RESUMO
Introduction: The ability to separate similar experiences into differentiated representations is proposed to be based on a computational process called pattern separation, and it is one of the key characteristics of episodic memory. Although pattern separation has been mainly studied in the dentate gyrus of the hippocampus, this cognitive function if thought to take place also in other regions of the brain. The perirhinal cortex is important for the acquisition and storage of object memories, and in particular for object memory differentiation. The present study was devoted to investigating the importance of the cellular mechanism of endocytosis for object memory differentiation in the perirhinal cortex and its association with brain-derived neurotrophic factor, which was previously shown to be critical for the pattern separation mechanism in this structure. Methods: We used a modified version of the object recognition memory task and intracerebral delivery of a peptide (Tat-P4) into the perirhinal cortex to block endocytosis. Results: We found that endocytosis is necessary for pattern separation in the perirhinal cortex. We also provide evidence from a molecular disconnection experiment that BDNF and endocytosis-related mechanisms interact for memory discrimination in both male and female rats. Discussion: Our experiments suggest that BDNF and endocytosis are essential for consolidation of separate object memories and a part of a time-restricted, protein synthesis-dependent mechanism of memory stabilization in Prh during storage of object representations.
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Upon retrieval, an aversive memory can undergo destabilization and reconsolidation. A traumatic-like memory, however, may be resistant to this process. The present study sought to contribute with a strategy to overcome this potential issue by investigating whether generalized fear retrieval is susceptible to destabilization-reconsolidation that can be pharmacologically modified. We hypothesized that exposure to a context that elicits moderate generalization levels would allow a malleable memory state. We developed a fear conditioning protocol in context A (cxt-A) paired with yohimbine administration to promote significant fear to a non-conditioned context B (cxt-B) in rats, mimicking the enhanced noradrenergic activity reported after traumatic events in humans. Next, we attempted to impair the reconsolidation phase by administering clonidine (CLO) immediately after exposure to cxt-A, cxt-B, or a third context C (cxt-C) neither conditioned nor generalized. CLO administered post-cxt-B exposure for two consecutive days subsequently resulted in decreased freezing levels in cxt-A. CLO after cxt-B only once, after cxt-A or cxt-C in two consecutive days, or independently of cxt-B exposures did not affect fear in a later test. A 6-h-delay in CLO treatment post-cxt-B exposures produced no effects, and nimodipine administered pre-cxt-B exposures precluded the CLO action. We then quantified the Egr1/Zif268 protein expression following cxt-B exposures and CLO treatments. We found that these factors interact to modulate this memory destabilization-reconsolidation mechanism in the basolateral amygdala but not the dorsal CA1 hippocampus. Altogether, memory destabilization can accompany generalized fear expression; thus, we may exploit it to potentiate reconsolidation blockers' action.
Assuntos
Medo/psicologia , Generalização Psicológica , Consolidação da Memória/fisiologia , Memória/fisiologia , Agonistas alfa-Adrenérgicos/farmacologia , Animais , Região CA1 Hipocampal/efeitos dos fármacos , Clonidina/farmacologia , Proteína 1 de Resposta de Crescimento Precoce/biossíntese , Proteína 1 de Resposta de Crescimento Precoce/genética , Extinção Psicológica , Masculino , Transtornos da Memória/induzido quimicamente , Transtornos da Memória/psicologia , Rememoração Mental , Ratos , Ratos Wistar , Simpatolíticos , IoimbinaRESUMO
Exposure to stressful conditions induces long-lasting neurobiological changes in selected brain areas, which could be associated with the emergence of negative emotional responses. Moreover, the interaction of a stressful experience and the retrieval of an established fear memory trace enhance both fear expression and fear retention. Related to this, the stimulation of the dorsolateral part of the mesencephalic periaqueductal gray matter (dlPAG) prior to retrieval potentiates a fear memory trace previously acquired. Therefore, the question that arises is whether the dlPAG mediates the increased fear expression and fear retention after retrieval. Rats were subjected to a contextual fear conditioning paradigm using a single footshock, and 1 day later, rats were subjected to a stressful situation. As previously reported, there was an increase of freezing response only in those rodents that were re-exposed to the associated context at 1 and 5 days after stress exposure. Muscimol intra-dlPAG prior to the restraint event prevented such increase. Conversely, Muscimol intra-dlPAG infusion immediately after the stress experience had no effect on the resulting fear memory. When the neuroendocrine response to stress was explored, intra-dlPAG infusion of muscimol prior to stress decreased Fos expression in the paraventricular nucleus and serum corticosterone levels. Moreover, this treatment prevented the enhancement of the density of hippocampal "mature" spines associated with fear memory. In conclusion, the present results suggest that the dlPAG is a key neural site for the negative valence instruction necessary to modulate the promoting influence of stress on fear memory.
