Memory consolidation and reconsolidation of an inhibitory avoidance response in mice: effects of i.c.v. injections of hemicholinium-3

Saffron (Crocus sativus L.) extract attenuates chronic scopolamine-induced cognitive impairment, amyloid beta, and neurofibrillary tangles accumulation in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Crocus sativus L. known as saffron, is a popular food condiment with a high aroma, deep colour, and long and thick threads (stigmas) cultivated in Iran, Morocco, Spain, Italy, China, Japan, France, Turkey, and India. In 'Ayurveda', saffron is acknowledged for its immunostimulant, aphrodisiac, cardiotonic, liver tonic, nervine tonic, carminative, diaphoretic, diuretic, emmenagogue, galactagogue, febrifuge, sedative, relaxant, and anxiolytic activities. The renowned Persian physician and philosopher, Avicenna, delineated saffron as an antidepressant, hypnotic, anti-inflammatory, hepatoprotective, bronchodilator, and aphrodisiac in his book, the Canon of Medicine. Within traditional Iranian Medicine (TIM), saffron is characterized as a mood elevator and a rejuvenator for the body and senses. Further, the ethnopharmacological evidence indicates that saffron has shown an effect against neurodegenerative disorders namely, dementia, Alzheimer's, and Parkinson's with its bioactive constituents i.e., carotenoids and apocarotenoids.

AIM

The present study aimed to investigate the potential of standardized (Kashmir Saffron, India) Crocus sativus extract (CSE) in chronic scopolamine-induced cognitive impairment, amyloid beta (Aβ) plaque, and neurofibrillary tangles (NFT) accumulation in rat brains by targeting AChE inhibition and scopolamine mechanistic effect.

METHODS

The experimental animals were divided into six groups: group 1: normal control, group 2: scopolamine, group 3,4 and 5 rivastigmine tartrate, CSE (p.o. 10 mg/kg, 15 mg/kg, and 20 mg/kg) respectively. Each treatment group received scopolamine after 20 min of dosing, till 4 weeks. The effects of different treatments on learning, acquisition, and reversal memory were performed using a Morris water maze test. In addition to behavioral assessments, biochemical parameters such as AChE, IL-6, and antioxidants were measured in isolated brains. Histological observations were also conducted to assess the presence of Aβ plaques and NFT. Furthermore, molecular docking was performed to explore the potential AChE inhibitory activity of the bioactive constituents of standardized CSE.

RESULTS

Scopolamine produces memory impairment, and its chronic administration forms Aβ plaque and NFT in rat brains. Supplementation with CSE in presence of scopolamine has shown remarkable effects on behavioural activity, special acquisition, and reversal memory. The CSE has also shown promising effects on AChE inhibition and antioxidant activity. The results of the docking study also indicate that trans-crocetin, i.e., a biologically active metabolite of Crocins, has strong AChE inhibitory activity, supported by an in vivo animal experiment.

CONCLUSION

Supplementation with CSE significantly attenuates the formation of Aβ plaque and NFT in the hippocampus at a dose of 20 mg/kg per day. In addition, CSE also counters scopolamine-induced neuroinflammation.

Biomarkers in Alzheimer's Disease: Are Olfactory Neuronal Precursors Useful for Antemortem Biomarker Research?

Alzheimer's disease (AD), as the main cause of dementia, affects millions of people around the world, whose diagnosis is based mainly on clinical criteria. Unfortunately, the diagnosis is obtained very late, when the neurodegenerative damage is significant for most patients. Therefore, the exhaustive study of biomarkers is indispensable for diagnostic, prognostic, and even follow-up support. AD is a multifactorial disease, and knowing its underlying pathological mechanisms is crucial to propose new and valuable biomarkers. In this review, we summarize some of the main biomarkers described in AD, which have been evaluated mainly by imaging studies in cerebrospinal fluid and blood samples. Furthermore, we describe and propose neuronal precursors derived from the olfactory neuroepithelium as a potential resource to evaluate some of the widely known biomarkers of AD and to gear toward searching for new biomarkers. These neuronal lineage cells, which can be obtained directly from patients through a non-invasive and outpatient procedure, display several characteristics that validate them as a surrogate model to study the central nervous system, allowing the analysis of AD pathophysiological processes. Moreover, the ease of obtaining and harvesting endows them as an accessible and powerful resource to evaluate biomarkers in clinical practice.

