Histone acetylation rescues contextual fear conditioning in nNOS KO mice and accelerates extinction of cued fear conditioning in wild type mice

Class 1 histone deacetylases differentially modulate memory and synaptic genes in a spatial and temporal manner in aged and APP/PS1 mice

Epigenetics plays a vital role in aging and Alzheimer's disease (AD); however, whether epigenetic alterations during aging can initiate AD and exacerbate AD progression remains unclear. In this study, using 3-, 12- and 18- month-old APP/PS1 mice and age matched WT littermates, we conducted a series of memory tests, measured synapse-related gene expression, class 1 histone deacetylases (HDACs) abundance, and H3K9ac levels at target gene promoters in the hippocampus and prefrontal cortex (PFC). Our results showed impaired recognition and long-term spatial memory in 18-month-old WT mice and impaired recognition, short-term working, and long-term spatial reference memory in 12-and 18- month-old APP/PS1 mice. These memory impairments are associated with changes of synapse-related gene (nr2a, glur1, glur2, psd95) expression, HDAC abundance, and H3K9ac levels; more specifically, increased HDAC2 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during aging and AD progression in the hippocampus. Conversely, increased HDAC3 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during AD progression in the PFC. These findings suggest memory impairments in aging and AD may associated with a differential HDAC modulation of synapse-related gene expression in the brain.

Epigenetics of Fear, Anxiety and Stress - Focus on Histone Modifications

Fear-, anxiety- and stress-related disorders are among the most frequent mental disorders. Given substantial rates of insufficient treatment response and often a chronic course, a better understanding of the pathomechanisms of fear-, anxiety- and stress-related disorders is urgently warranted. Epigenetic mechanisms such as histone modifications - positioned at the interface between the biological and the environmental level in the complex pathogenesis of mental disorders - might be highly informative in this context. The current state of knowledge on histone modifications, chromatin-related pharmacology and animal models modified for genes involved in the histone-related epigenetic machinery will be reviewed with respect to fear-, anxiety- and stress-related states. Relevant studies, published until 30th June 2022, were identified using a multi-step systematic literature search of the Pub- Med and Web of Science databases. Animal studies point towards histone modifications (e.g., H3K4me3, H3K9me1/2/3, H3K27me2/3, H3K9ac, H3K14ac and H4K5ac) to be dynamically and mostly brain region-, task- and time-dependently altered on a genome-wide level or gene-specifically (e.g., Bdnf) in models of fear conditioning, retrieval and extinction, acute and (sub-)chronic stress. Singular and underpowered studies on histone modifications in human fear-, anxiety- or stress-related phenotypes are currently restricted to the phenotype of PTSD. Provided consistent validation in human phenotypes, epigenetic biomarkers might ultimately inform indicated preventive interventions as well as personalized treatment approaches, and could inspire future innovative pharmacological treatment options targeting the epigenetic machinery improving treatment response in fear-, anxiety- and stressrelated disorders.

Targeting neuronal nitric oxide synthase and the nitrergic system in post-traumatic stress disorder

RATIONALE

Current pharmacological approaches to treatment of post-traumatic stress disorder (PTSD) lack adequate effectiveness. As a result, identifying new molecular targets for drug development is necessary. Furthermore, fear learning and memory in PTSD can undergo different phases, such as fear acquisition, consolidation, and extinction. Each phase may involve different cellular pathways and brain regions. As a result, effective management of PTSD requires mindfulness of the timing of drug administration. One of the molecular targets currently under intense investigation is the N-methyl-D-aspartate (NMDA)-type glutamate receptor (NMDAR). However, despite the therapeutic efficacy of drugs targeting NMDAR, their translation into clinical use has been challenging due to their various side effects. One possible solution to this problem is to target signaling proteins downstream to NMDAR to improve targeting specificity. One of these proteins is the neuronal nitric oxide synthase (nNOS), which is activated following calcium influx through the NMDAR.

OBJECTIVE

In this paper, we review the literature on the pharmacological modulation of nNOS in animal models of PTSD to evaluate its therapeutic potential. Furthermore, we attempt to decipher the inconsistencies observed between the findings of these studies based on the specific phase of fear learning which they had targeted.

RESULTS

Inhibition of nNOS may inhibit fear acquisition and recall, while not having a significant effect on fear consolidation and extinction. However, it may improve extinction consolidation or reconsolidation blockade.

CONCLUSIONS

Modulation of nNOS has therapeutic potential against PTSD and warrants further development for use in the clinical setting.

Twice Daily Prazosin and Valproic Acid in the Treatment of Flashbacks in PTSD

Posttraumatic stress disorder (PTSD) is a debilitating stress disorder occurring in the context of a traumatic event and is characterized by intrusive and avoidance symptoms, negative alterations in cognition and mood, and arousal and reactivity changes. Despite its representation throughout literature, the pathophysiology of PTSD remains incompletely understood, thus contributing to broad, variable, and at times, experimental treatment options. The authors present the first documented case of the rapid and successful management of PTSD using valproic acid and twice daily dosing of prazosin aimed at targeting symptoms of hyperarousal and both daily and nightly intrusive symptoms of flashbacks and nightmares, respectively. The authors also discuss postulations of the underlying mechanisms of action responsible for such symptom alleviation. Further investigation is needed to expand upon our knowledge of the use of such agents in the treatment of PTSD to improve upon existing clinical guidelines, especially in the acute setting, thus providing better overall prognosis.

