young and adult barrel cortex, axonal boutons existed for at least 1 week were less likely to be eliminated than those recently-formed boutons. In adulthood, 80% of axonal boutons persisted over 12 months and enriched sensory experience caused a slight but not significant increase in the turnover of axonal boutons over 2-4 weeks. Thus, similar to postsynaptic dendritic spines, presynaptic axonal boutons show remarkable stability after development ends. This long-term stability of synaptic connections is likely important for reliable sensory processing in the mature somatosensory cortex.

Phthalates and neurotoxic effects on hippocampal network plasticity

Phthalates are synthetically derived chemicals used as plasticizers in a variety of common household products. They are not chemically bound to plastic polymers and over time, easily migrate out of these products and into the environment. Experimental investigations evaluating the biological impact of phthalate exposure on developing organisms are critical given that estimates of phthalate exposure are considerably higher in infants and children compared to adults. Extensive growth and re-organization of neurocircuitry occurs during development leaving the brain highly susceptible to environmental insults. This review summarizes the effects of phthalate exposure on brain structure and function with particular emphasis on developmental aspects of hippocampal structural and functional plasticity. In general, it appears that widespread disruptions in hippocampal functional and structural plasticity occur following developmental (pre-, peri- and post-natal) exposure to phthalates. Whether these changes occur as a direct neurotoxic effect of phthalates or an indirect effect through disruption of endogenous endocrine functions is not fully understood. Comprehensive investigations that simultaneously assess the neurodevelopmental, neurotoxic, neuroendocrine and behavioral correlates of phthalate exposure are needed to provide an opportunity to thoroughly evaluate the neurotoxic potential of phthalates throughout the lifespan.

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.

Imbalance of HCN1 and HCN2 expression in hippocampal CA1 area impairs spatial learning and memory in rats with chronic morphine exposure

The hyperpolarization-activated cyclic-nucleotide-gated non-selective cation (HCN) channels play a vital role in the neurological basis underlying nervous system diseases. However, the role of HCN channels in drug addiction is not fully understood. In the present study, we investigated the expression of HCN1 and HCN2 subunits in hippocampal CA1 and the potential molecular mechanisms underlying the modulation of HCN channels in rats with chronic morphine exposure with approaches of electrophysiology, water maze, and Western blotting. We found that chronic morphine exposure (5 mg/kg, sc, for 7 days) caused an inhibition of long-term potentiation (LTP) and impairment of spatial learning and memory, which is associated with a decrease in HCN1, and an increase in HCN2 on cell membrane of hippocampal CA1 area. Additional experiments showed that the imbalance of cell membrane HCN1 and HCN2 expression under chronic morphine exposure was related to an increase in expression of TPR containing Rab8b interacting protein (TRIP8b) (1a-4) and TRIP8b (1b-2), and phosphorylation of protein kinase A (PKA) and adaptor protein 2 μ2 (AP2 μ2). Our results demonstrate the novel information that drug addiction-induced impairment of learning and memory is involved in the imbalance of HCN1 and HCN2 subunits, which is mediated by activation of TRIP8b (1a-4), TRIP8b (1b-2), PKA and AP2 μ2.

CB1 receptor affects cortical plasticity and response to physiotherapy in multiple sclerosis

OBJECTIVES

Therapeutic effects of physical therapy in neurologic disorders mostly rely on the promotion of use-dependent synaptic plasticity in damaged neuronal circuits. Genetic differences affecting the efficiency of synaptic plasticity mechanisms could explain why some patients do not respond adequately to the treatment. It is known that physical exercise activates the endocannabinoid system and that stimulation of cannabinoid CB1 receptors (CB1Rs) promotes synaptic plasticity in both rodents and humans. We thus tested whether CB1R genetic variants affect responsiveness to exercise therapy.

METHODS

We evaluated the effect of a genetic variant of the CB1R associated with reduced receptor expression (patients with long AAT trinucleotide short tandem repeats in the CNR1 gene) on long-term potentiation (LTP)-like cortical plasticity induced by transcranial magnetic theta burst stimulation (TBS) of the motor cortex and, in parallel, on clinical response to exercise therapy in patients with multiple sclerosis.

