Exposure to predator odor influences the relative use of multiple memory systems: role of basolateral amygdala

A dual-pathway architecture enables chronic stress to promote habit formation

Chronic stress can change how we learn and, thus, how we make decisions by promoting the formation of inflexible, potentially maladaptive, habits. Here we investigated the neuronal circuit mechanisms that enable this. Using a multifaceted approach in male and female mice, we reveal a dual pathway, amygdala-striatal, neuronal circuit architecture by which a recent history of chronic stress shapes learning to disrupt flexible goal-directed behavior in favor of inflexible habits. Chronic stress inhibits activity of basolateral amygdala projections to the dorsomedial striatum to impede the action-outcome learning that supports flexible, goal-directed decisions. Stress also increases activity in direct central amygdala projections to the dorsomedial striatum to promote the formation of rigid, inflexible habits. Thus, stress exerts opposing effects on two amygdala-striatal pathways to promote premature habit formation. These data provide neuronal circuit insights into how chronic stress shapes learning and decision making, and help understand how stress can lead to the disrupted decision making and pathological habits that characterize substance use disorders and other psychiatric conditions.

Neurocognitive effects of stress: a metaparadigm perspective

Stressful experiences, both physical and psychological, that are overwhelming (i.e., inescapable and unpredictable), can measurably affect subsequent neuronal properties and cognitive functioning of the hippocampus. At the cellular level, stress has been shown to alter hippocampal synaptic plasticity, spike and local field potential activity, dendritic morphology, neurogenesis, and neurodegeneration. At the behavioral level, stress has been found to impair learning and memory for declarative (or explicit) tasks that are based on cognition, such as verbal recall memory in humans and spatial memory in rodents, while facilitating those that are based on emotion, such as differential fear conditioning in humans and contextual fear conditioning in rodents. These vertically related alterations in the hippocampus, procedurally observed after subjects have undergone stress, are generally believed to be mediated by recurrently elevated circulating hypothalamic-pituitary-adrenal (HPA) axis effector hormones, glucocorticoids, directly acting on hippocampal neurons densely populated with corticosteroid receptors. The main purposes of this review are to (i) provide a synopsis of the neurocognitive effects of stress in a historical context that led to the contemporary HPA axis dogma of basic and translational stress research, (ii) critically reappraise the necessity and sufficiency of the glucocorticoid hypothesis of stress, and (iii) suggest an alternative metaparadigm approach to monitor and manipulate the progression of stress effects at the neural coding level. Real-time analyses can reveal neural activity markers of stress in the hippocampus that can be used to extrapolate neurocognitive effects across a range of stress paradigms (i.e., resolve scaling and dichotomous memory effects issues) and understand individual differences, thereby providing a novel neurophysiological scaffold for advancing future stress research.

Neural Index of Reinforcement Learning Predicts Improved Stimulus-Response Retention under High Working Memory Load

Human learning and decision-making are supported by multiple systems operating in parallel. Recent studies isolating the contributions of reinforcement learning (RL) and working memory (WM) have revealed a trade-off between the two. An interactive WM/RL computational model predicts that although high WM load slows behavioral acquisition, it also induces larger prediction errors in the RL system that enhance robustness and retention of learned behaviors. Here, we tested this account by parametrically manipulating WM load during RL in conjunction with EEG in both male and female participants and administered two surprise memory tests. We further leveraged single-trial decoding of EEG signatures of RL and WM to determine whether their interaction predicted robust retention. Consistent with the model, behavioral learning was slower for associations acquired under higher load but showed parametrically improved future retention. This paradoxical result was mirrored by EEG indices of RL, which were strengthened under higher WM loads and predictive of more robust future behavioral retention of learned stimulus-response contingencies. We further tested whether stress alters the ability to shift between the two systems strategically to maximize immediate learning versus retention of information and found that induced stress had only a limited effect on this trade-off. The present results offer a deeper understanding of the cooperative interaction between WM and RL and show that relying on WM can benefit the rapid acquisition of choice behavior during learning but impairs retention.SIGNIFICANCE STATEMENT Successful learning is achieved by the joint contribution of the dopaminergic RL system and WM. The cooperative WM/RL model was productive in improving our understanding of the interplay between the two systems during learning, demonstrating that reliance on RL computations is modulated by WM load. However, the role of WM/RL systems in the retention of learned stimulus-response associations remained unestablished. Our results show that increased neural signatures of learning, indicative of greater RL computation, under high WM load also predicted better stimulus-response retention. This result supports a trade-off between the two systems, where degraded WM increases RL processing, which improves retention. Notably, we show that this cooperative interplay remains largely unaffected by acute stress.

