Shedding light on cholecystokinin's role in hippocampal neuroplasticity and memory formation

The hippocampus is a crucial brain region involved in the process of forming and consolidating memories. Memories are consolidated in the brain through synaptic plasticity, and a key mechanism underlying this process is called long-term potentiation (LTP). Recent research has shown that cholecystokinin (CCK) plays a role in facilitating the formation of LTP, as well as learning and memory consolidation. However, the specific mechanisms by which CCK is involved in hippocampal neuroplasticity and memory formation are complicated or poorly understood. This literature review aims to explore the role of LTP in memory formation, particularly in relation to hippocampal memory, and to discuss the implications of CCK and its receptors in the formation of hippocampal memories. Additionally, we will examine the circuitry of CCK in the hippocampus and propose potential CCK-dependent mechanisms of synaptic plasticity that contribute to memory formation.

Cholecystokinin (CCK): a neuromodulator with therapeutic potential in Alzheimer's and Parkinson's disease

Cholecystokinin (CCK) is a neuropeptide modulating digestion, glucose levels, neurotransmitters and memory. Recent studies suggest that CCK exhibits neuroprotective effects in Alzheimer's disease (AD) and Parkinson's disease (PD). Thus, we review the physiological function and therapeutic potential of CCK. The neuropeptide facilitates hippocampal glutamate release and gates GABAergic basket cell activity, which improves declarative memory acquisition, but inhibits consolidation. Cortical CCK alters recognition memory and enhances audio-visual processing. By stimulating CCK-1 receptors (CCK-1Rs), sulphated CCK-8 elicits dopamine release in the substantia nigra and striatum. In the mesolimbic pathway, CCK release is triggered by dopamine and terminates reward responses via CCK-2Rs. Importantly, activation of hippocampal and nigral CCK-2Rs is neuroprotective by evoking AMPK activation, expression of mitochondrial fusion modulators and autophagy. Other benefits include vagus nerve/CCK-1R-mediated expression of brain-derived neurotrophic factor, intestinal protection and suppression of inflammation. We also discuss caveats and the therapeutic combination of CCK with other peptide hormones.

Perineuronal nets are associated with decision making under conditions of uncertainty in female but not male mice

Biological sex influences decision-making processes in significant ways, differentiating the responses animals choose when faced with a range of stimuli. The neurobiological underpinnings that dictate sex differences in decision-making tasks remains an important open question, yet single-sex studies of males form most studies in behavioural neuroscience. Here we used female and male BALB/c mice on two spatial learning and memory tasks and examined the expression of perineuronal nets (PNNs) and parvalbumin interneurons (PV) in regions correlated with spatial memory. Mice underwent the aversive active place avoidance (APA) task or the appetitive trial-unique nonmatching-to-location (TUNL) touchscreen task. Mice in the APA cohort learnt to avoid the foot-shock and no differences were observed on key measures of the task nor in the number and intensity of PNNs and PV. On the delay but not separation manipulation in the TUNL task, females received more incorrect trials and less correct trials compared to males. Furthermore, females in this cohort exhibited higher intensity PNNs and PV cells in the agranular and granular retrosplenial cortex, compared to males. These data show that female and male mice perform similarly on spatial learning tasks. However, sex differences in neural circuitry may underly differences in making decisions under conditions of uncertainty on an appetitive task. These data emphasise the importance of using mice of both sexes in studies of decision-making neuroscience.

Overreliance on inefficient computer-mediated information retrieval is countermanded by strategy advice that promotes memory-mediated retrieval

With ubiquitous computing, problems can be solved using more strategies than ever, though many strategies feature subpar performance. Here, we explored whether and how simple advice regarding when to use which strategy can improve performance. Specifically, we presented unfamiliar alphanumeric equations (e.g., A + 5 = F) and asked whether counting up the alphabet from the left letter by the indicated number resulted in the right letter. In an initial choice block, participants could engage in one of three cognitive strategies: (a) internal counting, (b) internal retrieval of previously generated solutions, or (c) computer-mediated external retrieval of solutions. Participants belonged to one of two groups: they were either instructed to first try internal retrieval before using external retrieval, or received no specific use instructions. In a subsequent internal block with identical instructions for both groups, external retrieval was made unavailable. The 'try internal retrieval first' instruction in the choice block led to pronounced benefits (d = .76) in the internal block. Benefits were due to facilitated creation and retrieval of internal memory traces and possibly also due to improved strategy choice. These results showcase how simple strategy advice can greatly help users navigate cognitive environments. More generally, our results also imply that uninformed use of external tools (i.e., technology) can bear the risk of not developing and using even more superior internal processing strategies.