Temporal prediction error triggers amygdala-dependent memory updating in appetitive operant conditioning in rats

Reinforcement learning theories postulate that prediction error, i.e., a discrepancy between the actual and expected outcomes, drives reconsolidation and new learning, inducing an updating of the initial memory. Pavlovian studies have shown that prediction error detection is a fundamental mechanism in triggering amygdala-dependent memory updating, where the temporal relationship between stimuli plays a critical role. However, in contrast to the well-established findings in aversive situations (e.g., fear conditioning), only few studies exist on prediction error in appetitive operant conditioning, and even less with regard to the role of temporal parameters. To explore if temporal prediction error in an appetitive operant paradigm could generate an updating and consequent reconsolidation and/or new learning of temporal association, we ran four experiments in adult male rats. Experiment 1 verified whether an unexpected delay in the time of reward's availability (i.e., a negative temporal prediction error) in a single session produces an updating in long-term memory of temporal expectancy in an appetitive operant conditioning. Experiment 2 showed that negative prediction errors, either due to the temporal change or through reward omission, increased in the basolateral amygdala nucleus (BLA) the activation of a protein that is critical for memory formation. Experiment 3 revealed that the presence of a protein synthesis inhibitor (anisomycin) in the BLA during the session when the reward was delayed (Error session) affected the temporal updating. Finally, Experiment 4 showed that anisomycin, when infused immediately after the Error session, interfered with the long-term memory of the temporal updating. Together, our study demonstrated an involvement of BLA after a change in temporal and reward contingencies, and in the resulting updating in long-term memory in appetitive operant conditioning.

Dissociating the involvement of muscarinic and nicotinic cholinergic receptors in object memory destabilization and reconsolidation

The content of long-term memory is neither fixed nor permanent. Reminder cues can destabilize consolidated memories, rendering them amenable to change before being reconsolidated. However, not all memories destabilize following reactivation. Characteristics of a memory, such as its age or strength, impose boundaries on destabilization. Previously, we demonstrated that presentation of salient novel information at the time of reactivation can readily destabilize resistant object memories in rats and this form of novelty-induced destabilization is dependent upon acetylcholine (ACh) activity at muscarinic receptors (mAChRs). In the present study, we sought to determine if this same mechanism for initiating destabilization of resistant object memories is present in mice and further expand our understanding of the mechanisms through which ACh modulates object memory destabilization by investigating the role of nicotinic receptors (nAChRs). We provide evidence that in mice mAChRs are necessary for destabilizing object memories that are readily destabilized and those that are resistant to destabilization. Conversely, nAChRs were found to be necessary only when memories are readily destabilized. We then investigated the role of both receptors in the reconsolidation of destabilized object memory traces and determined that nAChRs, but not mAChRs, are necessary for object memory reconsolidation. Together, these results suggest that nAChRs may play a more selective role in the re-storage of object memories following destabilization and that ACh acts through mAChRs to act as an override signal to initiate destabilization of resistant object memories following reactivation with novelty. These findings expand our current understanding of the role of ACh in the dynamic storage of long-term memory.

Oxytocin-Cholinergic Central Interaction: Implications for Non-Social Memory Formation

Oxytocin (OT) and vasopressin (AVP) are two closely related neuropeptides implicated in learning and memory processes, anxiety, nociception, addiction, feeding behavior and social information processing. Regarding learning and memory, OT has induced long-lasting impairment in different behaviors, while the opposite was observed with AVP. We have previously evaluated the effect of peripheral administration of OT or its antagonist (AOT) on the inhibitory avoidance response of mice and on the modulation of cholinergic mechanisms. Here, we replicate and validate those results, but this time through central administration of neuropeptides, considering their poor passage through the blood-brain barrier (BBB). When we delivered OT (0.10 ng/mouse) and its antagonist (0.10 ng/mouse) through intracerebroventricular (ICV) injections, the neuropeptide impaired and AOT enhanced the behavioral performance on an inhibitory avoidance response evaluated 48 h after training in a dose-dependent manner. On top of that, we investigated a possible central interaction between OT and the cholinergic system. Administration of anticholinesterases inhibitors with access to the central nervous system (CNS), the activation of muscarinic acetylcholine (Ach) receptors and the increase of evoked ACh release using linopirdine (Lino) (3-10 µg/kg, IP), reversed the impairment of retention performance induced by OT. Besides, either muscarinic or nicotinic antagonists with unrestricted access to the CNS reduced the magnitude of the performance-facilitating effect of AOT's central infusion. We suggest that OT might induce a cholinergic hypofunction state, resulting in an impairment of IA memory formation, a process for which the cholinergic system is crucially necessary.

Role of prediction error and the cholinergic system on memory reconsolidation processes in mice

The ability to make predictions based on stored information is a general coding strategy. A prediction error (PE) is a mismatch between expected and current events. Our memories, like ourselves, are subject to change. Thus, an acquired memory can become active and update its content or strength by a labilization-reconsolidation process. Within the reconsolidation framework, PE drives the updating of consolidated memories. In the past our lab has made key progresses showing that a blockade in the central cholinergic system during reconsolidation can cause memory impairment, while reinforcement of cholinergic activity enhances it. In the present work we determined that PE is a necessary condition for memory to reconsolidate in an inhibitory avoidance task using both male and female mice. Depending on the intensity of the unconditioned stimulus (US) used during training, a negative (higher US intensity) or positive (lower US intensity/no US) PE on a retrieval session modified the behavioral response on a subsequent testing session. Furthermore, we demonstrated that the cholinergic system modulates memory reconsolidation only when PE is detected. In this scenario administration of oxotremorine, scopolamine or nicotine after memory reactivation either enhanced or impaired memory reconsolidation in a sex-specific manner.

