Single session contextual fear conditioning remains dependent on the hippocampus despite an increase in the number of context-shock pairings during learning

Revisiting systems consolidation and the concept of consolidation

For more than 50 years, knowledge of memory processes has been based on the consolidation hypothesis, which postulates that new memories require time to become stabilized. Two forms of the consolidation model exist. The Cellular Consolidation concept is based upon retrograde amnesia induced by amnesic treatments, the severity of which decreases as the learning to treatment increases over minutes or hours. In contrast, The Systems Consolidation model is based on post-training hippocampal lesions, which produce more severe retrograde amnesia when induced after days than after weeks. Except for the temporal parameters, Cellular and Systems Consolidation show many similarities. Here we propose that Systems consolidation, much as Cellular Consolidation (see Gisquet- Verrier and Riccio, 2018), can be explained in terms of a form of state-dependency. Accordingly, lesions of the hippocampus induce a change in the internal state of the animal, which disrupts retrieval processes. But the effect of contextual change is known to decrease with the length of the retention intervals, consistent with time-dependent retrograde amnesia. We provide evidence supporting this new view.

Overtraining Strengthens the Visual Discrimination Memory Trace Outside the Hippocampus in Male Rats

The hippocampus (HPC) may compete with other memory systems when establishing a representation, a process termed overshadowing. However, this overshadowing may be mitigated by repeated learning episodes, making a memory resistant to post-training hippocampal damage. In the current study, we examined this overshadowing process for a hippocampal-dependent visual discrimination memory in rats. In Experiment 1, male rats were trained to criterion (80% accuracy on two consecutive days) on a visual discrimination and then given 50 additional trials distributed over 5 days or 10 weeks. Regardless of this additional learning, extensive damage to the HPC caused retrograde amnesia for the visual discrimination, suggesting that the memory remained hippocampal-dependent. In Experiment 2, rats received hippocampal damage before learning and required approximately twice as many trials to acquire the visual discrimination as control rats, suggesting that, when the overshadowing or competition is removed, the non-hippocampal memory systems only slowly acquires the discrimination. In Experiment 3, increasing the additional learning beyond criterion by 230 trials, the amount needed in Experiment 2 to train the non-hippocampal systems in absence of competition, successfully prevented the retrograde amnesic effects of post-training hippocampal damage. Combined, the findings suggest that a visual discrimination memory trace can be strengthened in non-hippocampal systems with overtraining and become independent of the HPC.

Distributed learning episodes create a context fear memory outside the hippocampus that depends on perirhinal and anterior cingulate cortices

Damage to the hippocampus (HPC) typically causes retrograde amnesia for contextual fear conditioning. Repeating the conditioning over several sessions, however, can eliminate the retrograde amnesic effects. This form of reinstatement thus permits modifications to networks that can support context memory retrieval in the absence of the HPC. The present study aims to identify cortical regions that support the nonHPC context memory. Specifically, the contribution of the perirhinal cortex (PRH) and the anterior cingulate cortex (ACC) were examined because of their established importance to context memory. The findings show that context memories established through distributed reinstatement survive damage limited only to the HPC, PRH, or ACC. Combined lesions of the HPC and PRH, as well as the HPC and ACC, caused retrograde amnesia, suggesting that network modifications in the PRH and ACC enable context fear memories to become resistant to HPC damage.

Intact Behavioral Expression of Contextual Fear, Context Discrimination, and Object Discrimination Memories Acquired in the Absence of the Hippocampus

