Complementary roles for the amygdala and hippocampus in aversive conditioning to explicit and contextual cues

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Robust and consistent measures of pattern separation based on information theory and demonstrated in the dentate gyrus

Pattern separation is a valuable computational function performed by neuronal circuits, such as the dentate gyrus, where dissimilarity between inputs is increased, reducing noise and increasing the storage capacity of downstream networks. Pattern separation is studied from both in vivo experimental and computational perspectives and, a number of different measures (such as orthogonalisation, decorrelation, or spike train distance) have been applied to quantify the process of pattern separation. However, these are known to give conclusions that can differ qualitatively depending on the choice of measure and the parameters used to calculate it. We here demonstrate that arbitrarily increasing sparsity, a noticeable feature of dentate granule cell firing and one that is believed to be key to pattern separation, typically leads to improved classical measures for pattern separation even, inappropriately, up to the point where almost all information about the inputs is lost. Standard measures therefore both cannot differentiate between pattern separation and pattern destruction, and give results that may depend on arbitrary parameter choices. We propose that techniques from information theory, in particular mutual information, transfer entropy, and redundancy, should be applied to penalise the potential for lost information (often due to increased sparsity) that is neglected by existing measures. We compare five commonly-used measures of pattern separation with three novel techniques based on information theory, showing that the latter can be applied in a principled way and provide a robust and reliable measure for comparing the pattern separation performance of different neurons and networks. We demonstrate our new measures on detailed compartmental models of individual dentate granule cells and a dentate microcircuit, and show how structural changes associated with epilepsy affect pattern separation performance. We also demonstrate how our measures of pattern separation can predict pattern completion accuracy. Overall, our measures solve a widely acknowledged problem in assessing the pattern separation of neural circuits such as the dentate gyrus, as well as the cerebellum and mushroom body. Finally we provide a publicly available toolbox allowing for easy analysis of pattern separation in spike train ensembles.

Enhancing Associative Learning in Rats With a Computationally Designed Training Protocol

Background

Learning requires the activation of protein kinases with distinct temporal dynamics. In Aplysia, nonassociative learning can be enhanced by a computationally designed learning protocol with intertrial intervals (ITIs) that maximize the interaction between fast-activated PKA (protein kinase A) and slow-activated ERK (extracellular signal-regulated kinase). Whether a similar strategy can enhance associative learning in mammals is unknown.

Methods

We simulated 1000 training protocols with varying ITIs to predict an optimal protocol based on empirical data for PKA and ERK dynamics in rat hippocampus. Adult male rats received the optimal protocol or control protocols in auditory fear conditioning and fear extinction experiments. Immunohistochemistry was performed to evaluate pCREB (phosphorylated cAMP response element binding)\protein levels in brain regions that have been implicated in fear acquisition.

Results

Rats exposed to the optimal conditioning protocol with irregular ITIs exhibited impaired extinction memory acquisition within the session using a standard footshock intensity, and stronger fear memory retrieval and spontaneous recovery with a weaker footshock intensity, compared with rats that received massed or spaced conditioning protocols with fixed ITIs. Rats exposed to the optimal extinction protocol displayed improved extinction of contextual fear memory and reduced spontaneous recovery compared with rats that received standard extinction protocols. Moreover, the optimal conditioning protocol increased pCREB levels in the dentate gyrus of the dorsal hippocampus, suggesting enhanced induction of long-term potentiation.

Conclusions

These findings demonstrate that a computational model-driven behavioral intervention can enhance associative learning in mammals and may provide insight into strategies to improve cognition in humans.

Drug memory reconsolidation: from molecular mechanisms to the clinical context

Since its rediscovery at the beginning of the 21st Century, memory reconsolidation has been proposed to be a therapeutic target for reducing the impact of emotional memories that can go awry in mental health disorders such as drug addiction (substance use disorder, SUD). Addiction can be conceptualised as a disorder of learning and memory, in which both pavlovian and instrumental learning systems become hijacked into supporting drug-seeking and drug-taking behaviours. The past two decades of research have characterised the details of the molecular pathways supporting the reconsolidation of pavlovian cue-drug memories, with more recent work indicating that the reconsolidation of instrumental drug-seeking memories also relies upon similar mechanisms. This narrative review considers what is known about the mechanisms underlying the reconsolidation of pavlovian and instrumental memories associated with drug use, how these approaches have translated to experimental medicine studies, and the challenges and opportunities for the clinical use of reconsolidation-based therapies.

Cortical and subcortical grey matter correlates of psychopathic traits in a Japanese community sample of young adults: sex and configurations of factors’ level matter!

While neuroimaging research has examined the structural brain correlates of psychopathy predominantly in clinical/forensic male samples from western countries, much less is known about those correlates in non-western community samples. Here, structural magnetic resonance imaging data were analyzed using voxel- and surface-based morphometry to investigate the neuroanatomical correlates of psychopathic traits in a mixed-sex sample of 97 well-functioning Japanese adults (45 males, 21–39 years; M = 27, SD = 5.3). Psychopathic traits were assessed using the Self-Report Psychopathy Scale (SRP-SF; 4th Edition). Multiple regression analysis showed greater Factor 1 scores were associated with higher gyrification in the lingual gyrus, and gray matter volume in the anterior cingulate cortex and amygdala/hippocampus border. Total psychopathy and Factor 1 scores interacted with sex to, respectively, predict cortical thickness in the precuneus and gyrification in the superior temporal gyrus. Finally, Factor 1 and Factor 2 traits interacted to predict gyrification in the posterior cingulate cortex. These preliminary data suggest that, while there may be commonalities in the loci of structural brain correlates of psychopathic traits in clinical/forensic and community samples, the nature of that association might be different (i.e. positive) and may vary according to sex and configurations of factors’ level.

Inbred Mice Again at Stake: How the Cognitive Profile of the Wild-Type Mouse Background Discloses Pathogenic Effects of APP Mutations

Increasing efforts have been made in the last decades to increase the face validity of Alzheimer's disease (AD) mouse models. Main advancements have consisted in generating AD mutations closer to those identified in humans, enhancing genetic diversity of wild-type backgrounds, and choosing protocols much apt to reveal AD-like cognitive dysfunctions. Nevertheless, two aspects remain less considered: the cognitive specialization of inbred strains used as recipient backgrounds of mutations and the heuristic importance of studying destabilization of memory circuits in pre-symptomatic mice facing cognitive challenges. This article underscores the relevance of these behavioral/experimental aspects by reviewing data which show that (i) inbred mice differ in their innate predisposition to rely on episodic vs. procedural memory, which implicates differential sensitivity to mutations aimed at disrupting temporal lobe-dependent memory, and that (ii) investigating training-driven neural alterations in asymptomatic mutants unveils early synaptic damage, which considerably anticipates detection of AD first signs.