Sodium butyrate enhances fear extinction and rescues hippocampal acetylcholinesterase activity in a rat model of posttraumatic stress disorder

It is believed that impaired extinction of fear memories is an underlying cause for the development of posttraumatic stress disorder (PTSD). Histone deacetylases (HDAC) are enzymes that modulate extinction by changing the chromatin structure and altering protein synthesis in the brain. Studies show that stress modifies both HDAC activity and cerebral cholinergic neurotransmission. The present work aims to evaluate the effect of sodium butyrate (NaBu), an HDAC inhibitor, on behavioral markers of extinction and biochemical changes in HDAC and acetylcholinesterase activity in the hippocampus. NaBu was administered for 7 days in a group of rats that were exposed to single prolonged stress (SPS), as a model for PTSD. Contextual fear conditioning was performed on the 8th day, and fear extinction was measured in the next 4 consecutive days. Other behavioral tests to measure anxiety, locomotor activity and working memory were performed for further interpretation of the results. Hippocampal acetylcholinesterase and HDAC activity were also measured through biochemical tests. Behavioral results showed that treatment with NaBu can reverse the SPS-induced extinction deficits. Biochemical data indicated that while SPS induced overactivity in hippocampal HDAC, it decreased acetylcholinesterase activity in the region. Both effects were reversed after NaBu treatment. It seems that at least part of extinction deficiency in SPS exposed rats is related to hypoacetylation of acetylcholinesterase in the hippocampus. Preemptive therapy with an HDAC inhibitor reverses this process and is worth further evaluation as a possible therapeutic approach in PTSD.

The Contribution of Epigenetic Inheritance Processes on Age-Related Cognitive Decline and Alzheimer's Disease

During the last years, epigenetic processes have emerged as important factors for many neurodegenerative diseases, such as Alzheimer’s disease (AD). These complex diseases seem to have a heritable component; however, genome-wide association studies failed to identify the genetic loci involved in the etiology. So, how can these changes be transmitted from one generation to the next? Answering this question would allow us to understand how the environment can affect human populations for multiple generations and explain the high prevalence of neurodegenerative diseases, such as AD. This review pays particular attention to the relationship among epigenetics, cognition, and neurodegeneration across generations, deepening the understanding of the relevance of heritability in neurodegenerative diseases. We highlight some recent examples of EI induced by experiences, focusing on their contribution of processes in learning and memory to point out new targets for therapeutic interventions. Here, we first describe the prominent role of epigenetic factors in memory processing. Then, we briefly discuss aspects of EI. Additionally, we summarize evidence of how epigenetic marks inherited by experience and/or environmental stimuli contribute to cognitive status offspring since better knowledge of EI can provide clues in the appearance and development of age-related cognitive decline and AD.

Optogenetic Stimulation of Prelimbic Pyramidal Neurons Maintains Fear Memories and Modulates Amygdala Pyramidal Neuron Transcriptome

Fear extinction requires coordinated neural activity within the amygdala and medial prefrontal cortex (mPFC). Any behavior has a transcriptomic signature that is modified by environmental experiences, and specific genes are involved in functional plasticity and synaptic wiring during fear extinction. Here, we investigated the effects of optogenetic manipulations of prelimbic (PrL) pyramidal neurons and amygdala gene expression to analyze the specific transcriptional pathways associated to adaptive and maladaptive fear extinction. To this aim, transgenic mice were (or not) fear-conditioned and during the extinction phase they received optogenetic (or sham) stimulations over photo-activable PrL pyramidal neurons. At the end of behavioral testing, electrophysiological (neural cellular excitability and Excitatory Post-Synaptic Currents) and morphological (spinogenesis) correlates were evaluated in the PrL pyramidal neurons. Furthermore, transcriptomic cell-specific RNA-analyses (differential gene expression profiling and functional enrichment analyses) were performed in amygdala pyramidal neurons. Our results show that the optogenetic activation of PrL pyramidal neurons in fear-conditioned mice induces fear extinction deficits, reflected in an increase of cellular excitability, excitatory neurotransmission, and spinogenesis of PrL pyramidal neurons, and associated to strong modifications of the transcriptome of amygdala pyramidal neurons. Understanding the electrophysiological, morphological, and transcriptomic architecture of fear extinction may facilitate the comprehension of fear-related disorders.

Epigenetic Mechanisms of Learning and Memory: Implications for Aging

The neuronal epigenome is highly sensitive to external events and its function is vital for producing stable behavioral outcomes, such as the formation of long-lasting memories. The importance of epigenetic regulation in memory is now well established and growing evidence points to altered epigenome function in the aging brain as a contributing factor to age-related memory decline. In this review, we first summarize the typical role of epigenetic factors in memory processing in a healthy young brain, then discuss the aspects of this system that are altered with aging. There is general agreement that many epigenetic marks are modified with aging, but there are still substantial inconsistencies in the precise nature of these changes and their link with memory decline. Here, we discuss the potential source of age-related changes in the epigenome and their implications for therapeutic intervention in age-related cognitive decline.

Effects of HDAC inhibitors on spatial memory and memory extinction in SPS-induced PTSD rats

Background and purpose

Neurobiological changes in memory processes seem to play a role in the pathophysiology of post-traumatic stress disorder (PTSD). Memory itself is influenced by PTSD, too. Histone deacetylase inhibitors (HDAIs) have shown promising results in the extinction of fear-related memories in animals and hence they seem to be important for the treatment of PTSD. Data are scarce about the effect of HDAIs in spatial memory formation/extinction in PTSD models. The main goal of the present work is to find the effect of sodium butyrate (NaBu), as an HDAI, on spatial memory and spatial memory extinction in rats exposed to single prolonged stress procedure (SPS).

Experimental approach

Different doses of NaBu were administered subcutaneously for 7 days in different groups of rats after SPS procedure. Learning, memory, and extinction of memory were evaluated in the Morris water maze test of spatial memory in 6 consecutive days.

Findings / Results

The results show that NaBu (0.5 mg/kg) alleviates impaired learning and memory in SPS rats. It also facilitates the extinction of newly formed memory in the animals.

Conclusion and implications

Our data suggest that the administration of HDAIs after a traumatic experience can prevent the aversive effects of SPS on spatial memory. It also reinforces the notion that extinction of spatial memory involves the same or similar brain circuitry that is involved in the extinction of fear memories in PTSD patients.