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.

Chronic corticosterone administration in adolescence enhances dorsolateral striatum-dependent learning in adulthood

Previous evidence indicates a link between early life stress (ELS) in humans and a predisposition to psychopathologies that are characterized in part by maladaptive habitual behaviors. Stress and anxiety influence the relative use of mammalian memory systems implicated in these disorders. Specifically, cognitive memory functions of the hippocampus are typically impaired by stress/anxiety, whereas habit memory functions of the dorsolateral striatum (DLS) are enhanced. A stress/anxiety bias toward habit memory has largely been demonstrated in adult rodents and humans, and the effects of ELS on the later use of DLS-dependent habit memory in adult rodents have not been extensively examined. The present study addressed this question by chronically elevating corticosterone (CORT) during adolescence, and investigated the effects of this treatment on DLS-dependent habit learning in adulthood. In experiment 1, adolescent rats received a single daily injection of either CORT (5 mg/kg) or vehicle (cVEH) over 5 days and then matured undisturbed before training as adults in a DLS-dependent water plus-maze task. Rats administered CORT injections during adolescence displayed a strong trend toward enhanced learning during adulthood relative to vehicle-treated rats. Adolescent CORT administration also increased anxiety-like behavior in adulthood in an elevated plus-maze. In experiment 2, adolescent CORT administration enhanced task acquisition in adulthood, and this effect was blocked by concurrent administration of the glucocorticoid antagonist mifepristone (30 mg/kg). Taken together, these findings suggest that chronic elevation of glucocorticoids during adolescence are sufficient to facilitate habit learning in adulthood, and indicate that glucocorticoid function may be a potential underlying mechanism by which ELS influences subsequent habitual behaviors.

Neural systems and the emotion-memory link

The present brief review for this Special Issue summarizes some of the original research on the emotional modulation of memory. The review begins by highlighting the pioneering research from James L. McGaugh and colleagues demonstrating modulatory effects of post-training drug administration on memory consolidation, in particular the stress hormone epinephrine. The subsequent discovery of a critical role for the basolateral amygdala in emotional modulation of memory is described. Within the context of a multiple systems approach to memory focusing on selective roles for the hippocampus and dorsolateral striatum in cognitive and habit memory, the original studies indicating that robust emotional arousal can bias animals and humans toward the predominant use of habit memory are reviewed. This effect of emotional arousal on the relative use of multiple memory systems depends on a modulatory role of the basolateral amygdala. Finally, we briefly consider how an emotion-induced enhancement of dorsolateral striatal-dependent memory may be relevant to understanding maladaptive habitual behaviors in certain human psychopathologies.

Analysis of memory modulation by conditioned stimuli

Conditioned stimuli (CS) have multiple psychological functions that can potentially contribute to their effect on memory formation. It is generally believed that CS-induced memory modulation is primarily due to conditioned emotional responses, however, well-learned CSs not only generate the appropriate behavioral and physiological reactions required to best respond to an upcoming unconditioned stimulus (US), but they also serve as signals that the US is about to occur. Therefore, it is possible that CSs can impact memory consolidation even when their ability to elicit conditioned emotional arousal is significantly reduced. To test this, male Sprague–Dawley rats trained on a signaled active avoidance task were divided into “Avoider” and “Non-Avoider” subgroups on the basis of percentage avoidance after 6 d of training. Subgroup differences in responding to the CS complex were maintained during a test carried out in the absence of the US. Moreover, the subgroups displayed significant differences in stress-induced analgesia (hot-plate test) immediately after this test, suggesting significant subgroup differences in conditioned emotionality. Importantly, using the spontaneous object recognition task, it was found that immediate post-sample exposure to the avoidance CS complex had a similar enhancing effect on object memory in the two subgroups. Therefore, to our knowledge, this is the first study to demonstrate that a significant conditioned emotional response is not necessary for the action of a predictive CS on modulation of memory consolidation.

