Pavlovian fear conditioning regulates Thr286 autophosphorylation of Ca2+/calmodulin-dependent protein kinase II at lateral amygdala synapses

Fear conditioning: Insights into learning, memory and extinction and its relevance to clinical disorders

Fear, whether innate or learned, is an essential emotion required for survival. The learning, and subsequent memory, of fearful events enhances our ability to recognise and respond to threats, aiding adaptation to new, ever-changing environments. Considerable research has leveraged associative learning protocols such as contextual or auditory forms of fear conditioning in rodents, to understand fear learning, memory consolidation and extinction phases of memory. Such assays have led to detailed characterisation of the underlying neurocircuitry and neurobiology supporting fear learning processes. Given fear processing is conserved across rodents and humans, fear conditioning experiments provide translational insights into fundamental memory processes and fear-related pathologies. This review examines associative learning protocols used to measure fear learning, memory and extinction, before providing an overview on the underlying complex neurocircuitry including the amygdala, hippocampus and medial prefrontal cortex. This is followed by an in-depth commentary on the neurobiology, particularly synaptic plasticity mechanisms, which regulate fear learning, memory and extinction. Next, we consider how fear conditioning assays in rodents can inform our understanding of disrupted fear memory in human disorders such as post-traumatic stress disorder (PTSD), anxiety and psychiatric disorders including schizophrenia. Lastly, we critically evaluate fear conditioning protocols, highlighting some of the experimental and theoretical limitations and the considerations required when conducting such assays, alongside recent methodological advancements in the field. Overall, rodent-based fear conditioning assays remain central to making progress in uncovering fundamental memory phenomena and understanding the aetiological mechanisms that underpin fear associated disorders, alongside the development of effective therapeutic strategies.

Cheonwangbosimdan mitigates post-traumatic stress disorder-like behaviors through GluN2B-containing NMDA receptor antagonism in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Cheonwangbosimdan (CWBSD), a herbal medicine traditionally used for anxiety, insomnia, depression, and heart palpitations, has been reported to have anti-anxiety, antidepressant, cognitive improvement, and neuroprotective effects.

AIM OF THE STUDY

The purpose of this study was to determine if CWBSD could affect post-traumatic stress disorder (PTSD)-like behaviors because it has prioritized clinical use over mechanism study.

MATERIALS AND METHODS

A single prolonged stress (SPS) mouse model, a well-established animal model of PTSD, was used to investigate whether standardized CWBSD could mitigate PTSD-like behaviors through robust behavioral tests, including the elevated plus-maze test and marble burying test for measuring anxiety-like behaviors, the splash test, forced swimming test, and tail suspension test for evaluating depression-like behaviors, and the Y-maze test and novel object recognition test for assessing cognitive function. Additionally, a fear extinction test was employed to determine whether CWBSD might reverse fear memory extinction deficits. Amygdala tissue was isolated from SPS-treated mouse brain and subjected to Western blotting or quantitative PCR to explore mechanisms by which CWBSD could mitigate PTSD-like behaviors.

RESULTS

CWBSD ameliorated emotional impairments and cognitive dysfunction in an SPS-induced PTSD-like mouse model. It also mitigated deficits in abnormal fear memory extinction. Protein expression levels of N-methyl-D-aspartate (NMDA) receptor subunit 2B (GluN2B) and phosphorylation levels of Ca2+/calmodulin-dependent protein kinase II in the amygdala were increased in SPS model mice and normalized by CWBSD. Additionally, co-administration of CWBSD and GluN2B-containing NMDA receptor antagonist, ifenprodil, at each sub-effective dose promoted fear memory extinction.

CONCLUSIONS

CWBSD can alleviate SPS-induced PTSD-like behaviors by normalizing GluN2B-containing NMDA receptor activity in the amygdala. Therefore, CWBSD could be a promising candidate for PTSD treatment with fewer adverse effects and better efficacy than existing therapies.

The Basolateral Amygdala: The Core of a Network for Threat Conditioning, Extinction, and Second-Order Threat Conditioning

Threat conditioning, extinction, and second-order threat conditioning studied in animal models provide insight into the brain-based mechanisms of fear- and anxiety-related disorders and their treatment. Much attention has been paid to the role of the basolateral amygdala (BLA) in such processes, an overview of which is presented in this review. More recent evidence suggests that the BLA serves as the core of a greater network of structures in these forms of learning, including associative and sensory cortices. The BLA is importantly regulated by hippocampal and prefrontal inputs, as well as by the catecholaminergic neuromodulators, norepinephrine and dopamine, that may provide important prediction-error or learning signals for these forms of learning. The sensory cortices may be required for the long-term storage of threat memories. As such, future research may further investigate the potential of the sensory cortices for the long-term storage of extinction and second-order conditioning memories.

Is PSD-95 entangled in the side effects of antidepressants?

PSD-95 is a component and a building block of an excitatory synapse. PSD-95 is a specialized protein that is part of a "combination lock" system responsible for plastic events at the synapse, such as receptor expression, which consequently induces changes in the PSD structure and thus affects synaptic plasticity. The possible involvement of PSD-95 in antidepressant side effects related to cognitive function and psychosis will be considered. An attempt will be made to trace the sequence of events in the proposed mechanism leading to these disorders, focusing mainly on NMDA receptors. Understanding the mechanisms of action of compounds with antidepressant potential may facilitate the design of safer drugs.

The ERK phosphorylation levels in the amygdala predict anxiety symptoms in humans and MEK/ERK inhibition dissociates innate and learned defensive behaviors in rats

We demonstrate that the rate of extracellular signal-related kinase phosphorylation (P-ERK1,2/Total-ERK1,2) in the amygdala is negatively and independently associated with anxiety symptoms in 23 consecutive patients with drug-resistant mesial temporal lobe epilepsy that was surgically treated. In naive Wistar rats, the P-ERK1,2/Total-ERK1,2 ratio in the amygdala correlates negatively with innate anxiety-related behavior on the elevated plus maze (n = 20) but positively with expression of defensive-learned behavior (i.e., freezing) on Pavlovian aversive (fear) conditioning (n = 29). The microinfusion of ERK1/2 inhibitor (FR180204, n = 8-13/group) or MEK inhibitor (U0126, n = 8-9/group) into the basolateral amygdala did not affect anxiety-related behavior but impaired the evocation (anticipation) of conditioned-defensive behavior (n = 9-11/group). In conclusion, the P-ERK1,2/Total-ERK1,2 ratio in the amygdala predicts anxiety in humans and the innate anxiety- and conditioned freezing behaviors in rats. However, the ERK1/2 in the basolateral AMY is only required for the expression of defensive-learned behavior. These results support a dissociate ERK-dependent mechanism in the amygdala between innate anxiety-like responses and the anticipation of learned-defensive behavior. These findings have implications for understanding highly prevalent psychiatric disorders related to the defensive circuit manifested by anxiety and fear. HIGHLIGHTS: The P-ERK1,2/Total-ERK1,2 ratio in the amygdala (AMY) correlates negatively with anxiety symptoms in patients with mesial temporal lobe epilepsy. The P-ERK1,2/Total-ERK1,2 in the amygdala correlates negatively with the anxiety-like behavior and positively with freezing-learned behavior in naive rats. ERK1,2 in the basolateral amygdala is required for learned-defensive but not for the anxiety-like behavior expression in rats.

Fear Memory Retrieval Is Associated With a Reduction in AMPA Receptor Density at Thalamic to Amygdala Intercalated Cell Synapses

The amygdala plays a crucial role in attaching emotional significance to environmental cues. Its intercalated cell masses (ITC) are tight clusters of GABAergic neurons, which are distributed around the basolateral amygdala complex. Distinct ITC clusters are involved in the acquisition and extinction of conditioned fear responses. Previously, we have shown that fear memory retrieval reduces the AMPA/NMDA ratio at thalamic afferents to ITC neurons within the dorsal medio-paracapsular cluster. Here, we investigate the molecular mechanisms underlying the fear-mediated reduction in the AMPA/NMDA ratio at these synapses and, in particular, whether specific changes in the synaptic density of AMPA receptors underlie the observed change. To this aim, we used a detergent-digested freeze-fracture replica immunolabeling technique (FRIL) approach that enables to visualize the spatial distribution of intrasynaptic AMPA receptors at high resolution. AMPA receptors were detected using an antibody raised against an epitope common to all AMPA subunits. To visualize thalamic inputs, we virally transduced the posterior thalamic complex with Channelrhodopsin 2-YFP, which is anterogradely transported along axons. Using face-matched replica, we confirmed that the postsynaptic elements were ITC neurons due to their prominent expression of μ-opioid receptors. With this approach, we show that, following auditory fear conditioning in mice, the formation and retrieval of fear memory is linked to a significant reduction in the density of AMPA receptors, particularly at spine synapses formed by inputs of the posterior intralaminar thalamic and medial geniculate nuclei onto identified ITC neurons. Our study is one of the few that has directly linked the regulation of AMPA receptor trafficking to memory processes in identified neuronal networks, by showing that fear-memory induced reduction in AMPA/NMDA ratio at thalamic-ITC synapses is associated with a reduced postsynaptic AMPA receptor density.