Reinforcement Learning Model With Dynamic State Space Tested on Target Search Tasks for Monkeys: Extension to Learning Task Events

Learning is a crucial basis for biological systems to adapt to environments. Environments include various states or episodes, and episode-dependent learning is essential in adaptation to such complex situations. Here, we developed a model for learning a two-target search task used in primate physiological experiments. In the task, the agent is required to gaze one of the four presented light spots. Two neighboring spots are served as the correct target alternately, and the correct target pair is switched after a certain number of consecutive successes. In order for the agent to obtain rewards with a high probability, it is necessary to make decisions based on the actions and results of the previous two trials. Our previous work achieved this by using a dynamic state space. However, to learn a task that includes events such as fixation to the initial central spot, the model framework should be extended. For this purpose, here we propose a "history-in-episode architecture." Specifically, we divide states into episodes and histories, and actions are selected based on the histories within each episode. When we compared the proposed model including the dynamic state space with the conventional SARSA method in the two-target search task, the former performed close to the theoretical optimum, while the latter never achieved target-pair switch because it had to re-learn each correct target each time. The reinforcement learning model including the proposed history-in-episode architecture and dynamic state scape enables episode-dependent learning and provides a basis for highly adaptable learning systems to complex environments.

Control of parallel hippocampal output pathways by amygdalar long-range inhibition

Projections from the basal amygdala (BA) to the ventral hippocampus (vH) are proposed to provide information about the rewarding or threatening nature of learned associations to support appropriate goal-directed and anxiety-like behaviour. Such behaviour occurs via the differential activity of multiple, parallel populations of pyramidal neurons in vH that project to distinct downstream targets, but the nature of BA input and how it connects with these populations is unclear. Using channelrhodopsin-2-assisted circuit mapping in mice, we show that BA input to vH consists of both excitatory and inhibitory projections. Excitatory input specifically targets BA- and nucleus accumbens-projecting vH neurons and avoids prefrontal cortex-projecting vH neurons, while inhibitory input preferentially targets BA-projecting neurons. Through this specific connectivity, BA inhibitory projections gate place-value associations by controlling the activity of nucleus accumbens-projecting vH neurons. Our results define a parallel excitatory and inhibitory projection from BA to vH that can support goal-directed behaviour.

Anterior thalamic NMDA-induced damage impairs extrapolation relying on serial stimulus patterns, in rats

Extrapolation of serial stimulus patterns seems to depend upon identification and application of patterns relating sequences of stimuli stored in memory, thus allowing prediction of pending events never experienced before. There have been proposals that such a "generator of predictions system" would include the subiculum, mammillary bodies, anteroventral thalamus and cingulate cortex (e.g., Gray, 1982). The anteroventral thalamus (AVT) seems to be in a strategic position, both hodologically and experimentally, to allow testing of this hypothesis. This study investigated the effect of NMDA-induced damage to the anteroventral thalamus [part of the anterodorsal (AD) thalamus was also damaged in some animals], following stereotaxic minute topic microinjections, on the ability of male Wistar rats to extrapolate relying on serial stimulus patterns. Corresponding sham-operated controls received phosphate-saline buffer microinjections at the same stereotaxic coordinates. The subjects were trained to run through a straight alleyway along 31 sessions, one session per day, to get rewarded. Each session included four successive trials. Subjects exposed to the monotonic serial pattern received 14, 7, 3, 1 sunflower seeds along trials. Subjects exposed to the non-monotonic serial pattern received 14, 3, 7, 1 sunflower seeds. On the 32nd testing session, a fifth trial, never experienced before, was included immediately after the fourth trial. Sham-operated control subjects exposed to the monotonic serial pattern were expected to exhibit longer running times, since the content of their prediction in the fifth trial should be "less than 1 sunflower seeds". In contrast, control subjects exposed to the non-monotonic serial pattern were expected to exhibit shorter running times, since the content of their prediction would be "more than 1 sunflower seeds". Confirming these predictions, control subjects exposed to the monotonic serial pattern exhibited longer running times as compared to both, their own running times in previous trials within the same session and control subjects exposed to the non-monotonic schedule, thus indicating the occurrence of extrapolation. In contrast, AVT/AD lesioned subjects exposed to the monotonic schedule did not exhibit this increase in running times on the fifth trial, indicating lack of extrapolation. These results indicate that extrapolation relying on serial stimulus patterns is disrupted following extensive NMDA-induced damage to AVT and part of the AD. This represents the first consistent demonstration that the anterior thalamic nuclei are required for extrapolation of serial stimulus patterns and generation of predictions.

Developing of Focal Ischemia in the Hippocampus or the Amygdala Reveals a Regional Compensation Rule for Fear Memory Acquisition

Circuit compensation is often observed in patients with acute ischemic stroke, suggesting the importance of the interaction between brain regions. Also, contextual fear memory is an association between multisensory contexts and fearful stimuli, for which the interaction between the hippocampus and the amygdala is believed to be critical. To understand how focal ischemia in one region could influence the other region, we used a modified photo-thrombosis to induce focal ischemia in the hippocampus or the amygdala or both in freely-moving rats. We found that the learning curve and short-term memory (STM) were not affected in the rats although focal ischemia was induced 5 h before learning in either the hippocampus or the amygdala; these were impaired by the induction of ischemia in both the regions. Furthermore, the learning curve and STM were impaired when ischemia was induced 24 h before learning in either the hippocampus or the amygdala when the synaptic transmission was altered in one region because of ischemia in the other region. These results suggest that the circuit compensation between the hippocampus and the amygdala is critical for fear memory acquisition.