Place vs. Response Learning: History, Controversy, and Neurobiology

The present article provides a historical review of the place and response learning plus-maze tasks with a focus on the behavioral and neurobiological findings. The article begins by reviewing the conflict between Edward C. Tolman's cognitive view and Clark L. Hull's stimulus-response (S-R) view of learning and how the place and response learning plus-maze tasks were designed to resolve this debate. Cognitive learning theorists predicted that place learning would be acquired faster than response learning, indicating the dominance of cognitive learning, whereas S-R learning theorists predicted that response learning would be acquired faster, indicating the dominance of S-R learning. Here, the evidence is reviewed demonstrating that either place or response learning may be dominant in a given learning situation and that the relative dominance of place and response learning depends on various parametric factors (i.e., amount of training, visual aspects of the learning environment, emotional arousal, et cetera). Next, the neurobiology underlying place and response learning is reviewed, providing strong evidence for the existence of multiple memory systems in the mammalian brain. Research has indicated that place learning is principally mediated by the hippocampus, whereas response learning is mediated by the dorsolateral striatum. Other brain regions implicated in place and response learning are also discussed in this section, including the dorsomedial striatum, amygdala, and medial prefrontal cortex. An exhaustive review of the neurotransmitter systems underlying place and response learning is subsequently provided, indicating important roles for glutamate, dopamine, acetylcholine, cannabinoids, and estrogen. Closing remarks are made emphasizing the historical importance of the place and response learning tasks in resolving problems in learning theory, as well as for examining the behavioral and neurobiological mechanisms of multiple memory systems. How the place and response learning tasks may be employed in the future for examining extinction, neural circuits of memory, and human psychopathology is also briefly considered.

Negative correlation between grey matter in the hippocampus and caudate nucleus in healthy aging

Neurobiological changes that occur with aging include a reduction in function and volume of the hippocampus. These changes were associated with corresponding memory deficits in navigation tasks. However, navigation can involve different strategies that are dependent on the hippocampus and caudate nucleus. The proportion of people using hippocampus-dependent spatial strategies decreases across the lifespan. As such, the decrease in spatial strategies, and corresponding increase in caudate nucleus-dependent response strategies with age, may play a role in the observed neurobiological changes in the hippocampus. Furthermore, we previously showed a negative correlation between grey matter in the hippocampus and caudate nucleus/striatum in mice, young adults, and in individuals diagnosed with Alzheimer's disease. As such, we hypothesized that this negative relationship between the two structures would be present during normal aging. The aim of the current study was to investigate this gap in the literature by studying the relationship between grey matter in the hippocampus and caudate nucleus of the striatum, in relation to each other and to navigation strategies, during healthy aging. Healthy older adults (N = 39) were tested on the Concurrent Spatial Discrimination Learning Task (CSDLT), a virtual radial task that dissociates between spatial and response strategies. A regression of strategies against structural MRIs showed for the first time in older adults that the response strategy was associated with higher amounts of grey matter in the caudate nucleus. As expected, the spatial strategy correlated with grey matter in the hippocampus, which was negatively correlated with grey matter in the caudate nucleus. Interestingly, a sex difference emerged showing that among older adult response learners, women have the least amount of grey matter in the hippocampus, which is a known risk for Alzheimer's disease. This difference was absent among spatial learners. These results are discussed in the context of the putative protective role of spatial memory against grey matter loss in the hippocampus, especially in women.

Opposing effects of cortisol on learning and memory in children using spatial versus response-dependent navigation strategies

Previous studies showed that healthy young adults who spontaneously use caudate nucleus-dependent strategies on a virtual navigation task, have significantly lower basal levels of cortisol compared with adults who use hippocampus-dependent spatial navigation strategies. In the current paper, we assessed the relation between basal cortisol levels and learning using a virtual navigation task in children. We show that basal cortisol level has a differential effect on learning and memory between children using spatial and response navigation strategies. Specifically, cortisol was found to be beneficial for learning performance in children using spatial strategies, such that higher levels of cortisol were associated with more efficient learning in a virtual maze. In contrast, cortisol had a deleterious effect on learning the virtual maze in children using response strategies, such that higher cortisol levels were associated with increased spatial working memory errors. Based on these results, individual differences in navigation strategy could help explain contradictory results in the literature showing that cortisol can have either a positive or negative association with learning and memory performance.