Mechanisms of higher-order learning in the amygdala

Adaptive behaviour is under the potent control of environmental cues. Such cues can acquire value by virtue of their associations with outcomes of motivational significance, be they appetitive or aversive. There are at least two ways through which an environmental cue can acquire value, through first-order and higher-order conditioning. In first-order conditioning, a neutral cue is directly paired with an outcome of motivational significance. In higher-order conditioning, a cue is indirectly associated with motivational events via a directly conditioned first-order stimulus. The present article reviews some of the associations that support learning in first- and higher-order conditioning, as well as the role of the BLA and the molecular mechanisms involved in these two types of learning.

The Calcium/Calmodulin-Dependent Kinases II and IV as Therapeutic Targets in Neurodegenerative and Neuropsychiatric Disorders

CaMKII and CaMKIV are calcium/calmodulin-dependent kinases playing a rudimentary role in many regulatory processes in the organism. These kinases attract increasing interest due to their involvement primarily in memory and plasticity and various cellular functions. Although CaMKII and CaMKIV are mostly recognized as the important cogs in a memory machine, little is known about their effect on mood and role in neuropsychiatric diseases etiology. Here, we aimed to review the structure and functions of CaMKII and CaMKIV, as well as how these kinases modulate the animals’ behavior to promote antidepressant-like, anxiolytic-like, and procognitive effects. The review will help in the understanding of the roles of the above kinases in the selected neurodegenerative and neuropsychiatric disorders, and this knowledge can be used in future drug design.

Hippocampal proteomic analysis reveals the disturbance of synaptogenesis and neurotransmission induced by developmental exposure to organophosphate flame retardant triphenyl phosphate

With the spread of organophosphorus flame retardants (OPFRs), the environmental and health risks they induce are attracting attention. Triphenyl phosphate (TPHP) is a popular alternative to brominated flame retardant and halogenated OPFRs. Neurodevelopmental toxicity is TPHP's primary adverse effect, whereas the biomarkers and the modes of action have yet to be elucidated. In the present study, 0.5, 5, and 50 mg/kg of TPHP were orally administered to mice from postnatal day 10 (P10) to P70. The behavioral tests showed a compromised learning and memory capability. Proteomic analysis of the hippocampus exposed to 0.5 or 50 mg/kg of TPHP identified 531 differentially expressed proteins that were mainly involved in axon guidance, synaptic function, neurotransmitter transport, exocytosis, and energy metabolism. Immunoblot and immunofluorescence analysis showed that exposure to TPHP reduced the protein levels of TUBB3 and SYP in the synapses of hippocampal neurons. TPHP exposure also downregulated the gene expression of neurotransmitter receptors including Grins, Htr1α, and Adra1α in a dose-dependent fashion. Moreover, the calcium-dependent synaptic exocytosis governed by synaptic vesicle proteins STX1A and SYT1 was inhibited in the TPHP-treated hippocampus. Our results reveal that TPHP exposure causes abnormal learning and memory behaviors by disturbing synaptogenesis and neurotransmission.

CaMKII antagonism in the ventral tegmental area impairs acquisition of conditioned approach learning in rats

This study investigated the role of calcium2+/calmodulin-dependent protein kinase II (CaMKII), a protein in the second messenger pathway of NMDA receptors, in the ventral tegmental area (VTA) in the acquisition and performance of conditioned approach learning. Male Long-Evans rats (N = 79) were exposed to 3 (to test acquisition) or 7 (to test performance) conditioning sessions in which they received 30 paired presentations of a light stimulus (CS) and a food pellet (US) on a random time schedule. These conditioning sessions were then followed by one 30-min session without the CS or US and lastly by a CS-only test session, where only the light stimulus was presented (without food) according to the same schedule as the conditioning sessions. Bilateral intra-VTA injections of KN93 (vehicle, 3.0, 4.5 or 6.0 μg/0.5 μL), a CaMKII inhibitor, were administered prior to each conditioning session to test effects on the acquisition of conditioned approach or prior to the CS-only test session to test effects on the performance of conditioned approach. KN93, when given prior to conditioning sessions, significantly reduced the number of conditioned approach responses emitted during CS presentations in the CS-only test. When KN93 was given prior to the CS-only test it had no effect. These results suggest that CaMKII activation in the VTA is necessary for the acquisition, but not the performance, of reward-related learning.

Males and Females Differ in the Subcellular and Brain Region Dependent Regulation of Proteasome Activity by CaMKII and Protein Kinase A

The ubiquitin-proteasome system (UPS) controls the degradation of ~90% of short-lived proteins in cells and is involved in activity- and learning-dependent synaptic plasticity in the brain. Calcium/calmodulin dependent protein kinase II (CaMKII) and Protein Kinase A (PKA) can regulate activity of the proteasome. However, there have been a number of conflicting reports regarding under what conditions CaMKII and PKA regulate proteasome activity in the brain. Furthermore, this work has been done exclusively in males, leaving questions about whether these kinases also regulate the proteasome in females. Here, using subcellular fractionation protocols in combination with in vitro pharmacology and proteasome activity assays, we investigated the conditions under which CaMKII and PKA regulate proteasome activity in the brains of male and female rats. In males, nuclear proteasome chymotrypsin activity was regulated by PKA in the amygdala but CaMKII in the hippocampus. Conversely, in females CaMKII regulated nuclear chymotrypsin activity in the amygdala, but not hippocampus. Additionally, in males CaMKII and PKA regulated proteasome trypsin activity in the cytoplasm of hippocampal, but not amygdala cells, while in females both CaMKII and PKA could regulate this activity in the nucleus of cells in both regions. Proteasome peptidylglutamyl activity was regulated by CaMKII and PKA activity in the nuclei of amygdala and hippocampus cells in males. However, in females PKA regulated nuclear peptidylglutamyl activity in the amygdala, but not hippocampus. Collectively, these results suggest that CaMKII- and PKA-dependent regulation of proteasome activity in the brain varies significantly across subcellular compartments and between males and females.

NPY2 Receptors Reduce Tonic Action Potential-Independent GABAB Currents in the Basolateral Amygdala

Although NPY has potent anxiolytic actions within the BLA, selective activation of BLA NPY Y₂ receptors (Y₂Rs) acutely increases anxiety by an unknown mechanism. Using ex vivo male rat brain slice electrophysiology, we show that the selective Y₂R agonist, [ahx^5-24]NPY, reduced the frequency of GABA_A-mediated mIPSCs in BLA principal neurons (PNs). [ahx^5-24]NPY also reduced tonic activation of GABA_B receptors (GABA_BR), which increased PN excitability through inhibition of a tonic, inwardly rectifying potassium current (K_IR ). Surprisingly, Y₂R-sensitive GABA_BR currents were action potential-independent, persisting after treatment with TTX. Additionally, the Ca²⁺-dependent, slow afterhyperpolarizing K⁺ current (I_sAHP ) was enhanced in approximately half of the Y₂R-sensitive PNs, possibly from enhanced Ca²⁺ influx, permitted by reduced GABA_BR tone. In male and female mice expressing tdTomato in Y₂R-mRNA cells (tdT-Y₂R mice), immunohistochemistry revealed that BLA somatostatin interneurons express Y₂Rs, as do a significant subset of BLA PNs. In tdT-Y₂R mice, [ahx^5-24]NPY increased excitability and suppressed the K_IR in nearly all BLA PNs independent of tdT-Y₂R fluorescence, consistent with presynaptic Y₂Rs on somatostatin interneurons mediating the above effects. However, only tdT-Y₂R-expressing PNs responded to [ahx^5-24]NPY with an enhancement of the I_sAHP Ultimately, increased PN excitability via acute Y₂R activation likely correlates with enhanced BLA output, consistent with reported Y₂R-mediated anxiogenesis. Furthermore, we demonstrate the following: (1) a novel mechanism whereby activity-independent GABA release can powerfully dampen BLA neuronal excitability via postsynaptic GABA_BRs; and (2) that this tonic inhibition can be interrupted by neuromodulation, here by NPY via Y₂Rs.SIGNIFICANCE STATEMENT Within the BLA, NPY is potently anxiolytic. However, selective activation of NPY₂ receptors (Y₂Rs) increases anxiety by an unknown mechanism. We show that activation of BLA Y₂Rs decreases tonic GABA release onto BLA principal neurons, probably from Y₂R-expressing somatostatin interneurons, some of which coexpress NPY. This increases principal neuron excitability by reducing GABA_B receptor (GABA_BR)-mediated activation of G-protein-coupled, inwardly rectifying K⁺ currents. Tonic, Y₂R-sensitive GABA_BR currents unexpectedly persisted in the absence of action potential firing, revealing, to our knowledge, the first report of substantial, activity-independent GABA_BR activation. Ultimately, we provide a plausible explanation for Y₂R-mediated anxiogenesis in vivo and describe a novel and modulatable means of damping neuronal excitability.

