Memory retrieval in response to partial cues requires NMDA receptor-dependent neurotransmission in the medial prefrontal cortex

Neuropharmacological Modulation of N-methyl-D-aspartate, Noradrenaline and Endocannabinoid Receptors in Fear Extinction Learning: Synaptic Transmission and Plasticity

Learning to recognize and respond to potential threats is crucial for survival. Pavlovian threat conditioning represents a key paradigm for investigating the neurobiological mechanisms of fear learning. In this review, we address the role of specific neuropharmacological adjuvants that act on neurochemical synaptic transmission, as well as on brain plasticity processes implicated in fear memory. We focus on novel neuropharmacological manipulations targeting glutamatergic, noradrenergic, and endocannabinoid systems, and address how the modulation of these neurobiological systems affects fear extinction learning in humans. We show that the administration of N-methyl-D-aspartate (NMDA) agonists and modulation of the endocannabinoid system by fatty acid amide hydrolase (FAAH) inhibition can boost extinction learning through the stabilization and regulation of the receptor concentration. On the other hand, elevated noradrenaline levels dynamically modulate fear learning, hindering long-term extinction processes. These pharmacological interventions could provide novel targeted treatments and prevention strategies for fear-based and anxiety-related disorders.

NMDA receptor blockade in the dorsolateral striatum impairs consolidation but not retrieval of habit memory

The present study investigated whether N-methyl-d-aspartate (NMDA) receptors in the dorsolateral striatum (DLS) mediate consolidation and retrieval of habit memory. Adult male Long-Evans rats were trained in a response learning version of a water plus-maze task in which rats were reinforced to make a habitual and consistent body-turn response at the maze choice point in order to mount a hidden escape platform. Prior research indicates that acquisition, consolidation, and retrieval in this task requires DLS function. The present study consisted of two experiments. In Experiment 1, rats received intra-DLS post-training injections of the NMDA receptor antagonist 2-amino-5- phosphonopentanoic acid (AP5; 2 µg/side) to examine the role of NMDA receptors in consolidation of habit memory. In Experiment 2, different groups of rats received a single pre-retrieval injection of AP5 in the DLS (AP5; 2 µg/side) during the last day of maze training to examine the potential role of NMDA receptors in retrieval of habit memory. Results indicated that post-training intra-DLS AP5 injections impaired memory consolidation. However, administration of AP5 at the same dose that impaired consolidation had no effect on memory retrieval. The findings are consistent with previous research indicating a role for NMDA receptors in the DLS in memory consolidation, and suggest that NMDA-dependent synaptic activity in the DLS may not be a critical component of habit memory retrieval.

The removal and addition of cues does not impair spatial retrieval and leads to a different metabolic activity of the limbic network in female rats

The retrieval of spatial memories does not always occur in an environment with the same stimuli configuration where the memory was first formed. However, re-exposure to a partial portion of the previously encountered cues can elicit memory successfully. Navigation with contextual changes has received little attention, especially in females. Thus, we aimed to assess memory retrieval using the Morris Water Maze spatial reference protocol in female adult Wistar rats. Rats were trained with five allocentric cues, and retrieval was explored one week later either with the same cues, or with four removed, or with three added cues. We studied the underlying brain oxidative metabolism of the hippocampus, prefrontal, parietal, retrosplenial, entorhinal, and perirhinal cortices through cytochrome c oxidase (CCO) histochemistry. Neither cue removal nor cue addition impaired retrieval performance. Retrieval with a degraded subset of cues led to increased prefrontal, hippocampal, retrosplenial, parietal, and perirhinal CCO activity. Retrieval with extra cues led to an enhancement of CCO activity in the hippocampus and retrosplenial cortex. Different patterns of network intercorrelations were found. The cue-removal group presented a closed reciprocal network, while the group with extra cues had separate parallel networks. Both groups showed a simpler network than the group with no cue modifications. Future research is needed to delve into behavioral and brain-related functions of spatial memory processes under modified environmental conditions.