Threat-induced modulation of hippocampal and striatal memory systems during navigation of a virtual environment

The brain is composed of multiple memory systems that mediate distinct types of navigation. The hippocampus is important for encoding and retrieving allocentric spatial cognitive maps, while the dorsal striatum mediates procedural memories based on stimulus-response (S-R) associations. These memory systems are differentially affected by emotional arousal. In particular, rodent studies show that stress typically impairs hippocampal spatial memory while it spares or sometimes enhances striatal S-R memory. The influence of emotional arousal on these separate navigational memory systems has received less attention in human subjects. We investigated the effect of dynamic changes in anticipatory anxiety on hippocampal spatial and dorsal striatal S-R memory systems while participants attempted to solve a virtual eight-arm radial maze. In Experiment 1, participants completed a hippocampus-dependent spatial version of the eight-arm radial maze that required allocentric spatial memory to successfully navigate the environment. In Experiment 2, participants completed a dorsal striatal S-R version of the maze where no allocentric spatial cues were present, requiring the use of S-R navigation. Anticipatory anxiety was modulated via threat of receiving an unpleasant electrical shock to the wrist during memory retrieval. Results showed that threat of shock was associated with more errors and increased use of non-spatial navigational strategies in the hippocampal spatial task, but did not influence memory performance in the striatal S-R task. Findings indicate a dissociation regarding the influence of anticipatory anxiety on memory systems that has implications for understanding how fear and anxiety contribute to memory-related symptoms in human psychopathologies.

Drug-Induced Glucocorticoids and Memory for Substance Use

The biological stress response of the body forms one of the foundations of adaptive behavior, including promoting (and impairing) different forms of memory. This response transcends stressful experiences and underlies reactions to challenges and even reinforcers such as addictive substances. Nevertheless, drug-induced stress responses are rarely incorporated into models of addiction. We propose here that drug-induced stress responses (particularly glucocorticoids) play a crucial role in addictive behavior by modulating the formation of memories for substance-use experiences. We review the contributions of amygdala-, striatum-, and hippocampus-based memory systems to addiction, and reveal common effects of addictive drugs and acute stress on these different memories. We suggest that the contributions of drug-induced stress responses to memory may provide insights into the mechanisms driving addictive behavior.

The Central Amygdala Corticotropin-releasing hormone (CRH) Neurons Modulation of Anxiety-like Behavior and Hippocampus-dependent Memory in Mice

The encoding, consolidation and retrieval of memories is a multifaceted process that depends strongly on the optimal level of arousal but high levels of arousal may trigger anxiety, which negatively impacts the memory processing by the brain. We investigated the role of CRH neurons in the central amygdala (CeA) for their capacity to modulate both, the anxiety-like behavior and hippocampus-dependent memory. First, we activated the CRH neurons in CeA using cre-dependent AAV-DREADD in CRH-cre mice. The activation of CeA CRH neurons increased the anxiety-like behavior in Elevated-O maze (O-maze) and Light-Dark box (LDB). The activation of the CeA CRH also decreased Y-maze memory performance and the discrimination index in novel object recognition test (NOR). The inhibition of CeA CRH neurons with AAV-DREADD had the opposite effects on the anxiety-like behavior and the memory tests. Next, we used a combination of retrograde cre virus injected into locus ceruleus (LC) and cre-dependent AAV-DREADD injected into the CeA. While the excitation of the CeA neurons that project to LC increased the anxiety-like behavior, it also led to a better performance on the memory tests. The behavioral and memory effects were accompanied by increased c-Fos expression in the LC region. Pretreatment with CRH1 receptor antagonist antalarmin hydrochloride blocked the effects that were observed after the activation of the CeA projections to LC. Our findings highlight the role of CeA CRH neuronal population not only as a generator of anxiety but also demonstrate their role in the control of hippocampus-dependent memory.

Acute stress, but not corticosterone, facilitates acquisition of paired associates learning in rats using touchscreen-equipped operant conditioning chambers

Acute stress influences learning and memory in humans and rodents, enhancing performance in some tasks while impairing it in others. Typically, subjects preferentially employ striatal-mediated stimulus-response strategies in spatial memory tasks following stress, making use of fewer hippocampal-based strategies which may be more cognitively demanding. Previous research demonstrated that the acquisition of rodent paired associates learning (PAL) relies primarily on the striatum, while task performance after extensive training is impaired by hippocampal disruption. Therefore, we sought to explore whether the acquisition of PAL, an operant conditioning task involving spatial stimuli, could be enhanced by acute stress. Male Long-Evans rats were trained to a predefined criterion in PAL and then subjected to either a single session of restraint stress (30 min) or injection of corticosterone (CORT; 3 mg/kg). Subsequent task performance was monitored for one week. We found that rats subjected to restraint stress, but not those rats injected with CORT, performed with higher accuracy and efficiency, when compared to untreated controls. These results suggest that while acute stress enhances the acquisition of PAL, CORT alone does not. This dissociation may be due to differences between these treatments and their ability to produce sufficient catecholamine release in the amygdala, a requirement for stress effects on memory.