Neurophysiological markers of successful learning in healthy aging

The capacity to learn and memorize is a key determinant for the quality of life but is known to decline to varying degrees with age. However, neural correlates of memory formation and the critical features that determine the extent to which aging affects learning are still not well understood. By employing a visual sequence learning task, we were able to track the behavioral and neurophysiological markers of gradual learning over several repetitions, which is not possible in traditional approaches that utilize a remember vs. forgotten comparison. On a neurophysiological level, we focused on two learning-related centro-parietal event-related potential (ERP) components: the expectancy-driven P300 and memory-related broader positivity (BP). Our results revealed that although both age groups showed significant learning progress, young individuals learned faster and remembered more stimuli than older participants. Successful learning was directly linked to a decrease of P300 and BP amplitudes. However, young participants showed larger P300 amplitudes with a sharper decrease during the learning, even after correcting for an observed age-related longer P300 latency and increased P300 peak variability. Additionally, the P300 amplitude predicted learning success in both age groups and showed good test-retest reliability. On the other hand, the memory formation processes, reflected by the BP amplitude, revealed a similar level of engagement in both age groups. However, this engagement did not translate into the same learning progress in the older participants. We suggest that the slower and more variable timing of the stimulus identification process reflected in the P300 means that despite the older participants engaging the memory formation process, there is less time for it to translate the categorical stimulus location information into a solidified memory trace. The results highlight the important role of the P300 and BP as a neurophysiological marker of learning and may enable the development of preventive measures for cognitive decline.

Anticipated nostalgia

Anticipated nostalgia is the foreseeing of looking back on life events and expecting to feel nostalgic about it in the future. I review recent findings on the content, affective profile, and psychological benefits of anticipated nostalgia. I also review neurological and cognitive evidence that may explain the mechanism underlying anticipated nostalgia. Finally, I discuss anticipated nostalgia in an applied setting, consumer behavior.

Dopamine neurotransmission in the VTA regulates aversive memory formation and persistence

Dopamine (DA) neurons in the ventral tegmental area (VTA) innervating several limbic and neocortical regions of the mammalian brain have long been implicated in motivation, rewarding and aversive behaviors, and memory processing. Recently, we demonstrated that somatodendritic release of DA in the VTA regulates the formation and maintenance of appetitive long-term memories (LTM). However, less is known about the impact of DA neurotransmission in the VTA on aversive LTM. Here, we describe the modulation of negative-valence memories by D1/D5-type DA (D1R)-receptor-mediated neurotransmission in the VTA. As aversive stimuli elicit both active and passive behavioral responses, we used two single-trial aversive training protocols: inhibitory avoidance task and conditioned place aversion. We bilaterally microinfused SCH23390, an antagonist of D1R, into the VTA immediately after training and found that DA neurotransmission in the VTA modulates LTM consolidation and persistence of aversive experiences. Together with previous findings demonstrating that D1R-mediated DA neurotransmission in the medial prefrontal cortex and hippocampus is involved in the formation and persistence of LTM for aversive events, our present results indicate that memory processing of environmental stimuli with negative-valence depends on the integration of information mediated by D1R activation in both the VTA region and in selected downstream target areas.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Neuromodulation of Memory Formation and Extinction

Memory retrieval is mediated by discharges of acetylcholine, glutamate, gammaaminobutyric acid, norepinephrine, and serotonin/5-hydroxytryptamine circuits. These projections and memory interact through engram circuits, neurobiological traces of memory. Increased excitability in engram circuits of the medial prefrontal cortex and hippocampus results in remote and recent memory retrievals, respectively. However, due to degenerated neurotransmitter projections, the excitability state of engram circuits is decreased in the patient with dementia; and thus, acquired- memory cannot be retrieved by natural cues. Here, we suggest that artificial neuropharmacological stimulations of the acquired-memory with an excitation potential higher than a natural cue can excite engram circuits in the medial prefrontal cortex, which results in the retrieval of lost memories in dementia. The neuropharmacological foundations of engram cell-mediated memory retrieval strategy in severe dementia, in line with this has also been explained. We particularly highlighted the close interactions between periaqueductal gray, locus coeruleus, raphe nuclei, and medial prefrontal cortex and basolateral amygdala as treatment targets for memory loss. Furthermore, the engram circuits projecting raphe nuclei, locus coeruleus, and pontomesencephalic tegmentum complex could be significant targets of memory editing and memory formation in the absence of experience, and a well-defined study of the neural events underlying the interaction of brain stem and memory will be relevant for such developments. We anticipate our perspective to be a starting point for more sophisticated in vivo models for neuropharmacological modulations of memory retrieval in Alzheimer's dementia.