Kinase and Phosphatase Engagement Is Dissociated Between Memory Formation and Extinction

Associative long-term memories (LTMs) support long-lasting behavioral changes resulting from sensory experiences. Retrieval of a stable LTM by means of a large number of conditioned stimulus (CS) alone presentations produces inhibition of the original memory through extinction. Currently, there are two opposing hypotheses to account for the neural mechanisms supporting extinction. The unlearning hypothesis posits that extinction affects the original memory trace by reverting the synaptic changes supporting LTM. On the contrary, the new learning hypothesis proposes that extinction is simply the formation of a new associative memory that inhibits the expression of the original one. We propose that detailed analysis of extinction-associated molecular mechanisms could help distinguish between these hypotheses. Here we will review experimental evidence regarding the role of protein kinases and phosphatases (K&P) on LTM formation and extinction. Even though K&P regulate both memory processes, their participation appears to be dissociated. LTM formation recruits kinases, but is constrained by phosphatases. Memory extinction presents a more diverse molecular landscape, requiring phosphatases and some kinases, but also being constrained by kinase activity. Based on the available evidence, we propose a new theoretical model for memory extinction: a neuronal segregation of K&P supports a combination of time-dependent reversible inhibition of the original memory [CS-unconditioned stimulus (US)], with establishment of a new associative memory trace (CS-noUS).

Mechanisms of fear learning and extinction: synaptic plasticity-fear memory connection

RATIONALE

The ability to memorize threat-associated cues and subsequently react to them, exhibiting escape or avoidance responses, is an essential, often life-saving behavioral mechanism that can be experimentally studied using the fear (threat) conditioning training paradigm. Presently, there is substantial evidence supporting the Synaptic Plasticity-Memory (SPM) hypothesis in relation to the mechanisms underlying the acquisition, retention, and extinction of conditioned fear memory.

OBJECTIVES

The purpose of this review article is to summarize findings supporting the SPM hypothesis in the context of conditioned fear control, applying the set of criteria and tests which were proposed as necessary to causally link lasting changes in synaptic transmission in corresponding neural circuits to fear memory acquisition and extinction with an emphasis on their pharmacological diversity.

RESULTS

The mechanisms of synaptic plasticity in fear circuits exhibit complex pharmacological profiles and satisfy all four SPM criteria-detectability, anterograde alteration, retrograde alteration, and mimicry.

CONCLUSION

The reviewed findings, accumulated over the last two decades, provide support for both necessity and sufficiency of synaptic plasticity in fear circuits for fear memory acquisition and retention, and, in part, for fear extinction, with the latter requiring additional experimental work.

Neurochemical and molecular mechanisms underlying the retrieval-extinction effect

Extinction within the reconsolidation window, or 'retrieval-extinction', has received much research interest as a possible technique for targeting the reconsolidation of maladaptive memories with a behavioural intervention. However, it remains to be determined whether the retrieval-extinction effect-a long-term reduction in fear behaviour, which appears resistant to spontaneous recovery, renewal and reinstatement-depends specifically on destabilisation of the original memory (the 'reconsolidation-update' account) or represents facilitation of an extinction memory (the 'extinction-facilitation' account). We propose that comparing the neurotransmitter systems, receptors and intracellular signalling pathways recruited by reconsolidation, extinction and retrieval-extinction will provide a way of distinguishing between these accounts.

Autophosphorylation of F-actin binding domain of CaMKIIβ is required for fear learning

CaMKII is a pivotal kinase that plays essential roles in synaptic plasticity. Apart from its signaling function, the structural function of CaMKII is becoming clear. CaMKII - F-actin interaction stabilizes actin cytoskeleton in a dendritic spine. A transient autophosphorylation at the F-actin binding region during LTP releases CaMKII from F-actin and opens a brief time-window of actin reorganization. However, the physiological relevance of this finding in learning and memory was not presented. Using a knock-in (KI) mouse carrying phosphoblock mutations in the actin-binding domain of CaMKIIβ, we demonstrate that proper regulation of CaMKII - F-actin interaction is important for fear conditioning memory tasks. The KI mice show poor performance in contextual and cued versions of fear conditioning test. These results suggest the importance of CaMKII - F-actin interactions in learning and memory.

The role of calcium-calmodulin-dependent protein kinase II in modulation of spatial memory in morphine sensitized rats

It has been shown that drug addiction and memory system are related but the signaling cascades underlying this interaction is not completely revealed yet. It has been demonstrated that binding of Calcium-calmodulin-dependent protein kinase II (CaMKII) to NMDA receptor is important in the memory process. The main objective of the study was to evaluate the role of CaMKII on the spatial memory of rats which previously were sensitized by morphine. The effect of CaMKII inhibitor (KN-93) on memory changes was investigated by hippocampal microinjection of KN-93 on the morphine-sensitized rats. Also, the role of the NMDA receptor in memory retention by KN-93 on the morphine sensitized rat was investigated with NMDA agonist and antagonist. Sensitization was induced by morphine injection (once daily for 3 days) followed by 5 days free of the drug before the trial phase. For the evaluation of spatial memory, the Morris Water Maze test (MWM) was used. Results showed that pre-trial administration of morphine, induced amnesia in MWM (p < 0.05). Also, three days pretreatment with morphine (20 mg/kg) followed by five days washout period, caused to enhance memory retrieval in confront with a pre-trial challenging dose of morphine (5 mg/kg). In addition, KN-93 administration during induction phase in morphine sensitization phenomena facilitated morphine-induced memory retention. In addition, inhibition of the NMDA receptor and KN-93 during the induction phase did not improve memory. However; intra-CA1 co-administration of KN-93 and NMDA during the induction phase of morphine sensitization resulted in improving spatial memory. It can be concluded that the effect of CaMKII on memory retention in morphine-sensitized rats depends on NMDA receptor.

Traumatic Stress Produces Distinct Activations of GABAergic and Glutamatergic Neurons in Amygdala

Posttraumatic stress disorder (PTSD) is an anxiety disorder characterized by intrusive recollections of a severe traumatic event and hyperarousal following exposure to the event. Human and animal studies have shown that the change of amygdala activity after traumatic stress may contribute to occurrences of some symptoms or behaviors of the patients or animals with PTSD. However, it is still unknown how the neuronal activation of different sub-regions in amygdala changes during the development of PTSD. In the present study, we used single prolonged stress (SPS) procedure to obtain the animal model of PTSD, and found that 1 day after SPS, there were normal anxiety behavior and extinction of fear memory in rats which were accompanied by a reduced proportion of activated glutamatergic neurons and increased proportion of activated GABAergic neurons in basolateral amygdala (BLA). About 10 days after SPS, we observed enhanced anxiety and impaired extinction of fear memory with increased activated both glutamatergic and GABAergic neurons in BLA and increased activated GABAergic neurons in central amygdala (CeA). These results indicate that during early and late phase after traumatic stress, distinct patterns of activation of glutamatergic neurons and GABAergic neurons are displayed in amygdala, which may be implicated in the development of PTSD.