Assuntos
Medo/fisiologia , Rememoração Mental/fisiologia , Substância Cinzenta Periaquedutal/fisiologia , Estresse Psicológico/fisiopatologia , Animais , Condicionamento Clássico , Espinhas Dendríticas/fisiologia , Hipocampo/fisiologia , Masculino , Núcleo Hipotalâmico Paraventricular/fisiologia , Ratos WistarRESUMO
Upon retrieval, aversive associative memories may engage alternative processes depending on the conditioned stimulus exposure length. Generally, a short session maintains it through reconsolidation, and a long session inhibits it because of extinction learning. However, various experimental interventions have produced no memory changes when given after intermediate conditioned stimulus exposure events. The lack of effectiveness in the latter case has been explained by a stage of transition from reconsolidation to extinction, during which both phases are engaged but neither prevails. Alternatively, it would represent a novel, intermediate phase between reconsolidation and extinction. By combining a varying time of exposure to the paired context with the amnesic agent midazolam, and the introduction of a reinstatement procedure in the protocol to investigate the occurrence of extinction and/or reconsolidation, we aimed at addressing this question in female rats. Midazolam disrupted the reconsolidation of the original aversive memory and the consolidation of extinction memory when given after short (2 or 5â¯min, but not 1â¯min) and long (30â¯min) exposure to the paired context, respectively. There was reinstatement in the latter case only. Midazolam produced no memory changes when given after a session of 7 or 10â¯min, with reinstatement data suggesting the absence of reconsolidation in both cases. Noteworthy, drug effects on reconsolidation or extinction and the lack of action on the intermediate process were similar across the estrous cycle. Altogether, it was possible to check and dissociate three retrieval-dependent contextual fear memory processes using a more nuanced approach in females.
Assuntos
Aprendizagem da Esquiva/fisiologia , Ciclo Estral/fisiologia , Medo/fisiologia , Medo/psicologia , Rememoração Mental/fisiologia , Animais , Feminino , Ratos , Ratos WistarRESUMO
Fear memory overgeneralization contributes to the genesis and persistence of anxiety disorders and is a central hallmark in the pathophysiology of post-traumatic stress disorder (PTSD). Recent findings suggest that fear generalization is closely related to hippocampal dependency during retrieval. The selective serotonin reuptake inhibitor (SSRI) fluoxetine has been used as a first-line treatment for PTSD; however, how it exerts its therapeutic effect remains a matter of debate. Here, using contextual fear conditioning in rats, we show that chronic fluoxetine treatment prevents fear generalization and enhances subsequent extinction. Moreover, fluoxetine treatment after extinction prevents spontaneous recovery. The mechanism through which fluoxetine affects generalization and extinction seems to be through the postponement of systems consolidation, thereby maintaining hippocampal involvement during retrieval. Such an effect relies on a remodeling of dendritic spines in the hippocampus, as well as the number of mature, mushroom-type spines promoted by fluoxetine treatment. In order to further investigate whether fear generalization is a potential predictor of extinction effectiveness, we categorized a large naive population according to their generalization rate. We found that discriminator rats showed a better extinction profile compared to generalizers, suggesting that the generalization rate predicts extinction effectiveness. Hence, we propose that the therapeutic strategy of choice should take into account the extension of memory generalization, in which therapies based on extinction could induce a better outcome in patients who present less fear overgeneralization. These results open new avenues for the development of interventions that prevent fear generalization by maintaining memory dependency of the hippocampus.