Danger Changes the Way the Mammalian Brain Stores Information About Innocuous Events: A Study of Sensory Preconditioning in Rats

The amygdala is a critical substrate for learning about cues that signal danger. Less is known about its role in processing innocuous or background information. The present study addressed this question using a sensory preconditioning protocol in male rats. In each experiment, rats were exposed to pairings of two innocuous stimuli in stage 1, S2 and S1, and then to pairings of S1 and shock in stage 2. As a consequence of this training, control rats displayed defensive reactions (freezing) when tested with both S2 and S1. The freezing to S2 is a product of two associations formed in training: an S2-S1 association in stage 1 and an S1-shock association in stage 2. We examined the roles of two medial temporal lobe (MTL) structures in consolidation of the S2-S1 association: the perirhinal cortex (PRh) and basolateral complex of the amygdala (BLA). When the S2-S1 association formed in a safe context, its consolidation required neuronal activity in the PRh (but not BLA), including activation of AMPA receptors and MAPK signaling. In contrast, when the S2-S1 association formed in a dangerous context, or when the context was rendered dangerous immediately after the association had formed, its consolidation required neuronal activity in the BLA (but not PRh), including activation of AMPA receptors and MAPK signaling. These roles of the PRh and BLA show that danger changes the way the mammalian brain stores information about innocuous events. They are discussed with respect to danger-induced changes in stimulus processing.

Impaired Spatial Memory and Enhanced Habit Memory in a Rat Model of Post-traumatic Stress Disorder

High levels of emotional arousal can impair spatial memory mediated by the hippocampus, and enhance stimulus-response (S-R) habit memory mediated by the dorsolateral striatum (DLS). The present study was conducted to determine whether these memory systems may be similarly affected in an animal model of post-traumatic stress disorder (PTSD). Sprague-Dawley rats were subjected to a “single-prolonged stress” (SPS) procedure and 1 week later received training in one of two distinct versions of the plus-maze: a hippocampus-dependent place learning task or a DLS-dependent response learning task. Results indicated that, relative to non-stressed control rats, SPS rats displayed slower acquisition in the place learning task and faster acquisition in the response learning task. In addition, extinction of place learning and response learning was impaired in rats exposed to SPS, relative to non-stressed controls. The influence of SPS on hippocampal spatial memory and DLS habit memory observed in the present study may be relevant to understanding some common features of PTSD, including hippocampal memory deficits, habit-like avoidance responses to trauma-related stimuli, and greater likelihood of developing drug addiction and alcoholism.

Acute Stress Time-dependently Modulates Multiple Memory Systems

Acute stress has been shown to modulate the engagement of different memory systems, leading to preferential expression of stimulus-response (SR) rather than episodic context memory when both types of memory can be used. However, questions remain regarding the cognitive mechanism that underlies this bias in humans-specifically, how each form of memory is individually influenced by stress in order for SR memory to be dominant. Here we separately measured context and SR memory and investigated how each was influenced by acute stress after learning (Experiment 1) and before retrieval (Experiment 2). We found that postlearning stress, in tandem with increased adrenergic activity during learning, impaired consolidation of context memory and led to preferential expression of SR rather than context memory. Preretrieval stress also impaired context memory, albeit transiently. Neither postlearning nor preretrieval stress changed the expression of SR memory. However, individual differences in cortisol reactivity immediately after learning were associated with variability in initial SR learning. These results reveal novel cognitive mechanisms by which stress can modulate multiple memory systems.

Beyond Risk and Protective Factors: An Adaptation-Based Approach to Resilience

How does repeated or chronic childhood adversity shape social and cognitive abilities? According to the prevailing deficit model, children from high-stress backgrounds are at risk for impairments in learning and behavior, and the intervention goal is to prevent, reduce, or repair the damage. Missing from this deficit approach is an attempt to leverage the unique strengths and abilities that develop in response to high-stress environments. Evolutionary-developmental models emphasize the coherent, functional changes that occur in response to stress over the life course. Research in birds, rodents, and humans suggests that developmental exposures to stress can improve forms of attention, perception, learning, memory, and problem solving that are ecologically relevant in harsh-unpredictable environments (as per the specialization hypothesis). Many of these skills and abilities, moreover, are primarily manifest in currently stressful contexts where they would provide the greatest fitness-relevant advantages (as per the sensitization hypothesis). This perspective supports an alternative adaptation-based approach to resilience that converges on a central question: “What are the attention, learning, memory, problem-solving, and decision-making strategies that are enhanced through exposures to childhood adversity?” At an applied level, this approach focuses on how we can work with, rather than against, these strengths to promote success in education, employment, and civic life.

