Between-systems memory interference during retrieval

Involvement of microglia in disturbed fear memory regulation: Possible microglial contribution to the pathophysiology of posttraumatic stress disorder

Microglia, immune cells in the brain, play a crucial role in brain inflammation and synaptic plasticity by releasing inflammatory mediators and neurotrophic factors as well as, phagocytosing synaptic elements. Recent studies have shown peripheral inflammation, immune alteration in the brain are associated with post-traumatic stress disorder (PTSD) in humans. Several preclinical studies using Pavlovian fear conditioning have suggested that microglia are involved in fear memory dysregulation and altered fear neuronal networks. Microglial priming resulting from previous stressful experiences may also have an effect. This review will introduce the current knowledge of microglial contribution to disturbed fear memory regulation, a fundamental feature of PTSD.

The Hippocampus and Dorsolateral Striatum Integrate Distinct Types of Memories through Time and Space, Respectively

Several decades of research have established that different kinds of memories result from the activity of discrete neural networks. Studying how these networks process information in experiments that target specific types of mnemonic representations has provided deep insights into memory architecture and its neural underpinnings. However, in natural settings reality confronts organisms with problems that are not neatly compartmentalized. Thus, a critical problem in memory research that still needs to be addressed is how distinct types of memories are ultimately integrated. Here we demonstrate how two memory networks, the hippocampus and dorsolateral striatum, may accomplish such a goal. The hippocampus supports memory for facts and events, collectively known as declarative memory and often studied as spatial memory in rodents. The dorsolateral striatum provides the basis for habits that are assessed in stimulus-response types of tasks. Expanding previous findings, the current work revealed that in male Long-Evans rats, the hippocampus and dorsolateral striatum use time and space in distinct and largely complementary ways to integrate spatial and habitual representations. Specifically, the hippocampus supported both types of memories when they were formed in temporal juxtaposition, even if the learning took place in different environments. In contrast, the lateral striatum supported both types of memories if they were formed in the same environment, even at temporally distinct points. These results reveal for the first time that by using fundamental aspects of experience in specific ways, the hippocampus and dorsolateral striatum can transcend their attributed roles in information storage.SIGNIFICANCE STATEMENT The current paradigm in memory research postulates that different types of memories reflected in separate types of behavioral strategies result from activity in distinct neural circuits. However, recent data have shown that when rats concurrently acquired in the same environment of hippocampal-dependent spatial navigation and striatal-dependent approach of a visual cue, each of the two types of memories became dependent on both the hippocampus and dorsolateral striatum. The current work reveals that the hippocampus and dorsolateral striatum use distinct and complementary principles to integrate different types of memories in time and space: the hippocampus integrates memories formed in temporal proximity, while the lateral striatum integrates memories formed in the same space.

Hippocampal Damage Causes Retrograde Amnesia and Slower Acquisition of a Cue-Place Discrimination in a Concurrent Cue-Place Water Task in Rats

Explanations of memory-guided navigation in rodents typically suggest that cue- and place-based navigations are independent aspects of behavior and neurobiology. The results of many experiments show that hippocampal damage causes both anterograde and retrograde amnesia (AA; RA) for place memory, but only RA for cue memory. In the present experiments, we used a concurrent cue-place water task (CWT) to study the effects of hippocampal damage before or after training on cue- and place-guided navigation, and how cue and place memory interact in damaged and control rats. We found that damaging the hippocampus before training caused a delay in the expression of cue-place navigation strategies relative to intact control animals; surprisingly, place navigation strategies emerged following pre-training hippocampal damage. With additional training, both control and damaged rats used local cues to navigate in the CWT. Damaged animals also show minor impairments in latency to navigate to the correct cue following a cue contingency reversal. By contrast to these anterograde effects, damage made after training causes RA for cue choice accuracy and latency to navigate to the correct cue. In addition, the extent of hippocampal damage predicted impairments in choice accuracy when lesions were made after training. These data extend previous work on the role of the hippocampus in cue and place memory-guided navigation, and show that the hippocampus plays an important role in both aspects of memory and navigation when present during the learning experience.