Age moderation of the association between negative subsequent memory effects and episodic memory performance

Negative subsequent memory effects in functional MRI studies of memory formation have been linked to individual differences in memory performance, yet the effect of age on this association is currently unclear. To provide insight into the brain systems related to memory across the lifespan, we examined functional neuroimaging data acquired during episodic memory formation and behavioral performance from a memory recognition task in a sample of 109 participants, including three developmental age groups (8-12, 13-17, 18-25 year-olds) and one additional group of older adults (55-85 year-olds). Young adults showed the highest memory performance and strongest negative subsequent memory effects, while older adults showed reduced negative subsequent memory effects relative to young adults. Across the sample, negative subsequent memory effects were associated with better memory performance, and there was a significant interaction between negative subsequent memory effects and memory performance by age group. Posthoc analyses revealed that this moderation effect was driven by a stronger association between negative subsequent memory effects and memory performance in young adults than children, and that neither children nor older adults showed a significant association. These findings suggest that negative subsequent memory effects may differentially support memory performance across a lifespan trajectory characterized by developmental maturation and support further investigation of this effect in aging.

Stress enhances emotional memory-related theta oscillations in the medial temporal lobe

Stressful events impact memory formation, in particular for emotionally arousing stimuli. Although these stress effects on emotional memory formation have potentially far-reaching implications, the underlying neural mechanisms are not fully understood. Specifically, the temporal processing dimension of the mechanisms involved in emotional memory formation under stress remains elusive. Here, we used magnetoencephalography (MEG) to examine the neural processes underlying stress effects on emotional memory formation with high temporal and spatial resolution and a particular focus on theta oscillations previously implicated in mnemonic binding. Healthy participants (n = 53) underwent a stress or control procedure before encoding emotionally neutral and negative pictures, while MEG was recorded. Memory for the pictures was probed in a recognition test 24 h after encoding. In this recognition test, stress did not modulate the emotional memory enhancement but led to significantly higher confidence in memory for negative compared to neutral stimuli. Our neural data revealed that stress increased memory-related theta oscillations specifically in medial temporal and occipito-parietal regions. Further, this stress-related increase in theta power emerged during memory formation for emotionally negative but not for neutral stimuli. These findings indicate that acute stress can enhance, in the medial temporal lobe, oscillations at a frequency that is ideally suited to bind the elements of an ongoing emotional episode, which may represent a mechanism to facilitate the storage of emotionally salient events that occurred in the context of a stressful encounter.

The temperature sensitivity of memory formation and persistence is altered by cold acclimation in a pond snail

There are reports on the inability of inbred, laboratory-reared Lymnaea stagnalis to perform feeding and aerial respiration in the cold. It has also been suggested that laboratory-bred snails have an inability to perform aerial respiration in winter months in the laboratory. Here, we used an inbred, laboratory-reared strain of Lymnaea (the S-strain) to demonstrate that the snails are capable of performing those behaviours in a cold (4°C) environment after a 2 day acclimation period. In addition, the inbred snails were able to perform aerial respiration during winter months at room temperature (20°C) in the laboratory. The persistence of long-term memory (LTM) was extended for at least 4 weeks by placing S-strain snails into a 4°C environment following training. Typically, the cold block (CB) procedure (1 h at 4°C) immediately after a training session blocks LTM formation in the S-strain but not in a freshly collected strain. Four weeks at 4°C transformed the S-strain phenotype into one resisting the CB procedure. Thus, with a 4 week cold spell snails gain a resistance to the CB procedure, and that would explain why freshly collected snails are resistant to the procedure. However, we found that F1 progeny of a freshly collected strain reared in the laboratory were resistant to the CB procedure. This suggests that an unknown selection resulted in the S-strain being susceptible to the CB procedure.