Critical role of hippocampal muscarinic acetylcholine receptors on memory reconsolidation in mice

Over the years, experimental and clinical evidence has given support to the idea that acetylcholine (Ach) plays an essential role in mnemonic phenomena. On the other hand, the Hippocampus is already known to have a key role in learning and memory. What is yet unclear is how the Ach receptors may contribute to this brain region role during memory retrieval. The Ach receptors are divided into two broad subtypes: the ionotropic nicotinic acetylcholine receptors and the metabotropic muscarinic acetylcholine receptors. Back in 2010, we demonstrated for the first time the critical role of hippocampal α7 nicotinic acetylcholine receptors in memory reconsolidation process of an inhibitory avoidance response in mice. In the present work, we further investigate the possible implication of hippocampal muscarinic Ach receptors (mAchRs) in this process using a pharmacological approach. By specifically administrating agonists and antagonists of the different mAchRs subtypes in the hippocampus, we found that M1 and M2 but not M3 subtype may be involved in memory reconsolidation processes in mice.

Investigating the efficacy of the reminder-extinction procedure to disrupt contextual threat memories in humans using immersive Virtual Reality

Upon reactivation, consolidated memories can enter a temporary labile state and require restabilisation, known as reconsolidation. Interventions during this reconsolidation period can disrupt the reactivated memory. However, it is unclear whether different kinds of memory that depend on distinct brain regions all undergo reconsolidation. Evidence for reconsolidation originates from studies assessing amygdala-dependent memories using cue-conditioning paradigms in rodents, which were subsequently replicated in humans. Whilst studies providing evidence for reconsolidation of hippocampus-dependent memories in rodents have predominantly used context conditioning paradigms, studies in humans have used completely different paradigms such as tests for wordlists or stories. Here our objective was to bridge this paradigm gap between rodent and human studies probing reconsolidation of hippocampus-dependent memories. We modified a recently developed immersive Virtual Reality paradigm to test in humans whether contextual threat-conditioned memories can be disrupted by a reminder-extinction procedure that putatively targets reconsolidation. In contrast to our hypothesis, we found comparable recovery of contextual conditioned threat responses, and comparable retention of subjective measures of threat memory, episodic memory and exploration behaviour between the reminder-extinction and standard extinction groups. Our result provide no evidence that a reminder before extinction can prevent the return of context conditioned threat memories in humans.

Relevance of ERK1/2 Post-retrieval Participation on Memory Processes: Insights in Their Particular Role on Reconsolidation and Persistence of Memories

Back in 1968, Misanin and his group posited that reactivation of consolidated memories could support changes in that trace, similar to what might happen during the consolidation process. Not until 2000, when Nader et al. (2000) studied the behavioral effect of a protein synthesis inhibitor on retrieved memories, could this previous statement be taken under consideration once again; suggesting that consolidated memories can become labile after reactivation. The process of strengthening after memory labilization was named memory reconsolidation. In recent years, many studies pointed towards a critical participation of the extracellular signal-regulated kinase (ERK)/mitogen activated protein kinases (MAPKs) pathway in different memory processes (e.g., consolidation, extinction, reconsolidation, among others). In this review article, we will focus on how this system might be modulating the processes triggered after retrieval of well-consolidated memories in mice.

Dopaminergic D1 receptor signalling is necessary, but not sufficient for cued fear memory destabilisation

RATIONALE

Pharmacological targeting of memory reconsolidation is a promising therapeutic strategy for the treatment of fear memory-related disorders. However, the success of reconsolidation-based approaches depends upon the effective destabilisation of the fear memory by memory reactivation.

OBJECTIVES

Here, we aimed to determine the functional involvement of dopamine D1 receptors in cued fear memory destabilisation, using systemic drug administration.

RESULTS

We observed that direct D1 receptor agonism was not sufficient to stimulate tone fear memory destabilisation to facilitate reconsolidation disruption by the glucocorticoid receptor antagonist mifepristone. Instead, administration of the nootropic nefiracetam did facilitate mifepristone-induced amnesia, in a manner that was dependent upon dopamine D1 receptor activation. Finally, while the combined treatment with nefiracetam and mifepristone did not confer fear-reducing effects under conditions of extinction learning, there was some evidence that mifepristone reduces fear expression irrespective of memory reactivation parameters.

CONCLUSIONS

The use of combination pharmacological treatment to stimulate memory destabilisation and impair reconsolidation has potential therapeutic benefits, without risking a maladaptive increase of fear.