Linking Social Cognition to Learning and Memory

Many mammals have evolved to be social creatures. In humans, the ability to learn from others' experiences is essential to survival; and from an early age, individuals are surrounded by a social environment that helps them develop a variety of skills, such as walking, talking, and avoiding danger. Similarly, in rodents, behaviors, such as food preference, exploration of novel contexts, and social approach, can be learned through social interaction. Social encounters facilitate new learning and help modify preexisting memories throughout the lifespan of an organism. Moreover, social encounters can help buffer stress or the effects of negative memories, as well as extinguish maladaptive behaviors. Given the importance of such interactions, there has been increasing work studying social learning and applying its concepts in a wide range of fields, including psychotherapy and medical sociology. The process of social learning, including its neural and behavioral mechanisms, has also been a rapidly growing field of interest in neuroscience. However, the term "social learning" has been loosely applied to a variety of psychological phenomena, often without clear definition or delineations. Therefore, this review gives a definition for specific aspects of social learning, provides an overview of previous work at the circuit, systems, and behavioral levels, and finally, introduces new findings on the social modulation of learning. We contextualize such social processes in the brain both through the role of the hippocampus and its capacity to process "social engrams" as well as through the brainwide realization of social experiences. With the integration of new technologies, such as optogenetics, chemogenetics, and calcium imaging, manipulating social engrams will likely offer a novel therapeutic target to enhance the positive buffering effects of social experiences or to inhibit fear-inducing social stimuli in models of anxiety and post-traumatic stress disorder.

Fear acquisition and extinction in elderly patients with depression

BACKGROUND

Depression in elderly patients is common and characterized by anxiety symptoms and cognitive impairment. To our knowledge, no studies have yet investigated the process of fear extinction in these patients. We investigated fear extinction with a paradigm consisting of habituation, acquisition and extinction.

METHODS

We included three age matched (mean age: 75.7 years) groups: Late Life Depression (LLD, n = 33), Mild Cognitive Impairment (MCI, n = 39), healthy controls (HC, n = 39). All participants were diagnosed with a standardized procedure including clinical examination, CERAD cognitive test battery, as well as magnetic resonance imaging. Participants underwent a fear conditioning paradigm consisting of habituation, acquisition, and extinction. During acquisition, a neutral face (conditioned stimulus, CS+) was paired with an electrical unconditioned stimulus, whereas another face (safety stimulus, CS-) was unpaired. Conditioned responses were measured by US-expectancy and valence ratings.

RESULTS

Compared to HC, both patient groups showed a significantly lower, differential (CS+ vs. CS-) fear acquisition across all measurements. Patients with cognitive impairment showed a significantly slower extinction, which is characterized by higher US-expectancy and reduced positive valence for CS+. Fear extinction was significantly less differential (CS+ vs. CS-) in patients with LLD.

LIMITATIONS

Due to the cross-sectional design we cannot distinguish whether the observed differences in fear extinction are state or trait markers in the LLD patients.

CONCLUSIONS

In this study, we demonstrate that fear extinction is impaired in elderly patients with depression. These results can have influence on treatment strategies.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

Human sleep consolidates allergic responses conditioned to the environmental context of an allergen exposure

Allergies are highly prevalent, and allergic responses can be triggered even in the absence of allergens due to Pavlovian conditioning to a specific cue. Here we show in humans suffering from allergic rhinitis that merely reencountering the environmental context in which an allergen was administered a week earlier is sufficient to trigger an allergic response-but only if participants had slept after allergen exposure. This context-conditioning effect was entirely absent when participants stayed awake the night after allergen exposure or were tested in a different context. Unlike in context conditioning, cue conditioning (to an odor stimulus) occurred independently of sleep, a differential pattern that is likewise observed for conditioning in the behavioral domain. Our findings provide evidence that allergic responses can be conditioned to contextual information alone, even after only a single-trial conditioning procedure, and that sleep is necessary to consolidate this rapidly acquired maladaptive response. The results unravel a mechanism that could explain part of the strong psychological impact on allergic responses.

Potential beneficial effects of caffeine administration in the neonatal period of an animal model of schizophrenia

Obstetric complications, like maternal hypertension and neonatal hypoxia, disrupt brain development, leading to psychiatry disorders later in life, like schizophrenia. The exact mechanisms behind this risk are not yet well known. Spontaneously hypertensive rats (SHR) are a well-established model to study neurodevelopment of schizophrenia since they exhibit behavioral alterations mimicking schizophrenia that can be improved with antipsychotic drugs. SHR mothers are hypertensive, and the SHR offspring develop in preeclampsia-like conditions. Hypoxic conditions increase levels of adenosine, which play an important role in brain development. The enhanced levels of adenosine at birth could be related to the future development of schizophrenia. To investigate this hypothesis adenosine levels of brain neonatal Wistar rats and SHR were quantified. After that, caffeine, an antagonist of adenosinergic system, was administrated on PND (postnatal day) 7 (neurodevelopmental age similar to a human at delivery) and rats were observed at adolescent and adult ages. We also investigated the acute effects of caffeine at adolescent and adult ages. SHR control adolescent and adult groups presented behavioral deficits like hyperlocomotion, deficit in social interaction (SI), and contextual fear conditioning (CFC). In SHR, neonatal caffeine treatment on PND 7 normalized hyperlocomotion, improved SI, and CFC observed at adolescent period and adult ages, showing a beneficial effect on schizophrenia-like behaviors. Wistar rats neonatally treated with caffeine exhibited hyperlocomotion, deficit in SI and CFC when observed at adolescent and adult ages. Acutely caffeine treatment administrated at adolescent and adult ages increased locomotion and decreased SI time of Wistar rats and impair CFC in adult Wistars. No effects were observed in SHR. In conclusion, caffeine can be suggested as a useful drug to prevent behavioral deficits observed in this animal model of prenatal hypoxia-induced schizophrenia profile when specifically administered on PND 7.

PTSD-related neuroimaging abnormalities in brain function, structure, and biochemistry

Although approximately 90% of the U.S. population will experience a traumatic event within their lifetime, only a fraction of those traumatized individuals will develop posttraumatic stress disorder (PTSD). In fact, approximately 7 out of 100 people in the U.S. will be afflicted by this debilitating condition, which suggests there is substantial inter-individual variability in susceptibility to PTSD. This uncertainty regarding who is susceptible to PTSD necessitates a thorough understanding of the neurobiological processes that underlie PTSD development in order to build effective predictive models for the disorder. In turn, these predictive models may lead to the development of improved diagnostic markers, early intervention techniques, and targeted treatment approaches for PTSD. Prior research has characterized a fear learning and memory network, centered on the prefrontal cortex, hippocampus, and amygdala, that plays a key role in the pathology of PTSD. Importantly, changes in the function, structure, and biochemistry of this network appear to underlie the cognitive-affective dysfunction observed in PTSD. The current review discusses prior research that has demonstrated alterations in brain function, structure, and biochemistry associated with PTSD. Further, the potential for future research to address current gaps in our understanding of the neural processes that underlie the development of PTSD is discussed. Specifically, this review emphasizes the need for multimodal neuroimaging research and investigations into the acute effects of posttraumatic stress. The present review provides a framework to move the field towards a comprehensive neurobiological model of PTSD.