N-Methyl-D-Aspartate (NMDA) Receptors in the Prelimbic Cortex Are Required for Short- and Long-Term Memory Formation in Trace Fear Conditioning

Accumulating evidence suggests that the medial prefrontal cortex (mPFC) has been implicated in the acquisition of fear memory during trace fear conditioning in which a conditional stimulus (CS) is paired with an aversive unconditional stimulus (UCS) separated by a temporal gap (trace interval, TI). However, little is known about the role of the prefrontal cortex for short- and long-term trace fear memory formation. Thus, we investigated how the prelimbic (PL) subregion within mPFC in rats contributes to short- and long-term trace fear memory formation using electrolytic lesions and d,l,-2-amino-5-phosphonovaleric acid (APV), an N-methyl-D-aspartate receptor (NMDAR) antagonist infusions into PL. In experiment 1, pre-conditioning lesions of PL impaired freezing to the CS as well as TI during the acquisition and retrieval sessions, indicating that PL is critically involved in trace fear memory formation. In experiment 2, temporary blockade of NMDA receptors in PL impaired the acquisition, but not the expression of short- and long-term trace fear memory. In addition, the inactivation of NMDAR in PL had little effect on locomotor activity, pre-pulse inhibition (PPI), or shock sensitivity. Taken together, these results suggest that NMDA receptor-mediated neurotransmission in PL is required for the acquisition of trace fear memory.

Limited contextual memory and transcriptional dysregulation in the medial prefrontal cortex of mice exposed to early protein malnutrition are intergenerationally transmitted

Memory contextualization is vital for the subsequent retrieval of relevant memories in specific situations and is a critical dimension of social cognition. The inability to properly contextualize information has been described as characteristic of psychiatric disorders like autism spectrum disorders, schizophrenia, and post-traumatic stress disorder. The exposure to early-life adversities, such as nutritional deficiency, increases the risk to trigger alterations in different domains of cognition related to those observed in mental diseases. In this work, we explored the consequences of exposure to perinatal protein malnutrition on contextual memory in a mouse model and assessed whether these consequences are transmitted to the next generation. Female mice were fed with a normal or hypoproteic diet during pregnancy and lactation. To evaluate contextual memory, the object-context mismatch test was performed in both sexes of F1 offspring and in the subsequent F2 generation. We observed that contextual memory was altered in mice of both sexes that had been subjected to maternal protein malnutrition and that the deficit in contextual memory was transmitted to the next generation. The basis of this alteration seems to be a transcriptional dysregulation of genes involved in the excitatory and inhibitory balance and immediate-early genes within the medial prefrontal cortex (mPFC) of both generations. The expression of genes encoding enzymes that regulate H3K27me3 levels was altered in the mPFC and partially in sperm of F1 malnourished mice. These results support the hypothesis that early nutritional deficiency represents a risk factor for the emergence of symptoms associated with mental disorders.

Hippocampus and cortex are involved in the retrieval of a spatial memory under full and partial cue availability

Retaking routes after a period of time usually occurs in an environment which has suffered from spatial configuration modifications. Thus, the original visual stimuli that allowed us to establish cognitive mapping using an allocentric strategy during the acquisition phase may not remain physically identical at the time of retrieval. However, in the standard experimental paradigms the cues are typically maintained constant. In this study, we explored memory retrieval with spatial modifications from learning in the Morris Water Maze. We trained rats on a reference memory protocol with five cues placed on black curtains that surrounded the pool, and seven days later, we tested memory retrieval under different conditions: maintenance of the five cues, removal of two and four of them, and the addition of three extra ones. Under full-cue and partial cue-conditions, rats showed successful memory retrieval, whereas adding extra cues resulted in impaired retrieval. Furthermore, we assessed brain oxidative metabolism through cytochrome c oxidase (CCO) histochemistry and found that, under full- and partial-cue conditions, there is an enhancement of the hippocampal, prefrontal, retrosplenial, parietal, and rhinal cortex metabolism. Rats that failed to retrieve spatial information in the extra cues condition showed similar or lower CCO activity than controls across many limbic areas. It is suggested that the presence of a partial portion of visual stimuli from learning makes it possible to reactivate the entire memory trace, but extra spatial information hinders retrieval, making it difficult to disengage the novel information from the older knowledge and establish a contextual generalization.