RESULTS

We found that patients with long AAT CNR1 repeats do not express TBS-induced LTP-like cortical plasticity and show poor clinical benefit after exercise therapy.

CONCLUSIONS

Our results provide the first evidence that genetic differences within the CB1R may influence clinical responses to exercise therapy, and they strengthen the hypothesis that CB1Rs are involved in the regulation of synaptic plasticity and in the control of spasticity in humans. This information might be of great relevance for patient stratification and personalized rehabilitation treatment programs.

Sleep, clocks, and synaptic plasticity

Sleep is widely believed to play an essential role in synaptic plasticity. However, the precise mechanisms governing this presumptive function are largely unknown. There is also evidence for independent circadian oscillations in synaptic strength and morphology. Therefore, synaptic changes observed after sleep reflect interactions between state-dependent (e.g., wake versus sleep) and state-independent (circadian) processes. In this review we consider how sleep and biological clocks influence synaptic plasticity. We discuss these findings in the context of current plasticity-based theories of sleep function and propose a new model that integrates circadian and brain-state influences on synaptic plasticity.

Prolonged ampakine exposure prunes dendritic spines and increases presynaptic release probability for enhanced long-term potentiation in the hippocampus

CX 546, an allosteric positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type ionotropic glutamate receptors (AMPARs), belongs to a drug class called ampakines. These compounds have been shown to enhance long-term potentiation (LTP), a cellular model of learning and memory, and improve animal learning task performance, and have augmented cognition in neurodegenerative patients. However, the chronic effect of CX546 on synaptic structures has not been examined. The structure and integrity of dendritic spines are thought to play a role in learning and memory, and their abnormalities have been implicated in cognitive disorders. In addition, their structural plasticity has been shown to be important for cognitive function, such that dendritic spine remodeling has been proposed as the morphological correlate for LTP. Here, we tested the effect of CX546 on dendritic spine remodeling following long-term treatment. We found that, with prolonged CX546 treatment, organotypic hippocampal slice cultures showed a significant reduction in CA3-CA1 excitatory synapse and spine density. Electrophysiological approaches revealed that the CA3-CA1 circuitry compensates for this synapse loss by increasing synaptic efficacy through enhancement of presynaptic release probability. CX546-treated slices showed prolonged and enhanced potentiation upon LTP induction. Furthermore, structural plasticity, namely spine head enlargement, was also more pronounced after CX546 treatment. Our results suggest a concordance of functional and structural changes that is enhanced with prolonged CX546 exposure. Thus, the improved cognitive ability of patients receiving ampakine treatment may result from the priming of synapses through increases in the structural plasticity and functional reliability of hippocampal synapses.

RhoGTPase-binding proteins, the exocyst complex and polarized vesicle trafficking

Cell polarity, the asymmetric distribution of proteins and lipids, is essential for a variety of cellular functions. One mechanism orchestrating cell polarity is polarized vesicle trafficking; whereby cargo loaded secretory vesicles are specifically transported to predetermined areas of the cell. The evolutionarily conserved exocyst complex and its small GTPase regulators play crucial roles in spatiotemporal control of polarized vesicle trafficking. In studies on neuronal membrane remodeling and synaptic plasticity, conserved mechanisms of exocyst regulation and cargo recycling during polarized vesicle trafficking are beginning to emerge as well. Recently, our lab demonstrated that RhoGTPase-binding proteins in both yeast (Bem3) and mammals (Ocrl1) are also required for the efficient traffic of secretory vesicles to sites of polarized growth and signaling. Together with our studies, we highlight the evolutionary conservation of the basic elements essential for polarized vesicle traffic across different cellular functions and model systems. In conclusion, we emphasize that studies on RhoGTPase-binding proteins in these processes should be included in the next level of investigation, for a more complete understanding of their hitherto unknown roles in polarized membrane traffic and exocyst regulation.