Differential Involvement of Kinase Activity of Ca2+/Calmodulin-Dependent Protein Kinase IIα in Hippocampus- and Amygdala-Dependent Memory Revealed by Kinase-Dead Knock-In Mouse

Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) is a key mediator of activity-dependent neuronal modifications and has been implicated in the molecular mechanisms of learning and memory. Indeed, several types of CaMKIIα knock-in (KI) and knock-out (KO) mice revealed impairments in hippocampal synaptic plasticity and behavioral learning. On the other hand, a similar role for CaMKIIα has been implicated in amygdala-dependent memory, but detailed analyses have not much been performed yet. To better understand its involvement in amygdala-dependent memory as compared to hippocampus-dependent memory, here we performed biochemical analyses and behavioral memory tests using the kinase-dead CaMKIIα (K42R)-KI mouse. In the Morris water maze tasks, homozygous mutants performed well in the visible platform trials, while they failed to form spatial memory in the hippocampus-dependent hidden platform trials. In fear conditioning, these mice were impaired but showed a certain level of amygdala-dependent cued fear memory, which lasted four weeks, while they showed virtually no hippocampus-dependent context discrimination. Neither stronger stimulation nor repetitive stimulation compensated for their memory deficits. The differential outcome of hippocampus- and amygdala-dependent memory in the mutant mouse was not due to differential expression of CaMKIIα between the hippocampus and the amygdala, because biochemical analyses revealed that both kinase activity and protein levels of CaMKII were indistinguishable between the two brain regions. These results indicate that kinase activity of CaMKIIα is indispensable for hippocampus-dependent memory, but not necessarily for amygdala-dependent memory. There may be a secondary, CaMKIIα activity-independent pathway, in addition to the CaMKIIα activity-dependent pathway, in the acquisition of amygdala-dependent memory.

The Ubiquitin-Proteasome System and Memory: Moving Beyond Protein Degradation

Cellular models of memory formation have focused on the need for protein synthesis. Recently, evidence has emerged that protein degradation mediated by the ubiquitin-proteasome system (UPS) is also important for this process. This has led to revised cellular models of memory formation that focus on a balance between protein degradation and synthesis. However, protein degradation is only one function of the UPS. Studies using single-celled organisms have shown that non-proteolytic ubiquitin-proteasome signaling is involved in histone modifications and DNA methylation, suggesting that ubiquitin and the proteasome can regulate chromatin remodeling independent of protein degradation. Despite this evidence, the idea that the UPS is more than a protein degradation pathway has not been examined in the context of memory formation. In this article, we summarize recent findings implicating protein degradation in memory formation and discuss various ways in which both ubiquitin signaling and the proteasome could act independently to regulate epigenetic-mediated transcriptional processes necessary for learning-dependent synaptic plasticity. We conclude by proposing comprehensive models of how non-proteolytic functions of the UPS could work in concert to control epigenetic regulation of the cellular memory consolidation process, which will serve as a framework for future studies examining the role of the UPS in memory formation.

Commonalities and Differences in the Substrates Underlying Consolidation of First- and Second-Order Conditioned Fear

Consolidation of newly formed fear memories requires a series of molecular events within the basolateral complex of the amygdala (BLA). Once consolidated, new information can be assimilated into these established associative networks to form higher-order associations. Much is known about the molecular events involved in consolidating newly acquired fear memories but little is known about the events that consolidate a secondary fear memory. Here, we show that, within the male rat BLA, DNA methylation and gene transcription are crucial for consolidating both the primary and secondary fear memories. We also show that consolidation of the primary, but not the secondary, fear memory requires de novo protein synthesis in the BLA. These findings show that consolidation of a fear memory and its updating to incorporate new information recruit distinct processes in the BLA, and suggest that DNA methylation in the BLA is fundamental to consolidation of both types of conditioned fear.SIGNIFICANCE STATEMENT Our data provide clear evidence that a different set of mechanisms mediate consolidation of learning about cues that signal learned sources of danger (i.e., second-order conditioned fear) compared with those involved in consolidation of learning about cues that signal innate sources of danger (i.e., first-order conditioned fear). These findings carry important implications because second-order learning could underlie aberrant fear-related behaviors (e.g., in anxiety disorders) as a consequence of neutral secondary cues being integrated into associative fear networks established through first-order pairings, and thereby becoming potent conditioned reinforcers and predictors of fear. Therefore, our data suggest that targeting such second-order conditioned triggers of fear may require pharmacological intervention different to that typically used for first-order conditioned cues.

Cue-induced reinstatement of alcohol-seeking behavior is associated with increased CaMKII T286 phosphorylation in the reward pathway of mice

Cue-induced reinstatement of alcohol-seeking is a hallmark behavioral pathology of addiction. Evidence suggests that reinstatement (e.g., relapse), may be regulated by cell signaling systems that underlie neuroplasticity. A variety of plasticity events require activation of calcium calmodulin-dependent protein kinase II (CaMKII) in components of the reward pathway, such as the nucleus accumbens and amygdala. We sought to determine if cue-induced reinstatement of alcohol-seeking behavior is associated with changes in the activation state (e.g., phosphorylation) of CaMKII-T286. Male C57BL/6J mice (n=14) were trained to lever press on a fixed-ratio-4 schedule of sweetened alcohol (2% sucrose+9% EtOH) reinforcement. After 14-d of extinction (no cues or reinforcers), mice underwent a response-contingent reinstatement (n=7) vs. an additional day of extinction (n=7). Brains were removed immediately after the test and processed for evaluation of pCaMKII-T286 immunoreactivity (IR). Number of pCaMKII-T286 positive cells/mm² was quantified from coronal brain sections using Bioquant Image Analysis software. Mice emitted significantly more responses on the alcohol vs. the inactive lever throughout the baseline phase with average alcohol intake of 1.1±0.03g/kg/1-h. During extinction, responses on the alcohol lever decreased to inactive lever levels by day 7. Significant cue-induced reinstatement of alcohol-seeking was observed during a single test with no effects on the inactive lever. Reinstatement was associated with increased pCaMKII-T286 IR specifically in amygdala (LA and BLA), nucleus accumbens (AcbSh), lateral septum, mediodorsal thalamus, and piriform cortex as compared to extinction control. Brain regions showing no change included the dorsal striatum, medial septum, cingulate cortex, habenula, paraventricular thalamus, and ventral hypothalamus. These results show response contingent cue-induced reinstatement of alcohol-seeking behavior is associated with selective increases in pCaMKII-T286 in specific reward- and memory-related brain regions of male C57BL/6J mice. Primary molecular mechanisms of associative learning and memory may regulate relapse in alcohol addiction.

From Structure to Behavior in Basolateral Amygdala-Hippocampus Circuits

Emotion influences various cognitive processes, including learning and memory. The amygdala is specialized for input and processing of emotion, while the hippocampus is essential for declarative or episodic memory. During emotional reactions, these two brain regions interact to translate the emotion into particular outcomes. Here, we briefly introduce the anatomy and functions of amygdala and hippocampus, and then present behavioral, electrophysiological, optogenetic and biochemical evidence from recent studies to illustrate how amygdala and hippocampus work synergistically to form long-term memory. With recent technological advances, the causal investigations of specific neural circuit between amygdala and hippocampus will help us understand the brain mechanisms of emotion-regulated memories and improve clinical treatment of emotion-associated memory disorders in patients.

Is Arc mRNA Unique: A Search for mRNAs That Localize to the Distal Dendrites of Dentate Gyrus Granule Cells Following Neural Activity

There have been several attempts to identify which RNAs are localized to dendrites; however, no study has determined which RNAs localize to the dendrites following the induction of synaptic activity. We sought to identify all RNA transcripts that localize to the distal dendrites of dentate gyrus granule cells following unilateral high frequency stimulation of the perforant pathway (pp-HFS) using Sprague Dawley rats. We then utilized laser microdissection (LMD) to very accurately dissect out the distal 2/3rds of the molecular layer (ML), which contains these dendrites, without contamination from the granule cell layer, 2 and 4 h post pp-HFS. Next, we purified and amplified RNA from the ML and performed an unbiased screen for 27,000 RNA transcripts using Affymetrix microarrays. We determined that Activity Regulated Cytoskeletal Protein (Arc/Arg3.1) mRNA, exhibited the greatest fold increase in the ML at both timepoints (2 and 4 h). In total, we identified 31 transcripts that increased their levels within the ML following pp-HFS across the two timepoints. Of particular interest is that one of these identified transcripts was an unprocessed micro-RNA (pri-miR132). Fluorescent in situ hybridization and qRT-PCR were used to confirm some of these candidate transcripts. Our data indicate Arc is a unique activity dependent gene, due to the magnitude that its activity dependent transcript localizes to the dendrites. Our study determined other activity dependent transcripts likely localize to the dendrites following neural activity, but do so with lower efficiency compared to Arc.

