NMDARs control object recognition memory destabilization and reconsolidation.

Object recognition memory (ORM) allows identification of previously encountered items and is therefore crucial for remembering episodic information. In rodents, reactivation during recall in the presence of a novel object destabilizes ORM and initiates a Zif268 and protein synthesis-dependent reconsolidation process in the hippocampus that links the memory of this object to the reactivated recognition trace. Hippocampal NMDA receptors (NMDARs) modulate Zif268 expression and protein synthesis and regulate memory stability but their possible involvement in the ORM destabilization/reconsolidation cycle has yet to be analyzed in detail. We found that, in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5, or of the GluN2A subunit-containing NMDAR antagonist TCN201, 5 min after an ORM reactivation session in the presence of a novel object carried out 24 h post-training impaired retention 24 h later. In contrast, pre-reactivation administration of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on ORM recall or retention but impeded the amnesia caused by Zif268 silencing and protein synthesis inhibition in dorsal CA1. Our results indicate that GluN2B-containing hippocampal NMDARs are necessary for ORM destabilization whereas GluN2A-containing NMDARs are involved in ORM reconsolidation, and suggest that modulation of the relative activity of these receptor subtypes during recall regulates ORM persistence.

On the effect of hippocampal c-Jun N-terminal kinase inhibition on object recognition memory.

c-Jun N-terminal kinase (JNK) phosphorylates the transcription factor c-Jun in response to stress stimuli and contributes to both hippocampal synaptic plasticity and memory processing in mammals. Object recognition memory (ORM) is essential for remembering facts and events. In rodents, ORM consolidation and reconsolidation require a functional hippocampus. However, the possible involvement of hippocampal JNK on ORM processing has not yet been studied. Here we show that when injected into dorsal CA1 5 min, but not 6 h, after training adult male rats in the novel object recognition learning task, the JNK inhibitor SP600125 impaired ORM for at least 7 days without affecting exploratory activity, short-term ORM retention, or the functional integrity of the hippocampus. SP600125 did not hinder ORM retention when given in CA1 after a memory reactivation session carried out 24 h post-training in the presence of the same two objects presented during the training session, but caused time-dependent amnesia when one of the objects presented at training was replaced by a different but behaviorally equivalent novel one. Taken together, our results indicate that hippocampal JNK activity is necessary for ORM consolidation and reconsolidation but not for ORM recall or short-term retention.

Optogenetic inactivation of the medial septum impairs long-term object recognition memory formation.

Theta is one of the most prominent extracellular synchronous oscillations in the mammalian brain. Hippocampal theta relies on an intact medial septum (MS) and has been consistently recorded during the training phase of some learning paradigms, suggesting that it may be implicated in hippocampus-dependent long-term memory processing. Object recognition memory (ORM) allows animals to identify familiar items and is essential for remembering facts and events. In rodents, long-term ORM formation requires a functional hippocampus but the involvement of the MS in this process remains controversial. We found that training adult male Wistar rats in a long-term ORM-inducing learning task involving exposure to two different, but behaviorally equivalent novel stimuli objects increased hippocampal theta power, and that suppressing theta via optogenetic MS inactivation caused amnesia. Importantly, the amnesia was specific to the object the animals were exploring when the MS was inactivated. Taken together, our results indicate that the MS is necessary for long-term ORM formation and suggest that hippocampal theta activity is causally linked to this process.

Reactivation-dependent amnesia for object recognition memory is contingent on hippocampal theta-gamma coupling during recall.

Hippocampal dopamine D1/D5 receptor-dependent destabilization is necessary for object recognition memory (ORM) updating through reconsolidation. Dopamine also regulates hippocampal theta and gamma oscillations, which are involved in novelty and memory processing. We found that, in adult male rats, ORM recall in the presence of a novel object, but not in the presence of a familiar one, triggers hippocampal theta-gamma coupling. Hippocampal theta-gamma coupling (hPAC) does not happen when ORM destabilization is prevented by blocking D1/D5 receptors, but artificial hPAC generation during recall in the presence of a familiar object enables the amnesic effect of reconsolidation inhibitors. Therefore, hPAC controls ORM destabilization, and its modulation could increase reconsolidation-based psychotherapy efficacy.

Avoidance memory requires CaMKII activity to persist after recall.

Avoidance memory is destabilized when recalled concurrently with conflicting information, and must undergo a hippocampus-dependent restabilization process called reconsolidation to persist. CaMKII is a serine/threonine protein kinase essential for memory processing; however, its possible involvement in avoidance memory reconsolidation has not yet been studied. Using pharmacological, electrophysiological and optogenetic tools, we found that in adult male Wistar rats hippocampal CaMKII is necessary to reconsolidate avoidance memory, but not to keep it stored while inactive, and that blocking reconsolidation via CaMKII inhibition erases learned avoidance responses.