Modulation of intrinsic excitability as a function of learning within the fear conditioning circuit

Experience-dependent neuronal plasticity is a fundamental substrate of learning and memory. Intrinsic excitability is a form of neuronal plasticity that can be altered by learning and indicates the pattern of neuronal responding to external stimuli (e.g. a learning or synaptic event). Associative fear conditioning is one form of learning that alters intrinsic excitability, reflecting an experience-dependent change in neuronal function. After fear conditioning, intrinsic excitability changes are evident in brain regions that are a critical part of the fear circuit, including the amygdala, hippocampus, retrosplenial cortex, and prefrontal cortex. Some of these changes are transient and/or reversed by extinction as well as learning-specific (i.e. they are not observed in neurons from control animals). This review will explore how intrinsic neuronal excitability changes within brain structures that are critical for fear learning, and it will also discuss evidence promoting intrinsic excitability as a vital mechanism of associative fear memories. This work has raised interesting questions regarding the role of fear learning in changes of intrinsic excitability within specific subpopulations of neurons, including those that express immediate early genes and thus demonstrate experience-dependent activity, as well as in neurons classified as having a specific firing type (e.g. burst-spiking vs. regular-spiking). These findings have interesting implications for how intrinsic excitability can serve as a neural substrate of learning and memory, and suggest that intrinsic plasticity within specific subpopulations of neurons may promote consolidation of the memory trace in a flexible and efficient manner.

Fear not: recent advances in understanding the neural basis of fear memories and implications for treatment development

Fear is a highly adaptive emotion that has evolved to promote survival and reproductive fitness. However, maladaptive expression of fear can lead to debilitating stressor-related and anxiety disorders such as post-traumatic stress disorder. Although the neural basis of fear has been extensively researched for several decades, recent technological advances in pharmacogenetics and optogenetics have allowed greater resolution in understanding the neural circuits that underlie fear. Alongside conceptual advances in the understanding of fear memory, this increased knowledge has clarified mechanisms for some currently available therapies for post-traumatic stress disorder and has identified new potential treatment targets.

Intraperitoneal 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues

The effects of the serotonergic (5-hydroxytryptamine, 5-HT) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 and 0.4 mg/kg i.p.) were examined in trace conditioning (Experiment 1) and overshadowing (Experiment 2) procedures. Both experiments used a fear conditioning procedure conducted off-the-baseline in water deprived male Wistar rats. 8-OH-DPAT was administered during conditioning and its effects were examined drug free as the suppression of an established licking response, both upon re-exposure to the cues provided by the conditioning chambers and upon presentation of experimental stimuli. There were no statistically significant effects of 8-OH-DPAT on conditioning to the discrete cue provided by a 5 s conditioned stimulus (CS), irrespective of the length of the trace interval used in Experiment 1, and irrespective of whether the CS took the form of a light alone, or a noise plus light compound in the Experiment 2 overshadowing procedure. The successful demonstration of overshadowing required the use of a second conditioning session which allowed further evaluation of the effects of 8-OH-DPAT in that neither a weak nor a strong overshadowing effect was modulated by either drug dose. Nonetheless conditioning to contextual cues was attenuated by treatment with 8-OH-DPAT at the 30 s trace interval. We therefore conclude that 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues. This pattern of results lends further support to the view that contextual cue conditioning and discrete cue conditioning are modulated by different neuropharmacological mechanisms.

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8-OH-DPAT (0.2 and 0.4 mg/kg i.p.) was tested in two fear conditioning procedures.

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8-OH-DPAT reduced conditioning to contextual cues at a 30 s trace.

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However, overshadowing produced by presentation of a compound cue was unaffected by 8-OH-DPAT.

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8-OH-DPAT reduced competition from contextual but not discrete conditioning cues.

The anxiolytic-like effects of ginsenoside Rg2 on an animal model of PTSD

Post traumatic stress disorder (PTSD) is one of the mental illness. The antidepressant-like properties of ginsenoside Rg2 (GRg2) have been shown, while little is known about its anti-PTSD-like effects. In the present study, the PTSD-associated behavioral deficits in rats were induced following exposure to single prolonged stress (SPS). The results showed that the decreased time and entries in the open arms in elevated plus maze test (EPMT) and increased freezing duration in contextual fear paradigm (CFP) were reversed by GRg2 (10 and 20 mg/kg) without affecting the locomotor activity. In addition, GRg2 (10 and 20 mg/kg) could block the decreased levels of progesterone, allopregnanolone, serotonin (5-HT), 5-Hydroxyindoleacetic acid (5-HIAA), corticotropin releasing hormone (CRH), corticosterone (Cort) and adrenocorticotropic hormone (ACTH) in the brain or serum. In summary, GRg2 alleviated the PTSD-associated behavioral deficits with biosynthesis of neurosteroids, normalization of serotonergic system and HPA axis dysfunction.

Conditioned contextual fear memory to assess natural forgetting and cognitive enhancement in rats

Aversively established contextual fear memory manifests itself in robust freezing behavior, often lasting several weeks or months. Therefore, this approach is amenable to investigate the underlying neural circuitries by lesion or inactivation of specific brain regions or to test efficacy of substances that disrupt either the ability to acquire the association or to retrieve memories. In contrast, investigation of memory enhancement using this technique is time intensive since the non-treated control group naturally forgets the learned association only weeks after acquisition. Pharmacological interventions have been used to overcome this time span by disrupting memory at any time point, however, limiting it a mechanistic model of reversal of impairments instead of studying memory enhancement. Here, we investigated several parameters of the cued and contextual fear conditioning (CFC) protocol such that, while memory acquisition is established, loss of fear association occurs within a shorter time frame, allowing studies of memory enhancement in the context of natural forgetting. We found that three predictive tone-cues, each separated from a 0.3 mA foot shock by an interstimulus interval of 2 s and a pre-exposure to the context enables the investigation of enhanced contextual memory 7 d post training without the necessity of inducing pharmacological lesions.

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu₂ and mGlu₃) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu₃ induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu₃-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu₃-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu₃ activation in vivo prevented the stress-induced maladaptive changes to amydalo-cortical physiology and motivated behavior. These data demonstrate that mGlu₃ mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu₃ modulation as a treatment for stress-related psychiatric disorders.