Volumetric Ca2+ Imaging in the Mouse Brain Using Hybrid Multiplexed Sculpted Light Microscopy

Calcium imaging using two-photon scanning microscopy has become an essential tool in neuroscience. However, in its typical implementation, the tradeoffs between fields of view, acquisition speeds, and depth restrictions in scattering brain tissue pose severe limitations. Here, using an integrated systems-wide optimization approach combined with multiple technical innovations, we introduce a new design paradigm for optical microscopy based on maximizing biological information while maintaining the fidelity of obtained neuron signals. Our modular design utilizes hybrid multi-photon acquisition and allows volumetric recording of neuroactivity at single-cell resolution within up to 1 × 1 × 1.22 mm volumes at up to 17 Hz in awake behaving mice. We establish the capabilities and potential of the different configurations of our imaging system at depth and across brain regions by applying it to in vivo recording of up to 12,000 neurons in mouse auditory cortex, posterior parietal cortex, and hippocampus.

Muscarinic activity in hippocampus and entorhinal cortex is crucial for spatial and fear memory retrieval

BACKGROUND

Hippocampus and entorhinal cortex are key players of learning and memory. Despite their established role in memory processes, the contribution of muscarinic receptor activity in these brain regions during memory retrieval remains elusive. This study was aimed to assess the role of hippocampal CA1 and medial entorhinal cortex muscarinic receptors in memory retrieval.

METHOD

Mice were implanted with bilateral cannulas in the hippocampus CA1 and medial entorhinal cortex. After recovery they were trained for Morris water maze test, novel object recognition test and contextual fear conditioning. Scopolamine was infused 10 min prior to retrieval test.

RESULTS

Pre-test scopolamine infusion in hippocampal CA1 and medial entorhinal cortex significantly reduced overall exploration of objects (p<0.001). Similarly, pre-retrieval inactivation dorsal hippocampal CA1 and medial entorhinal cortex muscarinic activity caused significant impairment of spatial and fear memories retrieval (p<0.05).

CONCLUSION

These findings showed vital role of muscarinic activity in retrieving hippocampal and entorhinal cortex dependent memories and suggest a possible target for treating retrograde amnesia.

Memory systems 2018 - Towards a new paradigm

The multiple memory systems theory (MMS) postulates that the brain stores information based on the independent and parallel activity of a number of modules, each with distinct properties, dynamics, and neural basis. Much of the evidence for this theory comes from dissociation studies indicating that damage to restricted brain areas cause selective types of memory deficits. MMS has been the prevalent paradigm in memory research for more than thirty years, even as it has been adjusted several times to accommodate new data. However, recent empirical results indicating that the memory systems are not always dissociable constitute a challenge to fundamental tenets of the current theory because they suggest that representations formed by individual memory systems can contribute to more than one type of memory-driven behavioral strategy. This problem can be addressed by applying a dynamic network perspective to memory architecture. According to this view, memory networks can reconfigure or transiently couple in response to environmental demands. Within this context, the neural network underlying a specific memory system can act as an independent unit or as an integrated component of a higher order meta-network. This dynamic network model proposes a way in which empirical evidence that challenges the idea of distinct memory systems can be incorporated within a modular memory architecture. The model also provides a framework to account for the complex interactions among memory systems demonstrated at the behavioral level. Advances in the study of dynamic networks can generate new ideas to experimentally manipulate and control memory in basic or clinical research.

Nucleus Reuniens Is Required for Encoding and Retrieving Precise, Hippocampal-Dependent Contextual Fear Memories in Rats