Dopamine controls whether new declarative information updates reactivated memories through reconsolidation.

Consolidation and reconsolidation are independent memory processes. Consolidation stabilizes new memories, whereas reconsolidation restabilizes memories destabilized when reactivated during recall. However, the biological role of the destabilization/reconsolidation cycle is still unknown. It has been hypothesized that reconsolidation links new information with reactivated memories, but some reports suggest that new and old memories are associated through consolidation mechanisms instead. Object-recognition memory (ORM) serves to judge the familiarity of items and is essential for remembering previous events. We took advantage of the fact that ORM consolidation, destabilization, and reconsolidation can be pharmacologically dissociated to demonstrate that, depending on the activation state of hippocampal dopamine D1/D5 receptors, the memory of a novel object presented during recall of the memory of a familiar one can be formed via reconsolidation or consolidation, but only reconsolidation can link them. We also found that recognition memories formed through reconsolidation can be destabilized even if indirectly reactivated. Our results indicate that dopamine couples novelty detection with memory destabilization to determine whether a new recognition trace is associated with an active network and suggest that declarative reminders should be used with caution during reconsolidation-based psychotherapeutic interventions.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation.

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

mTOR inhibition impairs extinction memory reconsolidation.

Fear-motivated avoidance extinction memory is prone to hippocampal brain-derived neurotrophic factor (BDNF)-dependent reconsolidation upon recall. Here, we show that extinction memory recall activates mammalian target of rapamycin (mTOR) in dorsal CA1, and that post-recall inhibition of this kinase hinders avoidance extinction memory persistence and recovers the learned aversive response. Importantly, coadministration of recombinant BDNF impedes the behavioral effect of hippocampal mTOR inhibition. Our results demonstrate that mTOR signaling is necessary for fear-motivated avoidance extinction memory reconsolidation and suggests that BDNF acts downstream mTOR in a protein synthesis-independent manner to maintain the reactivated extinction memory trace.

PERK, mTORC1 and eEF2 interplay during long term potentiation: An Editorial for 'Genetic removal of eIF2a kinase PERK in mice enables hippocampal L-LTP independent of mTORC1 activity' on page 133.

This Editorial highlights a study by Zimmermann and coworkers in the current issue of Journal of Neurochemistry. The authors' link suppression of PKR-like endoplasmatic reticulum kinase (PERK) activity to eukaryotic elongation factor 2 (eEF2) dephosphorylation and mTORC1-independent high-frequency stimulation (HFS)-induced long-term potentiation (LTP) in acute hippocampal slices from PERK forebrain conditional knockout mice. The results suggest that functional interaction between the signaling pathways controlling different phases of the mRNA translation process is necessary for long-term plasticity in the hippocampus.

Inactivation of the dorsal hippocampus or the medial prefrontal cortex impairs retrieval but has differential effect on spatial memory reconsolidation.

Active memories can incorporate new information through reconsolidation. However, the notion that memory retrieval is necessary for reconsolidation has been recently challenged. Non-reinforced retrieval induces hippocampus and medial prefrontal cortex (mPFC)-dependent reconsolidation of spatial memory in the Morris water maze (MWM). We found that the effect of protein synthesis inhibition on this process is abolished when retrieval of the learned spatial preference is hindered through mPFC inactivation but not when it is blocked by deactivation of dorsal CA1. Our results do not fully agree with the hypothesis that retrieval is unneeded for reconsolidation. Instead, they support the idea that a hierarchic interaction between the hippocampus and the mPFC controls spatial memory in the MWM, and indicate that this cortex is sufficient to retrieve the information essential to reconsolidate the spatial memory trace, even when the hippocampus is inactivated.

State-dependent effect of dopamine D1/D5 receptors inactivation on memory destabilization and reconsolidation.

Object recognition memories (ORM) can incorporate new information upon reactivation. This update initially involves destabilization of the original memory, which is followed by restabilization of the upgraded engram through a reconsolidation process that requires gene expression and protein synthesis in the hippocampus. We found that when given in dorsal CA1 either immediately after training or 15 min before ORM reactivation in the presence of a novel object, the dopamine D1/D5 receptor antagonist SCH23390 did not affect ORM consolidation, expression or retention but impeded the amnesia caused by the post-retrieval administration of the mRNA synthesis inhibitor α-amanitin or the protein synthesis blocker anisomycin. This anti-amnesic effect was not observed when SCH23390 was given immediately after training and again 15 min before memory reactivation. Our results demonstrate that hippocampal D1/D5 receptors are not needed for formation, retrieval or post-retrieval restabilization of the ORM trace but are essential for its destabilization when reactivation occurs together with the incorporation of new information into the original memory. Importantly, they also suggest that reenactment of the animal's post-learning neurochemical milieu at the moment of memory reactivation can be a boundary condition for reconsolidation.