Molecular and synaptic mechanisms regulating drug-associated memories: Towards a bidirectional treatment strategy

The successful treatment of substance use disorders is dependent on the establishment of a long-term abstinent state. Relapse can be suppressed by interfering with memories of drug use that are evoked by re-exposure to drug-associated contexts and cues. Two strategies for accomplishing this goal are either to prevent drug-memory reconsolidation or to induce the formation of a competing, extinction memory. However, clinical attempts to prolong abstinence by behavioral modification of drug-related memories have had limited success. One approach to improve behavioral treatment strategies is to identify the molecular mechanisms that regulate these memory processes and then use pharmacological tools as supplements to improve efficacy. Still, due to the involvement of several overlapping signaling cascades in both reconsolidation and extinction, it is difficult to specifically modify one of the two processes. For example, attempting to elicit extinction may instead initiate reconsolidation, resulting in the unintentional strengthening of drug-related memories. A better approach is to identify diverging components of the two processes, whereby a single medication would simultaneously weaken reconsolidation and enhance extinction. This review will provide an overview of the neural substrates that are involved in the regulation of drug-associated memories, and will discuss emerging approaches to pharmacologically weaken these memories, including recent efforts to precisely and bidirectionally target reconsolidation and extinction. Ultimately, pharmacologically-enhanced memory-based approaches have the potential to produce more informed relapse-prevention therapies.

Vitamin B6 prevents isocarbophos-induced vascular dementia in rats through N-methyl-D-aspartate receptor signaling

BACKGROUND

We have previously reported that the long-term exposure of organophosphorus induces vascular dementia (VD) in rats. As a coenzyme, vitamin B6 is mainly involved in the regulation of metabolisms. Whether vitamin B6 improves VD remains unknown.

METHODS

The model of VD was induced by feeding rats with isocarbophos (0.5 mg/kg per two day, 12 weeks). The blood flow of the posterior cerebral artery (PCA) in rat was assessed by transcranial Doppler (TCD). The learning and memory were evaluated by the Morris Water Maze (MWM) test.

RESULTS

Administration of vitamin B6 increased the blood flow in the right and left posterior cerebral arteries and improved the functions of learning and memory in isocarbophos-treated rats. Vitamin B6 increased the protein levels of N-methyl-D-aspartate receptor (NMDAR) 2B, postsynaptic densities (PSDs) protein 95, and calmodulin-dependent protein kinase II (CaMK-II) in the hippocampus, which were decreased by isocarbophos in rats. Morphological analysis by light microscope and electronic microscope indicated disruptions of the hippocampus caused by isocarbophos were normalized by vitamin B6. Importantly, the antagonist of NMDAR signaling by eliprodil abolished these beneficial effects produced by vitamin B6 on PCA blood flow, learning, memory, and hippocampus structure in rats, as well as the protein expression of NMDAR 2B, PSDs protein 95, and CaMK-II in the hippocampus.

CONCLUSION

Vitamin B6 activates NMDAR signaling to prevent isocarbophos-induced VD in rats.

Neurobiology of fear and specific phobias

Fear, which can be expressed innately or after conditioning, is triggered when a danger or a stimulus predicting immediate danger is perceived. Its role is to prepare the body to face this danger. However, dysfunction in fear processing can lead to psychiatric disorders in which fear outweighs the danger or possibility of harm. Although recognized as highly debilitating, pathological fear remains insufficiently treated, indicating the importance of research on fear processing. The neurobiological basis of normal and pathological fear reactions is reviewed in this article. Innate and learned fear mechanisms, particularly those involving the amygdala, are considered. These fear mechanisms are also distinguished in specific phobias, which can indeed be nonexperiential (implicating innate, learning-independent mechanisms) or experiential (implicating learning-dependent mechanisms). Poor habituation and poor extinction are presented as dysfunctional mechanisms contributing to persistence of nonexperiential and experiential phobias, respectively.

Phosphoproteomic Analysis Reveals a Novel Mechanism of CaMKIIα Regulation Inversely Induced by Cocaine Memory Extinction versus Reconsolidation

Successful addiction treatment depends on maintaining long-term abstinence, making relapse prevention an essential therapeutic goal. However, exposure to environmental cues associated with drug use often thwarts abstinence efforts by triggering drug using memories that drive craving and relapse. We sought to develop a dual approach for weakening cocaine memories through phosphoproteomic identification of targets regulated in opposite directions by memory extinction compared with reconsolidation in male Sprague-Dawley rats that had been trained to self-administer cocaine paired with an audiovisual cue. We discovered a novel, inversely regulated, memory-dependent phosphorylation event on calcium-calmodulin-dependent kinase II α (CaMKIIα) at serine (S)331. Correspondingly, extinction-associated S331 phosphorylation inhibited CaMKIIα activity. Intra-basolateral amygdala inhibition of CaMKII promoted memory extinction and disrupted reconsolidation, leading to a reduction in subsequent cue-induced reinstatement. CaMKII inhibition had no effect if the memory was neither retrieved nor extinguished. Therefore, inhibition of CaMKII represents a novel mechanism for memory-based addiction treatment that leverages both extinction enhancement and reconsolidation disruption to reduce relapse-like behavior.

SIGNIFICANCE STATEMENT

Preventing relapse to drug use is an important goal for the successful treatment of addictive disorders. Relapse-prevention therapies attempt to interfere with drug-associated memories, but are often hindered by unintentional memory strengthening. In this study, we identify phosphorylation events that are bidirectionally regulated by the reconsolidation versus extinction of a cocaine-associated memory, including a novel site on CaMKIIα. Additionally, using a rodent model of addiction, we show that CaMKII inhibition in the amygdala can reduce relapse-like behavior. Together, our data supports the existence of mechanisms that can be used to enhance current strategies for addiction treatment.

Age-dependent changes in autophosphorylation of alpha calcium/calmodulin dependent kinase II in hippocampus and amygdala after contextual fear conditioning

The hippocampus and amygdala are essential brain regions responsible for contextual fear conditioning (CFC). The autophosphorylation of alpha calcium-calmodulin kinase II (αCaMKII) at threonine-286 (T286) is a critical step implicated in long-term potentiation (LTP), learning and memory. However, the changes in αCaMKII levels with aging and training in associated brain regions are not fully understood. Here, we studied how aging and training affect the levels of phosphorylated (T286) and proportion of phosphorylated:total αCaMKII in the hippocampus and amygdala. Young and aged mice, naïve (untrained) and trained in CFC, were analysed by immunohistochemistry for the levels of total and phosphorylated αCaMKII in the hippocampus and amygdala. We found that two hours after CFC training, young mice exhibited a higher level of phosphorylated and increased ratio of phosphorylated:total αCaMKII in hippocampal CA3 stratum radiatum. Furthermore, aged untrained mice showed a higher ratio of phosphorylated:total αCaMKII in the CA3 region of the hippocampus when compared to the young untrained group. No effect of training or aging were seen in the central, lateral and basolateral amygdala regions, for both phosphorylated and ratio of phosphorylated:total αCaMKII. These results show that aging impairs the training-induced upregulation of autophosphorylated (T286) αCaMKII in the CA3 stratum radiatum of the hippocampus. This indicates that distinct age-related mechanisms underlie CFC that may rely more heavily on NMDA receptor-dependent plasticity in young age.

The role of Ca2+-calmodulin stimulated protein kinase II in ischaemic stroke - A potential target for neuroprotective therapies

Studies in multiple experimental systems show that Ca²⁺-calmodulin stimulated protein kinase II (CaMKII) is a major mediator of ischaemia-induced cell death and suggest that CaMKII would be a good target for neuroprotective therapies in acute treatment of stroke. However, as CaMKII regulates many cellular processes in many tissues any clinical treatment involving the inhibition of CaMKII would need to be able to specifically target the functions of ischaemia-activated CaMKII. In this review we summarise new developments in our understanding of the molecular mechanisms involved in ischaemia-induced CaMKII-mediated cell death that have identified ways in which such specificity of CaMKII inhibition after stroke could be achieved. We also review the mechanisms and phases of tissue damage in ischaemic stroke to identify where and when CaMKII-mediated mechanisms may be involved.

Statistical modeling implicates neuroanatomical circuit mediating stress relief by 'comfort' food

A history of eating highly palatable foods reduces physiological and emotional responses to stress. For instance, we have previously shown that limited sucrose intake (4 ml of 30 % sucrose twice daily for 14 days) reduces hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress. However, the neural mechanisms underlying stress relief by such 'comfort' foods are unclear, and could reveal an endogenous brain pathway for stress mitigation. As such, the present work assessed the expression of several proteins related to neuronal activation and/or plasticity in multiple stress- and reward-regulatory brain regions of rats after limited sucrose (vs. water control) intake. These data were then subjected to a series of statistical analyses, including Bayesian modeling, to identify the most likely neurocircuit mediating stress relief by sucrose. The analyses suggest that sucrose reduces HPA activation by dampening an excitatory basolateral amygdala-medial amygdala circuit, while also potentiating an inhibitory bed nucleus of the stria terminalis principle subdivision-mediated circuit, resulting in reduced HPA activation after stress. Collectively, the results support the hypothesis that sucrose limits stress responses via plastic changes to the structure and function of stress-regulatory neural circuits. The work also illustrates that advanced statistical methods are useful approaches to identify potentially novel and important underlying relationships in biological datasets.