Optogenetic Destabilization of the Memory Trace in CA1: Insights into Reconsolidation and Retrieval Processes

Reactivation of memory can cause instability necessitating the reconsolidation of the trace. This process can be blocked by amnestic treatments administered after memory reactivation resulting in subsequent memory deficits. While the basolateral amygdala (BLA) is known to be crucial for reconsolidation, evidence for a contribution of the hippocampal CA1 region has only started to accumulate. Moreover, the effect of a reconsolidation blockade in CA1 has only been evaluated behaviorally, and it is unknown whether this manipulation has a long-term effect on neuronal activity. We combined optogenetic and high-resolution molecular imaging techniques to inhibit cell firing in CA1 following the reactivation of a fear memory in mice, evaluated memory performance and imaged neuronal activity the next day upon reexposure to the conditioning context. Blocking memory reconsolidation led to severe memory impairments that were associated with reduced neuronal activity not only in CA1 but also in CA3 and the BLA. Thus, our results indicate that CA1 is necessary for reconsolidation and suggest the involvement of a CA3-CA1-BLA network in the retrieval of contextual fear memory. Further investigations of this network might contribute to the validation of new brain targets for the treatment of pathologies such as posttraumatic stress disorders.

APP/Go protein Gβγ-complex signaling mediates Aβ degeneration and cognitive impairment in Alzheimer's disease models

Deposition of amyloid-β (Aβ), the proteolytic product of the amyloid precursor protein (APP), might cause neurodegeneration and cognitive decline in Alzheimer's disease (AD). However, the direct involvement of APP in the mechanism of Aβ-induced degeneration in AD remains on debate. Here, we analyzed the interaction of APP with heterotrimeric Go protein in primary hippocampal cultures and found that Aβ deposition dramatically enhanced APP-Go protein interaction in dystrophic neurites. APP overexpression rendered neurons vulnerable to Aβ toxicity by a mechanism that required Go-Gβγ complex signaling and p38-mitogen-activated protein kinase activation. Gallein, a selective pharmacological inhibitor of Gβγ complex, inhibited Aβ-induced dendritic and axonal dystrophy, abnormal tau phosphorylation, synaptic loss, and neuronal cell death in hippocampal neurons expressing endogenous protein levels. In the 3xTg-AD mice, intrahippocampal application of gallein reversed memory impairment associated with early Aβ pathology. Our data provide further evidence for the involvement of APP/Go protein in Aβ-induced degeneration and reveal that Gβγ complex is a signaling target potentially relevant for developing therapies for halting Aβ degeneration in AD.

Altered Functional Connectivity of the Basal Nucleus of Meynert in Mild Cognitive Impairment: A Resting-State fMRI Study

Background: Cholinergic dysfunction plays an important role in mild cognitive impairment (MCI). The basal nucleus of Meynert (BNM) provides the main source of cortical cholinergic innervation. Previous studies have characterized structural changes of the cholinergic basal forebrain in individuals at risk of developing Alzheimer’s disease (AD). However, whether and how functional connectivity of the BNM (BNM-FC) is altered in MCI remains unknown.

Objective: The aim of this study was to identify alterations in BNM-FC in individuals with MCI as compared to healthy controls (HCs), and to examine the relationship between these alterations with neuropsychological measures in individuals with MCI.

Method: One-hundred-and-one MCI patients and 103 HCs underwent resting-state functional magnetic resonance imaging (rs-fMRI). Imaging data were processed with SPM8 and CONN software. BNM-FC was examined via correlation in low frequency fMRI signal fluctuations between the BNM and all other brain voxels. Group differences were examined with a covariance analysis with age, gender, education level, mean framewise displacement (FD) and global correlation (GCOR) as nuisance covariates. Pearson’s correlation was conducted to evaluate the relationship between the BNM-FC and clinical assessments.

Result: Compared with HCs, individuals with MCI showed significantly decreased BNM-FC in the left insula extending into claustrum (insula/claustrum). Furthermore, greater decrease in BNM-FC with insula/claustrum was associated with more severe impairment in immediate recall during Auditory Verbal Learning Test (AVLT) in MCI patients.

Conclusion: MCI is associated with changes in BNM-FC to the insula/claustrum in relation to cognitive impairments. These new findings may advance research of the cholinergic bases of cognitive dysfunction during healthy aging and in individuals at risk of developing AD.

Extinction and recovery of an avoidance memory impaired by scopolamine

Pre-training administration of scopolamine (SCP) resembles situations of cholinergic dysfunction, leading to memory impairment of mice trained in an inhibitory avoidance task. We suggest here that SCP does not impair memory formation, but acquisition is affected in a way that reduces the strength of the stored memory, thus making this memory less able to control behavior when tested. Hence, a memory trace is stored, but is poorly expressed during the test. Although weakly expressed, this memory shows extinction during successive tests, and can be strengthened by using a reminder. Our results indicate that memories stored under cholinergic dysfunction conditions seem absent or lost, but are in fact present and experience common memory processes, such as extinction, and could be even recovered by using appropriate protocols.

