The form and function of hippocampal context representations

Association between dietary zinc intake and epilepsy: findings from NHANES 2013-2018 and a Mendelian randomization study

Background

The association between dietary zinc intake and epilepsy remains unclear. This study aimed to investigate the relationship between zinc intake from the diet and epilepsy, employing Mendelian randomization (MR) to explore potential causal links between zinc and epilepsy.

Methods

The cross-sectional study utilized data from the National Health and Nutrition Examination Survey (NHANES) conducted between 2013 and 2018. Among the 4,434 participants included, 1.5% (67/4,434) reported having epilepsy. Restricted cubic spline models and logistic regression models were employed to examine the relationships between dietary zinc intakes and epilepsy. Subsequently, a 2-sample Mendelian randomization (MR) analysis was conducted using the inverse variance weighted (IVW) approach as the primary analysis.

Results

In the restricted cubic spline (RCS) analysis, the relationship between dietary zinc consumption and epilepsy displayed an L-shaped curve (nonlinear, p = 0.049). After multivariate adjustments, the adjusted odds ratios for epilepsy in T2 (5.0-11.0 mg/day) and T3 (≥11.0 mg/day) were 0.49 (95% confidence interval [CI]: 0.26-0.92, p = 0.026) and 0.60 (95% CI: 0.31-1.17, p = 0.132), respectively, compared to the lowest dietary zinc consumption tertile (T1, ≤5.0 mg/day). The IVW method indicated that genetically predicted zinc intake per standard-deviation increase was inversely associated with three types of epilepsy, including all types of epilepsy (OR = 1.06, 95% CI: 1.02-1.11, p = 0.008), generalized epilepsy (OR = 1.13, 95% CI: 1.01-1.25, p = 0.030), and focal epilepsy (documented hippocampal sclerosis) (OR = 1.01, 95% CI: 1.00-1.02, p = 0.025).

Conclusion

Our findings suggest that a daily zinc intake ranging from 5.0 to 11.0 mg is associated with the lowest risk of epilepsy. Furthermore, Mendelian randomization (MR) studies provide additional support for the existence of a causal relationship between zinc and epilepsy.

AMPA and NMDA receptors in dentate gyrus mediate memory for sucrose in two port discrimination task

Although the phenomenon of memory formation and recall associated with the use of psychotropic drugs has been extensively studied, mechanisms underlying memories for natural reward have not been clarified. Herein, we test the hypothesis that glutamatergic receptors in the dentate gyrus play a role in memories associated with sucrose. We used pellet self-administration protocol to generate memories in two-port nose-poke discrimination task using male Wistar rats. During non-rewarded probe trial, the conditioned animals readily discriminated the active port versus inactive port and showed massive increase in mRNA expression of AMPA receptor subunit genes (gria2, gria3) as well as c-Fos protein in the DG. Access to sweet pellet further enhanced c-Fos expression in the DG. However, animals pre-treated with AMPA receptor antagonist CNQX (intra-DG), on exposure to operant chamber (no pellet), showed decreased discrimination as well as c-Fos expression. We suggest that AMPA receptors in DG mediate recall and consolidation of memories associated with sucrose consumption. CNQX pre-treated animals, if presented with sweet pellet on nose poke, exhibited high discrimination index coupled with increased c-Fos expression. In these CNQX treated rats, the DI was again decreased following administration of NMDA receptor antagonist AP5. We suggest that, although AMPA receptors are blocked, the access to sweet pellet may induce surge of glutamate in the DG, which in turn may reinstate memories via activation of erstwhile silent synapses in NMDA dependant manner.

A comparison of hippocampal and retrosplenial cortical spatial and contextual firing patterns

The hippocampus (HPC) and retrosplenial cortex (RSC) are key components of the brain's memory and navigation systems. Lesions of either region produce profound deficits in spatial cognition and HPC neurons exhibit well-known spatial firing patterns (place fields). Recent studies have also identified an array of navigation-related firing patterns in the RSC. However, there has been little work comparing the response properties and information coding mechanisms of these two brain regions. In the present study, we examined the firing patterns of HPC and RSC neurons in two tasks which are commonly used to study spatial cognition in rodents, open field foraging with an environmental context manipulation and continuous T-maze alternation. We found striking similarities in the kinds of spatial and contextual information encoded by these two brain regions. Neurons in both regions carried information about the rat's current spatial location, trajectories and goal locations, and both regions reliably differentiated the contexts. However, we also found several key differences. For example, information about head direction was a prominent component of RSC representations but was only weakly encoded in the HPC. The two regions also used different coding schemes, even when they encoded the same kind of information. As expected, the HPC employed a sparse coding scheme characterized by compact, high contrast place fields, and information about spatial location was the dominant component of HPC representations. RSC firing patterns were more consistent with a distributed coding scheme. Instead of compact place fields, RSC neurons exhibited broad, but reliable, spatial and directional tuning, and they typically carried information about multiple navigational variables. The observed similarities highlight the closely related functions of the HPC and RSC, whereas the differences in information types and coding schemes suggest that these two regions likely make somewhat different contributions to spatial cognition.

Sex differences in the context dependency of episodic memory

Context contributes to multiple aspects of human episodic memory including segmentation and retrieval. The present studies tested if, in adult male and female mice, context influences the encoding of odors encountered in a single unsupervised sampling session of the type used for the routine acquisition of episodic memories. The three paradigms used differed in complexity (single vs. multiple odor cues) and period from sampling to testing. Results show that males consistently encode odors in a context-dependent manner: the mice discriminated novel from previously sampled cues when tested in the chamber of initial cue sampling but not in a distinct yet familiar chamber. This was independent of the interval between cue encounters or the latency from initial sampling to testing. In contrast, female mice acquired both single cues and the elements of multi-cue episodes, but recall of that information was dependent upon the surrounding context only when the cues were presented serially. These results extend the list of episodic memory features expressed by rodents and also introduce a striking and unexpected sex difference in context effects.

Gamma-oryzanol attenuates lipopolysaccharide-induced cognitive impairment by modulation of hippocampal inflammatory response and glial activation in mice

Systemic inflammation can cause chronic neuroinflammation, which is a significant risk factor for neurodegenerative disorders. Therefore, anti-inflammatory agents that reduce peripheral inflammation are potential targets for the prevention or treatment of these debilitating diseases. In the present study, we investigated whether gamma-oryzanol (ORY) could protect against chronic neuroinflammation induced by lipopolysaccharide (LPS) in adult male mice. Mice were injected with LPS (0.75 mg/kg/day) or saline for 7 consecutive days and orally received ORY (100 mg/kg) or vehicle for 14 days (7 days before LPS injections and 7 days co-treated with LPS). After two weeks, mice were subjected to behavioral assessments using the Morris water maze and Y-maze. Moreover, the expression level of several inflammatory mediators was measured in the hippocampus of treated animals. Also, neuronal loss, microglia, and astrocyte densities were evaluated in the CA1 and CA3 hippocampus. We found that ORY treatment significantly improved spatial and working memory in LPS-treated mice. This behavioral improvement was accompanied by a significant reduction in the number of microglia and astrocytes in the CA1 and CA3 hippocampus. Moreover, ORY treatment effectively prevented LPS-induced increases in the expression of inflammatory mediators and enhanced neuronal survival in the CA1 hippocampus. Our findings suggest that ORY treatment can be a therapeutic option to improve cognitive impairments and neuroinflammation induced by endotoxins.