Moderate Alcohol Drinking and the Amygdala Proteome: Identification and Validation of Calcium/Calmodulin Dependent Kinase II and AMPA Receptor Activity as Novel Molecular Mechanisms of the Positive Reinforcing Effects of Alcohol

BACKGROUND

Despite worldwide consumption of moderate amounts of alcohol, the neural mechanisms that mediate the transition from use to abuse are not fully understood.

METHODS

Here, we conducted a high-throughput screen of the amygdala proteome in mice after moderate alcohol drinking (n = 12/group) followed by behavioral studies (n = 6-8/group) to uncover novel molecular mechanisms of the positive reinforcing properties of alcohol that strongly influence the development of addiction.

RESULTS

Two-dimensional difference in-gel electrophoresis with matrix assisted laser desorption ionization tandem time-of-flight identified 29 differentially expressed proteins in the amygdala of nondependent C57BL/6J mice following 24 days of alcohol drinking. Alcohol-sensitive proteins included calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) and a network of functionally linked proteins that regulate neural plasticity and glutamate-mediated synaptic activity. Accordingly, alcohol drinking increased α-amino-3-hydroxy-5-methyl-4-isooxazole receptor (AMPAR) in central amygdala (CeA) and phosphorylation of AMPAR GluA1 subunit at a CaMKII locus (GluA1-Ser831) in CeA and lateral amygdala. Further, CaMKIIα-Thr286 and GluA1-Ser831 phosphorylation was increased in CeA and lateral amygdala of mice that lever-pressed for alcohol versus the nondrug reinforcer sucrose. Mechanistic studies showed that targeted pharmacologic inhibition of amygdala CaMKII or AMPAR activity specifically inhibited the positive reinforcing properties of alcohol but not sucrose.

CONCLUSIONS

Moderate alcohol drinking increases the activity and function of plasticity-linked protein networks in the amygdala that regulate the positive reinforcing effects of the drug. Given the prominence of positive reinforcement in the etiology of addiction, we propose that alcohol-induced adaptations in CaMKIIα and AMPAR signaling in the amygdala may serve as a molecular gateway from use to abuse.

CaMKIIα knockdown decreases anxiety in the open field and low serotonin-induced upregulation of GluA1 in the basolateral amygdala

Hyperactivation of the amygdala is implicated in anxiety and mood disorders, but the precise underlying mechanisms are unclear. We previously reported that depletion of serotonin (5-hydroxytryptamine, 5-HT) in the basolateral nucleus of the amygdala (BLA) using the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) potentiated learned fear and increased glutamate receptor (Glu) expression in BLA. Here we investigated the hypothesis that CaMKII facilitates anxiety-like behavior and increased Glu/AMPA receptor subunit A1 (GluA1) expression following depletion of 5-HT in the BLA. Infusion of 5,7-DHT into the BLA resulted in anxiety-like behavior in the open field test (OFT) and increased the phosphorylation of CaMKIIα (Thr-286) in the BLA. Knockdown of the CaMKIIα subunit using adeno-associated virus (AAV)-delivered shRNAi concomitantly attenuated anxiety-like behavior in the OFT and decreased GluA1 expression in the BLA. Our results suggest that the CaMKII signaling plays a key role in low 5-HT-induced anxiety and mood disturbances, potentially through regulation of GluA1 expression in the BLA.

CaMKII regulates proteasome phosphorylation and activity and promotes memory destabilization following retrieval

Numerous studies have suggested that memories "destabilize" and require de novo protein synthesis in order to reconsolidate following retrieval, but very little is known about how this destabilization process is regulated. Recently, ubiquitin-proteasome mediated protein degradation has been identified as a critical regulator of memory trace destabilization following retrieval, though the specific mechanisms controlling retrieval-induced changes in ubiquitin-proteasome activity remain equivocal. Here, we found that proteasome activity is increased in the amygdala in a CaMKII-dependent manner following the retrieval of a contextual fear memory. We show that in vitro inhibition of CaMKII reversed retrieval-induced increases in proteasome activity. Additionally, in vivo pharmacological blockade of CaMKII abolished increases in proteolytic activity and activity related regulatory phosphorylation in the amygdala following retrieval, suggesting that CaMKII was "upstream" of protein degradation during the memory reconsolidation process. Consistent with this, while inhibiting CaMKII in the amygdala did not impair memory following retrieval, it completely attenuated the memory impairments that resulted from post-retrieval protein synthesis blockade. Collectively, these results suggest that CaMKII controls the initiation of the memory reconsolidation process through regulation of the proteasome.

Stress exposure, food intake and emotional state

This manuscript summarizes the proceedings of the symposium entitled, "Stress, Palatable Food and Reward", that was chaired by Drs. Linda Rinaman and Yvonne Ulrich-Lai at the 2014 Neurobiology of Stress Workshop held in Cincinnati, OH. This symposium comprised research presentations by four neuroscientists whose work focuses on the biological bases for complex interactions among stress, food intake and emotion. First, Dr Ulrich-Lai describes her rodent research exploring mechanisms by which the rewarding properties of sweet palatable foods confer stress relief. Second, Dr Stephanie Fulton discusses her work in which excessive, long-term intake of dietary lipids, as well as their subsequent withdrawal, promotes stress-related outcomes in mice. Third, Dr Mark Wilson describes his group's research examining the effects of social hierarchy-related stress on food intake and diet choice in group-housed female rhesus macaques, and compared the data from monkeys to results obtained in analogous work using rodents. Finally, Dr Gorica Petrovich discusses her research program that is aimed at defining cortical-amygdalar-hypothalamic circuitry responsible for curbing food intake during emotional threat (i.e. fear anticipation) in rats. Their collective results reveal the complexity of physiological and behavioral interactions that link stress, food intake and emotional state, and suggest new avenues of research to probe the impact of genetic, metabolic, social, experiential and environmental factors on these interactions.

In search of a recognition memory engram

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The role of the perirhinal cortex in familiarity discrimination is reviewed.

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Behavioural, pharmacological and electrophysiological evidence is considered.

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The cortex is found to be essential for memory acquisition, retrieval and storage.

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The evidence indicates that perirhinal synaptic weakening is critically involved.

A large body of data from human and animal studies using psychological, recording, imaging, and lesion techniques indicates that recognition memory involves at least two separable processes: familiarity discrimination and recollection. Familiarity discrimination for individual visual stimuli seems to be effected by a system centred on the perirhinal cortex of the temporal lobe. The fundamental change that encodes prior occurrence within the perirhinal cortex is a reduction in the responses of neurones when a stimulus is repeated. Neuronal network modelling indicates that a system based on such a change in responsiveness is potentially highly efficient in information theoretic terms. A review is given of findings indicating that perirhinal cortex acts as a storage site for recognition memory of objects and that such storage depends upon processes producing synaptic weakening.

Protein degradation and protein synthesis in long-term memory formation

Long-term memory (LTM) formation requires transient changes in the activity of intracellular signaling cascades that are thought to regulate new gene transcription and de novo protein synthesis in the brain. Consistent with this, protein synthesis inhibitors impair LTM for a variety of behavioral tasks when infused into the brain around the time of training or following memory retrieval, suggesting that protein synthesis is a critical step in LTM storage in the brain. However, evidence suggests that protein degradation mediated by the ubiquitin-proteasome system (UPS) may also be a critical regulator of LTM formation and stability following retrieval. This requirement for increased protein degradation has been shown in the same brain regions in which protein synthesis is required for LTM storage. Additionally, increases in the phosphorylation of proteins involved in translational control parallel increases in protein polyubiquitination and the increased demand for protein degradation is regulated by intracellular signaling molecules thought to regulate protein synthesis during LTM formation. In some cases inhibiting proteasome activity can rescue memory impairments that result from pharmacological blockade of protein synthesis, suggesting that protein degradation may control the requirement for protein synthesis during the memory storage process. Results such as these suggest that protein degradation and synthesis are both critical for LTM formation and may interact to properly “consolidate” and store memories in the brain. Here, we review the evidence implicating protein synthesis and degradation in LTM storage and highlight the areas of overlap between these two opposing processes. We also discuss evidence suggesting these two processes may interact to properly form and store memories. LTM storage likely requires a coordinated regulation between protein degradation and synthesis at multiple sites in the mammalian brain.