Histone deacetylases inhibitor trichostatin A reverses anxiety-like symptoms and memory impairments induced by maternal binge alcohol drinking in mice

BACKGROUND

Alcohol exposure during development has detrimental effects, including a wide range of physical, cognitive and neurobehavioural anomalies known as foetal alcohol spectrum disorders. However, alcohol consumption among pregnant woman is an ongoing latent health problem.

AIM

In the present study, the effects of trichostatin A (TSA) on emotional and cognitive impairments caused by prenatal and lactational alcohol exposure were assessed. TSA is an inhibitor of class I and II histone deacetylases enzymes (HDAC), and for that, HDAC4 activity was determined. We also evaluated mechanisms underlying the behavioural effects observed, including the expression of brain-derived neurotrophic factor (BDNF) in discrete brain regions and newly differentiated neurons in the dentate gyrus (DG).

METHODS

C57BL/6 female pregnant mice were used, with limited access to a 20% v/v alcohol solution as a procedure to model binge alcohol drinking during gestation and lactation. Male offspring were treated with TSA during the postnatal days (PD28-35) and behaviourally evaluated (PD36-55).

RESULTS

Early alcohol exposure mice presented increased anxiogenic-like responses and memory deterioration - effects that were partially reversed with TSA. Early alcohol exposure produces a decrease in BDNF levels in the hippocampus (HPC) and prefrontal cortex, a reduction of neurogenesis in the DG and increased activity levels of the HDAC4 in the HPC.

CONCLUSIONS

Such findings support the participation of HDAC enzymes in cognitive and emotional alterations induced by binge alcohol consumption during gestation and lactation and would indicate potential benefits of HDAC inhibitors for some aspects of foetal alcohol spectrum disorders.

Regulation of HDAC1 and HDAC2 during consolidation and extinction of fear memory

Histone deacetylases (HDACs) regulate gene expression epigenetically through synchronized removal of acetyl groups from histones required towards memory consolidation. Moreover, dysregulated epigenetic machinery during fear or extinction learning may result in altered expression of some of these genes and result in Post Traumatic Stress Disorder (PTSD). In the present study, region-specific expression of Histone deacetylase 1 (HDAC1) and Histone deacetylase 2 (HDAC2) was correlated to the acetylation of histones H3 and H4 and the resultant conditioned response, in rats undergone fear and extinction learning. The neuronal activation, histone acetylation at H3/H4 and expression of HDAC1/HDAC2 in centrolateral amygdala (CeL) and centromedial amygdala (CeM) of central Amygdala (CeA) and prelimbic (PL) and infralimbic (IL) of Prefrontal cortex (PFC) were found to be associated in a differential manner following fear and extinction learning. Moreover in CeM, the main output of the fear circuitry, the level of HDAC1 was down-regulated following conditioning and up-regulated following extinction as opposed to which HDAC2 was down-regulated in CeM following conditioning but not following extinction. Furthermore, in CeL the HDAC1 was upregulated and HDAC2 was downregulated following conditioning and extinction. This has important implications in speculating of the role of HDACs in fear memory consolidation and its extinction.

Neuroepigenetic mechanisms underlying fear extinction: emerging concepts

An understanding of how memory is acquired and how it can be modified in fear-related anxiety disorders, with the enhancement of failing memories on one side and a reduction or elimination of traumatic memories on the other, is a key unmet challenge in the fields of neuroscience and neuropsychiatry. The latter process depends on an important form of learning called fear extinction, where a previously acquired fear-related memory is decoupled from its ability to control behaviour through repeated non-reinforced exposure to the original fear-inducing cue. Although simple in description, fear extinction relies on a complex pattern of brain region and cell-type specific processes, some of which are unique to this form of learning and, for better or worse, contribute to the inherent instability of fear extinction memory. Here, we explore an emerging layer of biology that may compliment and enrich the synapse-centric perspective of fear extinction. As opposed to the more classically defined role of protein synthesis in the formation of fear extinction memory, a neuroepigenetic view of the experience-dependent gene expression involves an appreciation of dynamic changes in the state of the entire cell: from a transient change in plasticity at the level of the synapse, to potentially more persistent long-term effects within the nucleus. A deeper understanding of neuroepigenetic mechanisms and how they influence the formation and maintenance of fear extinction memory has the potential to enable the development of more effective treatment approaches for fear-related neuropsychiatric conditions.

Role of Nitric Oxide and Hydrogen Sulfide in Ischemic Stroke and the Emergent Epigenetic Underpinnings

Nitric oxide (NO) and hydrogen sulfide (H₂S) are the key gasotransmitters with an imperious role in the maintenance of cerebrovascular homeostasis. A decline in their levels contributes to endothelial dysfunction that portends ischemic stroke (IS) or cerebral ischemia/reperfusion (CI/R). Nevertheless, their exorbitant production during CI/R is associated with exacerbation of cerebrovascular injury in the post-stroke epoch. NO-producing nitric oxide synthases are implicated in IS pathology and their activity is regulated, inter alia, by various post-translational modifications and chromatin-based mechanisms. These account for heterogeneous alterations in NO production in a disease setting like IS. Interestingly, NO per se has been posited as an endogenous epigenetic modulator. Further, there is compelling evidence for an ingenious crosstalk between NO and H₂S in effecting the canonical (direct) and non-canonical (off-target collateral) functions. In this regard, NO-mediated S-nitrosylation and H₂S-mediated S-sulfhydration of specific reactive thiols in an expanding array of target proteins are the principal modalities mediating the all-pervasive influence of NO and H₂S on cell fate in an ischemic brain. An integrated stress response subsuming unfolded protein response and autophagy to cellular stressors like endoplasmic reticulum stress, in part, is entrenched in such signaling modalities that substantiate the role of NO and H₂S in priming the cells for stress response. The precis presented here provides a comprehension on the multifarious actions of NO and H₂S and their epigenetic underpinnings, their crosstalk in maintenance of cerebrovascular homeostasis, and their "Janus bifrons" effect in IS milieu together with plausible therapeutic implications.

