Noradrenaline goes nuclear: epigenetic modifications during long-lasting synaptic potentiation triggered by activation of β-adrenergic receptors

Targeting epigenetic mechanisms in amyloid-β-mediated Alzheimer's pathophysiology: unveiling therapeutic potential

Alzheimer's disease is a prominent chronic neurodegenerative condition characterized by a gradual decline in memory leading to dementia. Growing evidence suggests that Alzheimer's disease is associated with accumulating various amyloid-β oligomers in the brain, influenced by complex genetic and environmental factors. The memory and cognitive deficits observed during the prodromal and mild cognitive impairment phases of Alzheimer's disease are believed to primarily result from synaptic dysfunction. Throughout life, environmental factors can lead to enduring changes in gene expression and the emergence of brain disorders. These changes, known as epigenetic modifications, also play a crucial role in regulating the formation of synapses and their adaptability in response to neuronal activity. In this context, we highlight recent advances in understanding the roles played by key components of the epigenetic machinery, specifically DNA methylation, histone modification, and microRNAs, in the development of Alzheimer's disease, synaptic function, and activity-dependent synaptic plasticity. Moreover, we explore various strategies, including enriched environments, exposure to non-invasive brain stimulation, and the use of pharmacological agents, aimed at improving synaptic function and enhancing long-term potentiation, a process integral to epigenetic mechanisms. Lastly, we deliberate on the development of effective epigenetic agents and safe therapeutic approaches for managing Alzheimer's disease. We suggest that addressing Alzheimer's disease may require distinct tailored epigenetic drugs targeting different disease stages or pathways rather than relying on a single drug.

Navigating glioblastoma complexity: the interplay of neurotransmitters and chromatin

Glioblastoma is the most aggressive brain cancer with an unfavorable prognosis for patient survival. Glioma stem cells, a subpopulation of cancer cells, drive tumor initiation, self-renewal, and resistance to therapy and, together with the microenvironment, play a crucial role in glioblastoma maintenance and progression. Neurotransmitters such as noradrenaline, dopamine, and serotonin have contrasting effects on glioblastoma development, stimulating or inhibiting its progression depending on the cellular context and through their action on glioma stem cells, perhaps changing the epigenetic landscape. Recent studies have revealed that serotonin and dopamine induce chromatin modifications related to transcriptional plasticity in the mammalian brain and possibly in glioblastoma; however, this topic still needs to be explored because of its potential implications for glioblastoma treatment. Also, it is essential to consider that neurotransmitters' effects depend on the tumor's microenvironment since it can significantly influence the response and behavior of cancer cells. This review examines the possible role of neurotransmitters as regulators of glioblastoma development, focusing on their impact on the chromatin of glioma stem cells.

Activation of β2-adrenergic receptors prevents AD-type synaptotoxicity via epigenetic mechanisms

We previously reported that prolonged exposure to an enriched environment (EE) enhances hippocampal synaptic plasticity, with one of the significant mechanistic pathways being activation of β2-adrenergic receptor (β2-AR) signaling, thereby mitigating the synaptotoxic effects of soluble oligomers of amyloid β-protein (oAβ). However, the detailed mechanism remained elusive. In this work, we recorded field excitatory postsynaptic potentials (fEPSP) in the CA1 region of mouse hippocampal slices treated with or without toxic Aβ-species. We found that pharmacological activation of β2-AR, but not β1-AR, selectively mimicked the effects of EE in enhancing LTP and preventing oAβ-induced synaptic dysfunction. Mechanistic analyses showed that certain histone deacetylase (HDAC) inhibitors mimicked the benefits of EE, but this was not seen in β2-AR knockout mice, suggesting that activating β2-AR prevents oAβ-mediated synaptic dysfunction via changes in histone acetylation. EE or activation of β-ARs each decreased HDAC2, whereas Aβ oligomers increased HDAC2 levels in the hippocampus. Further, oAβ-induced inflammatory effects and neurite degeneration were prevented by either β2-AR agonists or certain specific HDAC inhibitors. These preclinical results suggest that activation of β2-AR is a novel potential therapeutic strategy to mitigate oAβ-mediated features of AD.

Norepinephrine, beyond the Synapse: Coordinating Epigenetic Codes for Memory

The noradrenergic system is implicated in neuropathologies contributing to major disorders of the memory, including post-traumatic stress disorder and Alzheimer’s disease. Determining the impact of norepinephrine on cellular function and plasticity is thus essential for making inroads into our understanding of these brain conditions, while expanding our capacity for treating them. Norepinephrine is a neuromodulator within the mammalian central nervous system which plays important roles in cognition and associated synaptic plasticity. Specifically, norepinephrine regulates the formation of memory through the stimulation of β-ARs, increasing the dynamic range of synaptic modifiability. The mechanisms through which NE influences neural circuit function have been extended to the level of the epigenome. This review focuses on recent insights into how the noradrenergic recruitment of epigenetic modifications, including DNA methylation and post-translational modification of histones, contribute to homo- and heterosynaptic plasticity. These advances will be placed in the context of synaptic changes associated with memory formation and linked to brain disorders and neurotherapeutic applications.