Low lifetime stress exposure is associated with reduced stimulus-response memory

Exposure to stress throughout life can cumulatively influence later health, even among young adults. The negative effects of high cumulative stress exposure are well-known, and a shift from episodic to stimulus–response memory has been proposed to underlie forms of psychopathology that are related to high lifetime stress. At the other extreme, effects of very low stress exposure are mixed, with some studies reporting that low stress leads to better outcomes, while others demonstrate that low stress is associated with diminished resilience and negative outcomes. However, the influence of very low lifetime stress exposure on episodic and stimulus–response memory is unknown. Here we use a lifetime stress assessment system (STRAIN) to assess cumulative lifetime stress exposure and measure memory performance in young adults reporting very low and moderate levels of lifetime stress exposure. Relative to moderate levels of stress, very low levels of lifetime stress were associated with reduced use and retention (24 h later) of stimulus–response (SR) associations, and a higher likelihood of using context memory. Further, computational modeling revealed that participants with low levels of stress exhibited worse expression of memory for SR associations than those with moderate stress. These results demonstrate that very low levels of stress exposure can have negative effects on cognition.

Stress and Cognitive Flexibility: Cortisol Increases Are Associated with Enhanced Updating but Impaired Switching

Acute stress has frequently been shown to impair cognitive flexibility. Most studies have examined the effect of stress on cognitive flexibility by measuring how stress changes performance in paradigms that require participants to switch between different task demands. These processes typically implicate pFC function, a region known to be impaired by stress. However, cognitive flexibility is a multifaceted construct. Another dimension of flexibility, updating to incorporate relevant information, involves the dorsal striatum. Function in this region has been shown to be enhanced by stress. Using a within-subject design, we tested whether updating flexibility in a DMS task would be enhanced by an acute stress manipulation (cold pressor task). Participants' cortisol response to stress positively correlated with a relative increase in accuracy on updating flexibility (compared with trials with no working memory interference). In contrast, in line with earlier studies, cortisol responses correlated with worse performance when switching between trials with different task demands. These results demonstrate that stress-related increases in cortisol are associated with both increases and decreases in cognitive flexibility, depending on task demands.

Enhancement of striatum-dependent memory by conditioned fear is mediated by beta-adrenergic receptors in the basolateral amygdala

Emotional arousal can have a profound impact on various learning and memory processes. For example, unconditioned emotional stimuli (e.g., predator odor or anxiogenic drugs) enhance dorsolateral striatum (DLS)-dependent habit memory. These effects critically depend on a modulatory role of the basolateral complex of the amygdala (BLA). Recent work indicates that, like unconditioned emotional stimuli, exposure to an aversive conditioned stimulus (CS) (i.e., a tone previously paired with shock) can also enhance consolidation of DLS-dependent habit memory. The present experiments examined whether noradrenergic activity, particularly within the BLA, is required for a fear CS to enhance habit memory consolidation. First, rats underwent a fear conditioning procedure in which a tone CS was paired with an aversive unconditioned stimulus. Over the course of the next five days, rats received training in a DLS-dependent water plus-maze task, in which rats were reinforced to make a consistent body-turn response to reach a hidden escape platform. Immediately after training on days 1–3, rats received post-training systemic (Experiment 1) or intra-BLA (Experiment 2) administration of the β-adrenoreceptor antagonist, propranolol. Immediately after drug administration, half of the rats were re-exposed to the tone CS in the conditioning context (without shock). Post-training CS exposure enhanced consolidation of habit memory in vehicle-treated rats, and this effect was blocked by peripheral (Experiment 1) or intra-BLA (Experiment 2) propranolol administration. The present findings reveal that noradrenergic activity within the BLA is critical for the enhancement of DLS-dependent habit memory as a result of exposure to conditioned emotional stimuli.