Alpha/beta power decreases during episodic memory formation predict the magnitude of alpha/beta power decreases during subsequent retrieval

Episodic memory retrieval is characterised by the vivid reinstatement of information about a personally-experienced event. Growing evidence suggests that this reinstatement is supported by reductions in the spectral power of alpha/beta activity. Given that the amount of information that can be recalled depends on the amount of information that was originally encoded, information-based accounts of alpha/beta activity would suggest that retrieval-related alpha/beta power decreases similarly depend upon decreases in alpha/beta power during encoding. To test this hypothesis, seventeen human participants completed a sequence-learning task while undergoing concurrent MEG recordings. Regression-based analyses were then used to estimate how alpha/beta power decreases during encoding predicted alpha/beta power decreases during retrieval on a trial-by-trial basis. When subjecting these parameter estimates to group-level analysis, we find evidence to suggest that retrieval-related alpha/beta (7-15Hz) power decreases fluctuate as a function of encoding-related alpha/beta power decreases. These results suggest that retrieval-related alpha/beta power decreases are contingent on the decrease in alpha/beta power that arose during encoding. Subsequent analysis uncovered no evidence to suggest that these alpha/beta power decreases reflect stimulus identity, indicating that the contingency between encoding- and retrieval-related alpha/beta power reflects the reinstatement of a neurophysiological operation, rather than neural representation, during episodic memory retrieval.

An fMRI investigation of neural activation predicting memory formation in children with fetal alcohol spectrum disorders

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Expected brain regions activated during memory formation in FASD and Control children.

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Prenatal alcohol predicted differences in brain activations during memory formation.

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More extensive neural activation during successful memory formation in FAS/PFAS group.

Prenatal alcohol exposure (PAE) is associated with physical anomalies, growth restriction, and a range of neurobehavioral deficits. Although declarative memory impairment has been documented extensively in individuals with fetal alcohol spectrum disorders (FASD), this cognitive process has been examined in only one functional magnetic resonance imaging (fMRI) study, and mechanisms underlying this impairment are not well understood. We used an event-related fMRI design to examine neural activations during visual scene encoding that predict subsequent scene memory in 51 right-handed children (age range = 10–14 years, M = 11.3, SD = 1.3) whose mothers had been recruited and interviewed prospectively about their alcohol use during pregnancy. Following examination by expert dysmorphologists, children were assigned to one of three FASD diagnostic groups: FAS/PFAS (n_FAS = 7; n_PFAS = 4), nonsyndromal heavily exposed (HE; n = 14), and Controls (n = 26). Subsequent memory was assessed in a post-scan recognition test, and subsequent memory activations were examined by contrasting activations during encoding of scenes that were subsequently remembered (hits) to those for incorrectly judged as ‘new’ (misses). Recognition accuracy did not differ between groups. Pooled across groups, we observed extensive bilateral subsequent memory effects in regions including the hippocampal formation, posterior parietal cortex, and occipital cortex—a pattern consistent with previous similar studies of typically developing children. Critically, in the group of children with FAS or PFAS, we observed activations in several additional regions compared to HE and Control groups. Given the absence of between-group differences in recognition accuracy, these data suggest that in achieving similar memory compared to children in the HE and Control groups, children with FAS and PFAS recruit more extensive neural resources to achieve successful memory formation.

Morphine improved stress-induced amnesia and anxiety through interacting with the ventral hippocampal endocannabinoid system in rats

The present study aimed to investigate the possible role of the ventral hippocampal (VH) cannabinoid CB1 receptors in the improving effect of morphine on stress-induced memory formation impairment and anxiety. A step-through type passive avoidance task and a hole-board test were used to measure memory formation and anxiety-like exploratory behavior, respectively. The results showed that the exposure to 10-min stress immediately after the successful training phase impaired memory formation and also produced anxiogenic-like exploratory behaviour in adult male Wistar rats. Moreover, morphine administration before stress exposure improved the adverse effects of stress on memory formation and exploratory behaviour. After training, intra-VH microinjection of cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.01-0.05 μg/rat) enhanced the response of an ineffective dose of morphine (0.5 mg/kg for memory; 5 mg/kg for anxiety, i.p.) on memory impairment and anxiogenic-like exploratory behaviour induced by acute stress. Intra-VH microinjection of the higher dose of WIN 55,212-2 alone impaired memory formation. Post-training microinjection of a cannabinoid CB1 receptor antagonist/inverse agonist, AM-251 (100-150 ng/rat) into the VH attenuated the response of an effective dose of morphine (5 mg/kg for memory; 6 mg/kg for anxiety, i.p.) in stress-exposed rats. Taken together, the present results showed that morphine administration could improve stress-induced memory impairment and anxiety in the rats exposed to the inescapable acute stress. Interestingly, the improving effect of morphine on the adverse effect of stress on memory formation and anxiety-like exploratory behaviour may be mediated through the VH endocannabinoid CB1/CB2 receptors mechanism.