Recurrent binge ethanol is associated with significant loss of dentate gyrus granule neurons in female rats despite concomitant increase in neurogenesis

Binge drinking is becoming increasingly common among American women and girls. We have previously shown significant cell loss, downregulation of neurotrophins and microgliosis in female rats after a single 4-day ethanol exposure. To determine whether recurrent binge exposure would produce similar effects, we administered ethanol (5 g/kg) or iso-caloric control diet once-weekly for 11 weeks to adult female rats. As we have previously shown exercise neuroprotection against binge-induced damage, half the rats were given access to exercise wheels. Blood ethanol concentration (BEC) did not differ between sedentary and exercised groups, nor did it change across time. Using stereology, we quantified the number and/or size of neurons in the medial prefrontal cortex (mPFC) and hippocampal dentate gyrus (DG), as well as the number and activation state of microglia. Binged sedentary rats had significant cell loss in the dentate gyrus, but exercise eliminated this effect. Compared to sedentary controls, sedentary binged rats and all exercised rats showed increased neurogenesis in the DG. Number and nuclear volume of neurons in the mPFC were not changed. In the hippocampus and mPFC, the number of microglia with morphology indicative of partial activation was increased by recurrent binge ethanol and decreased by exercise. In summary, we show significant binge-induced loss of DG granule neurons despite increased neurogenesis, suggesting an unsuccessful compensatory response. Although exercise eliminated cell loss, our results indicate that infrequent, but recurrent exposure to clinically relevant BEC is neurotoxic.

Indoleamine 2,3-dioxygenase-dependent neurotoxic kynurenine metabolism mediates inflammation-induced deficit in recognition memory

Cognitive dysfunction in depression is a prevalent and debilitating symptom that is poorly treated by the currently available pharmacotherapies. Research over the past decade has provided evidence for proinflammatory involvement in the neurobiology of depressive disorders and symptoms associated with these disorders, including aspects of memory dysfunction. Recent clinical studies implicate inflammation-related changes in kynurenine metabolism as a potential pathogenic factor in the development of a range of depressive symptoms, including deficits in cognition and memory. Additionally, preclinical work has demonstrated a number of mood-related depressive-like behaviors to be dependent on indoleamine 2,3-dioxygenase-1 (IDO1), the inflammation-induced rate-limiting enzyme of the kynurenine pathway. Here, we demonstrate in a mouse model, that peripheral administration of endotoxin induced a deficit in recognition memory. Mice deficient in IDO were protected from cognitive impairment. Furthermore, endotoxin-induced inflammation increased kynurenine metabolism within the perirhinal/entorhinal cortices, brain regions which have been implicated in recognition memory. A single peripheral injection of kynurenine, the metabolic product of IDO1, was sufficient to induce a deficit in recognition memory in both control and IDO null mice. Finally, kynurenine monooxygenase (KMO) deficient mice were also protected from inflammation-induced deficits on novel object recognition. These data implicate IDO-dependent neurotoxic kynurenine metabolism as a pathogenic factor for cognitive dysfunction in inflammation-induced depressive disorders and a potential novel target for the treatment of these disorders.

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.

Finding the place without the whole: Timeline involvement of brain regions

Mastering the Morris water maze (MWM) requires the animal to consolidate, retain and retrieve spatial localizations of relevant visual cues. However, it is necessary to investigate whether a reorganization of the neural networks takes place when part of the spatial information is removed. We conducted four experiments using the MWM. A classical reference memory procedure was performed over five training days, RM5 (n=7), and eight days, RM8 (n=7), with the whole room and all the spatial cues presented. Another group of animals were trained in the same protocol, but they received an additional day of training with only partial cues, PC (n=8). Finally, a third group of animals performed the classical task, followed by an overtraining with partial cues for four more days, OPC (n=8). After completing these tasks, cytochrome c-oxidase activity (CO) in several brain limbic system structures was compared between groups. In addition, c-Fos positive cells were measured in the RM5, RM8, PC and OPC groups. No significant differences were found among the four groups in escape latencies or time spent in the target quadrant. CO revealed involvement of the prefrontal and parietal cortices, dorsal and ventral striatum, CA1 and CA3 subfields of the dorsal hippocampus, basolateral and lateral amygdala, and mammillary nuclei in the PC group, compared to the RM group. In the OPC group, involvement of the ventral striatum and anteroventral thalamus and the absence of amygdala involvement were revealed, compared to the PC group. C-Fos results highlighted the role of the prefrontal cortex, dorsal striatum, anterodorsal thalamus and CA3 in the PC group, compared to the OPC, RM5 and RM8 groups. The animals were able to find the escape platform even when only a portion of the space where the cues were placed was available. Although the groups did not differ behaviorally, energetic brain metabolism and immediate early gene expression revealed the engagement of different neural structures in the groups that received more training without the entire surrounding space.