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.

Dissociable roles for histone acetyltransferases p300 and PCAF in hippocampus and perirhinal cortex-mediated object memory

The importance of histone acetylation for certain types of memory is now well established. However, the specific contributions of the various histone acetyltransferases to distinct memory functions remain to be determined; therefore, we employed selective histone acetyltransferase protein inhibitors and short-interference RNAs to evaluate the roles of CREB-binding protein (CBP), E1A-binding protein (p300) and p300/CBP-associated factor (PCAF) in hippocampus and perirhinal cortex (PRh)-mediated object memory. Rats were tested for short- (STM) and long-term memory (LTM) in the object-in-place task, which relies on the hippocampus and PRh for spatial memory and object identity processing, respectively. Selective inhibition of these histone acetyltransferases by small-interfering RNA and pharmacological inhibitors targeting the HAT domain produced dissociable effects. In the hippocampus, CBP or p300 inhibition impaired long-term but not short-term object memory, while inhibition of PCAF impaired memory at both delays. In PRh, HAT inhibition did not impair STM, and only CBP and PCAF inhibition disrupted LTM; p300 inhibition had no effects. Messenger RNA analyses revealed findings consistent with the pattern of behavioral effects, as all three enzymes were upregulated in the hippocampus (dentate gyrus) following learning, whereas only CBP and PCAF were upregulated in PRh. These results demonstrate, for the first time, the necessity of histone acetyltransferase activity for PRh-mediated object memory and indicate that the specific mnemonic roles of distinctive histone acetyltransferases can be dissociated according to specific brain regions and memory timeframe.

Reconsolidation and extinction: Using epigenetic signatures to challenge conventional wisdom

Epigenetic mechanisms have the potential to give rise to lasting changes in cell function that ultimately can affect behavior persistently. This concept is especially interesting with respect to fear reconsolidation and fear memory extinction. These two behavioral approaches are used in the laboratory to investigate how fear memory can be attenuated, which becomes important when searching for therapeutic intervention to treat anxiety disorders and post-traumatic stress disorder. Here we review the role of several key epigenetic mechanisms in reconsolidation and extinction of learned fear and their potential to persistently alter behavioral responses to conditioned cues. We also briefly discuss how epigenetic mechanisms may establish persistent behaviors that challenge our definitions of extinction and reconsolidation.

Enhancing dopaminergic signaling and histone acetylation promotes long-term rescue of deficient fear extinction

Extinction-based exposure therapy is used to treat anxiety- and trauma-related disorders; however, there is the need to improve its limited efficacy in individuals with impaired fear extinction learning and to promote greater protection against return-of-fear phenomena. Here, using 129S1/SvImJ mice, which display impaired fear extinction acquisition and extinction consolidation, we revealed that persistent and context-independent rescue of deficient fear extinction in these mice was associated with enhanced expression of dopamine-related genes, such as dopamine D1 (Drd1a) and -D2 (Drd2) receptor genes in the medial prefrontal cortex (mPFC) and amygdala, but not hippocampus. Moreover, enhanced histone acetylation was observed in the promoter of the extinction-regulated Drd2 gene in the mPFC, revealing a potential gene-regulatory mechanism. Although enhancing histone acetylation, via administering the histone deacetylase (HDAC) inhibitor MS-275, does not induce fear reduction during extinction training, it promoted enduring and context-independent rescue of deficient fear extinction consolidation/retrieval once extinction learning was initiated as shown following a mild conditioning protocol. This was associated with enhanced histone acetylation in neurons of the mPFC and amygdala. Finally, as a proof-of-principle, mimicking enhanced dopaminergic signaling by L-dopa treatment rescued deficient fear extinction and co-administration of MS-275 rendered this effect enduring and context-independent. In summary, current data reveal that combining dopaminergic and epigenetic mechanisms is a promising strategy to improve exposure-based behavior therapy in extinction-impaired individuals by initiating the formation of an enduring and context-independent fear-inhibitory memory.

Swimming exercise ameliorates neurocognitive impairment induced by neonatal exposure to isoflurane and enhances hippocampal histone acetylation in mice

Isoflurane-induced neurocognitive impairment in the developing rodent brain is well documented, and regular physical exercise has been demonstrated to be a viable intervention for some types of neurocognitive impairment. This study was designed to investigate the potential protective effect of swimming exercise on both neurocognitive impairment caused by repeated neonatal exposure to isoflurane and the underlying molecular mechanism. Mice received 0.75% isoflurane exposures for 4h on postnatal days 7, 8, and 9. From the third month after anesthesia, the mice were subjected to regular swimming exercise for 4weeks, followed by a contextual fear condition (CFC) trial. We found that repeated neonatal exposure to isoflurane reduced freezing behavior during CFC testing and deregulated hippocampal histone H4K12 acetylation. Conversely, mice subjected to regular swimming exercise showed enhanced hippocampal H3K9, H4K5, and H4K12 acetylation levels, increased numbers of c-Fos-positive cells 1h after CFC training, and less isoflurane-induced memory impairment. We also observed increases in histone acetylation and of cAMP-response element-binding protein (CREB)-binding protein (CBP) during the swimming exercise program. The results suggest that neonatal isoflurane exposure-induced memory impairment was associated with dysregulation of H4K12 acetylation, which may lead to less hippocampal activation following learning tasks. Swimming exercise was associated with enhanced hippocampal histone acetylation and CBP expression. Exercise most likely ameliorated isoflurane-induced memory impairment by enhancing hippocampal histone acetylation and activating more neuron cells during memory formation.