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.

Maternal quercetin supplementation improved lipopolysaccharide-induced cognitive deficits and inflammatory response in a rat model of maternal immune activation

BACKGROUND

There is strong evidence that prenatal infection during a specific period of brain development increases the risk of neurodevelopmental disorders, partly through immune-inflammatory pathways. This suggests that anti-inflammatory agents could prevent these disorders by targeting the maternal inflammatory response. In the present study, we used a rat model of maternal immune activation (MIA) to examine whether maternal quercetin (QE) supplementation can alleviate behavioral deficits and inflammatory mediators in the prefrontal cortex (PFC) and hippocampus of adult male offspring.

METHODS

Pregnant rats were supplemented with QE (50 mg/kg) or vehicle throughout pregnancy and injected with either lipopolysaccharide (0.5 mg/kg) or saline on gestational days 15/16. At postnatal day 60, we evaluated the offspring's behavior, hippocampal and prefrontal cortex glial density, pro-inflammatory gene expression, and neuronal survival.

RESULTS

Our data showed that maternal QE supplementation can prevent working and recognition memory impairments in adult MIA offspring. This behavioral improvement correlates with the decrease in MIA-induced expression of pro-inflammatory genes, microglia, and astrocyte densities, without affecting neuronal survival, in both PFC and CA1 hippocampus areas.

CONCLUSION

Therefore, our study supports the potential preventive effect of QE on MIA-induced behavioral dysfunctions, at least in part, by suppressing the glial-mediated inflammatory response.

Hippocampal Engrams and Contextual Memory

Memories are not formed in a vacuum and often include rich details about the time and place in which events occur. Contextual stimuli promote the retrieval of events that have previously occurred in the encoding context and limit the retrieval of context-inappropriate information. Contexts that are associated with traumatic or harmful events both directly elicit fear and serve as reminders of aversive events associated with trauma. It has long been appreciated that the hippocampus is involved in contextual learning and memory and is central to contextual fear conditioning. However, little is known about the underlying neuronal mechanisms underlying the encoding and retrieval of contextual fear memories. Recent advancements in neuronal labeling methods, including activity-dependent tagging of cellular ensembles encoding memory ("engrams"), provide unique insight into the neural substrates of memory in the hippocampus. Moreover, these methods allow for the selective manipulation of memory ensembles. Attenuating or erasing fear memories may have considerable therapeutic value for patients with post-traumatic stress disorder or other trauma- or stressor-related conditions. In this chapter, we review the role of the hippocampus in contextual fear conditioning in rodents and explore recent work implicating hippocampal ensembles in the encoding and retrieval of aversive memories.

Partial or Complete Loss of Norepinephrine Differentially Alters Contextual Fear and Catecholamine Release Dynamics in Hippocampal CA1

Background

Contextual fear learning is heavily dependent on the hippocampus. Despite evidence that catecholamines contribute to contextual encoding and memory retrieval, the precise temporal dynamics of their release in the hippocampus during behavior is unknown. In addition, new animal models are required to probe the effects of altered catecholamine synthesis on release dynamics and contextual learning.

Methods

We generated 2 new mouse models of altered locus coeruleus-norepinephrine (NE) synthesis and utilized them together with GRABNE and GRABDA sensors and in vivo fiber photometry to investigate NE and dopamine (DA) release dynamics in the dorsal hippocampal CA1 during contextual fear conditioning.

Results

Aversive foot shock increased both NE and DA release in the dorsal CA1, while freezing behavior associated with recall of fear memory was accompanied by decreased release. Moreover, we found that freezing at the recent time point was sensitive to both partial and complete loss of locus coeruleus-NE synthesis throughout prenatal and postnatal development, similar to previous observations of mice with global loss of NE synthesis beginning postnatally. In contrast, freezing at the remote time point was compromised only by complete loss of locus coeruleus-NE synthesis beginning prenatally.

Conclusions

Overall, these findings provide novel insights into the role of NE in contextual fear and the precise temporal dynamics of both NE and DA during freezing behavior and highlight complex relationships between genotype, sex, and NE signaling.

Increased hippocampal CREB phosphorylation after retrieval of remote contextual fear memories in Carioca high-conditioned freezing rats

The participation of the hippocampal formation in consolidation and reconsolidation of contextual fear memories has been widely recognized and known to be dependent on the activation of the cAMP response element (CRE) binding protein (CREB) pathway. Recent findings have challenged the prevailing view that over time contextual fear memories migrate to neocortical circuits and no longer require the hippocampus for retrieval of remote fearful memories. It has also recently been found that this brain structure is important for the maintenance and recall of remote fear memories associated with aversive events, a common trait in stress-related disorders such as generalized anxiety disorder (GAD), major depression, and post-traumatic stress disorder. In view of these findings, here we examined the putative role of CREB in the hippocampus of an animal model of GAD during the retrieval of remote contextual fear memories. Specifically, we evaluated CREB phosphorylation in the hippocampus of male Carioca High- and Low-conditioned Freezing rats (CHF and CLF, respectively) upon re-exposure of animals to contextual cues associated to footshocks weeks after fear conditioning. Age-matched male rats from a randomized crossbreeding population served as controls (CTL). Adrenal catecholamine levels were also measured as a biological marker of stress response. Seven weeks after contextual fear conditioning, half of the sample of CHF (n = 9), CLF (n = 10) and CTL (n = 10) rats were randomly assigned to return to the same context chamber where footshocks were previously administrated (Context condition), while the remaining animals were individually placed in standard housing cages (Control condition). Western blot results indicated that pCREB levels were significantly increased in the hippocampus of CHF rats for both Context and Control conditions when compared to the other experimental groups. CHF rats in the Context condition also exhibited significant more freezing than that observed for both CLF and CTL rats. Lastly, CHF animals in the Context condition displayed significantly higher adrenal catecholamine levels than those in the Control condition, whereas no differences in catecholamine levels were observed between Context and Control conditions for CLF and CTL rats. These findings are discussed from a perspective in which the hippocampus plays a role in the maintenance and recall of remote contextual fear memories via the CREB pathway.

Loss of oxytocin receptors in hilar mossy cells impairs social discrimination

Hippocampal oxytocin receptor (OXTR) signaling is crucial for discrimination of social stimuli to guide social recognition, but circuit mechanisms and cell types involved remain incompletely understood. Here, we report a role for OXTR-expressing hilar mossy cells (MCs) of the dentate gyrus in social stimulus discrimination by regulating granule cell (GC) activity. Using a Cre-loxP recombination approach, we found that ablation of Oxtr from MCs impairs discrimination of social, but not object, stimuli in adult male mice. Ablation of MC Oxtr increases spontaneous firing rate of GCs, synaptic excitation to inhibition ratio of MC-to-GC circuit, and GC firing when temporally associated with the lateral perforant path inputs. Using mouse hippocampal slices, we found that bath application of OXTR agonist [Thr4,Gly7]-oxytocin causes membrane depolarization and increases MC firing activity. Optogenetic activation of MC-to-GC circuit ameliorates social discrimination deficit in MC OXTR deficient mice. Together, our results uncover a previously unknown role of MC OXTR signaling for discrimination of social stimuli and delineate a MC-to-GC circuit responsible for social information processing.