Immune dysfunction after spinal cord injury - A review of autonomic and neuroendocrine mechanisms

Infections impair neurological outcome and increase mortality after spinal cord injury (SCI). Emerging data show that pathogens more easily infect individuals with SCI because SCI disrupts neural and humoral control of immune cells, culminating with the development of "SCI-induced immune deficiency syndrome" (SCI-IDS). Here, we review data that implicate autonomic dysfunction and impaired neuroendocrine signaling as key determinants of SCI-IDS. Although it is widely appreciated that mature leukocyte dysfunction is a canonical feature of SCI-IDS, new data indicate that SCI impairs the development and mobilization of immune cell precursors in bone marrow. Thus, this review will also explore how the post-injury acquisition of a "bone marrow failure syndrome" may be the earliest manifestation of SCI-IDS.

Locus coeruleus in memory formation and Alzheimer's disease

Catecholamine neurons of the locus coeruleus (LC) in the dorsal pontine tegmentum innervate the entire neuroaxis, with signaling actions implicated in the regulation of attention, arousal, sleep-wake cycle, learning, memory, anxiety, pain, mood, and brain metabolism. The co-release of norepinephrine (NE) and dopamine (DA) from LC terminals in the hippocampus plays a role in all stages of hippocampal-memory processing. This catecholaminergic regulation modulates the encoding, consolidation, retrieval, and reversal of hippocampus-based memory. LC neurons in awake animals have two distinct firing modes: tonic firing (explorative) and phasic firing (exploitative). These two firing modes exert different modulatory effects on post-synaptic dendritic spines. In the hippocampus, the firing modes regulate long-term potentiation (LTP) and long-term depression, which differentially regulate the mRNA expression and transcription of plasticity-related proteins (PRPs). These proteins aid in structural alterations of dendritic spines, that is, structural long-term potentiation (sLTP), via expansion and structural long-term depression (sLTD) via contraction of post-synaptic dendritic spines. Given the LC's role in all phases of memory processing, the degeneration of 50% of the LC neuron population occurring in Alzheimer's disease (AD) is a clinically relevant aspect of disease pathology. The loss of catecholaminergic regulation contributes to dysfunction in memory processes along with impaired functions associated with attention and task completion. The multifaceted role of the LC in memory and general task performance and the close correlation of LC degeneration with neurodegenerative disease progression together implicate it as a target for new clinical assessment tools.

The Cholinergic System, the Adrenergic System and the Neuropathology of Alzheimer's Disease

Neurodegenerative diseases are a major public health problem worldwide with a wide spectrum of symptoms and physiological effects. It has been long reported that the dysregulation of the cholinergic system and the adrenergic system are linked to the etiology of Alzheimer’s disease. Cholinergic neurons are widely distributed in brain regions that play a role in cognitive functions and normal cholinergic signaling related to learning and memory is dependent on acetylcholine. The Locus Coeruleus norepinephrine (LC-NE) is the main noradrenergic nucleus that projects and supplies norepinephrine to different brain regions. Norepinephrine has been shown to be neuroprotective against neurodegeneration and plays a role in behavior and cognition. Cholinergic and adrenergic signaling are dysregulated in Alzheimer’s disease. The degeneration of cholinergic neurons in nucleus basalis of Meynert in the basal forebrain and the degeneration of LC-NE neurons were reported in Alzheimer’s disease. The aim of this review is to describe current literature on the role of the cholinergic system and the adrenergic system (LC-NE) in the pathology of Alzheimer’s disease and potential therapeutic implications.

Monoamines: Dopamine, Norepinephrine, and Serotonin, Beyond Modulation, "Switches" That Alter the State of Target Networks

How do monoamines influence the perceptual and behavioral aspects of brain function? A library of information regarding the genetic, molecular, cellular, and function of monoamines in the nervous system and other organs has accumulated. We briefly review monoamines' anatomy and physiology and discuss their effects on the target neurons and circuits. Monoaminergic cells in the brain stem receive inputs from sensory, limbic, and prefrontal areas and project extensively to the forebrain and hindbrain. We review selected studies on molecular, cellular, and electrophysiological effects of monoamines on the brain's target areas. The idea is that monoamines, by reversibly modulating the "primary" information processing circuits, regulate and switch the functions of brain networks and can reversibly alter the "brain states," such as consciousness, emotions, and movements. Monoamines, as the drivers of normal motor and sensory brain operations, including housekeeping, play essential roles in pathogenesis of neuropsychiatric diseases.

Spinal cord injury causes chronic bone marrow failure

Spinal cord injury (SCI) causes immune dysfunction, increasing the risk of infectious morbidity and mortality. Since bone marrow hematopoiesis is essential for proper immune function, we hypothesize that SCI disrupts bone marrow hematopoiesis. Indeed, SCI causes excessive proliferation of bone marrow hematopoietic stem and progenitor cells (HSPC), but these cells cannot leave the bone marrow, even after challenging the host with a potent inflammatory stimulus. Sequestration of HSPCs in bone marrow after SCI is linked to aberrant chemotactic signaling that can be reversed by post-injury injections of Plerixafor (AMD3100), a small molecule inhibitor of CXCR4. Even though Plerixafor liberates HSPCs and mature immune cells from bone marrow, competitive repopulation assays show that the intrinsic long-term functional capacity of HSPCs is still impaired in SCI mice. Together, our data suggest that SCI causes an acquired bone marrow failure syndrome that may contribute to chronic immune dysfunction.