Dexmedetomidine reduced sevoflurane-induced neurodegeneration and long-term memory deficits in neonatal rats

Exposure to sevoflurane and other inhalational anesthetics can induce neurodegeneration in the developing brain. Although dexmedetomidine (DEX) has provided neuroprotection against hypoxic ischemic injury, relatively little is known about whether it has the neuroprotective effects against anesthetic-induced neurodegeneration. This study examined whether DEX improves the long-term cognitive dysfunction observed after exposure of neonatal rats to 3% sevoflurane. Seven-day-old rats received intraperitoneal saline (DEX 0) or DEX (6.6, 12.5, 25 μg/kg) 30 min before exposure to 3% sevoflurane with 21% oxygen for 4 h (n = 10 per group). The pups in the control group received only DEX 25 μg/kg without anesthesia. The escape latency in the Morris water maze was significantly increased in the DEX 0 group compared with the sham and control group, and the escape latency, but not the swimming path length, was significantly shorter at post-natal day 47 in the DEX 25 than in the DEX 0 group. The percent time spent in the quadrant was significantly decreased in the DEX 0 group compared with the sham and control group, and the percent time spent in the quadrant was significantly increased in the DEX 25 group compared with the DEX 0 groups. The freezing times of the DEX 0 and 6.6 groups were significantly decreased compared with those in the sham, control and DEX 25 groups. The number of NeuN-positive cells in the CA1 region was significantly decreased in the DEX 0 and 6.6 groups compared with the sham, control and DEX 25 groups. These findings indicate pre-treatment with DEX may improve long-term cognitive function and ameliorate the neuronal degeneration induced by sevoflurane exposure in neonatal rats.

Increased anxiety-like behavior following circuit-specific catecholamine denervation in mice

Parkinson's disease (PD) presents with a constellation of non-motor symptoms, notably increased anxiety, which are currently poorly treated and underrepresented in animal models of the disease. Human post-mortem studies report loss of catecholaminergic neurons in the pre-symptomatic phases of PD when anxiety symptoms emerge, and a large literature from rodent and human studies indicate that catecholamines are important mediators of anxiety via their modulatory effects on limbic regions such as the amygdala. On the basis of these observations, we hypothesized that anxiety in PD could result from an early loss of catecholaminergic inputs to the amygdala and/or other limbic structures. To interrogate this hypothesis, we bilaterally injected the neurotoxin 6-OHDA in the mouse basolateral amygdala (BL). This produced a restricted pattern of catecholaminergic (tyrosine-hydroxylase-labeled) denervation in the BL, intercalated cell masses and ventral hippocampus, but not the central amygdala or prefrontal cortex. We found that this circuit-specific lesion did not compromise performance on multiple measures of motor function (home cage, accelerating rotarod, beam balance, pole climbing), but did increase anxiety-like behavior in the elevated plus-maze and light-dark exploration tests. Fear behavior in the pavlovian cued conditioning and passive avoidance assays was, by contrast, unaffected; possibly due to preservation of catecholamine innervation of the central amygdala from the periaqueductal gray. These data provide some of the first evidence implicating loss of catecholaminergic neurotransmission in midbrain-amygdala circuits to increased anxiety-like behavior. Our findings offer an initial step towards identifying the neural substrates for pre-motor anxiety symptoms in PD.

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.

Hippocampal Homer1b/c is necessary for contextual fear conditioning and group I metabotropic glutamate receptor mediated long-term depression

Coiled-coil forms of Homer1, including Homer1b and c (Homer1b/c) have been shown to play a role in hippocampal learning and memory and synaptic plasticity. We have previously found that overexpression of hippocampal Homer1c is sufficient to rescue learning and memory ability in aged learning impaired rats and in Homer1 knockout (KO) mice, and to rescue group I metabotropic glutamate receptor (mGluR1/5) mediated long-term potentiation in KO mice. Here, to determine if Homer1b/c is necessary for successful learning and memory we have utilized a rAAV5 vector expressing a Homer1b/c-targeting short hairpin RNA to knock down the expression of hippocampal Homer1b/c in adult 4-6-month old male Sprague Dawley rats. We have found that reduced hippocampal Homer1b/c expression elicits significant learning deficits in contextual fear conditioning, but not in the Morris water maze or novel object recognition tasks. Furthermore, we demonstrate that reduced hippocampal Homer1b/c is sufficient to completely block mGluR1/5 mediated long-term depression in the Schaffer collateral pathway. These results support a significant role for Homer1b/c in learning and synaptic plasticity; however, the exact role of each of these two protein isoforms in learning and memory remains elusive.

Self-efficacy modulates the neural correlates of craving in male smokers and ex-smokers: an fMRI study

The regulation of cue-induced craving for cigarettes is a key factor in smoking cessation. Outcomes of smoking cessation have been linked to self-efficacy, faith in one's own ability, in smokers. However, no study has examined the neural basis of self-efficacy during the control of craving. We examined whether self-efficacy can affect the neural response to smoking cues in smokers and ex-smokers using functional magnetic resonance imaging. During scanning, participants were instructed (1) to view smoking-related images passively, (2) to view the smoking-related images with a strategy focused on self-efficacy to control cue-induced craving or (3) to view neutral images. In smokers, the self-efficacy strategy significantly reduced self-reported craving. This strategy was related to increased activation in the rostral medial prefrontal cortex (rmPFC) and the pregenual anterior cingulate cortex in smokers compared with ex-smokers. Furthermore, smokers showed increased effective connectivity between rmPFC and hippocampus and between pregenual anterior cingulate cortex and parahippocampus gyrus when employing the self-efficacy strategy compared with ex-smokers. The magnitude of the rmPFC-hippocampus connectivity was positively correlated with self-reported self-efficacy. Our findings suggest that in smokers, self-efficacy is related to activation and connectivity in brain regions involved in regulating craving and self-assessment. The current study provides evidence for understanding the vunderlying cognitive and neurobiological mechanisms involved in the control of craving to smoke cigarettes.