Molecular and Functional Characterization of Bacopa monniera: A Retrospective Review

Over the last 50 years, laboratories around the world analyzed the pharmacological effect of Bacopa monniera extract in different dimensions, especially as a nerve tonic and memory enhancer. Studies in animal model evidenced that Bacopa treatment can attenuate dementia and enhances memory. Further, they demonstrate that Bacopa primarily either acts via antioxidant mechanism (i.e., neuroprotection) or alters different neurotransmitters (serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA), acetylcholine (ACh), γ-aminobutyric acid (GABA)) to execute the pharmacological effect. Among them, 5-HT has been shown to fine tune the neural plasticity, which is a substrate for memory formation. This review focuses on the studies which trace the effect of Bacopa treatment on serotonergic system and 5-HT mediated key molecular changes that are associated with memory formation.

HDAC inhibitors as cognitive enhancers in fear, anxiety and trauma therapy: where do we stand?

A novel strategy to treat anxiety and fear-related disorders such as phobias, panic and PTSD (post-traumatic stress disorder) is combining CBT (cognitive behavioural therapy), including extinction-based exposure therapy, with cognitive enhancers. By targeting and boosting mechanisms underlying learning, drug development in this field aims at designing CBT-augmenting compounds that help to overcome extinction learning deficits, promote long-term fear inhibition and thus support relapse prevention. Progress in revealing the role of epigenetic regulation of specific genes associated with extinction memory generation has opened new avenues in this direction. The present review examines recent evidence from pre-clinical studies showing that increasing histone acetylation, either via genetic or pharmacological inhibition of HDACs (histone deacetylases) by e.g. vorinostat/SAHA (suberoylanilide hydroxamic acid), entinostat/MS-275, sodium butyrate, TSA (trichostatin A) or VPA (valproic acid), or by targeting HATs (histone acetyltransferases), augments fear extinction and, importantly, generates a long-term extinction memory that can protect from return of fear phenomena. The molecular mechanisms and pathways involved including BDNF (brain-derived neurotrophic factor) and NMDA (N-methyl-D-aspartate) receptor signalling are just beginning to be revealed. First studies in healthy humans are in support of extinction-facilitating effects of HDAC inhibitors. Very recent evidence that HDAC inhibitors can rescue deficits in extinction-memory-impaired rodents indicates a potential clinical utility of this approach also for exposure therapy-resistant patients. Important future work includes investigation of the long-term safety aspects of HDAC inhibitor treatment, as well as design of isotype(s)-specific inhibitors. Taken together, HDAC inhibitors display promising potential as pharmacological adjuncts to augment the efficacy of exposure-based approaches in anxiety and trauma therapy.

Augmentation of Evidence-Based Psychotherapy for PTSD With Cognitive Enhancers

Exposure-based therapy has proven to be useful to treat various anxiety disorders as well as post-traumatic stress disorder (PTSD). Despite its efficacy, a fair proportion of patients remain symptomatic after treatment. Different lines of research have put considerable efforts to investigate ways to enhance the efficacy of exposure-based therapy, which could ultimately lead to better clinical outcomes for patients. Given that this type of therapy relies on extinction learning principles, neuroscience research has tested different adjuncts that could be used as cognitive enhancers through their impact on extinction learning and its consolidation. The current review will summarize some of the latest compounds that have received attention and show some promise to be used in clinical settings to improve the efficacy of exposure-based therapy.

Neonatal isoflurane exposure induces neurocognitive impairment and abnormal hippocampal histone acetylation in mice

Background

Neonatal exposure to isoflurane may induce long-term memory impairment in mice. Histone acetylation is an important form of chromatin modification that regulates the transcription of genes required for memory formation. This study investigated whether neonatal isoflurane exposure-induced neurocognitive impairment is related to dysregulated histone acetylation in the hippocampus and whether it can be attenuated by the histone deacetylase (HDAC) inhibitor trichostatin A (TSA).

Methods

C57BL/6 mice were exposed to 0.75% isoflurane three times (each for 4 h) at postnatal days 7, 8, and 9. Contextual fear conditioning (CFC) was tested at 3 months after anesthesia administration. TSA was intraperitoneally injected 2 h before CFC training. Hippocampal histone acetylation levels were analyzed following CFC training. Levels of the neuronal activation and synaptic plasticity marker c-Fos were investigated at the same time point.

Results

Mice that were neonatally exposed to isoflurane showed significant memory impairment on CFC testing. These mice also exhibited dysregulated hippocampal H4K12 acetylation and decreased c-Fos expression following CFC training. TSA attenuated isoflurane-induced memory impairment and simultaneously increased histone acetylation and c-Fos levels in the hippocampal cornu ammonis (CA)1 area 1 h after CFC training.

Conclusions

Memory impairment induced by repeated neonatal exposure to isoflurane is associated with dysregulated histone H4K12 acetylation in the hippocampus, which probably affects downstream c-Fos gene expression following CFC training. The HDAC inhibitor TSA successfully rescued impaired contextual fear memory, presumably by promoting histone acetylation and histone acetylation-mediated gene expression.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

The strength of aversive and appetitive associations and maladaptive behaviors

Certain maladaptive behaviors are thought to be acquired through classical Pavlovian conditioning. Exaggerated fear response, which can develop through Pavlovian conditioning, is associated with acquired anxiety disorders such as post-traumatic stress disorders (PTSDs). Inflated reward-seeking behavior, which develops through Pavlovian conditioning, underlies some types of addictive behavior (e.g., addiction to drugs, food, and gambling). These maladaptive behaviors are dependent on associative learning and the development of long-term memory (LTM). In animal models, an aversive reinforcer (fear conditioning) encodes an aversive contextual and cued LTM. On the other hand, an appetitive reinforcer results in conditioned place preference (CPP) that encodes an appetitive contextual LTM. The literature on weak and strong associative learning pertaining to the development of aversive and appetitive LTM is relatively scarce; thus, this review is particularly focused on the strength of associative learning. The strength of associative learning is dependent on the valence of the reinforcer and the salience of the conditioned stimulus that ultimately sways the strength of the memory trace. Our studies suggest that labile (weak) aversive and appetitive LTM may share similar signaling pathways, whereas stable (strong) aversive and appetitive LTM is mediated through different pathways. In addition, we provide some evidence suggesting that extinction of aversive fear memory and appetitive drug memory is likely to be mediated through different signaling molecules. We put forward the importance of studies aimed to investigate the molecular mechanisms underlying the development of weak and strong memories (aversive and appetitive), which would ultimately help in the development of targeted pharmacotherapies for the management of maladaptive behaviors that arise from classical Pavlovian conditioning.