Brain-wide neuron quantification toolkit reveals strong sexual dimorphism in the evolution of fear memory

Fear responses are functionally adaptive behaviors that are strengthened as memories. Indeed, detailed knowledge of the neural circuitry modulating fear memory could be the turning point for the comprehension of this emotion and its pathological states. A comprehensive understanding of the circuits mediating memory encoding, consolidation, and retrieval presents the fundamental technological challenge of analyzing activity in the entire brain with single-neuron resolution. In this context, we develop the brain-wide neuron quantification toolkit (BRANT) for mapping whole-brain neuronal activation at micron-scale resolution, combining tissue clearing, high-resolution light-sheet microscopy, and automated image analysis. The robustness and scalability of this method allow us to quantify the evolution of activity patterns across multiple phases of memory in mice. This approach highlights a strong sexual dimorphism in recruited circuits, which has no counterpart in the behavior. The methodology presented here paves the way for a comprehensive characterization of the evolution of fear memory.

Expectancy-related changes in firing of dopamine neurons depend on hippocampus

The orbitofrontal cortex (OFC) and hippocampus (HC) are both implicated in forming the cognitive or task maps that support flexible behavior. Previously, we used the dopamine neurons as a sensor or tool to measure the functional effects of OFC lesions (Takahashi et al., 2011). We recorded midbrain dopamine neurons as rats performed an odor-based choice task, in which errors in the prediction of reward were induced by manipulating the number or timing of the expected rewards across blocks of trials. We found that OFC lesions ipsilateral to the recording electrodes caused prediction errors to be degraded consistent with a loss in the resolution of the task states, particularly under conditions where hidden information was critical to sharpening the predictions. Here we have repeated this experiment, along with computational modeling of the results, in rats with ipsilateral HC lesions. The results show HC also shapes the map of our task, however unlike OFC, which provides information local to the trial, the HC appears to be necessary for estimating the upper-level hidden states based on the information that is discontinuous or separated by longer timescales. The results contrast the respective roles of the OFC and HC in cognitive mapping and add to evidence that the dopamine neurons access a rich information set from distributed regions regarding the predictive structure of the environment, potentially enabling this powerful teaching signal to support complex learning and behavior.

Effects of social context manipulation on dorsal and ventral hippocampal neuronal responses

The hippocampus is critical for contextual memory and has recently been implicated in various kinds of social memory. Traditionally, studies of hippocampal context coding have manipulated elements of the background environment, such as the shape and color of the apparatus. These manipulations produce large shifts in the spatial firing patterns, a phenomenon known as remapping. These findings suggest that the hippocampus encodes and differentiates contexts by generating unique spatial firing patterns for each environment a subject encounters. However, we do not know whether the hippocampus encodes social contexts defined by the presence of particular conspecifics. We examined this by exposing rats to a series of manipulations of the social context, including the presence of familiar male, unfamiliar male and female conspecifics, in order to determine whether remapping is a plausible mechanism for encoding socially-defined contexts. Because the dorsal and ventral regions of the hippocampus are thought to play different roles in spatial and social cognition, we recorded neurons in both regions. Surprisingly, we found little evidence of remapping in response to manipulation of the social context in either the dorsal or ventral hippocampus, although we saw typical remapping in response to changing the background color. This result suggests that remapping is not the primary mechanism for encoding different social contexts. However, we found that a subset of hippocampal neurons fired selectively near the cages that contained the conspecifics, and these responses were most prevalent in the ventral hippocampus. We also found a striking increase in the spatial information content of ventral hippocampal firing patterns. These results indicate that the ventral hippocampus is sensitive to changes in the social context and neurons that respond selectively near the conspecific cages could play an important, if not fully understood role in encoding the conjunction of conspecifics, their location and the environment.

Hippocampal conjunctive and complementary CA1 populations relate sensory events to movement

Hippocampal CA1 neurons respond to sensory stimuli during enforced immobility, movement, and their transitions in a new conveyor belt task. Head-fixed mice were exposed to light flashes or air streams while at rest, spontaneously moving, or running a fixed distance. Two-photon calcium imaging of CA1 neurons revealed that 62% of 3341 imaged cells were active during one or more of 20 sensorimotor events. Of these active cells, 17% were active for any given sensorimotor event, with a higher proportion during locomotion. The study found two types of cells: Conjunctive cells that were active across multiple events, and complementary cells that were active only during individual events, encoding novel sensorimotor events or their delayed repetitions. The configuration of these cells across changing sensorimotor events may signify the role of hippocampus in functional networks integrating sensory information with ongoing movement making it suitable for movement guidance.

Neuronal nitric oxide synthase inhibition accelerated the removal of fluoxetine's anxiogenic activity in an animal model of PTSD

While SSRIs are the current first-line pharmacotherapies against post-traumatic stress disorder (PTSD), they suffer from delayed onset of efficacy and low remission rates. One solution is to combine SSRIs with other treatments. Neuronal nitric oxide synthase (nNOS) has been shown to play a role in serotonergic signaling, and there is evidence of synergism between nNOS modulation and SSRIs in models of other psychiatric conditions. Therefore, in this study, we combined subchronic fluoxetine (Flx) with 7-nitroindazole (NI), a selective nNOS inhibitor, and evaluated their efficacy against anxiety-related behavior in an animal model of PTSD. We used the underwater trauma model to induce PTSD in rats. Animals underwent the open field (OFT) and elevated plus maze tests on days 14 (baseline) and 21 (post-treatment) after PTSD induction to assess anxiety-related behaviors. Between the two tests, the rats received daily intraperitoneal injections of 10 mg/kg Flx or saline, and were injected intraperitoneally before the second test with either 15 mg/kg NI or saline. The change in behaviors between the two tests was compared between treatment groups. Individual treatment with both Flx and NI had anxiogenic effects in the OFT. These effects were associated with modest increases in cFOS expression in the hippocampus. Combination therapy with Flx + NI did not show any anxiogenic effects, while causing even higher expression levels of cFOS. In conclusion, addition of NI treatment to subchronic Flx therapy accelerated the abrogation of Flx's anxiogenic properties. Furthermore, hippocampal activity, as evidenced by cFOS expression, was biphasically related to anxiety-related behavior.

Adolescent intermittent ethanol exposure enhances adult stress effects in male rats

Binge patterns of alcohol use, prevalent among adolescents, are associated with a higher probability of developing alcohol use disorders (AUD) and other psychiatric disorders, like anxiety and depression. Additionally, adverse life events strongly predict AUD and other psychiatric disorders. As such, the combined fields of stress and AUD have been well established, and animal models indicate that both binge-like alcohol exposure and stress exposure elevate anxiety-like behaviors. However, few have investigated the interaction of adolescent intermittent ethanol (AIE) and adult stressors. We hypothesized that AIE would increase vulnerability to restraint-induced stress (RS), manifested as increased anxiety-like behavior. After AIE exposure, in adulthood, animals were tested on forced swim (FST) and saccharin preference (SP) and then exposed to either RS (90 min/5 days) or home-cage control. Twenty-four hours after the last RS session, animals began testing on the elevated plus maze (EPM), and were re-tested on FST and SP. A separate group of animals were sacrificed in adulthood after AIE and RS, and brains were harvested for immunoblot analysis of dorsal and ventral hippocampus. Consistent with previous reports, AIE had no significant effect on closed arm time in the EPM (anxiety-like behavior). However, in male rats the interaction of AIE and adult RS increased time spent in the closed arms. No effect was observed among female animals. AIE and RS-specific alterations were found in glial and synaptic markers (GLT-1, FMRP and PSD-95) in male animals. These findings indicate AIE has sex-specific effects on both SP and the interaction of AIE and adult RS, which induces a propensity toward anxiety-like behavior in males. Also, AIE produces persistent hippocampal deficits that may interact with adult RS to cause increased anxiety-like behaviors. Understanding the mechanisms behind this AIE-induced increase in stress vulnerability may provide insight into treatment and prevention strategies for alcohol use disorders.