Memory-related process in physiological status and alzheimer's disease

Rejecting central dogma around static status of adult mammalian brain, CNS has the nascent neurons generated in subgranular zone of dentate gyrus in hippocampus which develop to novel glutamatergic granule cells, with the innate feature of transmuting to memory disks. Structural plasticity proceeds with synaptic plasticity to process all the developing stages required to successful maturation and functional integration, whereby the memory context is ready to leave the hippocampus toward cortex network through consolidation process, for being installed and run the memory disk forever. However, in Alzheimer's disease, brain deal with subtle deadly progressive loss of synapsis, neuronal dysfunction and ultimately network failure, resulting in memory decay and cognitive decline-concluding that AD destroys memory formation related-pathways.

Olfactory associative behavioral differences in three honey bee Apis mellifera L. races under the arid zone ecosystem of central Saudi Arabia

Apis mellifera jemenitica is the indigenous race of honey bees in the Arabian Peninsula and is tolerant to local drought conditions. Experiments were undertaken to determine the differences in associative learning and memory of honey bee workers living in the arid zone of Saudi Arabia, utilizing the proboscis extension response (PER). These experiments were conducted on the indigenous race (A. m. jemenitica) along with two introduced European races (A. m. carnica and A. m. ligustica). The data revealed that A. m. jemenitica is amenable to PER conditioning and may be used in conditioning experiments within the olfactory behavioral paradigm. The results also demonstrated that the three races learn and retain information with different capacities relative to each other during the experimental time periods. Native Arabian bees (A. m. jemenitica) exhibited significantly lower PER percentage during second and third conditioning trials when compared to exotic races. Apis mellifera jemenitica also exhibited reduced memory retention at 2 h and 24 h when compared to A. m. carnica and A. m. ligustica. Therefore, the native Arabian bees were relatively slow learners with reduced memory retention compared to the other two races that showed similar learning and memory retention. Three or five conditioning trials and monthly weather conditions (October and December) had no significant effects on learning and memory in A. m. jemenitica. These results emphasized a novel line of research to explore the mechanism and differences in associative learning as well as other forms of learning throughout the year among bee races in the harsh arid conditions of Saudi Arabia. This is the first study in Saudi Arabia to demonstrate inter-race differences regarding olfactory associative learning between native Arabian bees and two introduced European honey bee races.

NKG2D signaling certifies effector CD8 T cells for memory formation

BACKGROUND

The development of memory responses is an evolutionary function of the adaptive immune system. We propose that for the immune system to populate the memory compartment with the best-suited CD8 T cells it utilizes a process of certification or molecular accreditation mediated through Natural Killer Group 2D (NKG2D). This process of certification assures that the memory compartment is filled with CD8 T cells that have demonstrated their ability to kill their cognate targets through a two-step process that utilizes T cell receptor (TCR) and NKG2D signaling.

METHODS

One week after immunization with peptide-pulsed dendritic cells, NKG2D signaling was transiently blocked in vivo with a single injection of neutralizing antibodies. Under such conditions, we determined the importance of NKG2D signaling during the effector phase for memory formation without compromising NKG2D signaling at the memory phase. Both open (polyclonal) and closed (monoclonal) CD8 T cell repertoires were studied.

RESULTS

We show that signaling through NKG2D mediated this certification. Temporary blockade of NKG2D signaling during the effector phase resulted in the formation of highly defective memory CD8 T cells characterized by altered expression of the ribosomal protein S6 and epigenetic modifiers, suggesting modifications in the T cell translational machinery and epigenetic programming. Finally, these uncertified memory cells were not protective against a B16 tumor challenge.

CONCLUSION

Signaling through NKG2D during the effector phase (certification) favors the development of functional memory CD8 T cells, a previously undescribed role for NKG2D. Temporary blockade of NKG2D signaling during the effector phase results in the formation of highly defective memory CD8 T cells potentially by affecting the expression of the ribosomal protein S6 and epigenetic modifiers, suggesting alterations in T cell translational machinery and epigenetic programming.

Epigenetic regulation of Fgf1 transcription by CRTC1 and memory enhancement

Recent evidence demonstrates that epigenetic regulation of gene transcription is critically involved in learning and memory. Here, we discuss the role of histone acetylation and DNA methylation, which are two best understood epigenetic processes in memory processes. More specifically, we focus on learning-strength-dependent changes in chromatin on the fibroblast growth factor 1 (Fgf1) gene and on the molecular events that modulate regulation of Fgf1 transcription, required for memory enhancement, with the specific focus on CREB-regulated transcription coactivator 1 (CRTC1).