Hippocampal Wnt3a is Necessary and Sufficient for Contextual Fear Memory Acquisition and Consolidation

The Wnt signaling pathway plays critical roles in development. However, to date, the role of Wnts in learning and memory in adults is still not well understood. Here, we aimed to investigate the roles and mechanisms of Wnts in hippocampal-dependent contextual fear conditioning (CFC) memory formation in adult mice. CFC training induced the secretion and expression of Wnt3a and the activation of its downstream Wnt/Ca(2+) and Wnt/β-catenin signaling pathways in the dorsal hippocampus (DH). Intrahippocampal infusion of Wnt3a antibody impaired CFC acquisition and consolidation, but not expression. Using the Wnt antagonist sFRP1 or the canonical Wnt inhibitor Dkk1, we found that Wnt/Ca(2+) and Wnt/β-catenin signaling pathways were involved in acquisition and consolidation, respectively. Moreover, we found Wnt3a signaling is not only necessary but also sufficient for CFC memory. Intrahippocampal infusion of exogenous Wnt3a could enhance acquisition and consolidation of CFC. Overexpression of constitutively active β-catenin in the DH could rescue the deficit in CFC memory consolidation, but not acquisition induced by Wnt3a antibody injection, which suggests β-catenin signaling pathway acts downstream of Wnt3a to mediate CFC memory consolidation. Our study may help further the understanding of the precise regulation of Wnt3a in differential memory phases depending on divergent signaling pathways.

Adeno-associated viral serotypes produce differing titers and differentially transduce neurons within the rat basal and lateral amygdala

BACKGROUND

In recent years, there has been an increased interest in using recombinant adeno-associated viruses (AAV) to make localized genetic manipulations within the rodent brain. Differing serotypes of AAV possess divergent capsid protein sequences and these variations greatly influence each serotype's ability to transduce particular cell types and brain regions. We therefore aimed to determine the AAV serotype that is optimal for targeting neurons within the Basal and Lateral Amygdala (BLA) since the transduction efficiency of AAV has not been previously examined within the BLA. This region is desirable to genetically manipulate due to its role in emotion, learning & memory, and numerous psychiatric disorders. We accomplished this by screening 9 different AAV serotypes (AAV2/1, AAV2/2, AAV2/5, AAV2/7, AAV2/8, AAV2/9, AAV2/rh10, AAV2/DJ and AAV2/DJ8) designed to express red fluorescent protein (RFP) under the regulation of an alpha Ca2+/calmodulin-dependent protein kinase II promoter (αCaMKII).

RESULTS

We determined that these serotypes produce differing amounts of virus under standard laboratory production. Notably AAV2/2 consistently produced the lowest titers compared to the other serotypes examined. These nine serotypes were bilaterally infused into the rat BLA at the highest titers achieved for each serotype and at a normalized titer of 7.8E + 11 GC/ml. Twenty one days following viral infusion the degree of transduction was quantitated throughout the amygdala. These viruses exhibited differential transduction of neurons within the BLA. AAV2/7 exhibited a trend toward having the highest efficiency of transduction and AAV2/5 exhibited significantly lower transduction efficiency as compared to the serotypes examined. AAV2/5's decreased ability to transduce BLA neurons correlates with its significantly different capsid protein sequences as compared to the other serotypes examined.

CONCLUSIONS

For laboratories producing their own recombinant adeno-associated viruses, the use of AAV2/2 is likely less desirable since AAV2/2 produces significantly lower titers than many other serotypes of AAV. Numerous AAV serotypes appear to efficiently transduce BLA neurons, with the exception of AAV2/5. Taking into consideration the ability of certain serotypes to achieve high titers and transduce BLA neurons well, in our hands AAV2/DJ8 and AAV2/9 appear to be ideal serotypes to use when targeting neurons within the BLA.

Involvement of amygdalar protein kinase A, but not calcium/calmodulin-dependent protein kinase II, in the reconsolidation of cocaine-related contextual memories in rats

RATIONALE

Contextual control over drug relapse depends on the successful reconsolidation and retention of context-response-cocaine associations in long-term memory stores. The basolateral amygdala (BLA) plays a critical role in cocaine memory reconsolidation and subsequent drug context-induced cocaine-seeking behavior; however, less is known about the cellular mechanisms of this phenomenon.

OBJECTIVES

The present study evaluated the hypothesis that protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) activation in the BLA is necessary for the reconsolidation of context-response-cocaine memories that promote subsequent drug context-induced cocaine-seeking behavior.

METHODS

Rats were trained to lever-press for cocaine infusions in a distinct context, followed by extinction training in a different context. Rats were then briefly re-exposed to the previously cocaine-paired context or an unpaired context in order to reactivate cocaine-related contextual memories and initiate their reconsolidation or to provide a similar behavioral experience without explicit cocaine-related memory reactivation, respectively. Immediately after this session, rats received bilateral microinfusions of vehicle, the PKA inhibitor, Rp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS), or the CaMKII inhibitor, KN-93, into the BLA or the posterior caudate putamen (anatomical control region). Rats were then tested for cocaine-seeking behavior (responses on the previously cocaine-paired lever) in the cocaine-paired context and the extinction context.

RESULTS

Intra-BLA infusion of Rp-cAMPS, but not KN-93, following cocaine memory reconsolidation impaired subsequent cocaine-seeking behavior in a dose-dependent, site-specific, and memory reactivation-dependent fashion.

CONCLUSIONS

PKA, but not CaMKII, activation in the BLA is critical for cocaine memory re-stabilization processes that facilitate subsequent drug context-induced instrumental cocaine-seeking behavior.

Traits of fear resistance and susceptibility in an advanced intercross line

Genetic variability in the strength and precision of fear memory is hypothesised to contribute to the etiology of anxiety disorders, including post-traumatic stress disorder. We generated fear-susceptible (F-S) or fear-resistant (F-R) phenotypes from an F8 advanced intercross line (AIL) of C57BL/6J and DBA/2J inbred mice by selective breeding. We identified specific traits underlying individual variability in Pavlovian conditioned fear learning and memory. Offspring of selected lines differed in the acquisition of conditioned fear. Furthermore, F-S mice showed greater cued fear memory and generalised fear in response to a novel context than F-R mice. F-S mice showed greater basal corticosterone levels and hypothalamic corticotrophin-releasing hormone (CRH) mRNA levels than F-R mice, consistent with higher hypothalamic-pituitary-adrenal (HPA) axis drive. Hypothalamic mineralocorticoid receptor and CRH receptor 1 mRNA levels were decreased in F-S mice as compared with F-R mice. Manganese-enhanced magnetic resonance imaging (MEMRI) was used to investigate basal levels of brain activity. MEMRI identified a pattern of increased brain activity in F-S mice that was driven primarily by the hippocampus and amygdala, indicating excessive limbic circuit activity in F-S mice as compared with F-R mice. Thus, selection pressure applied to the AIL population leads to the accumulation of heritable trait-relevant characteristics within each line, whereas non-behaviorally relevant traits remain distributed. Selected lines therefore minimise false-positive associations between behavioral phenotypes and physiology. We demonstrate that intrinsic differences in HPA axis function and limbic excitability contribute to phenotypic differences in the acquisition and consolidation of associative fear memory. Identification of system-wide traits predisposing to variability in fear memory may help in the direction of more targeted and efficacious treatments for fear-related pathology.

GluA1 phosphorylation at serine 831 in the lateral amygdala is required for fear renewal

Fear renewal, a widely pursued model of post-traumatic stress disorder and phobias, refers to the context-specific relapse of conditioned fear after extinction. However, its molecular mechanisms are largely unknown. We found that renewal-inducing stimuli, generally believed to be insufficient to induce synaptic plasticity, enhanced excitatory synaptic strength, activity of synaptic GluA2-lacking AMPA receptors and Ser831 phosphorylation of synaptic surface GluA1 in the lateral nucleus of the amygdala (LAn) of fear-extinguished rats. Consistently, the induction threshold for LAn synaptic potentiation was considerably lowered after extinction, and renewal occluded this low-threshold potentiation. The low-threshold potentiation (a potential cellular substrate for renewal), but not long-term potentiation, was attenuated by dialysis into LAn neurons of a GluA1-derived peptide that competes with Ser831-phosphorylated GluA1. Microinjections of the same peptide into the LAn attenuated fear renewal, but not fear learning. Our findings suggest that GluA1 phosphorylation constitutes a promising target for clinical treatment of aberrant fear-related disorders.