A quadruple dissociation of reward-related behaviour in mice across excitatory inputs to the nucleus accumbens shell

The nucleus accumbens shell (NAcSh) is critically important for reward valuations, yet it remains unclear how valuation information is integrated in this region to drive behaviour during reinforcement learning. Using an optogenetic spatial self-stimulation task in mice, here we show that contingent activation of different excitatory inputs to the NAcSh change expression of different reward-related behaviours. Our data indicate that medial prefrontal inputs support place preference via repeated actions, ventral hippocampal inputs consistently promote place preferences, basolateral amygdala inputs produce modest place preferences but as a byproduct of increased sensitivity to time investments, and paraventricular inputs reduce place preferences yet do not produce full avoidance behaviour. These findings suggest that each excitatory input provides distinct information to the NAcSh, and we propose that this reflects the reinforcement of different credit assignment functions. Our finding of a quadruple dissociation of NAcSh input-specific behaviours provides insights into how types of information carried by distinct inputs to the NAcSh could be integrated to help drive reinforcement learning and situationally appropriate behavioural responses.

Dopaminergic regulation of hippocampal plasticity, learning, and memory

The hippocampus is responsible for encoding behavioral episodes into short-term and long-term memory. The circuits that mediate these processes are subject to neuromodulation, which involves regulation of synaptic plasticity and local neuronal excitability. In this review, we present evidence to demonstrate the influence of dopaminergic neuromodulation on hippocampus-dependent memory, and we address the controversy surrounding the source of dopamine innervation. First, we summarize historical and recent retrograde and anterograde anatomical tracing studies of direct dopaminergic projections from the ventral tegmental area and discuss dopamine release from the adrenergic locus coeruleus. Then, we present evidence of dopaminergic modulation of synaptic plasticity in the hippocampus. Plasticity mechanisms are examined in brain slices and in recordings from in vivo neuronal populations in freely moving rodents. Finally, we review pharmacological, genetic, and circuitry research that demonstrates the importance of dopamine release for learning and memory tasks while dissociating anatomically distinct populations of direct dopaminergic inputs.

DREADD-inactivation of dorsal CA1 pyramidal neurons in mice impairs retrieval of object and spatial memories

The hippocampus, a medial temporal lobe brain region, is critical for the consolidation of information from short-term memory into long-term episodic memory and for spatial memory that enables navigation. Hippocampal damage in humans has been linked to amnesia and memory loss, characteristic of Alzheimer's disease and other dementias. Numerous studies indicate that the rodent hippocampus contributes significantly to long-term memory for spatial and nonspatial information. For example, muscimol-induced depression of CA1 neuronal activity in the dorsal hippocampus impairs the encoding, consolidation, and retrieval of nonspatial object memory in mice. Here, a chemogenetic designer receptor exclusively activated by designer drugs (DREADDs) approach was used to test the selective involvement of CA1 pyramidal neurons in memory retrieval for objects and for spatial location in a cohort of male C57BL/6J mice. Activation of the inhibitory (hM4Di) DREADDs receptor expressed in CA1 neurons significantly impaired the retrieval of object memory in the spontaneous object recognition task and of spatial memory in the Morris water maze. Silencing of CA1 neuronal activity in hM4Di-expressing mice was confirmed by comparing Fos expression in vehicle- and clozapine-N-oxide-treated mice after exploration of a novel environment. Histological analyses revealed that expression of the hM4Di receptor was limited to CA1 neurons of the dorsal hippocampus. These results suggest that a common subset of CA1 neurons (i.e., those expressing hM4Di receptors) in mouse hippocampus contributed to the retrieval of long-term memory for nonspatial and spatial information. Our findings support the view that the contribution of the rodent hippocampus is like that of the primate hippocampus, specifically essential for global memory. Our results further validate mice as a suitable model system to study the neurobiological mechanisms of human episodic memory, but also in developing treatments and understanding the underlying causes of diseases affecting long-term memory, such as Alzheimer's disease.

The dorsal hippocampus' role in context-based timing in rodents

To act proactively, we must predict when future events will occur. Individuals generate temporal predictions using cues that indicate an event will happen after a certain duration elapses. Neural models of timing focus on how the brain represents these cue-duration associations. However, these models often overlook the fact that situational factors frequently modulate temporal expectations. For example, in realistic environments, the intervals associated with different cues will often covary due to a common underlying cause. According to the 'common cause hypothesis,' observers anticipate this covariance such that, when one cue's interval changes, temporal expectations for other cues shift in the same direction. Furthermore, as conditions will often differ across environments, the same cue can mean different things in different contexts. Therefore, updates to temporal expectations should be context-specific. Behavioral work supports these predictions, yet their underlying neural mechanisms are unclear. Here, we asked whether the dorsal hippocampus mediates context-based timing, given its broad role in context-conditioning. Specifically, we trained rats with either hippocampal or sham lesions that two cues predicted reward after either a short or long duration elapsed (e.g., tone-8 s/light-16 s). Then, we moved rats to a new context and extended the long cue's interval (e.g., light-32 s). This caused rats to respond later to the short cue, despite never being trained to do so. Importantly, when returned to the initial training context, sham rats shifted back toward both cues' original intervals. In contrast, lesion rats continued to respond at the long cue's newer interval. Surprisingly, they still showed contextual modulation for the short cue, responding earlier like shams. These data suggest the hippocampus only mediates context-based timing if a cue is explicitly paired and/or rewarded across distinct contexts. Furthermore, as lesions did not impact timing measures at baseline or acquisition for the long cue's new interval, our data suggests that the hippocampus only modulates timing when context is relevant.

Appetitive interoception, the hippocampus and western-style diet

Obesity, Type 2 diabetes and other metabolic disorders continue to pose serious challenges to human health and well-being. An important source of these challenges is the overconsumption of saturated fats and sugar, main staples of what has been called the Western-style diet (WD). The current paper describes a theoretical model and supporting evidence that links intake of a WD to interference with a specific brain substrate that underlies processing of interoceptive signals of hunger and satiety. We review findings from rats and humans that the capacity of these signals to modulate the strength of appetitive and eating behavior depends on the functional integrity of the hippocampus and the learning memory operations it performs. Important among these operations is the use of contextual information to retrieve memories that are associated with other events. Within our framework, satiety provides an interoceptive context that informs animals that food cues and appetitive behavior will not be followed by rewarding postingestive outcomes. This serves to prevent those cues and responses from retrieving those reward memories. The findings reviewed provide evidence that consuming a WD and the high amounts of saturated fat and sugar it contains (a) is associated with the emergence of pathophysiologies to which the hippocampus appears selectively vulnerable (b) impairs hippocampal-dependent learning and memory (HDLM) and (c) weakens behavioral control by interoceptive hunger and satiety contextual stimuli. It is hypothesized that these consequences of WD intake may establish the conditions for a vicious cycle of further WD intake, obesity, and potentially cognitive decline.