Norepinephrine stabilizes translation-dependent, homosynaptic long-term potentiation through mechanisms requiring the cAMP sensor Epac, mTOR and MAPK

Neuromodulators regulate higher-order cognitive processes including learning and memory through modulation of synaptic transmission and plasticity. Norepinephrine is a neuromodulator that is secreted throughout the brain in response to novelty or increased arousal, which alters neural circuits by increasing the modifiability of CNS synapses. Norepinephrine activates metabotropic receptors, initiating complex intracellular signalling cascades that can promote enduring changes in synaptic strength including long-term potentiation (LTP). In particular, activation of beta-adrenergic receptors (β-ARs) by norepinephrine enhances LTP through downstream engagement of signalling cascades which upregulate protein synthesis at synapses. Here, we sought to determine the select signalling pathways recruited by norepinephrine to promote homosynaptic LTP at hippocampal synapses in mice. Application of norepinephrine initiated a long-lasting form of homosynaptic LTP that requires protein synthesis. Norepinephrine-mediated enhancement of LTP was reduced by inhibition of mammalian target of rapamycin and exchange protein directly activated by cAMP (Epac) but not cAMP-dependent protein kinase A, suggesting that the endogenous β-AR ligand norepinephrine may preferentially recruit Epac signalling to promote enduring changes in synaptic strength. These findings advance our understanding of the mechanisms through which norepinephrine regulates synaptic plasticity associated with formation of new memories.

Noradrenergic Regulation of Hippocampus-Dependent Memory

Neuromodulation regulates critical functions of CNS synapses, ranging from neural circuit development to high-order cognitive processes, including learning and memory. This broad scope of action is generally mediated through alterations of the strength of synaptic transmission (i.e. synaptic plasticity). Changes in synaptic strength are widely considered to be a cellular representation of learned information. Noradrenaline is a neuromodulator that is secreted throughout the brain in response to novelty or increased arousal. Once released, noradrenaline activates metabotropic receptors, initiating intracellular signaling cascades that promote enduring changes in synaptic strength and facilitate memory storage. Here, we provide an overview of noradrenergic modulation of synaptic plasticity and memory formation within mammalian neural circuits, which has broad applicability within the neurotherapeutics community. Advances in our understanding of noradrenaline in the context of these processes may provide a foundation for refining treatment strategies for multiple brain diseases, ranging from post-traumatic stress disorder to Alzheimer's Disease.

Exploring the Stress Impact in the Paternal Germ Cells Epigenome: Can Catecholamines Induce Epigenetic Reprogramming?

Spermatogenesis is characterized by unique epigenetic programs that enable chromatin remodeling and transcriptional regulation for proper meiotic divisions and germ cells maturation. Paternal lifestyle stressors such as diet, drug abuse, or psychological trauma can directly impact the germ cell epigenome and transmit phenotypes to the next generation, pointing to the importance of epigenetic regulation during spermatogenesis. It is established that environmental perturbations can affect the development and behavior of the offspring through epigenetic inheritance, including changes in small non-coding RNAs, DNA methylation, and histones post-translational modifications. But how male germ cells react to lifestyle stressors and encode them in the paternal epigenome is still a research gap. Most lifestyle stressors activate catecholamine circuits leading to both acute and long-term changes in neural functions, and epigenetic mechanisms show strong links to both long-term and rapid, dynamic gene expression regulation during stress. Importantly, the testis shares a molecular and transcriptional signature with the brain tissue, including a rich expression of catecholaminergic elements in germ cells that seem to respond to stressors with similar epigenetic and transcriptional profiles. In this minireview, we put on stage the action of catecholamines as possible mediators between paternal stress responses and epigenetic marks alterations during spermatogenesis. Understanding the epigenetic regulation in spermatogenesis will contribute to unravel the coding mechanisms in the transmission of the biological impacts of stress between generations.

Neuromodulators and Long-Term Synaptic Plasticity in Learning and Memory: A Steered-Glutamatergic Perspective

The molecular pathways underlying the induction and maintenance of long-term synaptic plasticity have been extensively investigated revealing various mechanisms by which neurons control their synaptic strength. The dynamic nature of neuronal connections combined with plasticity-mediated long-lasting structural and functional alterations provide valuable insights into neuronal encoding processes as molecular substrates of not only learning and memory but potentially other sensory, motor and behavioural functions that reflect previous experience. However, one key element receiving little attention in the study of synaptic plasticity is the role of neuromodulators, which are known to orchestrate neuronal activity on brain-wide, network and synaptic scales. We aim to review current evidence on the mechanisms by which certain modulators, namely dopamine, acetylcholine, noradrenaline and serotonin, control synaptic plasticity induction through corresponding metabotropic receptors in a pathway-specific manner. Lastly, we propose that neuromodulators control plasticity outcomes through steering glutamatergic transmission, thereby gating its induction and maintenance.