Dopamine controls persistence of long-term memory storage.

The paradigmatic feature of long-term memory (LTM) is its persistence. However, little is known about the mechanisms that make some LTMs last longer than others. In rats, a long-lasting fear LTM vanished rapidly when the D1 dopamine receptor antagonist SCH23390 was injected into the dorsal hippocampus 12 hours, but not immediately or 9 hours, after the fearful experience. Conversely, intrahippocampal application of the D1 agonist SK38393 at the same critical post-training time converted a rapidly decaying fear LTM into a persistent one. This effect was mediated by brain-derived neurotrophic factor and regulated by the ventral tegmental area (VTA). Thus, the persistence of LTM depends on activation of VTA/hippocampus dopaminergic connections and can be specifically modulated by manipulating this system at definite post-learning time points.

The molecular cascades of long-term potentiation underlie memory consolidation of one-trial avoidance in the CA1 region of the dorsal hippocampus, but not in the basolateral amygdala or the neocortex.

Data accumulated through the past 15 years showed that memory consolidation of one-trial avoidance learning relies on a sequence of molecular events in the CA1 region of the hippocampus that is practically identical to that of long-term potentiation (LTP) in that area. Recent findings have indeed described CA1 LTP concomitant to the consolidation of this and other tasks. However, abundant evidence suggests that, in addition, other molecular events, involving some of the same steps but with different timing and in different sequence in the basolateral amygdala, entorhinal, parietal and cingulate cortex are as important as those of the hippocampus for memory consolidation. Here we review the hippocampal mechanisms involved and the possible interconnections between all these processes. Overall, the findings indicate that memory consolidation of even a task as deceivingly simple as one-trial avoidance relies on hippocampal LTP but also requires the concomitant participation of other brain systems and molecular events. Further, they point to the mechanisms that account for the enhanced consolidation usually seen for emotion-laden memories.

The role of the entorhinal cortex in extinction: influences of aging.

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.

Posttraining activation of CB1 cannabinoid receptors in the CA1 region of the dorsal hippocampus impairs object recognition long-term memory.

Evidence indicates that brain endocannabinoids are involved in memory processing. However, the participation of CB1 and CB2 cannabinoid receptors in recognition memory has not been yet conclusively determined. Therefore, we evaluated the effect of the posttraining activation of hippocampal cannabinoid receptors on the consolidation of object recognition memory. Rats with infusion cannulae stereotaxically aimed to the CA1 region of the dorsal hippocampus were trained in an object recognition learning task involving exposure to two different stimulus objects. Memory retention was assessed at different times after training. In the test sessions, one of the objects presented during training was replaced by a novel one. When infused in the CA1 region immediately after training, the non-selective cannabinoid receptor agonist WIN-55,212-2 and the endocannabinoid membrane transporter inhibitor VDM-11 blocked long-term memory retention in a dose-dependent manner without affecting short-term memory, exploratory behavior, anxiety state or the functionality of the hippocampus. The amnesic effect of WIN-55,212-2 and VDM-11 was not due to state-dependency and was completely reversed by co-infusion of the CB1 receptor antagonist AM-251 and mimicked by the CB1 receptor agonist ACEA but not by the CB2 receptor agonists JWH-015 and palmitoylethanolamide. Our data indicate that activation of hippocampal CB1 receptors early after training hampers consolidation of object recognition memory.

The evidence for hippocampal long-term potentiation as a basis of memory for simple tasks.

Long-term potentiation (LTP) is the enhancement of postsynaptic responses for hours, days or weeks following the brief repetitive afferent stimulation of presynaptic afferents. It has been proposed many times over the last 30 years to be the basis of long-term memory. Several recent findings finally supported this hypothesis: a) memory formation of one-trial avoidance learning depends on a series of molecular steps in the CA1 region of the hippocampus almost identical to those of LTP in the same region; b)hippocampal LTP in this region accompanies memory formation of that task and of another similar task. However, CA1 LTP and the accompanying memory processes can be dissociated, and in addition plastic events in several other brain regions(amygdala, entorhinal cortex, parietal cortex) are also necessary for memory formation of the one-trial task, and perhaps of many others.