Early Cognitive/Social Deficits and Late Motor Phenotype in Conditional Wild-Type TDP-43 Transgenic Mice

Frontotemporal Dementia (FTD) and amyotrophic lateral sclerosis (ALS) are two neurodegenerative diseases associated to mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43). To investigate in depth the behavioral phenotype associated with this proteinopathy, we used as a model transgenic (Tg) mice conditionally overexpressing human wild-type TDP 43 protein (hTDP-43-WT) in forebrain neurons. We previously characterized these mice at the neuropathological level and found progressive neurodegeneration and other features that evoke human TDP-43 proteinopathies of the FTD/ALS spectrum. In the present study we analyzed the behavior of mice at multiple domains, including motor, social and cognitive performance. Our results indicate that young hTDP-43-WT Tg mice (1 month after post-weaning transgene induction) present a normal motor phenotype compared to control littermates, as assessed by accelerated rotarod performance, spontaneous locomotor activity in the open field test and a mild degree of spasticity shown by a clasping phenotype. Analysis of social and cognitive behavior showed a rapid installment of deficits in social interaction, working memory (Y-maze test) and recognition memory (novel object recognition test) in the absence of overt motor abnormalities. To investigate if the motor phenotype worsen with age, we analyzed the behavior of mice after long-term (up to 12 months) transgene induction. Our results reveal a decreased performance on the rotarod test and in the hanging wire test, indicating a motor phenotype that was absent in younger mice. In addition, long-term hTDP-43-WT expression led to hyperlocomotion in the open field test. In sum, these results demonstrate a time-dependent emergence of a motor phenotype in older hTDP-43-WT Tg mice, recapitulating aspects of clinical FTD presentations with motor involvement in human patients, and providing a complementary animal model for studying TDP-43 proteinopathies.

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

Reversible behavioral phenotypes in a conditional mouse model of TDP-43 proteinopathies

Transactive response DNA-binding protein 43 (TDP-43) mislocalization and aggregation are hallmark features of amyotrophic lateral sclerosis and frontotemporal dementia (FTD). We have previously shown in mice that inducible overexpression of a cytoplasmically localized form of TDP-43 (TDP-43-ΔNLS) in forebrain neurons evokes neuropathological changes that recapitulate several features of TDP-43 proteinopathies. Detailed behavioral phenotyping could provide further validation for its usage as a model for FTD. In the present study, we performed a battery of behavioral tests to evaluate motor, cognitive, and social phenotypes in this model. We found that transgene (Tg) induction by doxycycline removal at weaning led to motor abnormalities including hyperlocomotion in the open field test, impaired coordination and balance in the rotarod test, and increased spasticity as shown by a clasping phenotype. Cognitive assessment demonstrated impaired recognition and spatial memory, measured by novel object recognition and Y-maze tests. Remarkably, TDP-43-ΔNLS mice displayed deficits in social behavior, mimicking a key aspect of FTD. To determine whether these symptoms were reversible, we suppressed Tg expression for 14 d in 1.5-month-old mice showing an established behavioral phenotype but modest neurodegeneration and found that motor and cognitive deficits were ameliorated; however, social performance remained altered. When Tg expression was suppressed in 6.5-month-old mice showing overt neurodegeneration, motor deficits were irreversible. These results indicate that TDP-43-ΔNLS mice display several core behavioral features of FTD with motor neuron disease, possibly due to functional changes in surviving neurons, and might serve as a valuable tool to unveil the underlying mechanisms of this and other TDP-43 proteinopathies.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and β-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

Neuropharmacology of memory consolidation and reconsolidation: Insights on central cholinergic mechanisms

Central cholinergic system is critically involved in all known memory processes. Endogenous acetylcholine release by cholinergic neurons is necessary for modulation of acquisition, encoding, consolidation, reconsolidation, extinction, retrieval and expression. Experiments from our laboratory are mainly focused on elucidating the mechanisms by which acetylcholine modulates memory processes. Blockade of hippocampal alpha-7-nicotinic receptors (α7-nAChRs) with the antagonist methyllycaconitine impairs memory reconsolidation. However, the administration of a α7-nAChR agonist (choline) produce a paradoxical modulation, causing memory enhancement in mice trained with a weak footshock, but memory impairment in animals trained with a strong footshock. All these effects are long-lasting, and depend on the age of the memory trace. This review summarizes and discusses some of our recent findings, particularly regarding the involvement of α7-nAChRs on memory reconsolidation.

Stress-dependent opioid and adrenergic modulation of newly retrieved fear memory

Recent studies on the effect of stress on modulation of fear memory in our laboratory have uncovered endogenous opioid and adrenergic based modulation systems, working in concert, that limit the strengthening or weakening of newly acquired fear memory during consolidation under conditions of mild or intense stress, respectively. The present study sought to determine if similar stress-dependent modulation, mediated by endogenous opioid and adrenergic systems, occurs during reconsolidation of newly retrieved fear memory. Rats underwent contextual fear conditioning followed 24h later by reactivation of fear memory; a retention test was administered the next day. Stress was manipulated by varying duration of recall of fear memory during reactivation. In the first experiment, vehicle or the opioid-receptor blocker naloxone was administered immediately after varied durations (30 or 120 s) of reactivation. The results indicate that (1) reactivation, in the absence of drug, has a marked effect on freezing behavior-as duration of reactivation increases from 30 to 120 s, freezing behavior and presumably fear-induced stress increases and (2) naloxone, administered immediately after 30 s (mild stress) or 120 s (intense stress) of reactivation, enhances or impairs retention, respectively, the next day. In the second experiment, naloxone and the ß-adrenergic blocker propranolol were administered either separately or in combination immediately after 120 s (intense stress) reactivation. The results indicate that separate administration of propranolol and naloxone impairs retention, while the combined administration fails to do so. Taken together the results of the two experiments are consistent with a protective mechanism, mediated by endogenous opioid and adrenergic systems working in concert, that limits enhancement and impairment of newly retrieved fear memory during reactivation in a stress-dependent manner.