Contribution of stress and sex hormones to memory encoding

Distinct stages of the menstrual cycle and the intake of oral contraceptives (OC) affect sex hormone levels, stress responses, and memory processes critically involved in the pathogenesis of mental disorders. To characterize the interaction of sex and stress hormones on memory encoding, 30 men, 30 women in the early follicular phase of the menstrual cycle (FO), 30 women in the luteal phase (LU), and 30 OC women were exposed to either a stress (socially evaluated cold-pressor test) or a control condition prior to memory encoding and immediate recall of neutral, positive, and negative words. On the next day, delayed free and cued recall was tested. Sex hormone levels verified distinct estradiol, progesterone, and testosterone levels between groups. Stress increased blood pressure, cortisol concentrations, and ratings of stress appraisal in all four groups as well as cued recall performance of negative words in men. Stress exposure in OC women led to a blunted cortisol response and rather enhanced cued recall of neutral words. Thus, pre-encoding stress facilitated emotional cued recall performance in men only, but not women with different sex hormone statuses pointing to the pivotal role of circulating sex hormones in modulation of learning and memory processes.

L1 retrotransposition alters the hippocampal genomic landscape enabling memory formation

Somatic LINE-1 (L1) retrotransposition is a source of genomic mosaicism and potential phenotypic diversity among neurons during brain development. In the adult brain, L1 expression can be triggered by different environmental alterations, but its functional role in this context remains unknown. Here we demonstrate a neural activation-dependent increase in the number of L1 retrotransposon insertions in the hippocampus. Using both pharmacologic and genetic approaches in mice, we demonstrate that L1 expression in the adult hippocampus enables long-term memory formation. These results provide experimental evidence that L1 retrotransposition-induced genomic mosaicism is involved in cognitive processes such as memory formation.

Dynamic changes in prefrontal cortex involvement during verbal episodic memory formation

During encoding, the neural activity immediately before or during an event can predict whether that event will be later remembered. The contribution of brain activity immediately after an event to memory formation is however less known. Here, we used repetitive Transcranial Magnetic Stimulation (rTMS) to investigate the temporal dynamics of episodic memory encoding with a focus on post-stimulus time intervals. At encoding, rTMS was applied during the online processing of the word, at its offset, or 100, 200, 300 or 400ms thereafter. rTMS was delivered to the left ventrolateral (VLPFC) or dorsolateral prefrontal cortex (DLPFC). VLPFC rTMS during the first few hundreds of milliseconds after word offset disrupted subsequent recognition accuracy. We did not observe effects of DLPFC rTMS at any time point. These results suggest that encoding-related VLPFC engagement starts at a relatively late processing stage, and may reflect brain processes related to the offset of the stimulus.

Stress in the zoo: Tracking the impact of stress on memory formation over time

Although stress is well known to modulate human memory, precisely how memory formation is altered by a stressful encounter remains unclear. Stress effects on cognition are mainly mediated by the rapidly acting sympathetic nervous system, resulting in the release of catecholamines, and the slower acting hypothalamus-pituitary-adrenal axis secreting cortisol, which induces its effects on cognition through fast, non-genomic actions and delayed, genomic actions. Importantly, these different waves of the physiological stress response are thought to dynamically alter neural processing in brain regions important for memory such as the amygdala and the hippocampus. However, the precise time course of stress effects on memory formation is still unclear. To track the development of stress effects on memory over time, we tested individuals who underwent a stressful experience or a control procedure before a 2-h walk through a zoo, while an automatic camera continuously photographed the events they encoded. In a recognition memory test one week later, participants were presented with target photographs of their own zoo tour and lure photographs from an alternate tour. Stressed participants showed better memory for the experimental treatment than control participants, and this memory enhancement for the stressful encounter itself was directly linked to the sympathetic stress response. Moreover, stress enhanced memory for events encoded 41-65min after stressor onset, which was associated with the cortisol stress response, most likely arising from non-genomic cortisol actions. However, memory for events encoded long after the stressor, when genomic cortisol actions had most likely developed, remained unchanged. Our findings provide novel insights into how stress effects on memory formation develop over time, depending on the activity of major physiological stress response systems.