Norepinephrine upregulates the expression of tyrosine hydroxylase and protects dopaminegic neurons against 6-hydrodopamine toxicity

As a classic neurotransmitter in the brain, norepinephrine (NE) also is an important modulator to other neuronal systems. Using primary cultures from rat ventral mesencephalon (VM) and dopaminergic cell line MN9D, the present study examined the neuroprotective effects of NE and its effects on the expression of tyrosine hydroxylase (TH). The results showed that NE protected both VM cultures and MN9D cells against 6-hydroxydopamine-caused apoptosis, with possible involvement of adrenal receptors. In addition, treatment with NE upregulated TH protein levels in dose- and time-dependent manner. Further experiments to investigate the potential mechanisms underlying this NE-induced upregulation of TH demonstrated a marked increase in protein levels of the brain-derived neurotrophic factor (BDNF) and the phosphorylated extracellular signal-regulated protein kinase 1 and 2 (pERK1/2) in VM cultures treated with NE. In MN9D cells, a significantly increase of TH and pERK1/2 protein levels were observed after their transfection with BDNF cDNA or exposure to BDNF peptides. Treatment of VM cultures with K252a, an antagonist of the tropomyosin-related kinase B, blocked the upregulatory effects of NE on TH, BDNF and pERK1/2. Administration of MEK1 & MEK2 inhibitors also reversed NE-induced upregulation of TH and pERK1/2. Moreover, ChIP assay showed that treatment with NE or BDNF increased H4 acetylation in the TH promoter. These results suggest that the neuroprotection and modulation of NE on dopaminergic neurons are mediated via BDNF and MAPK/ERK pathways, as well as through epigenetic histone modification, which may have implications for the improvement of therapeutic strategies for Parkinson's disease.

Recent developments in transcriptional and translational regulation underlying long-term synaptic plasticity and memory

Formation of long-term synaptic plasticity that underlies long-term memory requires new protein synthesis. Years of research has elucidated some of the transcriptional and translational mechanisms that contribute to the production of new proteins. Early research on transcription focused on the transcription factor cAMP-responsive element binding protein. Since then, other transcription factors, such as the Nuclear Receptor 4 family of proteins that play a role in memory formation and maintenance have been identified. In addition, several studies have revealed details of epigenetic mechanisms consisting of new types of chemical alterations of DNA such as hydroxymethylation, and various histone modifications in long-term synaptic plasticity and memory. Our understanding of translational control critical for memory formation began with the identification of molecules that impinge on the 5′ and 3′ untranslated regions of mRNAs and continued with the appreciation for local translation near synaptic sites. Lately, a role for noncoding RNAs such as microRNAs in regulating translation factors and other molecules critical for memory has been found. This review describes the past research in brief and mainly focuses on the recent work on molecular mechanisms of transcriptional and translational regulation that form the underpinnings of long-term synaptic plasticity and memory.

β-adrenergic modulation of discrimination learning and memory in the auditory cortex

Despite vast literature on catecholaminergic neuromodulation of auditory cortex functioning in general, knowledge about its role for long‐term memory formation is scarce. Our previous pharmacological studies on cortex‐dependent frequency‐modulated tone‐sweep discrimination learning of Mongolian gerbils showed that auditory‐cortical D_1/5‐dopamine receptor activity facilitates memory consolidation and anterograde memory formation. Considering overlapping functions of D_1/5‐dopamine receptors and β‐adrenoceptors, we hypothesised a role of β‐adrenergic signalling in the auditory cortex for sweep discrimination learning and memory. Supporting this hypothesis, the β_1/2‐adrenoceptor antagonist propranolol bilaterally applied to the gerbil auditory cortex after task acquisition prevented the discrimination increment that was normally monitored 1 day later. The increment in the total number of hurdle crossings performed in response to the sweeps per se was normal. Propranolol infusion after the seventh training session suppressed the previously established sweep discrimination. The suppressive effect required antagonist injection in a narrow post‐session time window. When applied to the auditory cortex 1 day before initial conditioning, β₁‐adrenoceptor‐antagonising and β₁‐adrenoceptor‐stimulating agents retarded and facilitated, respectively, sweep discrimination learning, whereas β₂‐selective drugs were ineffective. In contrast, single‐sweep detection learning was normal after propranolol infusion. By immunohistochemistry, β₁‐ and β₂‐adrenoceptors were identified on the neuropil and somata of pyramidal and non‐pyramidal neurons of the gerbil auditory cortex. The present findings suggest that β‐adrenergic signalling in the auditory cortex has task‐related importance for discrimination learning of complex sounds: as previously shown for D_1/5‐dopamine receptor signalling, β‐adrenoceptor activity supports long‐term memory consolidation and reconsolidation; additionally, tonic input through β₁‐adrenoceptors may control mechanisms permissive for memory acquisition.