CaMKII, but not protein kinase A, regulates Rpt6 phosphorylation and proteasome activity during the formation of long-term memories

CaMKII and Protein Kinase A (PKA) are thought to be critical for synaptic plasticity and memory formation through their regulation of protein synthesis. Consistent with this, numerous studies have reported that CaMKII, PKA and protein synthesis are critical for long-term memory formation. Recently, we found that protein degradation through the ubiquitin-proteasome system is also critical for long-term memory formation in the amygdala. However, the mechanism by which ubiquitin-proteasome activity is regulated during memory formation and how protein degradation interacts with known intracellular signaling pathways important for learning remain unknown. Recently, evidence has emerged suggesting that both CaMKII and PKA are capable of regulating proteasome activity in vitro through the phosphorylation of proteasome regulatory subunit Rpt6 at Serine-120, though whether they regulate Rpt6 phosphorylation and proteasome function in vivo remains unknown. In the present study we demonstrate for the first time that fear conditioning transiently modifies a proteasome regulatory subunit and proteasome catalytic activity in the mammalian brain in a CaMKII-dependent manner. We found increases in the phosphorylation of proteasome ATPase subunit Rpt6 at Serine-120 and an enhancement in proteasome activity in the amygdala following fear conditioning. Pharmacological manipulation of CaMKII, but not PKA, in vivo significantly reduced both the learning-induced increase in Rpt6 Serine-120 phosphorylation and the increase in proteasome activity without directly affecting protein polyubiquitination levels. These results indicate a novel role for CaMKII in memory formation through its regulation of protein degradation and suggest that CaMKII regulates Rpt6 phosphorylation and proteasome function both in vitro and in vivo.

Recognition memory and synaptic plasticity in the perirhinal and prefrontal cortices

Work is reviewed that relates recognition memory to studies of synaptic plasticity mechanisms in perirhinal and prefrontal cortices. The aim is to consider evidence that perirhinal cortex and medial prefrontal cortex store rather than merely transmit information necessary for recognition memory and, if so, to consider what mechanisms are potentially available within these cortices for producing such storage through synaptic change. Interventions with known actions on plasticity mechanisms are reviewed in relation to their effects on recognition memory processes. These interventions importantly include those involving antagonism of glutamatergic and cholinergic receptors but also inhibition of plasticity consolidation and expression mechanisms. It is concluded that there is strong evidence that perirhinal cortex is involved in information storage necessary for object recognition memory and, moreover, that such storage involves synaptic weakening mechanisms including the removal of AMPA glutamate receptors from synapses. There is good evidence that medial prefrontal cortex is necessary for associative and temporal order recognition memory and that this cortex expresses plasticity mechanisms that potentially allow the storage of information. However, the case for medial prefrontal cortex acting as a store requires further support.

The ubiquitin-proteasome system as a critical regulator of synaptic plasticity and long-term memory formation

Numerous studies have supported the idea that de novo protein synthesis is critical for synaptic plasticity and normal long-term memory formation. This requirement for protein synthesis has been shown for several different types of fear memories, exists in multiple brain regions and circuits, and is necessary for different stages of memory creation and storage. However, evidence has recently begun to accumulate suggesting that protein degradation through the ubiquitin-proteasome system is an equally important regulator of memory formation. Here we review those recent findings on protein degradation and memory formation and stability and propose a model explaining how protein degradation may be contributing to various aspects of memory and synaptic plasticity. We conclude that protein degradation may be the major factor regulating many of the molecular processes that we know are important for fear memory formation and stability in the mammalian brain.

Synaptic NMDA receptors in basolateral amygdala principal neurons are triheteromeric proteins: physiological role of GluN2B subunits

N-methyl-(D)-aspartate (NMDA) receptors are heteromultimeric ion channels that contain an essential GluN1 subunit and two or more GluN2 (GluN2A-GluN2D) subunits. The biophysical properties and physiological roles of synaptic NMDA receptors are dependent on their subunit composition. In the basolateral amygdala (BLA), it has been suggested that the plasticity that underlies fear learning requires activation of heterodimeric receptors composed of GluN1/GluN2B subunits. In this study, we investigated the subunit composition of NMDA receptors present at synapses on principal neurons in the BLA. Purification of the synaptic fraction showed that both GluN2A and GluN2B subunits are present at synapses, and co-immunoprecipitation revealed the presence of receptors containing both GluN2A and GluN2B subunits. The kinetics of NMDA receptor-mediated synaptic currents and pharmacological blockade indicate that heterodimeric GluN1/GluN2B receptors are unlikely to be present at glutamatergic synapses on BLA principal neurons. Selective RNA interference-mediated knockdown of GluN2A subunits converted synaptic receptors to a GluN1/GluN2B phenotype, whereas knockdown of GluN2B subunits had no effect on the kinetics of the synaptically evoked NMDA current. Blockade of GluN1/GluN2B heterodimers with ifenprodil had no effect, but knockdown of GluN2B disrupted the induction of CaMKII-dependent long-term potentiation at these synapses. These results suggest that, on BLA principal neurons, GluN2B subunits are only present as GluN1/GluN2A/GluN2B heterotrimeric NMDA receptors. The GluN2B subunit has little impact on the kinetics of the receptor, but is essential for the recruitment of signaling molecules essential for synaptic plasticity.

The tyrosine phosphatase STEP constrains amygdala-dependent memory formation and neuroplasticity

STriatal-Enriched protein tyrosine Phosphatase (STEP; PTPN5) is expressed in brain regions displaying adult neuroplasticity. STEP modulates neurotransmission by dephosphorylating regulatory tyrosine residues on its substrates. In this way, STEP inactivates extracellular-signal-regulated kinase 1/2 (ERK1/2), limiting the duration and spatial distribution of ERK signaling. Two additional substrates, the tyrosine kinase Fyn and the NR2B subunit of the N-methyl-d-aspartic acid receptor, link STEP to glutamate receptor internalization in the synapse. Thus, STEP may act through parallel pathways to oppose the development of experience-dependent synaptic plasticity. We examined the hypothesis that the absence of STEP facilitates amygdala-dependent behavioral and synaptic plasticity (i.e., fear conditioning and long-term potentiation) using STEP-deficient mice (STEP KO). These mice show no detectable expression of STEP in the brain along with increases in Tyr phosphorylation of STEP substrates. Here we demonstrate that STEP KO mice also display augmented fear conditioning as measured by an enhancement in conditioned suppression of instrumental response when a fear-associated conditioned stimulus was presented. Deletion of STEP also increases long-term potentiation and ERK phosphorylation in the lateral amygdala. The current experiments demonstrate that deletion of STEP can enhance experience-induced neuroplasticity and memory formation and identifies STEP as a target for pharmacological treatment aimed at improving the formation of long-term memories.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

What pharmacological interventions indicate concerning the role of the perirhinal cortex in recognition memory

Findings of pharmacological studies that have investigated the involvement of specific regions of the brain in recognition memory are reviewed. The particular emphasis of the review concerns what such studies indicate concerning the role of the perirhinal cortex in recognition memory. Most of the studies involve rats and most have investigated recognition memory for objects. Pharmacological studies provide a large body of evidence supporting the essential role of the perirhinal cortex in the acquisition, consolidation and retrieval of object recognition memory. Such studies provide increasingly detailed evidence concerning both the neurotransmitter systems and the underlying intracellular mechanisms involved in recognition memory processes. They have provided evidence in support of synaptic weakening as a major synaptic plastic process within perirhinal cortex underlying object recognition memory. They have also supplied confirmatory evidence that that there is more than one synaptic plastic process involved. The demonstrated necessity to long-term recognition memory of intracellular signalling mechanisms related to synaptic modification within perirhinal cortex establishes a central role for the region in the information storage underlying such memory. Perirhinal cortex is thereby established as an information storage site rather than solely a processing station. Pharmacological studies have also supplied new evidence concerning the detailed roles of other regions, including the hippocampus and the medial prefrontal cortex in different types of recognition memory tasks that include a spatial or temporal component. In so doing, they have also further defined the contribution of perirhinal cortex to such tasks. To date it appears that the contribution of perirhinal cortex to associative and temporal order memory reflects that in simple object recognition memory, namely that perirhinal cortex provides information concerning objects and their prior occurrence (novelty/familiarity).