Prediction of successful memory encoding based on single-trial rhinal and hippocampal phase information

Mediotemporal EEG characteristics are closely related to long-term memory formation. It has been reported that rhinal and hippocampal EEG measures reflecting the stability of phases across trials are better suited to distinguish subsequently remembered from forgotten trials than event-related potentials or amplitude-based measures. Theoretical models suggest that the phase of EEG oscillations reflects neural excitability and influences cellular plasticity. However, while previous studies have shown that the stability of phase values across trials is indeed a relevant predictor of subsequent memory performance, the effect of absolute single-trial phase values has been little explored. Here, we reanalyzed intracranial EEG recordings from the mediotemporal lobe of 27 epilepsy patients performing a continuous word recognition paradigm. Two-class classification using a support vector machine was performed to predict subsequently remembered vs. forgotten trials based on individually selected frequencies and time points. We demonstrate that it is possible to successfully predict single-trial memory formation in the majority of patients (23 out of 27) based on only three single-trial phase values given by a rhinal phase, a hippocampal phase, and a rhinal-hippocampal phase difference. Overall classification accuracy across all subjects was 69.2% choosing frequencies from the range between 0.5 and 50Hz and time points from the interval between -0.5s and 2s. For 19 patients, above chance prediction of subsequent memory was possible even when choosing only time points from the prestimulus interval (overall accuracy: 65.2%). Furthermore, prediction accuracies based on single-trial phase surpassed those based on single-trial power. Our results confirm the functional relevance of mediotemporal EEG phase for long-term memory operations and suggest that phase information may be utilized for memory enhancement applications based on deep brain stimulation.

Dorsal hippocampal NMDA receptors mediate the interactive effects of arachidonylcyclopropylamide and MDMA/ecstasy on memory retrieval in rats

A combination of cannabis and ecstasy may change the cognitive functions more than either drug alone. The present study was designed to investigate the possible involvement of dorsal hippocampal NMDA receptors in the interactive effects of arachidonylcyclopropylamide (ACPA) and ecstasy/MDMA on memory retrieval. Adult male Wistar rats were cannulated into the CA1 regions of the dorsal hippocampus (intra-CA1) and memory retrieval was examined using the step-through type of passive avoidance task. Intra-CA1 microinjection of a selective CB1 receptor agonist, ACPA (0.5-4ng/rat) immediately before the testing phase (pre-test), but not after the training phase (post-training), impaired memory retrieval. In addition, pre-test intra-CA1 microinjection of MDMA (0.5-1μg/rat) dose-dependently decreased step-through latency, indicating an amnesic effect of the drug by itself. Interestingly, pre-test microinjection of a higher dose of MDMA into the CA1 regions significantly improved ACPA-induced memory impairment. Moreover, pre-test intra-CA1 microinjection of a selective NMDA receptor antagonist, D-AP5 (1 and 2μg/rat) inhibited the reversal effect of MDMA on the impairment of memory retrieval induced by ACPA. Pre-test intra-CA1 microinjection of the same doses of D-AP5 had no effect on memory retrieval alone. These findings suggest that ACPA or MDMA consumption can induce memory retrieval impairment, while their co-administration improves this amnesic effect through interacting with hippocampal glutamatergic-NMDA receptor mechanism. Thus, it seems that the tendency to abuse cannabis with ecstasy may be for avoiding cognitive dysfunction.

Hippocampal signaling pathways are involved in stress-induced impairment of memory formation in rats

Stress is a potent modulator of hippocampal-dependent memory formation. The aim of the present study was to assess the role of hippocampal signaling pathways in stress-induced memory impairment in male Wistar rats. The animals were exposed to acute elevated platform (EP) stress and memory formation was measured by a step-through type passive avoidance task. The results indicated that post-training or pre-test exposure to EP stress impaired memory consolidation or retrieval respectively. Using western blot analysis, it was found that memory retrieval was associated with the increase in the levels of phosphorylated cAMP-responsive element binding protein (P-CREB), peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and its downstream targets in the hippocampus. In contrast, the stress exposure decreased the hippocampal levels of these proteins. In addition, stress-induced impairment of memory consolidation or retrieval was associated with the decrease in the P-CREB/CREB ratio and the PGC-1α level in the hippocampus. On the other hand, the hippocampal level of nuclear factor E2-related factor 2 (Nrf2) and gamma-glutamylcysteine synthetase (γ-GCS) which are the master regulators of defense system were decreased by the stress exposure. The increased hippocampal levels of Nrf2 and it׳s downstream was observed during memory retrieval, while stress-induced impairment of memory consolidation or retrieval inhibited this hippocampal signaling pathway. Overall, these findings suggest that down-regulation of CREB/PGC-1α signaling cascade and Nrf2 antioxidant pathways in the hippocampus may be associated with memory impairment induced by stress.