A requirement for epigenetic modifications during noradrenergic stabilization of heterosynaptic LTP in the hippocampus

Beta-adrenergic receptor (b-AR) activation by noradrenaline (NA) enhances memory formation and long-term potentiation (LTP), a form of synaptic plasticity characterized by an activity-dependent increase in synaptic strength. LTP is believed to be a cellular mechanism for contextual learning and memory. In the mammalian hippocampus, LTP can be observed at multiple synaptic pathways after strong stimulation of a single synaptic pathway. This heterosynaptic LTP is believed to involve synaptic tagging of active synapses and capture of plasticity-related proteins that enable heterosynaptic transfer of persistent potentiation. These processes may permit distinct neural pathways to associate information transmitted by separate, but convergent, synaptic inputs. We had previously shown that transcription and epigenetic modifications were necessary for stabilization of homosynaptic LTP. However, it is unclear whether transfer of LTP to a second, heterosynaptic pathway involves b-ARs signalling to the nucleus. Using electrophysiologic recordings in area CA1 of murine hippocampal slices, we show here that pharmacologically inhibiting b-AR activation, transcription, DNA methyltransferase or histone acetyltransferase activation, prevents stabilization of heterosynaptic LTP. Our data suggest that noradrenergic stabilization of heterosynaptic ("tagged") LTP requires not only transcription, but specifically, DNA methylation and histone acetylation. NA promotes stable heterosynaptic plasticity through engagement of nuclear processes that may contribute to prompt consolidation of short-term memories into resilient long-term memories under conditions when the brain's noradrenergic system is recruited.

Noradrenergic stabilization of heterosynaptic LTP requires activation of Epac in the hippocampus

Beta-adrenergic receptor (β-AR) activation by norepinephrine (NE) enhances memory and stabilizes long-term potentiation (LTP), a form of synaptic plasticity believed to underlie some forms of hippocampal memory. LTP can occur at multiple synaptic pathways as a result of strong stimulation to one pathway preceding milder stimulation of an adjacent, independent pathway. Synaptic tagging allows LTP to be transferred, or captured, at heterosynaptic pathways. Previous research has shown that β-AR activation promotes heterosynaptic LTP by engaging various signaling cascades. In particular, cyclic adenosine monophosphate (cAMP) activates cAMP-dependent protein kinase A (PKA) and guanine nucleotide exchange protein activated by cAMP (Epac), to enhance LTP. Epac activation can occlude subsequent induction of stable homosynaptic LTP after β-AR activation, but it is unclear whether Epac activation is required for heterosynaptic LTP following pairing of the natural transmitter, NE, with one 100 Hz train of stimulation ("NE-LTP"). Using electrophysiologic recordings of CA1 field excitatory postsynaptic potentials during stimulation of two independent synaptic pathways in murine hippocampal slices, we show that distinct inhibitors of Epac blocked stabilization of homo- and heterosynaptic NE-LTP. PKA inhibition also attenuated heterosynaptic transfer of NE-LTP, but only when a PKA inhibitor was applied during tetanization of a second, heterosynaptic pathway that was not treated with NE. Our data suggest that NE, paired with 100 Hz, activates Epac to stabilize homo- and heterosynaptic LTP. Epac may regulate the production of plasticity-related proteins and subsequent synaptic capture of NE-LTP at a heterosynaptic pathway. Epac activation under these conditions may enable behavioral experiences that engage noradrenergic inputs to hippocampal circuits to be transformed into stable long-term memories.

Locus Coeruleus Optogenetic Light Activation Induces Long-Term Potentiation of Perforant Path Population Spike Amplitude in Rat Dentate Gyrus

Norepinephrine (NE) in dentate gyrus (DG) produces NE-dependent long-term potentiation (NE-LTP) of the perforant path-evoked potential population spike both in vitro and in vivo. Chemical activators infused near locus coeruleus (LC), the source of DG NE, produce a NE-LTP that is associative, i.e., requires concurrent pairing with perforant path (PP) input. Here, we ask if LC optogenetic stimulation that allows us to activate only LC neurons can induce NE-LTP in DG. We use an adeno-associated viral vector containing a depolarizing channel (AAV8-Ef1a-DIO-eChR2(h134r)-EYFP-WPRE) infused stereotaxically into the LC of TH:Cre rats to produce light-sensitive LC neurons. A co-localization of ~62% in LC neurons was observed for these channels. Under urethane anesthesia, we demonstrated that 5-10 s 10 Hz trains of 30 ms light pulses in LC reliably activated neurons near an LC optoprobe. Ten minutes of the same train paired with 0.1 Hz PP electrical stimulation produced a delayed NE-LTP of population spike amplitude, but not EPSP slope. A leftward shift in the population spike input/output curve at the end of the experiment was also consistent with long-term population spike potentiation. LC neuron activity during the 10 min light train was unexpectedly transient. Increased LC neuronal firing was seen only for the first 2 min of the light train. NE-LTP was more delayed and less robust than reported with LC chemo-activation. Previous estimates of LC axonal conduction times suggest acute release of NE occurs 40-70 ms after an LC neuron action potential. We used single LC light pulses to examine acute effects of NE release and found potentiated population spike amplitude when a light pulse in LC occurred 40-50 ms, but not 20-30 ms, prior to a PP pulse, consistent with conduction estimates. These effects of LC optogenetic activation reinforce evidence for a continuum of NE potentiation effects in DG. The single pulse effects mirror an earlier report using LC electrical stimulation. These acute effects support an attentional role of LC activation. The LTP of PP responses induced by optogenetic LC activation is consistent with the role of LC in long-term learning and memory.