Hippocampal Egr1-Dependent Neuronal Ensembles Negatively Regulate Motor Learning

Motor skills learning is classically associated with brain regions including cerebral and cerebellar cortices and basal ganglia nuclei. Less is known about the role of the hippocampus in the acquisition and storage of motor skills. Here, we show that mice receiving a long-term training in the accelerating rotarod display marked hippocampal transcriptional changes and reduced pyramidal neurons activity in the CA1 region when compared with naive mice. Then, we use mice in which neural ensembles are permanently labeled in an Egr1 activity-dependent fashion. Using these mice, we identify a subpopulation of Egr1-expressing pyramidal neurons in CA1 activated in short-term (STT) and long-term (LTT) trained mice in the rotarod task. When Egr1 is downregulated in the CA1 or these neuronal ensembles are depleted, motor learning is improved whereas their chemogenetic stimulation impairs motor learning performance. Thus, Egr1 organizes specific CA1 neuronal ensembles during the accelerating rotarod task that limit motor learning. These evidences highlight the role of the hippocampus in the control of this type of learning and we provide a possible underlying mechanism.SIGNIFICANCE STATEMENT It is a major topic in neurosciences the deciphering of the specific circuits underlying memory systems during the encoding of new information. However, the potential role of the hippocampus in the control of motor learning and the underlying mechanisms has been poorly addressed. In the present work we show how the hippocampus responds to motor learning and how the Egr1 molecule is one of the major responsible for such phenomenon controlling the rate of motor coordination performances.

Activation of a Locus Coeruleus to Dorsal Hippocampus Noradrenergic Circuit Facilitates Associative Learning

Processing of contextual information during a new episodic event is crucial for learning and memory. Neuromodulation in the hippocampus and prefrontal cortex plays an important role in the formation of associations between environmental cues and an aversive experience. Noradrenergic neurons in the locus coeruleus send dense projections to both regions, but their contribution to contextual associative learning has not been established. Here, we utilize selective optogenetic and pharmacological manipulations to control noradrenergic transmission in the hippocampus during the encoding of a contextual fear memory. We find that boosting noradrenergic terminal release in the dorsal CA1 enhances the acquisition of contextual associative learning and that this effect requires local activation of β-adrenenergic receptors. Moreover, we show that increasing norepinephrine release can ameliorate contextual fear learning impairments caused by dopaminergic dysregulation in the hippocampus. Our data suggest that increasing of hippocampal noradrenergic activity can have important implications in the treatment of cognitive disorders that involve problems in contextual processing.

The limbic memory circuit and the neural basis of contextual memory

The hippocampus, retrosplenial cortex and anterior thalamus are key components of a neural circuit known to be involved in a variety of memory functions, including spatial, contextual and episodic memory. In this review, we focus on the role of this circuit in contextual memory processes. The background environment, or context, is a powerful cue for memory retrieval, and neural representations of the context provide a mechanism for efficiently retrieving relevant memories while avoiding interference from memories that belong to other contexts. Data from experimental lesions and neural manipulation techniques indicate that each of these regions is critical for contextual memory. Neurophysiological evidence from the hippocampus and retrosplenial cortex suggest that contextual information is represented within this circuit by population-level neural firing patterns that reliably differentiate each context a subject encounters. These findings indicate that encoding contextual information to support context-dependent memory retrieval is a key function of this circuit.

Episodic memory enhancement versus impairment is determined by contextual similarity across events

For over a century, stability of spatial context across related episodes has been considered a source of memory interference, impairing memory retrieval. However, contemporary memory integration theory generates a diametrically opposite prediction. Here, we aimed to resolve this discrepancy by manipulating local context similarity across temporally disparate but related episodes and testing the direction and underlying mechanisms of memory change. A series of experiments show that contextual stability produces memory integration and marked reciprocal strengthening. Variable context, conversely, seemed to result in competition such that new memories become enhanced at the expense of original memories. Interestingly, these patterns were virtually inverted in an additional experiment where context was reinstated during recall. These observations 1) identify contextual similarity across original and new memories as an important determinant in the volatility of memory, 2) present a challenge to classic and modern theories on episodic memory change, and 3) indicate that the sensitivity of context-induced memory changes to retrieval conditions may reconcile paradoxical predictions of interference and integration theory.

Complementary roles of differential medial entorhinal cortex inputs to the hippocampus for the formation and integration of temporal and contextual memory (Systems Neuroscience)

In humans and rodents, the entorhinal cortical (EC)-hippocampal (HPC) circuit is crucial for the formation and recall of memory, preserving both spatial information and temporal information about the occurrence of past events. Both modeling and experimental studies have revealed circuits within this network that play crucial roles in encoding space and context. However, our understanding about the time-related aspects of memory is just beginning to be understood. In this review, we first describe updates regarding recent anatomical discoveries for the EC-HPC network, as several important neural circuits critical for memory formation have been discovered by newly developed neural tracing technologies. Second, we examine the complementary roles of multiple medial entorhinal cortical inputs, including newly discovered circuits, into the hippocampus for the temporal and spatial aspects of memory. Finally, we will discuss how temporal and contextual memory information is integrated in HPC cornu ammonis 1 cells. We provide new insights into the neural circuit mechanisms for anatomical and functional segregation and integration of the temporal and spatial aspects of memory encoding in the EC-HPC networks.

Maintenance of the Amygdala-Hippocampal Circuit Function with Safe and Feasible Shaking Exercise Therapy in SAMP-10 Mice

INTRODUCTION

Patients with dementia show reduced adaptive, behavioral, and physiological responses to environmental threats. Physical exercise is expected to delay brain aging, maintain cognitive function and, consequently, help dementia patients face threats and protect themselves skillfully.

METHODS

To confirm this, we aimed to investigate the effects of the shaking exercise on the avoidance function in the senescence-accelerated mouse-prone strain-10 (SAMP-10) model at the behavioral and tissue levels. SAMP-10 mice were randomized into 2 groups: a control group and a shaking group. The avoidance response (latency) of the mice was evaluated using a passive avoidance task. The degree of amygdala and hippocampal aging was evaluated based on the brain morphology. Subsequently, the association between avoidance response and the degree of amygdala-hippocampal aging was evaluated.

RESULTS

Regarding the passive avoidance task, the shaking group showed a longer latency period than the control group (p < 0.05), even and low intensity staining of ubiquitinated protein, and had a higher number of and larger neurons than those of the control group. The difference between the groups was more significant in the BA region of the amygdala and the CA1 region of the hippocampus (staining degree: p < 0.05, neuron size: p < 0.01, neuron counts: p < 0.01) than in other regions.

CONCLUSIONS

The shaking exercise prevents nonfunctional protein (NFP) accumulation, neuron atrophy, and neuron loss; delays the aging of the amygdala and hippocampus; and maintains the function of the amygdala-hippocampal circuit. It thus enhances emotional processing and cognition functions, the memory of threats, the skillful confrontation of threats, and proper self-protection from danger.