Memory Systems and the Addicted Brain

The view that anatomically distinct memory systems differentially contribute to the development of drug addiction and relapse has received extensive support. The present brief review revisits this hypothesis as it was originally proposed 20 years ago (1) and highlights several recent developments. Extensive research employing a variety of animal learning paradigms indicates that dissociable neural systems mediate distinct types of learning and memory. Each memory system potentially contributes unique components to the learned behavior supporting drug addiction and relapse. In particular, the shift from recreational drug use to compulsive drug abuse may reflect a neuroanatomical shift from cognitive control of behavior mediated by the hippocampus/dorsomedial striatum toward habitual control of behavior mediated by the dorsolateral striatum (DLS). In addition, stress/anxiety may constitute a cofactor that facilitates DLS-dependent memory, and this may serve as a neurobehavioral mechanism underlying the increased drug use and relapse in humans following stressful life events. Evidence supporting the multiple systems view of drug addiction comes predominantly from studies of learning and memory that have employed as reinforcers addictive substances often considered within the context of drug addiction research, including cocaine, alcohol, and amphetamines. In addition, recent evidence suggests that the memory systems approach may also be helpful for understanding topical sources of addiction that reflect emerging health concerns, including marijuana use, high-fat diet, and video game playing.

Glucocorticoid enhancement of dorsolateral striatum-dependent habit memory requires concurrent noradrenergic activity

Previous findings indicate that post-training administration of glucocorticoid stress hormones can interact with the noradrenergic system to enhance consolidation of hippocampus- or amygdala-dependent cognitive/emotional memory. The present experiments were designed to extend these findings by examining the potential interaction of glucocorticoid and noradrenergic mechanisms in enhancement of dorsolateral striatum (DLS)-dependent habit memory. In experiment 1, different groups of adult male Long-Evans rats received training in two DLS-dependent memory tasks. In a cued water maze task, rats were released from various start points and were reinforced to approach a visibly cued escape platform. In a response-learning version of the water plus-maze task, animals were released from opposite starting positions and were reinforced to make a consistent egocentric body-turn to reach a hidden escape platform. Immediately post-training, rats received peripheral injections of the glucocorticoid corticosterone (1 or 3 mg/kg) or vehicle solution. In both tasks, corticosterone (3 mg/kg) enhanced DLS-dependent habit memory. In experiment 2, a separate group of animals received training in the response learning version of the water plus-maze task and were given peripheral post-training injections of corticosterone (3 mg/kg), the β-adrenoreceptor antagonist propranolol (3 mg/kg), corticosterone and propranolol concurrently, or control vehicle solution. Corticosterone injections again enhanced DLS-dependent memory, and this effect was blocked by concurrent administration of propranolol. Propranolol administration by itself (3 mg/kg) did not influence DLS-dependent memory. Taken together, the findings indicate an interaction between glucocorticoid and noradrenergic mechanisms in DLS-dependent habit memory. Propranolol administration may be useful in treating stress-related human psychopathologies associated with a dysfunctional DLS-dependent habit memory system.

The influence of cannabinoids on learning and memory processes of the dorsal striatum

Extensive evidence indicates that the mammalian endocannabinoid system plays an integral role in learning and memory. Our understanding of how cannabinoids influence memory comes predominantly from studies examining cognitive and emotional memory systems mediated by the hippocampus and amygdala, respectively. However, recent evidence suggests that cannabinoids also affect habit or stimulus-response (S-R) memory mediated by the dorsal striatum. Studies implementing a variety of maze tasks in rats indicate that systemic or intra-dorsolateral striatum infusions of cannabinoid receptor agonists or antagonists impair habit memory. In mice, cannabinoid 1 (CB1) receptor knockdown can enhance or impair habit formation, whereas Δ(9)THC tolerance enhances habit formation. Studies in human cannabis users also suggest an enhancement of S-R/habit memory. A tentative conclusion based on the available data is that acute disruption of the endocannabinoid system with either agonists or antagonists impairs, whereas chronic cannabinoid exposure enhances, dorsal striatum-dependent S-R/habit memory. CB1 receptors are required for multiple forms of striatal synaptic plasticity implicated in memory, including short-term and long-term depression. Interactions with the hippocampus-dependent memory system may also have a role in some of the observed effects of cannabinoids on habit memory. The impairing effect often observed with acute cannabinoid administration argues for cannabinoid-based treatments for human psychopathologies associated with a dysfunctional habit memory system (e.g. post-traumatic stress disorder and drug addiction/relapse). In addition, the enhancing effect of repeated cannabinoid exposure on habit memory suggests a novel neurobehavioral mechanism for marijuana addiction involving the dorsal striatum-dependent memory system.