Anticipation of electric shocks modulates low beta power and event-related fields during memory encoding

In humans, the temporal and oscillatory dynamics of pain anticipation and its effects on long-term memory are largely unknown. Here, we investigated this open question by using a previously established behavioral paradigm in combination with magnetoencephalography (MEG). Healthy human subjects encoded a series of scene images, which was combined with cues predicting an aversive electric shock with different probabilities (0.2, 0.5 or 0.8). After encoding, memory for the studied images was tested using a remember/know recognition task. Behaviorally, pain anticipation did not modulate recollection-based recognition memory per se, but interacted with the perceived unpleasantness of the electric shock [visual analogue scale rating from 1 (not unpleasant) to 10 (highly unpleasant)]. More precisely, the relationship between pain anticipation and recollection followed an inverted u-shaped function the more unpleasant the shocks were rated by a subject. At the physiological level, this quadratic effect was mimicked in the event-related magnetic fields associated with successful memory formation ('DM-effect') ∼450ms after image onset at left frontal sensors. Importantly, across all subjects, shock anticipation modulated oscillatory power in the low beta frequency range (13-20Hz) in a linear fashion at left temporal sensors. Taken together, our findings indicate that beta oscillations provide a generic mechanism underlying pain anticipation; the effect on subsequent long-term memory, on the other hand, is much more variable and depends on the level of individual pain perception. As such, our findings give new and important insights into how aversive motivational states can drive memory formation.

Systems biology of synaptic plasticity: a review on N-methyl-D-aspartate receptor mediated biochemical pathways and related mathematical models

Synaptic plasticity, an emergent property of synaptic networks, has shown strong correlation to one of the essential functions of the brain, memory formation. Through understanding synaptic plasticity, we hope to discover the modulators and mechanisms that trigger memory formation. In this paper, we first review the well understood modulators and mechanisms underlying N-methyl-D-aspartate receptor dependent synaptic plasticity, a major form of synaptic plasticity in hippocampus, and then comment on the key mathematical modelling approaches available in the literature to understand synaptic plasticity as the integration of the established functionalities of synaptic components.

The role of epigenetic regulation in learning and memory

The formation of long-term memory involves a series of molecular and cellular changes, including gene transcription, protein synthesis and synaptic plasticity dynamics. Some of these changes arise during learning and are subsequently retained throughout life. 'Epigenetic' regulation, which involves DNA methylation and histone modifications, plays a critical role in retaining long-term changes in post-mitotic cells. Accumulating evidence suggests that the epigenetic machinery might regulate the formation and stabilization of long-term memory in two ways: a 'gating' role of the chromatin state to regulate activity-triggered gene expression; and a 'stabilizing' role of the chromatin state to maintain molecular and cellular changes induced by the memory-related event. The neuronal activation regulates the dynamics of the chromatin status under precise timing, with subsequent alterations in the gene expression profile. This review summarizes the existing literature, focusing on the involvement of epigenetic regulation in learning and memory. We propose that the identification of different epigenetic regulators and signaling pathways involved in memory-related epigenetic regulations will provide mechanistic insights into the formation of long-term memory.

Neuroepigenetics of memory formation and impairment: the role of microRNAs

MicroRNAs (miRNAs) are a class of short non-coding RNAs that primarily regulate protein synthesis through reversible translational repression or mRNA degradation. MiRNAs can act by translational control of transcription factors or via direct action on the chromatin, and thereby contribute to the non-genetic control of gene-environment interactions. MiRNAs that regulate components of pathways required for learning and memory further modulate the influence of epigenetics on cognition in the normal and diseased brain. This review summarizes recent data exemplifying the known roles of miRNAs in memory formation in different model organisms, and describes how neuronal plasticity regulates miRNA biogenesis, activity and degradation. It also examines the relevance of miRNAs for memory impairment in human, using recent clinical observations related to neurodevelopmental and neurodegenerative diseases, and discusses the potential mechanisms by which these miRNAs may contribute to memory disorders.

Nuclear gating of a Drosophila dCREB2 activator is involved in memory formation

The transcription factor CREB is an important regulator of many adaptive processes in neurons, including sleep, cellular homeostasis, and memory formation. The Drosophila dCREB2 family includes multiple protein isoforms generated from a single gene. Overexpression of an activator or blocker isoform has been shown to enhance or block memory formation, but the molecular mechanisms underlying these phenomena remain unclear. In the present study, we generate isoform-specific antibodies and new transgenic flies to track and manipulate the activity of different dCREB2 isoforms during memory formation. We find that nuclear accumulation of a dCREB2 activator-related species, p35+, is dynamically regulated during memory formation. Furthermore, various dCREB2 genetic manipulations that enhance or block memory formation correspondingly increase or decrease p35+ levels in the nucleus. Finally, we show that overexpression of S6K can enhance memory formation and increase p35+ nuclear abundance. Taken together, these results suggest that regulation of dCREB2 localization may be a key molecular convergence point in the coordinated host of events that lead to memory formation.