We test the hypothesis that the stability and precision of context and visual discrimination memories depend on interactions between the hippocampus (HPC) and other memory storage networks. In four experiments we tested the properties of memories acquired in the absence of the HPC. Long-Evans male rats were exclusively used in all experiments. Experiment 1 evaluated acquisition and retention of context fear memories in rats with prior partial or complete HPC damage. Confirming an earlier report (Zelikowsky et al., 2012) a very small but statistically reliable slowing in a single session of context fear conditioning was found after HPC damage. In contrast, retention of context fear memory was normal after HPC damage up to 30 d after learning. In experiment 2, we found that discrimination between a context paired with foot shocks and a different context never paired with foot shock was retained normally for 15 d. In experiment 3, we replicated the finding of intact context discrimination for at least 15 d in rats who display a significant impairment in acquisition of place learning in the Morris water task (MWT). In final experiment using an appetitive object discrimination task, we showed normal retention of the discrimination for at least 30 d after training in rats with complete HPC damage. These finding score against the idea that non HPC memory storage requires a period of interaction with HPC to establish a stable, precise memory.SIGNIFICANCE STATEMENT Contrary to expectations from systems memory consolidation, we find that in the absence of a functional hippocampus (HPC) context and visual memories are formed rapidly and exhibit normal persistence and precision. The findings suggest that the HPC is not obligatory for these features of long-term memories.

A contextual binding theory of episodic memory: systems consolidation reconsidered

Episodic memory reflects the ability to recollect the temporal and spatial context of past experiences. Episodic memories depend on the hippocampus but have been proposed to undergo rapid forgetting unless consolidated during offline periods such as sleep to neocortical areas for long-term storage. Here, we propose an alternative to this standard systems consolidation theory (SSCT) - a contextual binding account - in which the hippocampus binds item-related and context-related information. We compare these accounts in light of behavioural, lesion, neuroimaging and sleep studies of episodic memory and contend that forgetting is largely due to contextual interference, episodic memory remains dependent on the hippocampus across time, contextual drift produces post-encoding activity and sleep benefits memory by reducing contextual interference.

Effects of Repetition Learning on Associative Recognition Over Time: Role of the Hippocampus and Prefrontal Cortex

When stimuli are learned by repetition, they are remembered better and retained for a longer time. However, current findings are lacking as to whether the medial temporal lobe (MTL) and cortical regions are involved in the learning effect when subjects retrieve associative memory, and whether their activations differentially change over time due to learning experience. To address these issues, we designed an fMRI experiment in which face-scene pairs were learned once (L1) or six times (L6). Subjects learned the pairs at four retention intervals, 30-min, 1-day, 1-week and 1-month, after which they finished an associative recognition task in the scanner. The results showed that compared to learning once, learning six times led to stronger activation in the hippocampus, but weaker activation in the perirhinal cortex (PRC) as well as anterior ventrolateral prefrontal cortex (vLPFC). In addition, the hippocampal activation was positively correlated with that of the parahippocampal place area (PPA) and negatively correlated with that of the vLPFC when the L6 group was compared to the L1 group. The hippocampal activation decreased over time after L1 but remained stable after L6. These results clarified how the hippocampus and cortical regions interacted to support associative memory after different learning experiences.

The neurocircuitry of remote cued fear memory

Memories of threatening, fear-evoking events can persist even over a lifetime. While fear memory is widely considered to be a highly persistent and durable form of memory, its circuits are not. This article reviews the dynamic temporal representation of remote fear memory in the brain, at the level of local circuits and distributed networks. Data from the study of Pavlovian cued fear conditioning suggests memory retrieval remains amygdala-dependent, even over protracted time scales, all the while interconnected cortical and subcortical circuits are newly recruited and progressively reorganized. A deeper understanding into how the neurocircuitry of cued fear memory reorganizes with the passage of time will advance our ongoing search for the elusive physical changes representing fear memories in the brain. Considering that persistent, pathological fear memories are a hallmark feature of post-traumatic stress disorder (PTSD), the behavioral and circuit-level study of remote cued fear memory retrieval adds a key element towards a systems understanding of PTSD.

The effect of AMPA receptor blockade on spatial information acquisition, consolidation and expression in juvenile rats