On the resilience of remote traumatic memories against exposure therapy-mediated attenuation

How to attenuate traumatic memories has long been the focus of intensive research efforts, as traumatic memories are extremely persistent and heavily impinge on the quality of life. Despite the fact that traumatic memories are often not readily amenable to immediate intervention, surprisingly few studies have investigated treatment options for remote traumata in animal models. The few that have unanimously concluded that exposure therapy-based approaches, the most successful behavioral intervention for the attenuation of recent forms of traumata in humans, fail to effectively reduce remote fear memories. Here, we provide an overview of these animal studies with an emphasis on why remote traumatic memories might be refractory to behavioral interventions: A lack of neuroplasticity in brain areas relevant for learning and memory emerges as a common denominator of such resilience. We then outline the findings of a recent study in mice showing that by combining exposure therapy-like approaches with small molecule inhibitors of histone deacetylases (HDACis), even remote memories can be persistently attenuated. This pharmacological intervention reinstated neuroplasticity to levels comparable to those found upon successful attenuation of recent memories. Thus, HDACis-or any other agent capable of heightening neuroplasticity-in conjunction with exposure therapy-based treatments might constitute a promising approach to overcome remote traumata.

Repression of contexual fear memory induced by isoflurane is accompanied by reduction in histone acetylation and rescued by sodium butyrate

BACKGROUND

Isoflurane produces amnesia in mice during contextual fear conditioning (CFC) trials. Histone acetylation is a form of chromatin modification involved in the transcriptional regulation underlying memory formation. We investigated whether isoflurane-induced repression of contextual fear memory is related to altered histone acetylation in the hippocampus, and whether it can be rescued by the histone deacetylases inhibitor sodium butyrate (SB).

METHODS

Adult C57BL/6 mice were chronically given intraperitoneal injections of SB or vehicle for 28 days. Immediately before CFC training, the mice were exposed to isoflurane or air for 30 min and CFC testing was performed the next day. Hippocampal histone acetylation was analysed 1 h after CFC training. c-Fos, an immediate early gene (IEG) suggested to participate in learning and memory formation, was also investigated at the same timepoint.

RESULTS

Mice exposed to isoflurane showed a reduction in freezing time during the CFC test. These mice also exhibited reduced hippocampal H3K14, H4K5, and H4K12 acetylation 1 h after CFC training, and also decreased c-Fos expression. All of these changes were attenuated in isoflurane-exposed mice that were chronically treated with SB.

CONCLUSIONS

Isoflurane suppresses histone acetylation and down-regulates c-Fos gene expression in CA1 of the hippocampus after CFC training. These changes are associated with isoflurane-induced amnesia. The HDAC inhibitor SB prevented repressed contextual fear memory, presumably by promoting histone acetylation and histone acetylation-mediated gene expression in response to CFC training.

Epigenetic priming of memory updating during reconsolidation to attenuate remote fear memories

Traumatic events generate some of the most enduring forms of memories. Despite the elevated lifetime prevalence of anxiety disorders, effective strategies to attenuate long-term traumatic memories are scarce. The most efficacious treatments to diminish recent (i.e., day-old) traumata capitalize on memory updating mechanisms during reconsolidation that are initiated upon memory recall. Here, we show that, in mice, successful reconsolidation-updating paradigms for recent memories fail to attenuate remote (i.e., month-old) ones. We find that, whereas recent memory recall induces a limited period of hippocampal neuroplasticity mediated, in part, by S-nitrosylation of HDAC2 and histone acetylation, such plasticity is absent for remote memories. However, by using an HDAC2-targeting inhibitor (HDACi) during reconsolidation, even remote memories can be persistently attenuated. This intervention epigenetically primes the expression of neuroplasticity-related genes, which is accompanied by higher metabolic, synaptic, and structural plasticity. Thus, applying HDACis during memory reconsolidation might constitute a treatment option for remote traumata.

Standardised extract of Bacopa monniera (CDRI-08) improves contextual fear memory by differentially regulating the activity of histone acetylation and protein phosphatases (PP1α, PP2A) in hippocampus

Contextual fear conditioning is a paradigm for investigating cellular mechanisms involved in hippocampus-dependent memory. Earlier, we showed that standardised extract of Bacopa monniera (CDRI-08) improves hippocampus-dependent learning in postnatal rats by elevating the level of serotonin (5-hydroxytryptamine, 5-HT), activate 5-HT3A receptors, and cyclic adenosine monophosphate (cAMP) response element binding (CREB) protein. In this study, we have further examined the molecular mechanism of CDRI-08 in hippocampus-dependent memory and compared to the histone deacetylase (HDACs) inhibitor sodium butyrate (NaB). To assess the hippocampus-dependent memory, wistar rat pups were subjected to contextual fear conditioning (CFC) following daily (postnatal days 15-29) administration of vehicle solution (0.5 % gum acacia + 0.9 % saline)/CDRI-08 (80 mg/kg, p.o.)/NaB (1.2 g/kg in PBS, i.p.). CDRI-08/NaB treated group showed enhanced freezing behavior compared to control group when re-exposed to the same context. Administration of CDRI-08/NaB resulted in activation of extracellular signal-regulated kinase ERK/CREB signaling cascade and up-regulation of p300, Ac-H3 and Ac-H4 levels, and down-regulation of HDACs (1, 2) and protein phosphatases (PP1α, PP2A) in hippocampus following CFC. This would subsequently result in an increased brain-derived neurotrophic factor (Bdnf) (exon IV) mRNA in hippocampus. Altogether, our results indicate that CDRI-08 enhances hippocampus-dependent contextual memory by differentially regulating histone acetylation and protein phosphatases in hippocampus.