Oxytocin facilitates adaptive fear and attenuates anxiety responses in animal models and human studies-potential interaction with the corticotropin-releasing factor (CRF) system in the bed nucleus of the stria terminalis (BNST)

Despite its relatively well-understood role as a reproductive and pro-social peptide, oxytocin (OT) tells a more convoluted story in terms of its modulation of fear and anxiety. This nuanced story has been obscured by a great deal of research into the therapeutic applications of exogenous OT, driving more than 400 ongoing clinical trials. Drawing from animal models and human studies, we review the complex evidence concerning OT's role in fear learning and anxiety, clarifying the existing confusion about modulation of fear versus anxiety. We discuss animal models and human studies demonstrating the prevailing role of OT in strengthening fear memory to a discrete signal or cue, which allows accurate and rapid threat detection that facilitates survival. We also review ostensibly contrasting behavioral studies that nonetheless provide compelling evidence of OT attenuating sustained contextual fear and anxiety-like behavior, arguing that these OT effects on the modulation of fear vs. anxiety are not mutually exclusive. To disambiguate how endogenous OT modulates fear and anxiety, an understudied area compared to exogenous OT, we survey behavioral studies utilizing OT receptor (OTR) antagonists. Based on emerging evidence about the role of OTR in rat dorsolateral bed nucleus of stria terminalis (BNST) and elsewhere, we postulate that OT plays a critical role in facilitating accurate discrimination between stimuli representing threat and safety. Supported by human studies, we demonstrate that OT uniquely facilitates adaptive fear but reduces maladaptive anxiety. Last, we explore the limited literature on endogenous OT and its interaction with corticotropin-releasing factor (CRF) with a special emphasis on the dorsolateral BNST, which may hold the key to the neurobiology of phasic fear and sustained anxiety.

Contextual Fear Memory Formation and Destabilization Induce Hippocampal RyR2 Calcium Channel Upregulation

Hippocampus-dependent spatial and aversive memory processes entail Ca2+ signals generated by ryanodine receptor (RyR) Ca2+ channels residing in the endoplasmic reticulum membrane. Rodents exposed to different spatial memory tasks exhibit significant hippocampal RyR upregulation. Contextual fear conditioning generates robust hippocampal memories through an associative learning process, but the effects of contextual fear memory acquisition, consolidation, or extinction on hippocampal RyR protein levels remain unreported. Accordingly, here we investigated if exposure of male rats to contextual fear protocols, or subsequent exposure to memory destabilization protocols, modified the hippocampal content of type-2 RyR (RyR2) channels, the predominant hippocampal RyR isoforms that hold key roles in synaptic plasticity and spatial memory processes. We found that contextual memory retention caused a transient increase in hippocampal RyR2 protein levels, determined 5 h after exposure to the conditioning protocol; this increase vanished 29 h after training. Context reexposure 24 h after training, for 3, 15, or 30 min without the aversive stimulus, decreased fear memory and increased RyR2 protein levels, determined 5 h after reexposure. We propose that both fear consolidation and extinction memories induce RyR2 protein upregulation in order to generate the intracellular Ca2+ signals required for these distinct memory processes.

Neonatal pain and stress disrupts later-life pavlovian fear conditioning and sensory function in rats: Evidence for a two-hit model

Early life trauma has been linked to increased risks for anxiety, depression, and chronic pain. We used rodent models of acute and inflammatory neonatal pain to explore effects on fear conditioning and somatosensory function. Hindpaw needle pricks or handling on postnatal days (PNDs) 1-7 caused lasting impacts on affective and somatosensory function when assessed at later ages, PNDs 24 (postweaning), 45 (adolescence), or 66 (adulthood). First, auditory, but not contextual, freezing was mildly disrupted regardless of age. Second, a profound postfear conditioning tactile hypersensitivity was observed in neonatally stressed, postweaning rats. In the absence of fear conditioning, the mechanical hypersensitivity was not observed, consistent with a two-hit model of psychopathology. Injections of 2% α-carrageenan did not have the same lasting impact but was slightly protective against observed effects of neonatal vehicle injections. Basal and elicited corticosterone levels postweaning were not altered by neonatal pain or handling. These data demonstrate that neonatal adversity can have lasting impacts on affective and somatosensory function that differs regardless of age.

Using New Approaches in Neurobiology to Rethink Stress-Induced Amnesia

Purpose of Review

Psychological stress can impact memory systems in several different ways. In individuals with healthy defense and coping systems, stress results in the formation of negatively valenced memories whose ability to induce emotional and somatic distress subsides with time. Vulnerable individuals, however, go on to develop stress-related disorders such as post-traumatic stress disorder (PTSD) and suffer from significant memory abnormalities. Whether expressed as intrusive trauma memories, partial amnesia, or dissociative amnesia, such abnormalities are thought to be the core source of patients' symptoms, which are often debilitating and implicate an entire socio-cognitive-affective spectrum.

Recent Findings

With this in mind, and focusing on stress-responsive hippocampal microcircuits, this article highlights recent advances in the neurobiology of memory that allow us to (1) isolate and visualize memory circuits, (2) change their activity using genetic tools and state-dependent manipulations, and (3) directly examine their impact on socio-affective circuits and global network connectivity. By integrating these approaches, we are now in a position to address important questions that have troubled psychiatry for a long time-questions such as are traumatic memories special, and why are stress effects on memory diverse.

Summary

Furthering our fundamental understanding of memory in the framework of adaptive and maladaptive stress responses has the potential to boost the development of new treatments that can benefit patients suffering from psychological trauma.

Adar3 Is Involved in Learning and Memory in Mice

The amount of regulatory RNA encoded in the genome and the extent of RNA editing by the post-transcriptional deamination of adenosine to inosine (A-I) have increased with developmental complexity and may be an important factor in the cognitive evolution of animals. The newest member of the A-I editing family of ADAR proteins, the vertebrate-specific ADAR3, is highly expressed in the brain, but its functional significance is unknown. In vitro studies have suggested that ADAR3 acts as a negative regulator of A-I RNA editing but the scope and underlying mechanisms are also unknown. Meta-analysis of published data indicates that mouse Adar3 expression is highest in the hippocampus, thalamus, amygdala, and olfactory region. Consistent with this, we show that mice lacking exon 3 of Adar3 (which encodes two double stranded RNA binding domains) have increased levels of anxiety and deficits in hippocampus-dependent short- and long-term memory formation. RNA sequencing revealed a dysregulation of genes involved in synaptic function in the hippocampi of Adar3-deficient mice. We also show that ADAR3 transiently translocates from the cytoplasm to the nucleus upon KCl-mediated activation in SH-SY5Y cells. These results indicate that ADAR3 contributes to cognitive processes in mammals.