Improvement on spatial tasks in rats is observed during a late, postnatal developmental period (post-natal day (PND) 18 - PND 20). The developmental emergence of this spatial function occurs in conjunction with hippocampal connectivity changes and enhanced hippocampal-AMPA receptor-mediated synaptic responses. The current work investigated the effect of AMPAr blockade on the emergence and long-term storage of spatial information in juvenile rats and associated neural activity patterns in the dorsal hippocampus CA1 region. Male, Long Evans rats between the ages of PND 18 and PND 20 were systemically (i.p.) administered the AMPAr antagonist, NBQX, (0, 5 or 10mg/kg) every day prior to hidden platform water maze training (PND 18, 19 and 20), every day immediately post-training or immediately before the probe test (PND 41). NBQX administration prior to training prolonged latencies, pathlength and increased thigmotaxis during the acquisition phase. Administration of NBQX immediately posttraining had no effect on the day-to-day performance. When given a probe test 3weeks later, the saline group across all conditions spent more time in the target quadrant. Rats treated with pretraining 5mg NBQX dose showed a preference for the target quadrant while the posttraining and pretesting 5mg NBQX doses impaired the target quadrant preference. Groups injected with 10mg of NBQX pretraining, posttraining or pretesting did not show a preference for the target quadrant. c-Fos labeling in the CA1 reflected these differences in probe performance in that groups showing greater than chance dwell time in the target quadrant showed more c-Fos labeling in the CA1 region than groups that did not show a target quadrant preference. These findings provide support for the critical role of AMPA receptor-mediated function in the organization and long-term storage of spatial memories acquired during the juvenile period.

Inactivation of the anterior cingulate reveals enhanced reliance on cortical networks for remote spatial memory retrieval after sequential memory processing

One system consolidation model suggests that as time passes, ensembles of cortical neurons form strong connections to represent remote memories. In this model, the anterior cingulate cortex (ACC) serves as a cortical region that represents remote memories. However, there is debate as to whether remote spatial memories go through this systems consolidation process and come to rely on the ACC. The present experiment examined whether increasing the processing demand on the hippocampus, by sequential training on two spatial tasks, would more fully engage the ACC during retrieval of a remote spatial memory. In this scenario, inactivation of the ACC at a remote time point was hypothesized to produce a severe memory deficit if rats had been trained on two, sequential spatial tasks. Rats were trained on a water maze (WM) task only or a WM task followed by a radial arm maze task. A WM probe test was given recently or remotely to all rats. Prior to the probe test, rats received an injection of saline or muscimol into the ACC. A subtle deficit in probe performance was found at the remote time point in the group trained on only one spatial task and treated with muscimol. In the group trained on two spatial tasks and treated with muscimol, a subtle deficit in probe performance was noted at the recent time point and a substantial deficit in probe performance was observed at the remote time point. c-Fos labeling in the hippocampus revealed more labeling in the CA1 region in all remotely tested groups than recently tested groups. Findings suggest that spatial remote memories come to rely more fully on the ACC when hippocampal processing requirements are increased. Results also suggest continued involvement of the hippocampus in spatial memory retrieval along with a progressive strengthening of cortical connections as time progresses.

The impact of multiple memory formation on dendritic complexity in the hippocampus and anterior cingulate cortex assessed at recent and remote time points

Consolidation processes, involving synaptic and systems level changes, are suggested to stabilize memories once they are formed. At the synaptic level, dendritic structural changes are associated with long-term memory storage. At the systems level, memory storage dynamics between the hippocampus and anterior cingulate cortex (ACC) may be influenced by the number of sequentially encoded memories. The present experiment utilized Golgi-Cox staining and neuron reconstruction to examine recent and remote structural changes in the hippocampus and ACC following training on three different behavioral procedures. Rats were trained on one hippocampal-dependent task only (a water maze task), two hippocampal-dependent tasks (a water maze task followed by a radial arm maze task), or one hippocampal-dependent and one non-hippocampal-dependent task (a water maze task followed by an operant conditioning task). Rats were euthanized recently or remotely. Brains underwent Golgi-Cox processing and neurons were reconstructed using Neurolucida software (MicroBrightField, Williston, VT, USA). Rats trained on two hippocampal-dependent tasks displayed increased dendritic complexity compared to control rats, in neurons examined in both the ACC and hippocampus at recent and remote time points. Importantly, this behavioral group showed consistent, significant structural differences in the ACC compared to the control group at the recent time point. These findings suggest that taxing the demand placed upon the hippocampus, by training rats on two hippocampal-dependent tasks, engages synaptic and systems consolidation processes in the ACC at an accelerated rate for recent and remote storage of spatial memories.