The nucleus reuniens (RE) is a ventral midline thalamic nucleus that interconnects the medial prefrontal cortex (mPFC) and hippocampus (HPC). Considerable data indicate that HPC-mPFC circuits are involved in contextual and spatial memory; however, it is not clear whether the RE mediates the acquisition or retrieval of these memories. To examine this question, we inactivated the RE with muscimol before either the acquisition or retrieval of pavlovian fear conditioning in rats; freezing served as the index of fear. We found that RE inactivation before conditioning impaired the acquisition of contextual freezing, whereas inactivation of the RE before retrieval testing increased the generalization of freezing to a novel context; inactivation of the RE did not affect either the acquisition or expression of auditory fear conditioning. Interestingly, contextual conditioning impairments were absent when retrieval testing was also conducted after RE inactivation. Contextual memories acquired under RE inactivation were hippocampal independent, insofar as contextual freezing in rats conditioned under RE inactivation was insensitive to intrahippocampal infusions of the NMDA receptor antagonist aminophosphonovalerate. Together, these data reveal that the RE supports hippocampal-dependent encoding of precise contextual memories that allow discrimination of dangerous contexts from safe contexts. When the RE is inactive, however, alternate neural systems acquire an impoverished contextual memory that is expressed only when the RE is off-line.SIGNIFICANCE STATEMENT The midline thalamic nucleus reuniens (RE) coordinates communication between the hippocampus and medial prefrontal cortex, brain areas that are critical for contextual and spatial memory. Here we show that temporary pharmacological inactivation of RE impairs the acquisition and precision of contextual fear memories after pavlovian fear conditioning in rats. However, inactivating the RE before retrieval testing restored contextual memory in rats conditioned after RE inactivation. Critically, we show that imprecise contextual memories acquired under RE inactivation are learned independently of the hippocampus. These data reveal that the RE is required for hippocampal-dependent encoding of precise contextual memories to support the discrimination of safe and dangerous contexts.

Hypobaric hypoxia impairs cued and contextual fear memory in rats

Fear memory is essential for survival, and its dysregulation leads to disorders. High altitude hypobaric hypoxia (HH) is known to induce cognitive decline. However, its effect on fear memory is still an enigma. We aimed to investigate the temporal effect of HH on fear conditioning and the underlying mechanism. Adult male Sprague-Dawley rats were trained for fear conditioning and exposed to simulated HH equivalent to 25,000 ft for different durations (1, 3, 7, 14 and 21 days). Subsequently, rats were tested for cued and contextual fear conditioning. Neuronal morphology, apoptosis and DNA fragmentation were studied in the medial prefrontal cortex (mPFC), hippocampus and basolateral amygdala (BLA). We observed significant deficit in cued and contextual fear acquisition (at 1, 3 and 7 days) and consolidation (cued at 1 and 3 days and contextual fear at 1, 3 and 7 days) under HH. HH exposure with retraining showed the earlier restoration of contextual fear memory. Further, we found a gradual increase in the number of pyknotic and apoptotic neurons together with the increase in DNA fragmentation in mPFC, hippocampus, and BLA up to 7 days of HH exposure. The present study concludes that HH exposure equivalent to 25,000 ft induced cued and contextual fear memory deficit (acquisition and consolidation) which is found to be correlated with the neurodegenerative changes in the limbic brain regions.

Hippocampal damage causes retrograde but not anterograde memory loss for context fear discrimination in rats

There is a substantial body of evidence that the hippocampus (HPC) plays and essential role in context discrimination in rodents. Studies reporting anterograde amnesia (AA) used repeated, alternating, distributed conditioning and extinction sessions to measure context fear discrimination. In addition, there is uncertainty about the extent of damage to the HPC. Here, we induced conditioned fear prior to discrimination tests and rats sustained extensive, quantified pre- or post-training HPC damage. Unlike previous work, we found that extensive HPC damage spares context discrimination, we observed no AA. There must be a non-HPC system that can acquire long-term memories that support context fear discrimination. Post-training HPC damage caused retrograde amnesia (RA) for context discrimination, even when rats are fear conditioned for multiple sessions. We discuss the implications of these findings for understanding the role of HPC in long-term memory.