Sodium butyrate-induced histone acetylation strengthens the expression of cocaine-associated contextual memory

The conditioned place preference (CPP) paradigm entails Pavlovian conditioning and allows evaluating the acquisition and extinction of drug-associated memory. Epigenetic regulation of chromatin structure by acetylation and deacetylation of histone proteins is critical for formation of long-term memory (LTM). We have recently shown that a single administration of the histone deacetylase (HDAC) inhibitor sodium butyrate (NaB) facilitated extinction of fear-associated memory in mice. Using the CPP paradigm, the present study investigated the effect of NaB on cocaine-associated memory. C57B/6 mice were conditioned by either fixed daily doses of cocaine (5mg/kg×4 and 15mg/kg×4days) or an escalating schedule (3, 6, 12 and 24mg/kg). Acute administration of NaB (1.2g/kg) prior to conditioning by fixed doses of cocaine increased the expression and impaired the extinction of place preference compared to control subjects. Subjects that were conditioned by 15mg/kg×4 cocaine and received a single injection of NaB following the first or the second CPP test showed impaired extinction compared to control mice that received saline instead of NaB. Subjects that were conditioned by escalating schedule of cocaine and subsequently received repeated injections of NaB during daily reexposure to nonreinforced context showed either enhancement or no effect on place preference. Acute administration of NaB (1.2g/kg) to naïve mice resulted in marked increase in acetylation of histone H3 lysine 14 (H3K14) and histone H4 lysine 8 (H4K8) in hippocampus but not amygdala. Results suggest that regardless of the scheduling of either cocaine or NaB administration, NaB-induced histone hyperacetylation in the hippocampus may strengthen cocaine-associated contextual memory.

The histone deacetylase inhibitor sodium butyrate modulates acquisition and extinction of cocaine-induced conditioned place preference

Despite decades of research on treatments for cocaine dependence, relapse rates following many behavioral and drug-based therapies remain high. This may be in part because cocaine-associated cues and contexts can invoke powerful drug cravings years after quitting. Recent studies suggest that drugs that promote cognitive function can enhance the formation of memories involving cocaine and other substances. One target of these drugs is facilitating histone acetylation to promote learning by increasing gene transcription that supports memory formation. Here, we investigate the effects of the histone deacetylase (HDAC) inhibitor sodium butyrate (NaBut) on cocaine-induced conditioned place preference (CPP) in C57BL/6 mice. After establishing a graded dose-response curve (2, 5, & 20 mg/kg) for cocaine-induced CPP, we examined the effects of different doses of NaBut (0, 0.3, 0.6, & 1.2 g/kg) on conditioning, extinction, and post-extinction reconditioning of CPP. A high dose of NaBut (1.2 g/kg) enhanced initial acquisition of cocaine CPP, but there were no effects of NaBut on reconditioning of extinguished CPP. Effects of NaBut on extinction were more complex, with a low-dose (0.3 g/kg) facilitating extinction and a high dose (1.2 g/kg) weakening extinction evident by preference at a retention test. These findings suggest that HDAC inhibition may have dose dependent effects on different components of cocaine CPP, with implications for (1) involvement of histone acetylation in context-drug learning, (2) interpretation of acute and chronic drug effects, and (3) the targeting of different types of learning in therapeutic application of HDAC inhibitors.

Histone acetylation: molecular mnemonics on the chromatin

Long-lasting memories require specific gene expression programmes that are, in part, orchestrated by epigenetic mechanisms. Of the epigenetic modifications identified in cognitive processes, histone acetylation has spurred considerable interest. Whereas increments in histone acetylation have consistently been shown to favour learning and memory, a lack thereof has been causally implicated in cognitive impairments in neurodevelopmental disorders, neurodegeneration and ageing. As histone acetylation and cognitive functions can be pharmacologically restored by histone deacetylase inhibitors, this epigenetic modification might constitute a molecular memory aid on the chromatin and, by extension, a new template for therapeutic interventions against cognitive frailty.

Epigenetic regulation of memory formation and maintenance

Understanding the cellular and molecular mechanisms underlying the formation and maintenance of memories is a central goal of the neuroscience community. It is well regarded that an organism's ability to lastingly adapt its behavior in response to a transient environmental stimulus relies on the central nervous system's capability for structural and functional plasticity. This plasticity is dependent on a well-regulated program of neurotransmitter release, post-synaptic receptor activation, intracellular signaling cascades, gene transcription, and subsequent protein synthesis. In the last decade, epigenetic markers like DNA methylation and post-translational modifications of histone tails have emerged as important regulators of the memory process. Their ability to regulate gene transcription dynamically in response to neuronal activation supports the consolidation of long-term memory. Furthermore, the persistent and self-propagating nature of these mechanisms, particularly DNA methylation, suggests a molecular mechanism for memory maintenance. In this review, we will examine the evidence that supports a role of epigenetic mechanisms in learning and memory. In doing so, we hope to emphasize (1) the widespread involvement of these mechanisms across different behavioral paradigms and distinct brain regions, (2) the temporal and genetic specificity of these mechanisms in response to upstream signaling cascades, and (3) the functional outcome these mechanisms may have on structural and functional plasticity. Finally, we consider the future directions of neuroepigenetic research as it relates to neuronal storage of information.