Remote context fear conditioning remains hippocampus-dependent irrespective of training protocol, training-surgery interval, lesion size, and lesion method

Systems consolidation involves the reorganization of brain circuits that support long-term memory. It is a prolonged process that can take days, weeks, or longer. An animal model of systems consolidation was established in the early 1990s and provided compelling support for the initial observations in humans, that hippocampal damage disproportionally impairs recent memory compared to remote memory. Context fear conditioning was the most frequently and successfully used task to study systems consolidation and demonstrate temporally graded retrograde amnesia. However, recent studies have failed to support these early findings of temporal gradients and instead reported that both recent and remote memories are equally impaired. Thus, the status of context fear conditioning as method to study the process of systems consolidation is at present uncertain. Accordingly, we evaluated classically conditioned fear memory in large groups of rats with hippocampal damage by manipulating several procedural variables including the training protocol, the training-surgery interval, the extent of hippocampal damage, and the method of damaging the hippocampus. The results indicate that hippocampal damage profoundly impairs context fear conditioning. These findings are unambiguous and independent of any particular procedural manipulation we evaluated. We suggest that the preponderance of currently available evidence indicates that context fear memory remains hippocampus-dependent indefinitely.

Loss of Acid sensing ion channel-1a and bicarbonate administration attenuate the severity of traumatic brain injury

Traumatic brain injury (TBI) is a common cause of morbidity and mortality in people of all ages. Following the acute mechanical insult, TBI evolves over the ensuing minutes and days. Understanding the secondary factors that contribute to TBI might suggest therapeutic strategies to reduce the long-term consequences of brain trauma. To assess secondary factors that contribute to TBI, we studied a lateral fluid percussion injury (FPI) model in mice. Following FPI, the brain cortex became acidic, consistent with data from humans following brain trauma. Administering HCO₃⁻ after FPI prevented the acidosis and reduced the extent of neurodegeneration. Because acidosis can activate acid sensing ion channels (ASICs), we also studied ASIC1a^−/− mice and found reduced neurodegeneration after FPI. Both HCO₃⁻ administration and loss of ASIC1a also reduced functional deficits caused by FPI. These results suggest that FPI induces cerebral acidosis that activates ASIC channels and contributes to secondary injury in TBI. They also suggest a therapeutic strategy to attenuate the adverse consequences of TBI.

Suppression of neurotoxic lesion-induced seizure activity: evidence for a permanent role for the hippocampus in contextual memory

Damage to the hippocampus (HPC) using the excitotoxin N-methyl-D-aspartate (NMDA) can cause retrograde amnesia for contextual fear memory. This amnesia is typically attributed to loss of cells in the HPC. However, NMDA is also known to cause intense neuronal discharge (seizure activity) during the hours that follow its injection. These seizures may have detrimental effects on retrieval of memories. Here we evaluate the possibility that retrograde amnesia is due to NMDA-induced seizure activity or cell damage per se. To assess the effects of NMDA induced activity on contextual memory, we developed a lesion technique that utilizes the neurotoxic effects of NMDA while at the same time suppressing possible associated seizure activity. NMDA and tetrodotoxin (TTX), a sodium channel blocker, are simultaneously infused into the rat HPC, resulting in extensive bilateral damage to the HPC. TTX, co-infused with NMDA, suppresses propagation of seizure activity. Rats received pairings of a novel context with foot shock, after which they received NMDA-induced, TTX+NMDA-induced, or no damage to the HPC at a recent (24 hours) or remote (5 weeks) time point. After recovery, the rats were placed into the shock context and freezing was scored as an index of fear memory. Rats with an intact HPC exhibited robust memory for the aversive context at both time points, whereas rats that received NMDA or NMDA+TTX lesions showed a significant reduction in learned fear of equal magnitude at both the recent and remote time points. Therefore, it is unlikely that observed retrograde amnesia in contextual fear conditioning are due to disruption of non-HPC networks by propagated seizure activity. Moreover, the memory deficit observed at both time points offers additional evidence supporting the proposition that the HPC has a continuing role in maintaining contextual memories.