Word Imageability Enhances Association-memory by Increasing Hippocampal Engagement

The hippocampus is thought to support association-memory, particularly when tested with cued recall. One of the most well-known and studied factors that influences accuracy of verbal association-memory is imageability; participants remember pairs of high-imageability words better than pairs of low-imageability words. High-imageability words are also remembered better in tests of item-memory. However, we previously found that item-memory effects could not explain the enhancement in cued recall, suggesting that imageability enhances association-memory strength. Here we report an fMRI study designed to ask, what is the role of the hippocampus in the memory advantage for associations due to imageability? We tested two alternative hypotheses: (1) Recruitment Hypothesis: High-imageability pairs are remembered better because they recruit the underlying hippocampal association-memory function more effectively. Alternatively, (2) Bypassing Hypothesis: Imageability functions by making the association-forming process easier, enhancing memory in a way that bypasses the hippocampus, as has been found, for example, with explicit unitization imagery strategies. Results found, first, hippocampal BOLD signal was greater during study and recall of high- than low-imageability word pairs. Second, the difference in activity between recalled and forgotten pairs showed a main effect, but no significant interaction with imageability, challenging the bypassing hypothesis, but consistent with the predictions derived from the recruitment hypothesis. Our findings suggest that certain stimulus properties, like imageability, may leverage, rather than avoid, the associative function of the hippocampus to support superior association-memory.

Stress and Fear Extinction

Stress has a critical role in the development and expression of many psychiatric disorders, and is a defining feature of posttraumatic stress disorder (PTSD). Stress also limits the efficacy of behavioral therapies aimed at limiting pathological fear, such as exposure therapy. Here we examine emerging evidence that stress impairs recovery from trauma by impairing fear extinction, a form of learning thought to underlie the suppression of trauma-related fear memories. We describe the major structural and functional abnormalities in brain regions that are particularly vulnerable to stress, including the amygdala, prefrontal cortex, and hippocampus, which may underlie stress-induced impairments in extinction. We also discuss some of the stress-induced neurochemical and molecular alterations in these brain regions that are associated with extinction deficits, and the potential for targeting these changes to prevent or reverse impaired extinction. A better understanding of the neurobiological basis of stress effects on extinction promises to yield novel approaches to improving therapeutic outcomes for PTSD and other anxiety and trauma-related disorders.

Relative contributions of CA3 and medial entorhinal cortex to memory in rats

The hippocampal CA1 field processes spatial information, but the relative importance of intra- vs. extra-hippocampal sources of input into CA1 for spatial behavior is unclear. To characterize the relative roles of these two sources of input, originating in the hippocampal field CA3 and in the medial entorhinal cortex (MEC), we studied effects of discrete neurotoxic lesions of CA3 or MEC on concurrent spatial and nonspatial navigation tasks, and on synaptic transmission in afferents to CA1. Lesions in CA3 or MEC regions that abolished CA3-CA1, or reduced MEC-CA1 synaptic transmission, respectively, impaired spatial navigation and unexpectedly interfered with cue response, suggesting that in certain conditions of training regimen, hippocampal activity may influence behavior otherwise supported by nonhippocampal neural networks. MEC lesions had milder and temporary behavioral effects, but also markedly amplified transmission in the CA3-CA1 pathway. Extensive behavioral training had a similar, but more modest effect on CA3-CA1 transmission. Thus, cortical input to the hippocampus modulates CA1 activity both directly and indirectly, through heterosynaptic interaction, to control information flow in the hippocampal loop. Following damage to hippocampal cortical input, the functional coupling of separate intra- and extra-hippocampal inputs to CA1 involved in normal learning may initiate processes that support recovery of behavioral function. Such a process may explain how CA3 lesions, which do not significantly modify the basic features of CA1 neural activity, nonetheless impair spatial recall, whereas lesions of EC input to CA1, which reduce the spatial selectivity of CA1 firing in foraging rats, have only mild effects on spatial navigation.

How does a specific learning and memory system in the mammalian brain gain control of behavior?