Neural representations of time-linked memory

Many cognitive processes, such as episodic memory and decision making, rely on the ability to form associations between two events that occur separately in time. The formation of such temporal associations depends on neural representations of three types of information: what has been presented (trace holding), what will follow (temporal expectation), and when the following event will occur (explicit timing). The present review seeks to link these representations with firing patterns of single neurons recorded while rodents and non-human primates associate stimuli, outcomes, and motor responses over time intervals. Across these studies, two distinct firing patterns were observed in the hippocampus, neocortex, and striatum: some neurons change firing rates during or shortly after the stimulus presentation and sustain the firing rate stably or sidlingly during the subsequent intervals (tonic firings). Other neurons transiently change firing rates during a specific moment within the time intervals (phasic firings), and as a group, they form a sequential firing pattern that covers the entire interval. Clever task designs used in some of these studies collectively provide evidence that both tonic and phasic firing responses represent trace holding, temporal expectation, and explicit timing. Subsequently, we applied machine-learning based classification approaches to the two firing patterns within the same dataset collected from rat medial prefrontal cortex during trace eyeblink conditioning. This quantitative analysis revealed that phasic-firing patterns showed greater selectivity for stimulus identity and temporal position than tonic-firing patterns. Our summary illuminates distributed neural representations of temporal association in the forebrain and generates several ideas for future investigations.

The role of prefrontal-subcortical circuitry in negative bias in anxiety: Translational, developmental and treatment perspectives

Anxiety disorders are the most common cause of mental ill health in the developed world, but our understanding of symptoms and treatments is not presently grounded in knowledge of the underlying neurobiological mechanisms. In this review, we discuss accumulating work that points to a role for prefrontal-subcortical brain circuitry in driving a core psychological symptom of anxiety disorders - negative affective bias. Specifically, we point to converging work across humans and animal models, suggesting a reciprocal relationship between dorsal and ventral prefrontal-amygdala circuits in promoting and inhibiting negative bias, respectively. We discuss how the developmental trajectory of these circuits may lead to the onset of anxiety during adolescence and, moreover, how effective pharmacological and psychological treatments may serve to shift the balance of activity within this circuitry to ameliorate negative bias symptoms. Together, these findings may bring us closer to a mechanistic, neurobiological understanding of anxiety disorders and their treatment.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Cognitive avoidance and aversive cues related to tobacco in male smokers

BACKGROUND

Treatment using aversive conditioning has been suggested for smoking cessation. The efficacy of this method is thought to be associated with cognitive avoidance. We compare differences in avoidance traits and patterns of associated brain activation in response to cues that induce cravings versus aversion between smokers and non-smokers.

METHODS

Fifteen smokers and fifteen non-smokers completed cue reactivity tasks while undergoing functional magnetic resonance imaging (fMRI) to examine brain responses to craving-inducing cues (Cr) and aversion-inducing cues (Av). Participant avoidant traits were also assessed.

RESULTS

Activation of the left frontal subcallosal gyrus in response to Cr was greater in smokers than in non-smokers. Smokers showed less activation in the right temporal lobe in response to Av than did non-smokers. Brain activation in response to Cr in the left frontal subcallosal gyrus was positively correlated with Fagerstrom Test for Nicotine Dependence (FTND) scores in smokers. Brain activation in response to Av in the right temporal lobe was negatively correlated with the Korean Version of the Cognitive Avoidance Questionnaire (KCAQ) scores in non-smokers.

CONCLUSIONS

Cognitive avoidance in smokers during aversive stimulation might result in sustaining addictive behaviors. On the other hand, non-smokers may be able to emotionally confront the adverse effects of smoking.

Emotional learning, stress, and development: An ever-changing landscape shaped by early-life experience

The capacity to learn to associate cues with negative outcomes is a highly adaptive process that appears to be conserved across species. However, when the cue is no longer a valid predictor of danger, but the emotional response persists, this can result in maladaptive behaviors, and in humans contribute to debilitating emotional disorders. Over the past several decades, work in neuroscience, psychiatry, psychology, and biology have uncovered key processes underlying, and structures governing, emotional responding and learning, as well as identified disruptions in the structural and functional integrity of these brain regions in models of pathology. In this review, we highlight some of this elegant body of work as well as incorporate emerging findings from the field of developmental neurobiology to emphasize how development contributes to changes in the ability to learn and express emotional responses, and how early experiences, such as stress, shape the development and functioning of these circuits.

Involvement of the prelimbic cortex in contextual fear conditioning with temporal and spatial discontinuity

Time plays an important role in conditioning, it is not only possible to associate stimuli with events that overlap, as in delay fear conditioning, but it is also possible to associate stimuli that are discontinuous in time, as shown in trace conditioning for a discrete stimuli. The environment itself can be a powerful conditioned stimulus (CS) and be associated to unconditioned stimulus (US). Thus, the aim of the present study was to determine the parameters in which contextual fear conditioning occurs by the maintenance of a contextual representation over short and long time intervals. The results showed that a contextual representation can be maintained and associated after 5s, even in the absence of a 15s re-exposure to the training context before US delivery. The same effect was not observed with a 24h interval of discontinuity. Furthermore, optimal conditioned response with a 5s interval is produced only when the contexts (of pre-exposure and shock) match. As the pre-limbic cortex (PL) is necessary for the maintenance of a continuous representation of a stimulus, the involvement of the PL in this temporal and contextual processing was investigated. The reversible inactivation of the PL by muscimol infusion impaired the acquisition of contextual fear conditioning with a 5s interval, but not with a 24h interval, and did not impair delay fear conditioning. The data provided evidence that short and long intervals of discontinuity have different mechanisms, thus contributing to a better understanding of PL involvement in contextual fear conditioning and providing a model that considers both temporal and contextual factors in fear conditioning.

Brain-wide maps of Fos expression during fear learning and recall

Fos induction during learning labels neuronal ensembles in the hippocampus that encode a specific physical environment, revealing a memory trace. In the cortex and other regions, the extent to which Fos induction during learning reveals specific sensory representations is unknown. Here we generate high-quality brain-wide maps of Fos mRNA expression during auditory fear conditioning and recall in the setting of the home cage. These maps reveal a brain-wide pattern of Fos induction that is remarkably similar among fear conditioning, shock-only, tone-only, and fear recall conditions, casting doubt on the idea that Fos reveals auditory-specific sensory representations. Indeed, novel auditory tones lead to as much gene induction in visual as in auditory cortex, while familiar (nonconditioned) tones do not appreciably induce Fos anywhere in the brain. Fos expression levels do not correlate with physical activity, suggesting that they are not determined by behavioral activity-driven alterations in sensory experience. In the thalamus, Fos is induced more prominently in limbic than in sensory relay nuclei, suggesting that Fos may be most sensitive to emotional state. Thus, our data suggest that Fos expression during simple associative learning labels ensembles activated generally by arousal rather than specifically by a particular sensory cue.