Assuntos
Espinhas Dendríticas/efeitos dos fármacos , Medo/efeitos dos fármacos , Fluoxetina/administração & dosagem , Hipocampo/efeitos dos fármacos , Aprendizagem/efeitos dos fármacos , Plasticidade Neuronal/efeitos dos fármacos , Inibidores Seletivos de Recaptação de Serotonina/administração & dosagem , Animais , Condicionamento Clássico , Espinhas Dendríticas/fisiologia , Extinção Psicológica/efeitos dos fármacos , Extinção Psicológica/fisiologia , Medo/fisiologia , Generalização Psicológica/efeitos dos fármacos , Generalização Psicológica/fisiologia , Hipocampo/fisiologia , Aprendizagem/fisiologia , Masculino , Consolidação da Memória/efeitos dos fármacos , Consolidação da Memória/fisiologia , Ratos WistarRESUMO
Fear extinction is defined as a decline in fear-conditioned responses following non-reinforced exposure to a fear conditioned stimulus, therefore the conditioned stimulus gains new predictive properties. Patients with anxiety related disorders (e.g.: PTSD) subjected to extinction-like exposure treatments often experience a relapse of symptoms. Stress is a risk factor for those psychiatric disorders and a critical modulator of fear learning that turns the memory resistant to the extinction process. Dendritic spines are the anatomical sites where neuronal activity reshapes brain networks during learning and memory processes. Thus, we planned to characterize the dynamics of synaptic remodeling before and after contextual fear extinction in the dorsal hippocampus (DH), and how this process is affected by a previous stress experience. Animals with or without previous stress were contextually fear conditioned and one day later trained in an extinction paradigm. Rats were sacrificed one day after conditioning (pre-extinction) or one day after extinction for spine density analysis in the DH. We confirmed that stress exposure induced a deficit in extinction learning. Further, a higher density of dendritic spines, particularly mature ones, was observed in the DH of non-stressed conditioned animals at pre-extinction. Interestingly, after extinction, the spine levels returned to the control values. Conversely, stressed animals did not show such spines boost (pre-extinction) or any other change (post-extinction). In contrast, such standard dynamics of dendritic changes as well as the behavioral extinction was recovered when stressed animals received an intra-basolateral amygdala infusion of midazolam prior to stress. Altogether, these findings suggest that stress hinders the normal dynamic of dendritic remodeling after fear extinction and this could be part of the neurobiological substrate that makes those memories resistant to be extinguished.
Assuntos
Espinhas Dendríticas/fisiologia , Extinção Psicológica/fisiologia , Medo/fisiologia , Hipocampo/fisiologia , Memória/fisiologia , Plasticidade Neuronal , Estresse Psicológico/psicologia , Animais , Complexo Nuclear Basolateral da Amígdala/fisiologia , Condicionamento Clássico , Masculino , Rememoração Mental/fisiologia , Ratos WistarRESUMO
The main κ opioid receptors (κORs) subtypes already described (κ1ORs and κ2ORs) are expressed in brain regions involved in aversive memory consolidation, including the dorsal hippocampus (DH). However, the role of DH κORs in consolidation of aversive memories with varied intensity and specificity is still uncertain. The present study aimed to investigate this question using pharmacological agents in rats subjected to a weak, moderate or strong contextual aversive conditioning (CAC) protocol. Antagonizing DH κORs with nor-binaltorphimine (nor-BNI), immediately after, but not 6â¯h later, a moderate CAC leads to intensified freezing behavior in the re-exposure to the paired context. Thus, indicating that DH κORs have an inhibitory role in the consolidation of an aversive memory. Increased DH κORs expression 1â¯h and 3â¯h after the moderate CAC was also observed. This up-regulation was absent in animals only exposed to the shock or to the context, indicating that this phenomenon requires a shock-context pairing to occur. Intra-DH nor-BNI infusion induced no changes following a weak CAC, but it was able to potentiate the expression of freezing behavior in novel and unpaired context after a strong CAC, indicating that DH κORs also modulate the consolidation of a more intense and generalized memory. Moreover, infusing the κ2ORs agonist GR 89696, but not the κ1ORs agonist U-69593, into the DH reduced the conditioned freezing expression. Nor-BNI pretreatment in a sub-effective dose prevented the κ2ORs agonist effects. Altogether, the present findings provide convergent evidence that κORs activation negatively modulates contextual aversive memory consolidation in rat dorsal hippocampus.
Assuntos
Aprendizagem da Esquiva/fisiologia , Hipocampo/metabolismo , Consolidação da Memória/fisiologia , Receptores Opioides kappa/metabolismo , Analgésicos Opioides/farmacologia , Animais , Associação , Aprendizagem da Esquiva/efeitos dos fármacos , Benzenoacetamidas/farmacologia , Condicionamento Psicológico/efeitos dos fármacos , Condicionamento Psicológico/fisiologia , Eletrochoque , Reação de Congelamento Cataléptica/efeitos dos fármacos , Reação de Congelamento Cataléptica/fisiologia , Hipocampo/efeitos dos fármacos , Masculino , Consolidação da Memória/efeitos dos fármacos , Naltrexona/análogos & derivados , Naltrexona/farmacologia , Antagonistas de Entorpecentes/farmacologia , Piperazinas/farmacologia , Psicotrópicos/farmacologia , Pirrolidinas/farmacologia , Distribuição Aleatória , Ratos Wistar , Receptores Opioides kappa/agonistas , Receptores Opioides kappa/antagonistas & inibidoresRESUMO
It is known that a consolidated memory can return to a labile state and become transiently malleable following reactivation. This instability is followed by a restabilization phase termed reconsolidation. In this work, we explored whether an unrelated appetitive experience (voluntary consumption of diluted sucrose) can affect a contextual fear memory in rats during the reactivation-induced destabilization phase. Our findings show that exposure to an appetitive experience following reactivation can diminish fear retention. This effect persisted after 1 wk. Importantly, it was achieved only under conditions that induced fear memory destabilization. This result could not be explained as a potentiated extinction, because sucrose was unable to promote extinction. Since GluN2B-containing NMDA receptors in the basolateral amygdala complex (BLA) have been implicated in triggering fear memory destabilization, we decided to block pharmacologically these receptors to explore the neurobiological bases of the observed effect. Intra-BLA infusion with ifenprodil, a GluN2B-NMDA antagonist, prevented the fear reduction caused by the appetitive experience. In sum, these results suggest that the expression of a fear memory can be dampened by an unrelated appetitive experience, as long as memory destabilization is achieved during reactivation. Possible mechanisms behind this effect and its clinical implications are discussed.