Chronic scopolamine-injection-induced cognitive deficit on reward-directed instrumental learning in rat is associated with CREB signaling activity in the cerebral cortex and dorsal hippocampus

RATIONALE

Scopolamine, a nonselective muscarinic receptor antagonist, has been used in experimental animal models of dementia. It has been demonstrated to disrupt performances in a battery of behavioral tests. However, no attempt has been made to determine how scopolamine-treated animals would respond to a series of reward-directed instrumental learning (RDIL) tasks.

OBJECTIVES

The present study was designed to investigate the effects of chronic intraperitoneal injection of scopolamine in Wistar rats on RDIL, as well as on the expression of memory-related molecules in the dorsal hippocampus (DH) and cerebral cortex (CCx).

METHODS

The effects of the pretraining injection of scopolamine on the acquisition of instrumental response (experiment 1) were first investigated. Then, the effects of post-training manipulation on the maintenance of instrumental response and the responses to changes in contingency degradation and signal discrimination were assessed (experiment 2). Finally, the expression of cyclic AMP response element-binding protein (CREB), phosphorylated CREB, and brain-derived neurotrophic factor in the DH and CCx were examined using Western blotting and enzyme-linked immunosorbent assay.

RESULTS

The acquisition of instrumental conditioning is more vulnerable than its maintenance. The 3.0-mg/kg dose of scopolamine rendered rats unable to make adaptive changes in facing contingency degradation and correct responses in signal discrimination tasks. Furthermore, CREB signaling was inactivated by pretraining scopolamine treatment in both the DH and CCx. Nevertheless, this pathway was selectively suppressed by post-training treatment only in the CCx during memory reconsolidation.

CONCLUSIONS

The results suggest that scopolamine-induced cognitive deficits on RDIL are related to the distinguishing alteration of CREB signaling in the DH and CCx.

Positive and negative emotional arousal increases duration of memory traces: common and independent mechanisms

We compared the ability of positive and negative emotional arousal to increase the duration of consolidated memory traces. Positive arousal was modulated by manipulating the motivational salience of the testing cage of an object recognition test. Negative emotional arousal was modulated by manipulating shock levels in a step-through inhibitory avoidance (IA). Mice trained in either a high (chocolate-associated) or a low (inedible object-associated) emotionally arousing cage showed discrimination of a novel object 24 h after training, but only mice trained in the more arousing cage showed retention 96 h after training. Mice trained with either low (0.35 mA) or high (0.7 mA) shock intensities showed increased step-through latencies when tested 24 h after training, but only mice trained with the higher shock intensity showed retention of the IA learning 1 week after training. Administration of the phosphodiesterase type IV inhibitor Rolipram immediately after training in the two low arousing conditions increases duration of both responses.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

Does reconsolidation occur in humans?

Evidence for reconsolidation in non-human animals has accumulated rapidly in the last decade, providing compelling` demonstration for this phenomenon across species and memory paradigms. In vast contrast, scant evidence exists for human reconsolidation to date. A major reason for this discrepancy is the invasive nature of current techniques used to investigate reconsolidation, which are difficult to apply in humans. Pharmacological blockade of reconsolidation, for example, has been typically used in animals as a proof of concept. However, most compounds used in these studies are toxic for humans, and those compounds that are safe target related, but not direct mechanisms of reconsolidation. Thus, although human reconsolidation has been hypothesized, there is limited evidence it actually exists. The best evidence for human reconsolidation emerges from non-invasive techniques that “update” memory during reconsolidation rather than block it, a technique only rarely used in animal research. Here we discuss the current state of human reconsolidation and the challenges ahead. We review findings on reconsolidation of emotional associative, episodic, and procedural memories, using invasive and non-invasive techniques. We discuss the possible interpretation of these results, attempt to reconcile some inconsistencies, and suggest a conceptual framework for future research.