Unravelling the Neural Basis of Spatial Delusions After Stroke

OBJECTIVE

Knowing explicitly where we are is an interpretation of our spatial representations. Reduplicative paramnesia is a disrupting syndrome in which patients present a firm belief of spatial mislocation. Here, we studied the largest sample of patients with delusional misidentifications of space (ie, reduplicative paramnesia) after stroke to shed light on their neurobiology.

METHODS

In a prospective, cumulative, case-control study, we screened 400 patients with acute right-hemispheric stroke. We included 64 cases and 233 controls. First, lesions were delimited and normalized. Then, we computed structural and functional disconnection maps using methods of lesion-track and network-mapping. The maps were compared, controlling for confounders. Second, we built a multivariate logistic model, including clinical, behavioral, and neuroimaging data. Finally, we performed a nested cross-validation of the model with a support-vector machine analysis.

RESULTS

The most frequent misidentification subtype was confabulatory mislocation (56%), followed by place reduplication (19%), and chimeric assimilation (13%). Our results indicate that structural disconnection is the strongest predictor of the syndrome and included 2 distinct streams, connecting right fronto-thalamic and right occipitotemporal structures. In the multivariate model, the independent predictors of reduplicative paramnesia were the structural disconnection map, lesion sparing of right dorsal fronto-parietal regions, age, and anosognosia. Good discrimination accuracy was demonstrated (area under the curve = 0.80 [0.75-0.85]).

INTERPRETATION

Our results localize the anatomic circuits that may have a role in the abnormal spatial-emotional binding and in the defective updating of spatial representations underlying reduplicative paramnesia. This novel data may contribute to better understand the pathophysiology of delusional syndromes after stroke. ANN NEUROL 2021;89:1181-1194.

The anterior thalamic nuclei and cognition: A role beyond space?

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Anterior thalamic nuclei important for specific classes of temporal discriminations.

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Anterior thalamic nuclei required for hippocampal-dependent contextual processes.

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Critical role for anterior thalamic nuclei in selective attention.

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Significance of anterior thalamic – anterior cingulate interactions.

The anterior thalamic nuclei are a vital node within hippocampal-diencephalic-cingulate circuits that support spatial learning and memory. Reflecting this interconnectivity, the overwhelming focus of research into the cognitive functions of the anterior thalamic nuclei has been spatial processing. However, there is increasing evidence that the functions of the anterior thalamic nuclei extend beyond the spatial realm. This work has highlighted how these nuclei are required for certain classes of temporal discrimination as well as their importance for processing other contextual information; revealing parallels with the non-spatial functions of the hippocampal formation. Yet further work has shown how the anterior thalamic nuclei may be important for other forms of non-spatial learning, including a critical role for these nuclei in attentional mechanisms. This evidence signals the need to reconsider the functions of the anterior thalamic within the framework of their wider connections with sites including the anterior cingulate cortex that subserve non-spatial functions.

Multiple memories can be simultaneously reactivated during sleep as effectively as a single memory

Memory consolidation involves the reactivation of memory traces during sleep. If different memories are reactivated each night, how much do they interfere with one another? We examined whether reactivating multiple memories incurs a cost to sleep-related benefits by contrasting reactivation of multiple memories versus single memories during sleep. First, participants learned the on-screen location of different objects. Each object was part of a semantically coherent group comprised of either one, two, or six items (e.g., six different cats). During sleep, sounds were unobtrusively presented to reactivate memories for half of the groups (e.g., "meow"). Memory benefits for cued versus non-cued items were independent of the number of items in the group, suggesting that reactivation occurs in a simultaneous and promiscuous manner. Intriguingly, sleep spindles and delta-theta power modulations were sensitive to group size, reflecting the extent of previous learning. Our results demonstrate that multiple memories may be consolidated in parallel without compromising each memory's sleep-related benefit. These findings highlight alternative models for parallel consolidation that should be considered in future studies.

Cell-Type-Specific Optogenetic Techniques Reveal Neural Circuits Crucial for Episodic Memories

The formation and maintenance of episodic memories are important for our daily life. Accumulating evidence from extensive studies with pharmacological, electrophysiological, and molecular biological approaches has shown that both entorhinal cortex (EC) and hippocampus (HPC) are crucial for the formation and recall of episodic memory. However, to further understand the neural mechanisms of episodic memory processes in the EC-HPC network, cell-type-specific manipulation of neural activity with high temporal resolution during memory process has become necessary. Recently, the technological innovation of optogenetics combined with pharmacological, molecular biological, and electrophysiological approaches has significantly advanced our understanding of the circuit mechanisms for learning and memory. Optogenetic techniques with transgenic mice and/or viral vectors enable us to manipulate the neural activity of specific cell populations as well as specific neural projections with millisecond-scale temporal control during animal behavior. Integrating optogenetics with drug-regulatable activity-dependent gene expression systems has identified memory engram cells, which are a subpopulation of cells that encode a specific episode. Finally, millisecond pulse stimulation of neural activity by optogenetics has further achieved (a) identification of synaptic connectivity between targeted pairs of neural populations, (b) cell-type-specific single-unit electrophysiological recordings, and (c) artificial induction and modification of synaptic plasticity in targeted synapses. In this chapter, we summarize technological and conceptual advancements in the field of neurobiology of learning and memory as revealed by optogenetic approaches in the rodent EC-HPC network for episodic memories.

Dual-Factor Representation of the Environmental Context in the Retrosplenial Cortex

The retrosplenial cortex (RSC) is thought to be involved in a variety of spatial and contextual memory processes. However, we do not know how contextual information might be encoded in the RSC or whether the RSC representations may be distinct from context representations seen in other brain regions such as the hippocampus. We recorded RSC neuronal responses while rats explored different environments and discovered 2 kinds of context representations: one involving a novel rate code in which neurons reliably fire at a higher rate in the preferred context regardless of spatial location, and a second involving context-dependent spatial firing patterns similar to those seen in the hippocampus. This suggests that the RSC employs a unique dual-factor representational mechanism to support contextual memory.

Hippocampal subfield volumes in abstinent men and women with a history of alcohol use disorder

Alcohol Use Disorder (AUD) has been associated with abnormalities in hippocampal volumes, but these relationships have not been fully explored with respect to sub-regional volumes, nor in association with individual characteristics such as age, gender differences, drinking history, and memory. The present study examined the impact of those variables in relation to hippocampal subfield volumes in abstinent men and women with a history of AUD. Using Magnetic Resonance Imaging at 3 Tesla, we obtained brain images from 67 participants with AUD (31 women) and 64 nonalcoholic control (NC) participants (31 women). The average duration of the most recent period of sobriety for AUD participants was 7.1 years. We used Freesurfer 6.0 to segment the hippocampus into 12 regions. These were imputed into statistical models to examine the relationships of brain volume with AUD group, age, gender, memory, and drinking history. Interactions with gender and age were of particular interest. Compared to the NC group, the AUD group had approximately 5% smaller subiculum, CA1, molecular layer, and hippocampal tail regions. Age was negatively associated with volumes for the AUD group in the subiculum and the hippocampal tail, but no significant interactions with gender were identified. The relationships for delayed and immediate memory with hippocampal tail volume differed for AUD and NC groups: Higher scores on tests of immediate and delayed memory were associated with smaller volumes in the AUD group, but larger volumes in the NC group. Length of sobriety was associated with decreasing CA1 volume in women (0.19% per year) and increasing volume size in men (0.38% per year). The course of abstinence on CA1 volume differed for men and women, and the differential relationships of subfield volumes to age and memory could indicate a distinction in the impact of AUD on functions of the hippocampal tail. These findings confirm and extend evidence that AUD, age, gender, memory, and abstinence differentially impact volumes of component parts of the hippocampus.