GANEing traction: The broad applicability of NE hotspots to diverse cognitive and arousal phenomena

The GANE (glutamate amplifies noradrenergic effects) model proposes that local glutamate-norepinephrine interactions enable "winner-take-more" effects in perception and memory under arousal. A diverse range of commentaries addressed both the nature of this "hotspot" feedback mechanism and its implications in a variety of psychological domains, inspiring exciting avenues for future research.

Adrenergic receptor β3 is involved in the memory consolidation process in mice

Attention and emotion have a positive impact on memory formation, which is related to the activation of the noradrenergic system in the brain. The hippocampus and amygdala are fundamental structures in memory acquisition, which is modulated by noradrenaline through the noradrenergic receptors. Pharmacological studies suggest that memory acquisition depends on the action of both the β₃ (β₃-AR) and β₂ (β₂-AR) receptor subtypes. However, the use of animal models with specific knockout for the β₃-AR receptor only (β₃-ARKO) allows researchers to more accurately assess its role in memory formation processes. In the present study, we evaluated short- and long-term memory acquisition capacity in β₃-ARKO mice and wild-type mice at approximately 60 days of age. The animals were submitted to the open field test, the elevated plus maze, object recognition, and social preference. The results showed that the absence of the β₃-AR receptor caused no impairment in locomotion and did not cause anxious behavior, but it caused significant impairment of short- and long-term memory compared to wild-type animals. We also evaluated the expression of genes involved in memory consolidation. The mRNA levels for GLUT3, a glucose transporter expressed in the central nervous system, were significantly reduced in the amygdala, but not in the hippocampus of the β₃-ARKO animals. Our results showed that β₃-AR was involved in the process of acquisition of declarative memory, and its action may be due to the facilitation of glucose absorption in the amygdala.

EZH2 Methyltransferase Activity Controls Pten Expression and mTOR Signaling during Fear Memory Reconsolidation

Memory retrieval induces a transient period of increased transcriptional and translational regulation in neurons called reconsolidation, which is regulated by the protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway. However, it is currently unknown how activation of the AKT-mTOR pathway is regulated during the reconsolidation process. Here, we found that in male rats retrieval of a contextual fear memory transiently increased Enhancer of Zeste Homolog 2 (EZH2) levels along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, which correlated with decreased levels of phosphatase and tensin homolog (PTEN), a potent inhibitor of AKT-mTOR-dependent signaling in the hippocampus. Further experiments found increased H3K27me3 levels and DNA methylation across the Pten promoter and coding regions, indicating transcriptional silencing of the Pten gene. Pten H3K27me3 levels did not change following training or after the retrieval of a remote (old) fear memory, suggesting that this mechanism of Pten repression was specific to the reconsolidation of a new memory. In vivo siRNA-mediated knockdown of Ezh2 in the hippocampus abolished retrieval-induced increases in H3K27me3 and prevented decreases in PTEN levels. Ezh2 knockdown attenuated increases in the phosphorylation of AKT and mTOR following retrieval, which could be restored by simultaneously reducing Pten, suggesting that H3K27me3 regulates AKT-mTOR phosphorylation via repression of Pten Consistent with these results, knockdown of Ezh2 in area CA1 before retrieval impaired memory on later tests. Collectively, these results suggest that EZH2-mediated H3K27me3 plays a critical role in the repression of Pten transcription necessary for AKT-mTOR activation and memory reconsolidation following retrieval.SIGNIFICANCE STATEMENT Understanding how critical translation pathways, like mTOR-mediated protein synthesis, are regulated during the memory storage process is necessary for improving memory impairments. This study tests whether mTOR activation is coupled to epigenetic mechanisms in the hippocampus following the retrieval of a contextual fear memory. Specifically, this study evaluates the role of epigenetic modifications in the form of histone methylation in downstream mTOR translational control during learning-dependent synaptic plasticity in neurons. Considering the broad implications of transcriptional and translational mechanisms in synaptic plasticity, psychiatric, and neurological and neurodegenerative disorders, these data are of interest to the neuroscience community due to the robust and specific regulation of mTOR signaling we found to be dependent on repressive histone methylation.

Acupuncture Prevents the Impairment of Hippocampal LTP Through β1-AR in Vascular Dementia Rats

It is widely accepted that the synaptic dysfunction and synapse loss contribute to the cognitive deficits of vascular dementia (VD) patients. We have previously reported that acupuncture improved cognitive function in rats with VD. However, the mechanisms involved in acupuncture improving cognitive ability remain to be elucidated. The present study aims to investigate the pathways and molecules involved in the neuroprotective effect of acupuncture. We assessed the effects of acupuncture on hippocampal long-term potentiation (LTP), the most prominent cellular model of memory formation. Acupuncture enhanced LTP and norepinephrine (NE) levels in the hippocampus. Inhibition of the β-adrenergic receptor (AR), but not the α-AR, was able to block the effects of acupuncture on hippocampal LTP. Furthermore, inhibition of β1-AR, not β2-AR, abolished the enhanced LTP induced by acupuncture. The expression analysis revealed a significant upregulation of β1-AR and unchanged β2-AR with acupuncture, which supported the above findings. Specifically, increased β1-ARs in the dentate gyrus were expressed on neurons exclusively. Taken together, the present data supports a beneficial role of acupuncture in synaptic plasticity challenged with VD. A likely mechanism is the increase of NE and activation of β1-AR in the hippocampus.