15-Deoxy-Δ12,14-prostaglandin J2 induced neurotoxicity via suppressing phosphoinositide 3-kinase

15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂) induces neuronal cell death via apoptosis independently of its receptors. 15d-PGJ₂ inhibits growth factor-induced cell proliferation of primary astrocytes via down-regulating phosphoinositide 3-kinase (PI3K)-Akt pathway. Although 15d-PGJ₂-reduced cell viability is accompanied with attenuation of the PI3K signaling in neuroblastoma, it has not been sufficiently clarified how 15d-PGJ₂ induces cell death in primary neurons. Here, we found that 15d-PGJ₂ exhibited neurotoxicity via inhibiting the PI3K signaling in the primary culture of rat cortical neurons. A PI3K inhibitor induced neuronal cell death regardless serum throughout maturation, confirming that PI3K is required for neuronal cell survival. The inhibitor disrupted neuronal cell bodies, shortened neurites thinly, damaged plasma membranes and activated caspase-3 similarly to 15d-PGJ₂. Little additive or synergistic neurotoxicity was detected between 15d-PGJ₂ and the PI3K inhibitor. A PI3K activator prevented neurons from undergoing the 15d-PGJ₂-induced cell death in vitro. In vivo, the PI3K signaling is required for contextual memory retrieval, which was impaired by bilateral injection of 15d-PGJ₂ into hippocampus. The activator suppressed the 15d-PGJ₂-impaired memory retrieval significantly. In neurons as well as primary astrocytes and neuroblastomas, 15d-PGJ₂ exhibited cytotoxicity via suppressing the PI3K-Akt pathway in vivo and in vitro.

Individual difference in prepulse inhibition does not predict spatial learning and memory performance in C57BL/6 mice

The startle reflex to an intense acoustic pulse stimulus is attenuated if the pulse stimulus is shortly preceded by a weak non-startling prepulse stimulus. This attenuation of the startle reflex represents a form of pre-attentional sensory gating known as prepulse inhibition (PPI). Although PPI does not require learning, its expression is regulated by higher cognitive processes. PPI deficits have been detected in several psychiatric conditions including schizophrenia where they co-exist with cognitive deficits. A potential link between PPI expression and cognitive performance has therefore been suggested such that poor PPI may predict, or may be mechanistically linked to, overt cognitive impairments. A positive relationship between PPI and strategy formation, planning efficiency, and execution speed has been observed in healthy humans. However, parallel studies in healthy animals are rare. It thus remains unclear what cognitive domains may be associated with, or orthogonal to, sensory gating in the form of PPI in healthy animals. The present study evaluated a potential link between the magnitude of PPI and spatial memory performance by comparing two subgroups of animals differing substantially in baseline PPI expression (low-PPI vs high-PPI) within a homogenous cohort of 100 male adult C57BL/6 mice. Assessment of spatial reference memory in the Morris water maze and spatial recognition memory in the Y-maze failed to reveal any difference between low-PPI and high-PPI subjects. These negative findings contrast with our previous reports that individual difference in PPI correlated with sustained attention and working memory performance in C57BL/6 mice.

Effects of dopamine D1 modulation of the anterior cingulate cortex in a fear conditioning procedure

The anterior cingulate cortex (AC) component of the medial prefrontal cortex (mPFC) has been implicated in attention and working memory as measured by trace conditioning. Since dopamine (DA) is a key modulator of mPFC function, the present study evaluated the role of DA receptor agents in rat AC, using trace fear conditioning. A conditioned stimulus (CS, noise) was followed by an unconditioned stimulus (US, shock) with or without a 10s trace interval interposed between these events in a between-subjects design. Conditioned suppression of drinking was assessed in response to presentation of the CS or an experimental background stimulus (flashing lights, previously presented for the duration of the conditioning session). The selective D1 agonist SKF81297 (0.05μg/side) or D1 antagonist SCH23390 (0.5μg/side) was administered by intra-cerebral microinfusion directly into AC. It was predicted that either of these manipulations should be sufficient to impair trace (but not delay) conditioning. Counter to expectation, there was no effect of DA D1 modulation on trace conditioning as measured by suppression to the noise CS. However, rats infused with SKF81297 acquired stronger conditioned suppression to the experimental background stimulus than those infused with SCH23390 or saline. Thus, the DA D1 agonist SKF81297 increased conditioned suppression to the contextual background light stimulus but was otherwise without effect on fear conditioning.

Distinct pathways for norepinephrine- and opioid-triggered antinociception from the amygdala

BACKGROUND

The amygdala has an important role in pain and pain modulation. We showed previously in animal studies that α2 -adrenoreceptor activation in the central nucleus of the amygdala (CeA) mediates hypoalgesia produced by restraint stress, and that direct application of an α2 -agonist in this region produces analgesia.

AIMS

In the present animal experiments, we investigated the pathways through which α2 -sensitive systems in the CeA produce behavioural analgesia. The CeA has dense connections to a descending pain modulatory network, centred in the midbrain periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM), which is implicated in various forms of stress-related hypoalgesia and which mediates the antinociceptive effect of morphine applied in the basolateral amygdala. We investigated whether this circuit mediates the hypoalgesic effects of α2 -adrenergic agonist administration into the CeA as well as the contribution of endogenous opioids and cannabinoids. We also tested the possibility that activation of α2 -receptors in the CeA produces antinociception by recruitment of noradrenergic pathways projecting to the spinal cord.

RESULTS

Hypoalgesia resulting from bilateral application of the α2 -adrenergic agonist clonidine in the CeA was not reversed by chemical inactivation of the RVM or by systemic injections of naloxone (μ-opioid antagonist) or rimonabant (CB1 antagonist). By contrast, spinal α2 -receptor blockade (intrathecal idazoxan) completely prevented the hypoalgesic effect of clonidine in the CeA, and unmasked a small but significant hyperalgesia.

CONCLUSION

In rats, adrenergic actions in the CeA mediating hypoalgesia require spinal adrenergic neurotransmission but not the PAG-RVM pain modulatory network, or opiate or cannabinoid systems.