Sildenafil, a selective phosphodiesterase type 5 inhibitor, enhances memory reconsolidation of an inhibitory avoidance task in mice

Intracellular levels of the second messengers cAMP and cGMP are maintained through a balance between production, carried out by adenyl cyclase (AC) and guanylyl cyclase (GC), and degradation, carried out by phosphodiesterases (PDEs). Recently, PDEs have gained increased attention as potential new targets for cognition enhancement, with particular reference to phosphodiesterase type 5 (PDE5A). It is accepted that once consolidation is completed memory becomes permanent, but it has also been suggested that reactivation (memory retrieval) of the original memory makes it sensitive to the same treatments that affect memory consolidation when given after training. This new period of sensitivity coined the term reconsolidation. Sildenafil (1, 3, and 10mg/kg, ip), a cGMP-PDE5 inhibitor, facilitated retention performance of a one-trial step-through inhibitory avoidance task, when administered to CF-1 male mice immediately after retrieval. The effects of sildenafil (1mg/kg, ip) were time-dependent, long-lasting and inversely correlated with memory age. The administration of sildenafil (1mg/kg, ip) 30 min prior to the 2nd retention test did not affect retention of mice given post-retrieval injections of either vehicle or sildenafil (1mg/kg, ip). Finally, an enhancement of retention was also observed in CF-1 female mice receiving sildenafil (1mg/kg, ip) immediately, but not 180 min after retrieval. In the present paper we reported for the first time that systemic administration of sildenafil after memory reactivation enhances retention performance of the original learning. Our results indirectly point out cGMP, a component of the NO/cGMP/PKG pathway, as a necessary factor for memory reconsolidation.

Scopolamine prevents retrograde memory interference between two different learning tasks

Subjects exposed to learning experiences could store the new information through memory consolidation process. If consolidation is interfered by exposing the experimental subjects to another novel stimulus, memory of the first learning situation is sometimes disrupted. The cholinergic system is critically involved in acquisition of new information. Here, we use low doses of the muscarinic cholinergic receptor antagonist scopolamine (SCOP) to disrupt acquisition of new information, but sparing memory consolidation of previous memories. Mice were consecutively exposed to two learning situations: the inhibitory avoidance (IA) and the nose-poke habituation (NPH) tasks. The exposure of mice to the NPH task, after being trained in the IA apparatus, impairs consolidation of the avoidance memory in a manner related to the duration of the exposure to the NPH task. If the exposure to the NPH task occurred after reactivation of the avoidance memory, reconsolidation was impaired. Blockade of acquisition of the NPH task by SCOP allowed consolidation and reconsolidation of the avoidance memory. Results indicate that cholinergic system blockade by SCOP impairs acquisition but is less able to affect memory consolidation. The mere exposure and perception of a novel situation are not sufficient conditions to cause impairment of retention performance about previously learned information, but effective processing leading to acquisition of the NPH task information is necessary to cause the interference between both learning situations.

Novel long-term anticonvulsant treatment with gabapentin without causing memory impairment in mice

We previously reported that administration of a single dose of gabapentin (GBP) immediately after training improves memory of mice in an inhibitory avoidance task (IA), whereas GBP administered repeatedly for 7 days impairs memory. This is in accordance with the observation that long-term clinical treatment with GBP may be associated with adverse cognitive side effects. In the present work we used a GBP-loaded poly(epsilon-caprolactone) implant, allowing controlled release of the drug and maintenance of constant plasma levels over 1 week. When GBP-loaded implants were inserted subcutaneously into mice, immediately after training in the IA task, memory consolidation was enhanced. Moreover, GBP released from implants had an anticonvulsant action against pentylenetetrazole-induced seizures. These results suggest that maintenance of stable GBP plasma levels could protect against seizures without causing memory impairment. Hence, the adverse cognitive effects might be avoided by stabilizing plasma levels of the drug.

A synaptic reinforcement-based model for transient amnesia following disruptions of memory consolidation and reconsolidation

The observation of memory recovery following post-training amnestic interventions has historically caused controversy over the meaning of this finding, leading some authors to question the paradigm of a consolidation period for memories. Similarly, recent demonstrations of transient amnesia caused by interventions following memory reactivation have been used to question the existence of a retrieval-driven reconsolidation process. The present work aims to approach the phenomenon of transient amnesia following disruptions of consolidation and reconsolidation, discussing how memory recovery might be explained within a framework of systems consolidation, persistent synaptic reinforcement, and multiple memory traces. With these concepts in mind, we propose that long-term consolidation processes can underlie recovery from amnesia, demonstrating the feasibility of such a hypothesis in a two-structure computational model of learning in which consolidation is dependent upon synaptic reentry reinforcement. On the basis of this, we suggest that prolonged consolidation can account for experimental findings of transient amnesia, in a way that explains differences between disruptions of consolidation and reconsolidation without the need to dwell into the discussion between storage- and retrieval-based explanations for memory impairment.

Pharmacological manipulation of memory reconsolidation: towards a novel treatment of pathogenic memories

Well-consolidated memories, when retrieved, may return to a transiently fragile state, and need to be consolidated again in order to be maintained. This process has been referred to as memory reconsolidation and presumably serves to modify or strengthen memory traces. In recent years, our understanding of the neurobiological mechanisms underlying this phenomenon has increased rapidly. Here, we will briefly review some of the pharmacological evidence, stressing a crucial role for the brain's major neurotransmitter systems, such as glutamate and noradrenaline, in memory reconsolidation. Pharmacological intervention of reconsolidation processes may have clinical relevance, especially for the treatment of psychiatric disorders that are characterized by pathological memories, including post-traumatic stress disorder and addictive behaviour.