Administration of a Histone Deacetylase Inhibitor into the Basolateral Amygdala Enhances Memory Consolidation, Delays Extinction, and Increases Hippocampal BDNF Levels

Gene expression related to the formation and modification of memories is regulated epigenetically by chromatin remodeling through histone acetylation. Memory formation and extinction can be enhanced by treatment with inhibitors of histone deacetylases (HDACs). The basolateral amygdala (BLA) is a brain area critically involved in regulating memory for inhibitory avoidance (IA). However, previous studies have not examined the effects of HDAC inhibition in the amygdala on memory for IA. Here we show that infusion of an HDAC inhibitor (HDACi), trichostatin A (TSA), into the BLA, enhanced consolidation of IA memory in rats when given at 1.5, 3, or 6 h posttraining, but not when the drug was infused immediately after training. In addition, intra-BLA administration of TSA immediately after retrieval delayed extinction learning. Moreover, we show that intra-BLA TSA in rats given IA training increased the levels of brain-derived neurotrophic factor in the dorsal hippocampus, but not in the BLA itself. These findings reveal novel aspects of the regulation of fear memory by epigenetic mechanisms in the amygdala.

The Longevity of Hippocampus-Dependent Memory Is Orchestrated by the Locus Coeruleus-Noradrenergic System

The locus coeruleus is connected to the dorsal hippocampus via strong fiber projections. It becomes activated after arousal and novelty, whereupon noradrenaline is released in the hippocampus. Noradrenaline from the locus coeruleus is involved in modulating the encoding, consolidation, retrieval, and reversal of hippocampus-based memory. Memory storage can be modified by the activation of the locus coeruleus and subsequent facilitation of hippocampal long-term plasticity in the forms of long-term depression and long-term potentiation. Recent evidence indicates that noradrenaline and dopamine are coreleased in the hippocampus from locus coeruleus terminals, thus fostering neuromodulation of long-term synaptic plasticity and memory. Noradrenaline is an inductor of epigenetic modifications regulating transcriptional control of synaptic long-term plasticity to gate the endurance of memory storage. In conclusion, locus coeruleus activation primes the persistence of hippocampus-based long-term memory.

Arousal From Sleep and Sympathetic Excitation During Wakefulness

Obstructive apnea during sleep elevates the set point for efferent sympathetic outflow during wakefulness. Such resetting is attributed to hypoxia-induced upregulation of peripheral chemoreceptor and brain stem sympathetic function. Whether recurrent arousal from sleep also influences daytime muscle sympathetic nerve activity is unknown. We therefore tested, in a cohort of 48 primarily nonsleepy, middle-aged, male (30) and female (18) volunteers (age: 59±1 years, mean±SE), the hypothesis that the frequency of arousals from sleep (arousal index) would relate to daytime muscle sympathetic burst incidence, independently of the frequency of apnea or its severity. Polysomnography identified 24 as having either no or mild obstructive sleep apnea (apnea–hypopnea index <15 events/h) and 24 with moderate-to-severe obstructive sleep apnea (apnea–hypopnea index >15 events/h). Burst incidence correlated significantly with arousal index ( r =0.53; P <0.001), minimum oxygen saturation ( r =−0.43; P =0.002), apnea–hypopnea index ( r =0.41; P =0.004), age ( r =0.36; P =0.013), and body mass index ( r =0.33; P =0.022) but not with oxygen desaturation index ( r =0.28; P =0.056). Arousal index was the single strongest predictor of muscle sympathetic nerve activity burst incidence, present in all best subsets regression models. The model with the highest adjusted R 2 (0.456) incorporated arousal index, minimum oxygen saturation, age, body mass index, and oxygen desaturation index but not apnea–hypopnea index. An apnea- and hypoxia-independent effect of sleep fragmentation on sympathetic discharge during wakefulness could contribute to intersubject variability, age-related increases in muscle sympathetic nerve activity, associations between sleep deprivation and insulin resistance or insomnia and future cardiovascular events, and residual adrenergic risk with persistence of hypertension should therapy eliminate obstructive apneas but not arousals.

Astrocytic calcium activation in a mouse model of tDCS-Extended discussion

Transcranial direct current stimulation (tDCS) has been reported to be effective for alleviation of neuropsychiatric and neurological conditions as well as enhancement of memory and cognition. Despite the positive effects of tDCS in humans, its mechanism of action remains poorly understood. Recently, we reported that astrocytes, a major glial cell type in the brain, show an increase in intracellular Ca²⁺ levels during tDCS in the cerebral cortex of the awake mouse. This tDCS-induced elevation in astrocytic Ca²⁺ has subsequently been demonstrated to be important for cortical plasticity. In this commentary article, we discuss possible interpretations and implications of our findings from the viewpoint of neuron-glia interactions.

Locus Ceruleus Norepinephrine Release: A Central Regulator of CNS Spatio-Temporal Activation?