Assuntos
Comportamento Apetitivo , Complexo Nuclear Basolateral da Amígdala/fisiologia , Medo , Consolidação da Memória/fisiologia , Receptores de N-Metil-D-Aspartato/fisiologia , Retenção Psicológica/fisiologia , Animais , Antagonistas de Aminoácidos Excitatórios/administração & dosagem , Masculino , Piperidinas/administração & dosagem , Ratos Wistar , Receptores de N-Metil-D-Aspartato/antagonistas & inibidoresRESUMO
The retrosplenial cortex (RSC) is one of the largest cortical areas in rodents, and is subdivided in two main regions, A29 and A30, according to their cytoarchitectural organization and connectivities. However, very little is known about the functional activity of each RSC subdivision during the execution of complex cognitive tasks. Here, we used a well-established fear learning protocol that induced long-lasting contextual fear memory and showed that during evocation of the fear memory, the expression of early growth response gene 1 was up-regulated in A30, and in other brain areas implicated in fear and spatial memory, however, was down-regulated in A29, including layers IV and V. To search for the participation of A29 on fear memory, we triggered selective degeneration of neurons within cortical layers IV and V of A29 by using a non-invasive protocol that takes advantage of the vulnerability that these neurons have MK801-toxicity and the modulation of this neurodegeneration by testosterone. Application of 5 mg/kg MK801 in intact males induced negligible neuronal degeneration of A29 neurons and had no impact on fear memory retrieval. However, in orchiectomized rats, 5 mg/kg MK801 induced overt degeneration of layers IV-V neurons of A29, significantly impairing fear memory recall. Degeneration of A29 neurons did not affect exploratory or anxiety-related behavior nor altered unconditioned freezing. Importantly, protecting A29 neurons from MK801-toxicity by testosterone preserved fear memory recall in orchiectomized rats. Thus, neurons within cortical layers IV-V of A29 are critically required for efficient retrieval of contextual fear memory.
Assuntos
Medo/fisiologia , Giro do Cíngulo/fisiologia , Rememoração Mental/fisiologia , Neurônios/fisiologia , Animais , Condicionamento Clássico/efeitos dos fármacos , Condicionamento Clássico/fisiologia , Maleato de Dizocilpina/administração & dosagem , Proteína 1 de Resposta de Crescimento Precoce/metabolismo , Antagonistas de Aminoácidos Excitatórios/administração & dosagem , Medo/efeitos dos fármacos , Giro do Cíngulo/efeitos dos fármacos , Masculino , Rememoração Mental/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Neurônios/patologia , Ratos , Ratos Wistar , Testosterona/administração & dosagemRESUMO
GABAergic signaling in the basolateral amygdala complex (BLA) plays a crucial role on the modulation of the stress influence on fear memory. Moreover, accumulating evidence suggests that the dorsal hippocampus (DH) is a downstream target of BLA neurons in contextual fear. Given that hippocampal structural plasticity is proposed to provide a substrate for the storage of long-term memories, the main aim of this study is to evaluate the modulation of GABA neurotransmission in the BLA on spine density in the DH following stress on contextual fear learning. The present findings show that prior stressful experience promoted contextual fear memory and enhanced spine density in the DH. Intra-BLA infusion of midazolam, a positive modulator of GABAa sites, prevented the facilitating influence of stress on both fear retention and hippocampal dendritic spine remodeling. Similarly to the stress-induced effects, the blockade of GABAa sites within the BLA ameliorated fear memory emergence and induced structural remodeling in the DH. These findings suggest that GABAergic transmission in BLA modulates the structural changes in DH associated to the influence of stress on fear memory.