Varieties of graded forgetting

Forgetting is typically viewed as counterproductive in everyday life. However, it may mainly be harmful when it is complete, that is, all-encompassing and permanent, and not when it is graded, that is, partial and fluctuating. I propose that forgetting is in fact mostly graded, and that this is an essential reason that it is often helpful. I delineate three ways in which forgetting may be graded. First, it may occur with respect to one, but not another, part of a memory. Second, it may occur in one context, but not in another. Third, forgetting may be present at one point in time, but not at another. Also, I propose that different levels of forgetting are possible, based on whether an engram or a context is unavailable, silent, restricted, latent, or potent. Overall, I hypothesize that forgetting is often helpful because it can be flexible and tailored to the circumstances.

Hippocampal state transitions at the boundaries between trial epochs

The hippocampus encodes distinct contexts with unique patterns of activity. Representational shifts with changes in context, referred to as remapping, have been extensively studied. However, less is known about transitions between representations. In this study, we leverage a large dataset of neuronal recordings taken while rats performed an olfactory memory task with a predictable temporal structure involving trials and intertrial intervals (ITIs), separated by salient boundaries at the trial start and trial end. We found that trial epochs were associated with stable hippocampal representations despite moment-to-moment variability in stimuli and behavior. Representations of trial and ITI epochs were far more distinct than spatial factors would predict and the transitions between the two were abrupt. The boundary was associated with a large spike in multiunit activity, with many individual cells specifically active at the start or end of each trial. Both epochs and boundaries were encoded by hippocampal populations, and these representations carried information on orthogonal axes readily identified using principal component analysis. We suggest that the hippocampus orthogonalizes representations of the trial and ITI epochs and the activity spike at trial boundaries might serve to drive hippocampal activity from one stable state to the other.

From Isolated Emotional Memories to Their Competition During Conflict

Aversive or rewarding experiences are remembered better than those of lesser survival significance. These emotional memories, whether negative or positive, leave traces in the brain which can later be retrieved and strongly influence how we perceive, how we form associations with environmental stimuli and, ultimately, guide our decision-making. In this review aticle, we outline what constitutes an emotional memory by focusing on threat- and reward-related memories and describe how they are formed in the brain during learning and reformed during retrieval. Finally, we discuss how the field is moving from understanding emotional memory brain circuits separately, towards studying how these two opposing brain systems interact to guide choices during conflict. Here, we outline two novel tasks in rodents that model opposing binary choices (approach or avoid) guided by competing emotional memories. The prefrontal cortex (PFC) is a major integration hub of emotional information which is also known to be critical for decision-making. Consequently, brain circuits that involve this brain region may be key for understanding how the retrieval of emotional memories flexibly orchestrates adaptive choice behavior. Because several mental disorders (e.g., drug addiction and depression) are characterized by deficits in decision-making in the face of conflicting emotional memories (maladaptively giving more weight to one memory over the other), the development of choice-based animal models for emotional regulation could give rise to new approaches for the treatment of these disorders in humans.

Threat-induced anxiety during goal pursuit disrupts amygdala-prefrontal cortex connectivity in posttraumatic stress disorder

To investigate how unpredictable threat during goal pursuit impacts fronto-limbic activity and functional connectivity in posttraumatic stress disorder (PTSD), we compared military veterans with PTSD (n = 25) vs. trauma-exposed control (n = 25). Participants underwent functional magnetic resonance imaging (fMRI) while engaged in a computerized chase-and-capture game task that involved optimizing monetary rewards obtained from capturing virtual prey while simultaneously avoiding capture by virtual predators. The game was played under two alternating contexts-one involving exposure to unpredictable task-irrelevant threat from randomly occurring electrical shocks, and a nonthreat control condition. Activation in and functional connectivity between the amygdala and ventromedial prefrontal cortex (vmPFC) was tested across threat and nonthreat task contexts with generalized psychophysiological interaction (gPPI) analyses. PTSD patients reported higher anxiety than controls across contexts. Better task performance represented by successfully avoiding capture by predators under threat compared with nonthreat contexts was associated with stronger left amygdala-vmPFC functional connectivity in controls and greater vmPFC activation in PTSD patients. PTSD symptom severity was negatively correlated with vmPFC activation in trauma-exposed controls and with right amygdala-vmPFC functional connectivity across all participants in the threat relative to nonthreat contexts. The findings showed that veterans with PTSD have disrupted amygdala-vmPFC functional connectivity and greater localized vmPFC processing under threat modulation of goal-directed behavior, specifically related to successfully avoiding loss of monetary rewards. In contrast, trauma survivors without PTSD relied on stronger threat-modulated left amygdala-vmPFC functional connectivity during goal-directed behavior, which may represent a resilience-related functional adaptation.

Long-term memory is formed immediately without the need for protein synthesis-dependent consolidation in Drosophila

It is believed that long-term memory (LTM) cannot be formed immediately because it must go through a protein synthesis-dependent consolidation process. However, the current study uses Drosophila aversive olfactory conditioning to show that such processes are dispensable for context-dependent LTM (cLTM). Single-trial conditioning yields cLTM that is formed immediately in a protein-synthesis independent manner and is sustained over 14 days without decay. Unlike retrieval of traditional LTM, which requires only the conditioned odour and is mediated by mushroom-body neurons, cLTM recall requires both the conditioned odour and reinstatement of the training-environmental context. It is mediated through lateral-horn neurons that connect to multiple sensory brain regions. The cLTM cannot be retrieved if synaptic transmission from any one of these centres is blocked, with effects similar to those of altered encoding context during retrieval. The present study provides strong evidence that long-term memory can be formed easily without the need for consolidation.

New protein synthesis is known to be indispensable for the consolidation of long-term memory. Here, the authors report that an olfactory memory can be successfully recalled after 14 days without protein synthesis when the training context is also provided in addition to the conditioned odor.

Time-dependent effects of psychosocial stress on the contextualization of neutral memories

Memories about stressful experiences need to be both specific and generalizable to adequately guide future behavior. Memory strength is influenced by emotional significance, and contextualization (i.e., encoding experiences with their contextual details) enables selective context-dependent retrieval and protects against overgeneralization. The current randomized-controlled study investigated how the early and late phase of the endogenous stress response affects the contextualization of neutral and negative information. One hundred healthy male participants were randomly divided into three experimental groups that performed encoding either 1) without stress (control), 2) immediately after acute stress (early) or 3) two hours after acute stress (late). Stress was induced via the Trier Social Stress Test and salivary alpha-amylase and cortisol levels were measured throughout the experiment. In the Memory Contextualization Task, neutral and angry faces (items) were depicted against unique context pictures during encoding. During testing 24 h later, context-dependent recognition memory of the items was assessed by presenting these in either congruent or incongruent contexts (relative to encoding). Multilevel analyses revealed that neutral information was more contextualized when encoding took place two hours after psychosocial stress, than immediately after the stressor. Results suggest that the late effects in the unique, time-dependent sequence of a healthy endogenous stress response, could complement reduced contextualization immediately after stress. The contextualization of negative information was not influenced by psychosocial stress, as opposed to earlier reported effects of exogenous hydrocortisone administration. An imbalance between the early and late effects of the endogenous stress response could increase vulnerability for stress-related psychopathology.