The evidence for hippocampal long-term potentiation as a basis of memory for simple tasks

Long-term potentiation (LTP) is the enhancement of postsynaptic responses for hours, days or weeks following the brief repetitive afferent stimulation of presynaptic afferents. It has been proposed many times over the last 30 years to be the basis of long-term memory. Several recent findings finally supported this hypothesis: a) memory formation of one-trial avoidance learning depends on a series of molecular steps in the CA1 region of the hippocampus almost identical to those of LTP in the same region; b)hippocampal LTP in this region accompanies memory formation of that task and of another similar task. However, CA1 LTP and the accompanying memory processes can be dissociated, and in addition plastic events in several other brain regions(amygdala, entorhinal cortex, parietal cortex) are also necessary for memory formation of the one-trial task, and perhaps of many others.

A potenciação de longa duração (LTP) é o aumento de respostas pós-sinápticas durante horas, dias ou semanas após a breve estimulação repetitiva de aferentes pre-sinápticos. Foi proposto durante 30 anos ser a base da memória de longa duração. Vários achados recentes finalmente apoiaram esta hipótese: a) a formação da memória de esquiva inibitória adquirida numa sessão depende de uma cadeia de processos moleculares na região CA1 do hipocampo quase idêntica à da LTP nessa mesma região; b) LTP hipocampal nessa região acompanha a formação da memóría dessa tarefa e de outra semelhante. No entanto, a LTP de CA1 e os processos de memória podem ser dissociados e, fora disso, processos plásticos em outras regiões cerebrais (amígdala, córtex entorrinal, córtex parietal) também são necessários para a formação da memória da tarefa de uma sessão e talvez de muitas outras.

Inhibition of mRNA synthesis in the hippocampus impairs consolidation and reconsolidation of spatial memory.

Using two different mRNA synthesis inhibitors, we show that blockade of hippocampal gene expression during restricted posttraining or postretrieval time windows hinders retention of long-term spatial memory for the Morris water maze task, without affecting short-term memory, nonspatial learning, or the functionality of the hippocampus. Our results indicate that spatial memory consolidation induces the activation of the hippocampal transcriptional machinery and suggest the existence of a gene expression-dependent reconsolidation process that operates in the dorsal hippocampus at the moment of retrieval to stabilize the reactivated mnemonic trace.

On the participation of mTOR in recognition memory.

Evidence indicates that activation of the neuronal protein synthesis machinery is required in areas of the brain relevant to memory for consolidation and persistence of the mnemonic trace. Here, we report that inhibition of hippocampal mTOR, a protein kinase involved in the initiation of mRNA translation, immediately or 180min but not 540min after training impairs consolidation of long-term object recognition memory without affecting short-term memory retention or exploratory behavior. When infused into dorsal CA1 after long-term memory reactivation in the presence of familiar objects the mTOR inhibitor rapamycin (RAP) did not affect retention. However, when given immediately after exposing animals to a novel and a familiar object, RAP impaired memory for both of them. The amnesic effect of the post-retrieval administration of RAP was long-lasting, did not happen after exposure to two novel objects or following exploration of the training arena in the absence of other stimuli, suggesting that it was contingent with reactivation of the consolidated trace in the presence of a behaviorally relevant and novel cue. Our results indicate that mTOR activity is required in the dorsal hippocampus for consolidation of object recognition memory and suggest that inhibition of this kinase after memory retrieval in the presence of a particular set of cues hinders persistence of the original recognition memory trace.

Different effect of high fat diet and physical exercise in the hippocampal signaling.

Obesity is an epidemic disease that may affect brain function. The present study examined the effect of high fat diet (HF) and physical exercise on peripheral tissue and hippocampal signaling. CF-1 mice (n = 4, per cage) were divided into groups receiving high fat (HF) or control (CD) diets for 5 months, with or without voluntary exercise. Serum triacylglycerol, total cholesterol, HDLc, liver triacylglycerol and glycogen concentrations were evaluated (n = 6). Also, the phosphorylation state of the AKT --> ERK 1/2 --> CREB pathway (AKT, pAKTser473, ERK 1/2, pERK 1/2, CREB and pCREB, n = 4-6) was analyzed in the hippocampus. HF diet caused an increase in AKT phosphorylation at ser473 (P < 0.05), while exercise increased the phosphorylation of ERK 1/2 (P < 0.05) and CREB (P < 0.05). As expected, exercise reversed some of the harmful effects of HF, i.e., increased liver deposition of fat (P < 0.05) and fat gain in the abdominal region (P < 0.05). In conclusion, the effects of exercise and HF diet on brain signaling appear to affect the hippocampal AKT --> ERK 1/2 --> CREB pathway in independent ways: HF intake caused increased phosphorylation of AKTser473, while exercise increased ERK 1/2 --> CREB signaling. The physiological relevance of these findings in brain function remains to be elucidated.