This review addresses a fundamental, yet poorly understood set of issues in systems neuroscience. The issues revolve around conceptualizations of the organization of learning and memory in the mammalian brain. One intriguing, and somewhat popular, conceptualization is the idea that there are multiple learning and memory systems in the mammalian brain and they interact in different ways to influence and/or control behavior. This approach has generated interesting empirical and theoretical work supporting this view. One issue that needs to be addressed is how these systems influence or gain control of voluntary behavior. To address this issue, we clearly specify what we mean by a learning and memory system. We then review two types of processes that might influence which memory system gains control of behavior. One set of processes are external factors that can affect which system controls behavior in a given situation including task parameters like the kind of information available to the subject, types of training experience, and amount of training. The second set of processes are brain mechanisms that might influence what memory system controls behavior in a given situation including executive functions mediated by the prefrontal cortex; switching mechanisms mediated by ascending neurotransmitter systems, the unique role of the hippocampus during learning. The issue of trait differences in control of different learning and memory systems will also be considered in which trait differences in learning and memory function are thought to potentially emerge from differences in level of prefrontal influence, differences in plasticity processes, differences in ascending neurotransmitter control, differential access to effector systems like motivational and motor systems. Finally, we present scenarios in which different mechanisms might interact. This review was conceived to become a jumping off point for new work directed at understanding these issues. The outcome of this work, in combination with other approaches, might improve understanding of the mechanisms of volition in human and non-human animals.

Double dissociation between the contributions of the septal and temporal hippocampus to spatial learning: the role of prior experience

The mammalian hippocampus is anatomically heterogeneous along its longitudinal axis, and there is evidence that distinct functions are executed by different septotemporal subregions. The best documented example is the dependency of spatial learning on the septal, but not the temporal, hippocampus. Here, we carried out a watermaze memory task in rats with partial lesions of the septal or temporal hippocampus made either before or after training. We then studied memory retention, reversal, and new spatial learning in a novel environment. This resulted in the surprising finding that spatial learning in a new environment is dependent on the temporal hippocampus in rats with preoperative experience of a different pool. Rats with septal hippocampal lesions made after learning not only retained the focused search strategy that was acquired during preoperative training, but were also capable of rapid spatial learning in a second pool. This demonstrates that once spatial information has been acquired in one context, related new learning in a different context can be mediated by the temporal hippocampus, a result that challenges the widely held view that spatial memory is an exclusive function of the septal hippocampus.

Competitive Trace Theory: A Role for the Hippocampus in Contextual Interference during Retrieval

Much controversy exists regarding the role of the hippocampus in retrieval. The two dominant and competing accounts have been the Standard Model of Systems Consolidation (SMSC) and Multiple Trace Theory (MTT), which specifically make opposing predictions as to the necessity of the hippocampus for retrieval of remote memories. Under SMSC, memories eventually become independent of the hippocampus as they become more reliant on cortical connectivity, and thus the hippocampus is not required for retrieval of remote memories, only recent ones. MTT on the other hand claims that the hippocampus is always required no matter the age of the memory. We argue that this dissociation may be too simplistic, and a continuum model may be better suited to address the role of the hippocampus in retrieval of remote memories. Such a model is presented here with the main function of the hippocampus during retrieval being "recontextualization," or the reconstruction of memory using overlapping traces. As memories get older, they are decontextualized due to competition among partially overlapping traces and become more semantic and reliant on neocortical storage. In this framework dubbed the Competitive Trace Theory (CTT), consolidation events that lead to the strengthening of memories enhance conceptual knowledge (semantic memory) at the expense of contextual details (episodic memory). As a result, remote memories are more likely to have a stronger semantic representation. At the same time, remote memories are also more likely to include illusory details. The CTT is a novel candidate model that may provide some resolution to the memory consolidation debate.

Interfering with post-learning hippocampal activity does not affect long-term consolidation of a context fear memory outside the hippocampus

There are still uncertainties about the role of the hippocampus (HPC) in memory consolidation. One theory, systems consolidation, states that the HPC is required for the initial storage of certain memories that subsequently become established in non-HPC networks through a lengthy process, involving an interaction with the HPC. A similar process may underlie the ability of multiple, distributed learning episodes of contextual fear conditioning to create a HPC-independent context fear memory, in a memory task that does not undergo systems consolidation with the mere passage of time [5]. The current study examined whether post-learning HPC activity is necessary to establish these HPC-independent context memories through distributed learning episodes. Rats received either three or six context conditioning sessions over the course of three days. The HPC-dependence of context memories was assessed using multisite, temporary inactivation of the HPC using ropivacaine during retention testing. We established that six conditioning sessions, but not three, created a memory that could be retrieved while the HPC was inactive. To directly test our hypothesis, HPC was inactivated after half of the six context-shock pairings in an independent group of rats. The rats were then tested for retention of context fear in the absence of HPC activity. Post-learning inactivation of the HPC did not affect the establishment of a HPC-independent context memory. These results favor the idea that at least one memory system outside the HPC can acquire context memories independently.