Post-training re-exposure to fear conditioned stimuli enhances memory consolidation and biases rats toward the use of dorsolateral striatum-dependent response learning

In a dual-solution task that can be acquired using either hippocampus-dependent "place" or dorsolateral striatum-dependent "response" learning, emotional arousal induced by unconditioned stimuli (e.g. anxiogenic drug injections or predator odor exposure) biases rats toward response learning. In the present experiments emotionally-arousing conditioned stimuli were used to modulate the relative use of multiple memory systems. In Experiment 1, adult male Long-Evans rats initially received three standard fear-conditioning trials in which a tone (2 kHz, 75 dB) was paired with a brief electrical shock (1 mA, 2s). On day 2, the rats were trained in a dual-solution plus-maze task to swim from the same start arm (South) to a hidden escape platform always located in the same goal arm (East). Immediately following training, rats received post-training re-exposure to the fear-conditioned stimuli (i.e. tone and context) without shock. On day 3, the relative use of place or response learning was assessed on a probe trial in which rats were started from the opposite start arm (North). Post-training re-exposure to fear-conditioned stimuli produced preferential use of a response strategy. In Experiment 2, different rats received fear conditioning and were then trained in a single-solution task that required the use of response learning. Immediately following training, rats received post-training re-exposure to the fear-conditioned stimuli without shock. Re-exposure to fear-conditioned stimuli enhanced memory consolidation in the response learning task. Thus, re-exposure to fear-conditioned stimuli biases rats toward the use of dorsolateral striatum-dependent response learning and enhances memory consolidation of response learning.

Peripheral and intra-dorsolateral striatum injections of the cannabinoid receptor agonist WIN 55,212-2 impair consolidation of stimulus-response memory

The endocannabinoid system plays a major role in modulating memory. In the present study, we examined whether cannabinoid agonists influence the consolidation of stimulus-response/habit memory, a form of memory dependent upon the dorsolateral striatum (DLS). In Experiment 1, rats were trained in a cued platform water maze task in which animals were released from different start points and in order to escape had to find a cued platform which was moved to various spatial locations across trials. Immediately following training, rats received an i.p. injection of the cannabinoid receptor agonist WIN 55,212-2 (1 or 3mg/kg) or a vehicle solution. In Experiment 2, rats were trained in a forced-response version of the water plus-maze task in which a consistent body-turn response was reinforced across trials. Immediately following training, rats received an i.p. injection of WIN 55,212-2 (3 mg/kg) or vehicle. In Experiment 3, rats were trained in the cued platform task and after training received bilateral intra-DLS WIN 55,212-2 (100 ng/.5 μL or 200 ng/.5 μL) or vehicle. In Experiments 1-3, the higher doses of WIN 55,212-2 were associated with significant memory impairments, relative to vehicle-treated controls. The results indicate that peripheral or intra-DLS administration of a cannabinoid receptor agonist impairs consolidation of DLS-dependent memory. The findings are discussed within the context of previous research encompassing cannabinoids and DLS-dependent learning and memory processes, and the possibility that cannabinoids may be used to treat some habit-like human psychopathologies (e.g. posttraumatic stress disorder) is considered.

Olfactory systems and neural circuits that modulate predator odor fear

When prey animals detect the odor of a predator a constellation of fear-related autonomic, endocrine, and behavioral responses rapidly occur to facilitate survival. How olfactory sensory systems process predator odor and channel that information to specific brain circuits is a fundamental issue that is not clearly understood. However, research in the last 15 years has begun to identify some of the essential features of the sensory detection systems and brain structures that underlie predator odor fear. For instance, the main (MOS) and accessory olfactory systems (AOS) detect predator odors and different types of predator odors are sensed by specific receptors located in either the MOS or AOS. However, complex predator chemosignals may be processed by both the MOS and AOS, which complicate our understanding of the specific neural circuits connected directly and indirectly from the MOS and AOS to activate the physiological and behavioral components of unconditioned and conditioned fear. Studies indicate that brain structures including the dorsal periaqueductal gray (DPAG), paraventricular nucleus (PVN) of the hypothalamus, and the medial amygdala (MeA) appear to be broadly involved in predator odor induced autonomic activity and hypothalamic-pituitary-adrenal (HPA) stress hormone secretion. The MeA also plays a key role in predator odor unconditioned fear behavior and retrieval of contextual fear memory associated with prior predator odor experiences. Other neural structures including the bed nucleus of the stria terminalis and the ventral hippocampus (VHC) appear prominently involved in predator odor fear behavior. The basolateral amygdala (BLA), medial hypothalamic nuclei, and medial prefrontal cortex (mPFC) are also activated by some but not all predator odors. Future research that characterizes how distinct predator odors are uniquely processed in olfactory systems and neural circuits will provide significant insights into the differences of how diverse predator odors activate fear.