Maternal dietary loads of alpha-tocopherol increase synapse density and glial synaptic coverage in the hippocampus of adult offspring

An increased intake of the antioxidant α-Tocopherol (vitamin E) is recommended in complicated pregnancies, to prevent free radical damage to mother and fetus. However, the anti-PKC and antimitotic activity of α-Tocopherol raises concerns about its potential effects on brain development. Recently, we found that maternal dietary loads of α-Tocopherol through pregnancy and lactation cause developmental deficit in hippocampal synaptic plasticity in rat offspring. The defect persisted into adulthood, with behavioral alterations in hippocampus-dependent learning. Here, using the same rat model of maternal supplementation, ultrastructural morphometric studies were carried out to provide mechanistic interpretation to such a functional impairment in adult offspring by the occurrence of long-term changes in density and morphological features of hippocampal synapses. Higher density of axo-spinous synapses was found in CA1 stratum radiatum of α-Tocopherol-exposed rats compared to controls, pointing to a reduced synapse pruning. No morphometric changes were found in synaptic ultrastructural features, i.e., perimeter of axon terminals, length of synaptic specializations, extension of bouton-spine contact. Glia-synapse anatomical relationship was also affected. Heavier astrocytic coverage of synapses was observed in Tocopherol-treated offspring, notably surrounding axon terminals; moreover, the percentage of synapses contacted by astrocytic endfeet at bouton-spine interface (tripartite synapses) was increased. These findings indicate that gestational and neonatal exposure to supranutritional tocopherol intake can result in anatomical changes of offspring hippocampus that last through adulthood. These include a surplus of axo-spinous synapses and an aberrant glia-synapse relationship, which may represent the morphological signature of previously described alterations in synaptic plasticity and hippocampus-dependent learning.

In vivo activation of Wnt signaling pathway enhances cognitive function of adult mice and reverses cognitive deficits in an Alzheimer's disease model

The role of the Wnt signaling pathway during synaptic development has been well established. In the adult brain, different components of Wnt signaling are expressed, but little is known about its role in mature synapses. Emerging in vitro studies have implicated Wnt signaling in synaptic plasticity. Furthermore, activation of Wnt signaling has shown to protect against amyloid-β-induced synaptic impairment. The present study provides the first evidence that in vivo activation of Wnt signaling improves episodic memory, increases excitatory synaptic transmission, and enhances long-term potentiation in adult wild-type mice. Moreover, the activation of Wnt signaling also rescues memory loss and improves synaptic dysfunction in APP/PS1-transgenic mice that model the amyloid pathology of Alzheimer's diseases. These findings indicate that Wnt signaling modulates cognitive function in the adult brain and could be a novel promising target for Alzheimer's disease therapy.

Transcriptomic analysis of the effects of a fish oil enriched diet on murine brains

The health benefits of fish oil enriched with high omega-3 polyunsaturated fatty acids (n-3 PUFA) are widely documented. Fish oil as dietary supplements, however, show moderate clinical efficacy, highlighting an immediate scope of systematic in vitro feedback. Our transcriptomic study was designed to investigate the genomic shift of murine brains fed on fish oil enriched diets. A customized fish oil enriched diet (FD) and standard lab diet (SD) were separately administered to two randomly chosen populations of C57BL/6J mice from their weaning age until late adolescence. Statistical analysis mined 1,142 genes of interest (GOI) differentially altered in the hemibrains collected from the FD- and SD-fed mice at the age of five months. The majority of identified GOI (∼40%) encodes proteins located in the plasma membrane, suggesting that fish oil primarily facilitated the membrane-oriented biofunctions. FD potentially augmented the nervous system's development and functions by selectively stimulating the Src-mediated calcium-induced growth cascade and the downstream PI3K-AKT-PKC pathways. FD reduced the amyloidal burden, attenuated oxidative stress, and assisted in somatostatin activation—the signatures of attenuation of Alzheimer's disease, Parkinson's disease, and affective disorder. FD induced elevation of FKBP5 and suppression of BDNF, which are often linked with the improvement of anxiety disorder, depression, and post-traumatic stress disorder. Hence we anticipate efficacy of FD in treating illnesses such as depression that are typically triggered by the hypoactivities of dopaminergic, adrenergic, cholinergic, and GABAergic networks. Contrastingly, FD's efficacy could be compromised in treating illnesses such as bipolar disorder and schizophrenia, which are triggered by hyperactivities of the same set of neuromodulators. A more comprehensive investigation is recommended to elucidate the implications of fish oil on disease pathomechanisms, and the result-driven repositioning of fish oil utilization may revitalize its therapeutic efficacy.