Novel odour recognition memory is independent of the hippocampus in rats

We examined the effects of hippocampal (HPC) damage on odour recognition memory, using a novel odour recognition task that was adapted from the more common novel object recognition task. Three separate experiments were conducted. In Experiment 1, we tested rats in novel odour recognition across different retention intervals (i.e. 15 min, 24 h, 1 week, 5 weeks). Given a single acquisition session, rats' performance deteriorated after 24 h, but given multiple acquisition sessions (i.e. four sessions over 2 days), rats were able to perform well after retention intervals up to 5 weeks. In Experiment 2, we examined the possible anterograde amnesic effects of HPC damage on novel odour recognition, finding that pre-training damage to the entire HPC failed to cause amnesia for retention delays extending up to 5 weeks. In Experiment 3, we examined whether post-training HPC damage would cause retrograde amnesia, but failed to find any evidence of an impairment. The combined results suggest that the neural network supporting odour recognition is independent of the HPC.

Context, emotion, and the strategic pursuit of goals: interactions among multiple brain systems controlling motivated behavior

Motivated behavior exhibits properties that change with experience and partially dissociate among a number of brain structures. Here, we review evidence from rodent experiments demonstrating that multiple brain systems acquire information in parallel and either cooperate or compete for behavioral control. We propose a conceptual model of systems interaction wherein a ventral emotional memory network involving ventral striatum (VS), amygdala, ventral hippocampus, and ventromedial prefrontal cortex triages behavioral responding to stimuli according to their associated affective outcomes. This system engages autonomic and postural responding (avoiding, ignoring, approaching) in accordance with associated stimulus valence (negative, neutral, positive), but does not engage particular operant responses. Rather, this emotional system suppresses or invigorates actions that are selected through competition between goal-directed control involving dorsomedial striatum (DMS) and habitual control involving dorsolateral striatum (DLS). The hippocampus provides contextual specificity to the emotional system, and provides an information rich input to the goal-directed system for navigation and discriminations involving ambiguous contexts, complex sensory configurations, or temporal ordering. The rapid acquisition and high capacity for episodic associations in the emotional system may unburden the more complex goal-directed system and reduce interference in the habit system from processing contingencies of neutral stimuli. Interactions among these systems likely involve inhibitory mechanisms and neuromodulation in the striatum to form a dominant response strategy. Innate traits, training methods, and task demands contribute to the nature of these interactions, which can include incidental learning in non-dominant systems. Addition of these features to reinforcement learning models of decision-making may better align theoretical predictions with behavioral and neural correlates in animals.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Dorsal hippocampus is necessary for novel learning but sufficient for subsequent similar learning

Our current understanding of brain mechanisms involved in learning and memory has been derived largely from studies using experimentally naïve animals. However, it is becoming increasingly clear that not all identified mechanisms may generalize to subsequent learning. For example, N-methyl-D-aspartate glutamate (NMDA) receptors in the dorsal hippocampus are required for contextual fear conditioning in naïve animals but not in animals previously trained in a similar task. Here we investigated how animals learn contextual fear conditioning for a second time-a response which is not due to habituation or generalization. We found that dorsal hippocampus infusions of voltage-dependent calcium channel blockers or the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) agonist impaired the first, not the second contextual learning. Only manipulations of the entire hippocampus led to an impairment in second learning. Specifically, inactivation of either the dorsal or ventral hippocampus caused the remaining portion of the hippocampus to acquire and consolidate the second learning. Thus, dorsal hippocampus seems necessary for initial contextual fear conditioning, but either the dorsal or ventral hippocampus is sufficient for subsequent conditioning in a different context. Together, these findings suggest that prior training experiences can change how the hippocampus processes subsequent similar learning.