Epigenetic mechanisms in learned fear: implications for PTSD

One of the most exciting discoveries in the learning and memory field in the past two decades is the observation that active regulation of gene expression is necessary for experience to trigger lasting functional and behavioral change, in a wide variety of species, including humans. Thus, as opposed to the traditional view of 'nature' (genes) being separate from 'nurture' (environment and experience), it is now clear that experience actively drives alterations in central nervous system (CNS) gene expression in an ongoing fashion, and that the resulting transcriptional changes are necessary for experience to trigger altered long-term behavior. In parallel over the past decade, epigenetic mechanisms, including regulation of chromatin structure and DNA methylation, have been shown to be potent regulators of gene transcription in the CNS. In this review, we describe data supporting the hypothesis that epigenetic molecular mechanisms, especially DNA methylation and demethylation, drive long-term behavioral change through active regulation of gene transcription in the CNS. Specifically, we propose that epigenetic molecular mechanisms underlie the formation and stabilization of context- and cue-triggered fear conditioning based in the hippocampus and amygdala, a conclusion reached in a wide variety of studies using laboratory animals. Given the relevance of cued and contextual fear conditioning to post-traumatic stress, by extension we propose that these mechanisms may contribute to post-traumatic stress disorder (PTSD) in humans. Moreover, we speculate that epigenetically based pharmacotherapy may provide a new avenue of drug treatment for PTSD-related cognitive and behavioral function.

Regulating critical period plasticity: insight from the visual system to fear circuitry for therapeutic interventions

Early temporary windows of heightened brain plasticity called critical periods developmentally sculpt neural circuits and contribute to adult behavior. Regulatory mechanisms of visual cortex development - the preeminent model of experience-dependent critical period plasticity-actively limit adult plasticity and have proved fruitful therapeutic targets to reopen plasticity and rewire faulty visual system connections later in life. Interestingly, these molecular mechanisms have been implicated in the regulation of plasticity in other functions beyond vision. Applying mechanistic understandings of critical period plasticity in the visual cortex to fear circuitry may provide a conceptual framework for developing novel therapeutic tools to mitigate aberrant fear responses in post traumatic stress disorder. In this review, we turn to the model of experience-dependent visual plasticity to provide novel insights for the mechanisms regulating plasticity in the fear system. Fear circuitry, particularly fear memory erasure, also undergoes age-related changes in experience-dependent plasticity. We consider the contributions of molecular brakes that halt visual critical period plasticity to circuitry underlying fear memory erasure. A major molecular brake in the visual cortex, perineuronal net formation, recently has been identified in the development of fear systems that are resilient to fear memory erasure. The roles of other molecular brakes, myelin-related Nogo receptor signaling and Lynx family proteins - endogenous inhibitors for nicotinic acetylcholine receptor, are explored in the context of fear memory plasticity. Such fear plasticity regulators, including epigenetic effects, provide promising targets for therapeutic interventions.

Exploring epigenetic regulation of fear memory and biomarkers associated with post-traumatic stress disorder

This review examines recent work on epigenetic mechanisms underlying animal models of fear learning as well as its translational implications in disorders of fear regulation, such as Post-traumatic Stress Disorder (PTSD). Specifically, we will examine work outlining roles of differential histone acetylation and DNA-methylation associated with consolidation, reconsolidation, and extinction in Pavlovian fear paradigms. We then focus on the numerous studies examining the epigenetic modifications of the Brain-derived neurotrophin factor (BDNF) pathway and the extension of these findings from animal models to recent work in human clinical populations. We will also review recently published data on FKBP5 regulation of glucocorticoid receptor function, and how this is modulated in animal models of PTSD and in human clinical populations via epigenetic mechanisms. As glucocorticoid regulation of memory consolidation is well established in fear models, we examine how these recent data contribute to our broader understanding of fear memory formation. The combined recent progress in epigenetic modulation of memory with the advances in fear neurobiology suggest that this area may be critical to progress in our understanding of fear-related disorders with implications for new approaches to treatment and prevention.

The Plasticity of Extinction: Contribution of the Prefrontal Cortex in Treating Addiction through Inhibitory Learning

Theories of drug addiction that incorporate various concepts from the fields of learning and memory have led to the idea that classical and operant conditioning principles underlie the compulsiveness of addictive behaviors. Relapse often results from exposure to drug-associated cues, and the ability to extinguish these conditioned behaviors through inhibitory learning could serve as a potential therapeutic approach for those who suffer from addiction. This review will examine the evidence that extinction learning alters neuronal plasticity in specific brain regions and pathways. In particular, subregions of the prefrontal cortex (PFC) and their projections to other brain regions have been shown to differentially modulate drug-seeking and extinction behavior. Additionally, there is a growing body of research demonstrating that manipulation of neuronal plasticity can alter extinction learning. Therefore, the ability to alter plasticity within areas of the PFC through pharmacological manipulation could facilitate the acquisition of extinction and provide a novel intervention to aid in the extinction of drug-related memories.

Individual differences in recovery from traumatic fear

Although exposure to major psychological trauma is unfortunately common, risk for related neuropsychiatric conditions, such as post-traumatic stress disorder (PTSD), varies greatly among individuals. Fear extinction offers a tractable and translatable behavioral readout of individual differences in learned recovery from trauma. Studies in rodent substrains and subpopulations are providing new insights into neural system dysfunctions associated with impaired fear extinction. Rapid progress is also being made in identifying key molecular circuits, epigenetic mechanisms, and gene variants associated with differences in fear extinction. Here, we discuss how this research is informing understanding of the etiology and pathophysiology of individual differences in risk for trauma-related anxiety disorders, and how future work can help identify novel diagnostic biomarkers and pharmacotherapeutics for these disorders.