Activation of hippocampal nuclear factor-kappa B by retrieval is required for memory reconsolidation

Initially, memory is labile and requires consolidation to become stable. However, several studies support that consolidated memories can undergo a new period of lability after retrieval. The mechanistic differences of this process, termed reconsolidation, with the consolidation process are under debate, including the participation of hippocampus. Up to this point, few reports describe molecular changes and, in particular, transcription factor (TF) involvement in memory restabilization. Increasing evidence supports the participation of the TF nuclear factor-kappaB (NF-kappaB) in memory consolidation. Here, we demonstrate that the inhibition of NF-kappaB after memory reactivation impairs retention of a hippocampal-dependent inhibitory avoidance task in mice. We used two independent disruptive strategies to reach this conclusion. First, we administered intracerebroventricular or intrahippocampal sulfasalazine, an inhibitor of IKK (IkappaB kinase), the kinase that activates NF-kappaB. Second, we infused intracerebroventricular or intrahippocampal kappaB decoy, a direct inhibitor of NF-kappaB consisting of a double-stranded DNA oligonucleotide that contains the kappaB consensus sequence. When injected immediately after memory retrieval, sulfasalazine or kappaB decoy (Decoy) impaired long-term retention. In contrast, a one base mutated kappaB decoy (mDecoy) had no effect. Furthermore, we also found NF-kappaB activation in the hippocampus, with a peak 15 min after memory retrieval. This activation was earlier than that found during consolidation. Together, these results indicate that NF-kappaB is an important transcriptional regulator in memory consolidation and reconsolidation in hippocampus, although the temporal kinetics of activation differs between the two processes.

Sleep-dependent memory consolidation and reconsolidation

Molecular, cellular, and systems-level processes convert initial, labile memory representations into more permanent ones, available for continued reactivation and recall over extended periods of time. These processes of memory consolidation and reconsolidation are not all-or-none phenomena, but rather a continuing series of biological adjustments that enhance both the efficiency and utility of stored memories over time. In this chapter, we review the role of sleep in supporting these disparate but related processes.

Molecular mechanisms of memory reconsolidation

Memory reconsolidation has been argued to be a distinct process that serves to maintain, strengthen or modify memories. Specifically, the retrieval of a previously consolidated memory has been hypothesized to induce an additional activity-dependent labile period during which the memory can be modified. Understanding the molecular mechanisms of reconsolidation could provide crucial insights into the dynamic aspects of normal mnemonic function and psychiatric disorders that are characterized by exceptionally strong and salient emotional memories.

Evolutionarily-conserved role of the NF-kappaB transcription factor in neural plasticity and memory

NF-kappaB is an evolutionarily conserved family of transcription factors (TFs) critically involved in basic cellular mechanisms of the immune response, inflammation, development and apoptosis. In spite of the fact that it is expressed in the central nervous system, particularly in areas involved in memory processing, and is activated by signals such as glutamate and Ca2+, its role in neural plasticity and memory has only recently become apparent. A surprising feature of this molecule is its presence within the synapse. An increasing number of reports have called attention to the role of this TF in processes that require long-term regulation of the synaptic function underlying memory and neural plasticity. Here we review the evidence regarding a dual role for NF-kappaB, as both a signalling molecule after its activation at the synapse and a transcriptional regulator upon reaching the nucleus. The specific role of this signal, as well as the general transcriptional mechanism, in the process of memory formation is discussed. Converging lines of evidence summarized here point to a pivotal role for the NF-kappaB transcription factor as a direct signalling mechanism in the regulation of gene expression involved in long-term memory.

Poly(Lactic-co-glycolic Acid) Microspheres for the Controlled Release of Huperzine A: In Vitro and in Vivo Studies and the Application in the Treatment of the Impaired Memory of Mice

Huperzine A loaded poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres were prepared by an oil/water (o/w) solvent evaporation technique. With a decrease of the ratio of o/w from 1 : 100 to 1 : 50, the encapsulation efficiency was reduced about 4%. Increasing the PVA concentration from 0.5 to 2% reduced the percentage encapsulation efficiency of huperzine A from 60.7 to 47.4% and the particle size of microspheres from 84.2 to 26.2 microm. The addition of stearic acid improved the encapsulation efficiency, but also accelerated the in vitro release of hupezine A from microspheres. After i.m. administration of huperzine A loaded microspheres in mice, huperzine A was sustained released from the PLGA microspheres up to 12 d with a low initial burst. Passive avoidance test of mice showed that the microspheres formulation offered an improved therapeutic efficiency in the treatment of the impaired memory of the mice superior to injection gastric (i.g.) administration of huperzine A suspension at the same dose, whose therapeutic efficiency was similar as that of a 50% reduced dose of the microspheres formulation.