In a Temporally Segmented Experience Hippocampal Neurons Represent Temporally Drifting Context But Not Discrete Segments

There is widespread agreement that episodic memory is organized into a timeline of past experiences. Recent work suggests that the hippocampus may parse the flow of experience into discrete episodes separated by event boundaries. A complementary body of work suggests that context changes gradually as experience unfolds. We recorded from hippocampal neurons as male Long-Evans rats performed 6 blocks of an object discrimination task in sets of 15 trials. Each block was separated by removal from the testing chamber for a delay to enable segmentation. The reward contingency reversed from one block to the next to incentivize segmentation. We expected animals to hold two distinct, recurring representations of context to match the two distinct rule contingencies. Instead, we found that overtrained rats began each block neither above nor below chance but by guessing randomly. While many units had clear firing fields selective to the conjunction of objects in places, a significant population also reflected a continuously drifting code both within block and across blocks. Despite clear boundaries between blocks, we saw no neural evidence for event segmentation in this experiment. Rather, the hippocampal ensemble drifted continuously across time. This continuous drift in the neural representation was consistent with the lack of segmentation observed in behavior.SIGNIFICANCE STATEMENT The neuroscience literature yet to reach consensus on how the hippocampus supports the organization of events across time in episodic memory. Initial studies reported stable hippocampal maps segmented by remapping events. However, it remains unclear whether segmentation is an artifact of cue responsivity. Recently, research has shown that the hippocampal code exhibits continuous drift. Drift may represent a continually evolving context; however, it is unclear whether this is an artifact of changing experiences. We recorded dCA1 in rats performing an object discrimination task designed to segment time. Overtrained rats could not anticipate upcoming context switches but used context boundaries to their advantage. Hippocampal ensembles showed neither evidence of alternating between stable contexts nor sensitivity to boundaries, but showed robust temporal drift.

Neural and neuronal discoordination in schizophrenia: From ensembles through networks to symptoms

Despite the substantial knowledge accumulated by past research, the exact mechanisms of the pathogenesis of schizophrenia and causal treatments still remain unclear. Deficits of cognition and information processing in schizophrenia are today often viewed as the primary and core symptoms of this devastating disorder. These deficits likely result from disruptions in the coordination of neuronal and neural activity. The aim of this review is to bring together convergent evidence of discoordinated brain circuits in schizophrenia at multiple levels of resolution, ranging from principal cells and interneurons, neuronal ensembles and local circuits, to large-scale brain networks. We show how these aberrations could underlie deficits in cognitive control and other higher order cognitive-behavioural functions. Converging evidence from both animal models and patients with schizophrenia is presented in an effort to gain insight into common features of deficits in the brain information processing in this disorder, marked by disruption of several neurotransmitter and signalling systems and severe behavioural outcomes.

Differential role of interleukin-1β in neuroinflammation-induced impairment of spatial and nonspatial memory in hyperammonemic rats

Activated microglia and increased brain IL-1β play a main role in cognitive impairment in much pathology. We studied the role of IL-1β in neuroinflammation-induced impairment of the following different types of learning and memory: novel object recognition (NOR), novel object location (NOL), spatial learning, reference memory (RM), and working memory (WM). All these processes are impaired in hyperammonemic rats. We assessed which of these types of learning and memory are restored by blocking the IL-1 receptor in vivo in hyperammonemic rats and the possible mechanisms involved. Blocking the IL-1 receptor reversed microglial activation in the hippocampus, perirhinal cortex, and prefrontal cortex but not in the postrhinal cortex. This was associated with the restoration of NOR and WM but not of tasks involving a spatial component (NOL and RM). This suggests that IL-1β would be involved in neuroinflammation-induced nonspatial memory impairment, whereas spatial memory impairment would be IL-1β-independent and would be mediated by other proinflammatory factors.-Taoro-González, L., Cabrera-Pastor, A., Sancho-Alonso, M., Arenas, Y. M., Meseguer-Estornell, F., Balzano, T., ElMlili, N., Felipo, V. Differential role of interleukin-1β in neuroinflammation-induced impairment of spatial and nonspatial memory in hyperammonemic rats.

Image-guided cranial irradiation-induced ablation of dentate gyrus neurogenesis impairs extinction of recent morphine reward memories

Dentate gyrus adult neurogenesis is implicated in the formation of hippocampal-dependent contextual associations. However, the role of adult neurogenesis during reward-based context-dependent paradigms-such as conditioned place preference (CPP)-is understudied. Therefore, we used image-guided, hippocampal-targeted X-ray irradiation (IG-IR) and morphine CPP to explore whether dentate gyrus adult neurogenesis plays a role in reward memories created in adult C57BL/6J male mice. In addition, as adult neurogenesis appears to participate to a greater extent in retrieval and extinction of recent (<48 hr posttraining) versus remote (>1 week posttraining) memories, we specifically examined the role of adult neurogenesis in reward-associated contextual memories probed at recent and remote timepoints. Six weeks post-IG-IR or Sham treatment, mice underwent morphine CPP. Using separate groups, retrieval of recent and remote reward memories was found to be similar between IG-IR and Sham treatments. Interestingly, IG-IR mice showed impaired extinction-or increased persistence-of the morphine-associated reward memory when it was probed 24-hr (recent) but not 3-weeks (remote) postconditioning relative to Sham mice. Taken together, these data show that hippocampal-directed irradiation and the associated decrease in dentate gyrus adult neurogenesis affect the persistence of recently-but not remotely-probed reward memory. These data indicate a novel role for adult neurogenesis in reward-based memories and particularly the extinction rate of these memories. Consideration of this work may lead to better understanding of extinction-based behavioral interventions for psychiatric conditions characterized by dysregulated reward processing.

Anterior thalamic nuclei, but not retrosplenial cortex, lesions abolish latent inhibition in rats

The present study examined the effects of excitotoxic lesions in 2 closely related structures, the anterior thalamic nuclei and the retrosplenial cortex, on latent inhibition. Latent inhibition occurs when nonreinforced preexposure to a stimulus retards the subsequent acquisition of conditioned responding to that stimulus. Latent inhibition was assessed in a within-subject procedure with auditory stimuli and food reinforcement. As expected, sham-operated animals were slower to acquire conditioned responding to a stimulus that had previously been experienced without consequence, relative to a non-preexposed stimulus. This latent inhibition effect was absent in rats with excitotoxic lesions in the anterior thalamic nuclei, as these animals conditioned to both stimuli at equivalent rates. The retrosplenial lesions appeared to spare latent inhibition, as these animals displayed a robust stimulus preexposure effect. The demonstration here that anterior thalamic nuclei lesions abolish latent inhibition is consistent with emerging evidence of the importance of these thalamic nuclei for attentional control. (PsycINFO Database Record (c) 2018 APA, all rights reserved).