A Special Extract of Bacopa monnieri (CDRI-08) Restores Learning and Memory by Upregulating Expression of the NMDA Receptor Subunit GluN2B in the Brain of Scopolamine-Induced Amnesic Mice

In the present communication, we have investigated effects of the CDRI-08, a well characterized extract of Bacopa monnieri, on expression of the GluN2B subunit of NMDAR in various brain regions of the scopolamine-induced amnesic mice. Our behavioral data reveal that scopolamine-treated amnesic mice exhibit significant decline in the spatial memory compared to the normal control mice. Our RT-PCR and immunoblotting data revealed that the scopolamine treatment resulted in a significant downregulation of the NMDAR GluN2B subunit expression in prefrontal cortex and hippocampus. Our enzyme assay data revealed that scopolamine caused a significant increase in the acetylcholinesterase activity in both the brain regions. Further, oral administration of the CDRI-08 to scopolamine-treated amnesic mice restored the spatial memory which was found to be associated with significant upregulation of the GluN2B subunit expression and decline in the acetylcholinesterase activity in prefrontal cortex as well as hippocampus towards their levels in the normal control mice. Our study provides the evidence for the mechanism underlying role of the Bacopa monnieri extract (CDRI-08) in restoring spatial memory in amnesic mice, which may have therapeutic implications.

Glucose, relational memory, and the hippocampus

RATIONALE

Many studies suggest that glucose can temporarily enhance hippocampal-dependent memories. As the hippocampus plays a key role in associative learning, we examined the influence of glucose on verbal paired associate memory.

OBJECTIVE

This study examines how glucose modifies performance on a relational memory task by examining its influence on learning, subsequent forgetting and relearning.

METHODS

A selective reminding procedure was used to show high and low imagability paired associates to 80 participants, who were seen twice. On the first session, they received 25 g glucose pre-learning, 25 g glucose post-learning or placebo. On the second session, 1 week later, they received 25 g glucose or placebo. Cued-recall was evaluated after each learning trial, 1 week later to assess forgetting and after an opportunity to relearn the material forgotten.

RESULTS

Glucose did not influence paired associate acquisition. Those given glucose pre-learning tended to forget less material the following week, and independently, glucose at retrieval facilitated cued-recall. Both forms of facilitation were equally apparent on low and high imagability pairs. The benefit of glucose pre-learning was eliminated once the paired associates had been seen again, but the benefit of glucose at retrieval extended into the second relearning trial.

CONCLUSIONS

The discussion considers the cognitive processes and hippocampal basis for paired associate learning and retention and the implications for glucose's mode of action. It is proposed that glucose during encoding serves to make the delayed memories initially more available, whereas its influence during delayed retrieval makes available memories temporarily more accessible.

Optogenetic stimulation of mPFC pyramidal neurons as a conditioned stimulus supports associative learning in rats

It is generally accepted that the associative learning occurs when a behaviorally neutral conditioned stimulus (CS) is paired with an aversive unconditioned stimulus (US) in close temporal proximity. Eyeblink conditioning (EBC) is a simple form of associative learning for motor responses. Specific activation of a population of cells may be an effective and sufficient CS for establishing EBC. However, there has been no direct evidence to support this hypothesis. Here, we show in rats that optogenetic activation of the right caudal mPFC pyramidal neurons as a CS is sufficient to support the acquisition of delay eyeblink conditioning (DEC). Interestingly, the associative memory was not stably expressed during the initial period of daily conditioning session even after the CR acquisition reached the asymptotic level. Finally, the intensity and consistency of the CS were found to be crucial factors in regulating the retrieval of the associative memory. These results may be of importance in understanding the neural cellular mechanisms underlying associative learning and the mechanisms underlying retrieval process of memory.