Norepinephrine (NE) is synthesized in the Locus Coeruleus (LC) of the brainstem, from where it is released by axonal varicosities throughout the brain via volume transmission. A wealth of data from clinics and from animal models indicates that this catecholamine coordinates the activity of the central nervous system (CNS) and of the whole organism by modulating cell function in a vast number of brain areas in a coordinated manner. The ubiquity of NE receptors, the daunting number of cerebral areas regulated by the catecholamine, as well as the variety of cellular effects and of their timescales have contributed so far to defeat the attempts to integrate central adrenergic function into a unitary and coherent framework. Since three main families of NE receptors are represented-in order of decreasing affinity for the catecholamine-by: α2 adrenoceptors (α2Rs, high affinity), α1 adrenoceptors (α1Rs, intermediate affinity), and β adrenoceptors (βRs, low affinity), on a pharmacological basis, and on the ground of recent studies on cellular and systemic central noradrenergic effects, we propose that an increase in LC tonic activity promotes the emergence of four global states covering the whole spectrum of brain activation: (1) sleep: virtual absence of NE, (2) quiet wake: activation of α2Rs, (3) active wake/physiological stress: activation of α2- and α1-Rs, (4) distress: activation of α2-, α1-, and β-Rs. We postulate that excess intensity and/or duration of states (3) and (4) may lead to maladaptive plasticity, causing-in turn-a variety of neuropsychiatric illnesses including depression, schizophrenic psychoses, anxiety disorders, and attention deficit. The interplay between tonic and phasic LC activity identified in the LC in relationship with behavioral response is of critical importance in defining the short- and long-term biological mechanisms associated with the basic states postulated for the CNS. While the model has the potential to explain a large number of experimental and clinical findings, a major challenge will be to adapt this hypothesis to integrate the role of other neurotransmitters released during stress in a centralized fashion, like serotonin, acetylcholine, and histamine, as well as those released in a non-centralized fashion, like purines and cytokines.

Dendritic diameter influences the rate and magnitude of hippocampal cAMP and PKA transients during β-adrenergic receptor activation

In the hippocampus, cyclic-adenosine monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) form a critical signaling cascade required for long-lasting synaptic plasticity, learning and memory. Plasticity and memory are known to occur following pathway-specific changes in synaptic strength that are thought to result from spatially and temporally coordinated intracellular signaling events. To better understand how cAMP and PKA dynamically operate within the structural complexity of hippocampal neurons, we used live two-photon imaging and genetically-encoded fluorescent biosensors to monitor cAMP levels or PKA activity in CA1 neurons of acute hippocampal slices. Stimulation of β-adrenergic receptors (isoproterenol) or combined activation of adenylyl cyclase (forskolin) and inhibition of phosphodiesterase (IBMX) produced cAMP transients with greater amplitude and rapid on-rates in intermediate and distal dendrites compared to somata and proximal dendrites. In contrast, isoproterenol produced greater PKA activity in somata and proximal dendrites compared to intermediate and distal dendrites, and the on-rate of PKA activity did not differ between compartments. Computational models show that our observed compartmental difference in cAMP can be reproduced by a uniform distribution of PDE4 and a variable density of adenylyl cyclase that scales with compartment size to compensate for changes in surface to volume ratios. However, reproducing our observed compartmental difference in PKA activity required enrichment of protein phosphatase in small compartments; neither reduced PKA subunits nor increased PKA substrates were sufficient. Together, our imaging and computational results show that compartment diameter interacts with rate-limiting components like adenylyl cyclase, phosphodiesterase and protein phosphatase to shape the spatial and temporal components of cAMP and PKA signaling in CA1 neurons and suggests that small neuronal compartments are most sensitive to cAMP signals whereas large neuronal compartments accommodate a greater dynamic range in PKA activity.

Priming locus coeruleus noradrenergic modulation of medial perforant path-dentate gyrus synaptic plasticity

Priming phenomenon, in which an earlier exposure to a stimulus or condition alters synaptic plasticity in response to a subsequent stimulus or condition, known as a challenge, is an example of metaplasticity. In this review, we make the case that the locus coeruleus noradrenergic system-medial perforant path-dentate gyrus pathway is a neural ensemble amenable to studying priming-challenge effects on synaptic plasticity. Accumulating evidence points to a tyrosine hydroxylase-dependent priming effect achieved by pharmacological (nicotine and antipsychotics) or physiological (septal theta driving) manipulations of the locus coeruleus noradrenergic system that can facilitate noradrenaline-induced synaptic plasticity in the dentate gyrus of the hippocampus. The evidence suggests the hypothesis that behavioural experiences inducing tyrosine hydroxylase expression in the locus coeruleus may be sufficient to prime this form of metaplasticity. We propose exploring this phenomenon of priming and challenge physiologically, to determine whether behavioural experiences are sufficient to prime the locus coeruleus, enabling subsequent pharmacological or behavioural challenge conditions that increase locus coeruleus firing to release sufficient noradrenaline to induce long-lasting potentiation in the dentate gyrus. Such an approach may contribute to unravelling mechanisms underlying this form of metaplasticity and its importance in stress-related mnemonic processes.