The activity of thalamic nucleus reuniens is critical for memory retrieval, but not essential for the early phase of "off-line" consolidation

The ventral midline thalamus and long-term memory: What consolidation, what retrieval, what plasticity in rodents?

The ventral midline thalamus, including the reuniens and rhomboid (ReRh) nuclei, connects bidirectionally with the medial prefrontal cortex (mPFC) and hippocampus (Hip), both essential for memory processes. This review compiles and discusses studies on a role for the ReRh nuclei in the system consolidation of memories, also considering their potentially limited participation in memory retrieval or early phases of consolidation. It also examines scientific literature on short- and long-term plasticity in ReRh-mPFC and ReRh-Hip connections, emphasizing plasticity's importance in understanding these nuclei's role in memory. The idea that the two nuclei are at the crossroads of information exchange between the mPFC and the Hip is not new, but the relationship between this status and the plasticity of their connections remains elusive. Since this perspective is relatively recent, our concluding section suggests conceptual and practical avenues for future research, aiming perhaps to bring more order to the apparently multi-functional implication of the ventral midline thalamus in cognition.

Isolated theta waves originating from the midline thalamus trigger memory reactivation during NREM sleep in mice

During non-rapid eye movement (NREM) sleep, neural ensembles in the entorhinal-hippocampal circuit responsible for encoding recent memories undergo reactivation to facilitate the process of memory consolidation. This reactivation is widely acknowledged as pivotal for the formation of stable memory and its impairment is closely associated with memory dysfunction. To date, the neural mechanisms driving the reactivation of neural ensembles during NREM sleep remain poorly understood. Here, we show that the neural ensembles in the medial entorhinal cortex (MEC) that encode spatial experiences exhibit reactivation during NREM sleep. Notably, this reactivation consistently coincides with isolated theta waves. In addition, we found that the nucleus reuniens (RE) in the midline thalamus exhibits typical theta waves during NREM sleep, which are highly synchronized with those occurring in the MEC in male mice. Closed-loop optogenetic inhibition of the RE-MEC pathway specifically suppressed these isolated theta waves, resulting in impaired reactivation and compromised memory consolidation following a spatial memory task in male mice. The findings suggest that theta waves originating from the ventral midline thalamus play a role in initiating memory reactivation and consolidation during sleep.

Coupling between the prelimbic cortex, nucleus reuniens, and hippocampus during NREM sleep remains stable under cognitive and homeostatic demands

The interplay between the medial prefrontal cortex and hippocampus during non-rapid eye movement (NREM) sleep contributes to the consolidation of contextual memories. To assess the role of the thalamic nucleus reuniens (Nre) in this interaction, we investigated the coupling of neuro-oscillatory activities among prelimbic cortex, Nre, and hippocampus across sleep states and their role in the consolidation of contextual memories using multi-site electrophysiological recordings and optogenetic manipulations. We showed that ripples are time-locked to the Up state of cortical slow waves, the transition from UP to DOWN state in thalamic slow waves, the troughs of cortical spindles, and the peaks of thalamic spindles during spontaneous sleep, rebound sleep and sleep following a fear conditioning task. In addition, spiking activity in Nre increased before hippocampal ripples, and the phase-locking of hippocampal ripples and thalamic spindles during NREM sleep was stronger after acquisition of a fear memory. We showed that optogenetic inhibition of Nre neurons reduced phase-locking of ripples to cortical slow waves in the ventral hippocampus whilst their activation altered the preferred phase of ripples to slow waves in ventral and dorsal hippocampi. However, none of these optogenetic manipulations of Nre during sleep after acquisition of fear conditioning did alter sleep-dependent memory consolidation. Collectively, these results showed that Nre is central in modulating hippocampus and cortical rhythms during NREM sleep.

Nucleus reuniens inactivation does not impair consolidation or reconsolidation of fear extinction

Recent data reveal that the thalamic nucleus reuniens (RE) has a critical role in the extinction of conditioned fear. Muscimol (MUS) infusions into the RE impair within-session extinction of conditioned freezing and result in poor long-term extinction memories in rats. Although this suggests that RE inactivation impairs extinction learning, it is also possible that it is involved in the consolidation of extinction memories. To examine this possibility, we examined the effects of RE inactivation on the consolidation and reconsolidation of fear extinction in male and female rats. Twenty-four hours after auditory fear conditioning, rats underwent an extinction procedure (45 CS-alone trials) in a novel context and were infused with saline (SAL) or MUS within minutes of the final extinction trial. Twenty-four hours later, conditioned freezing to the extinguished CS was assessed in the extinction context. Postextinction inactivation of the RE did not affect extinction retrieval. In a second experiment, rats underwent extinction training and, 24 h later, were presented with a single CS to reactivate the extinction memory; rats were infused with SAL or MUS immediately after the reactivation session. Pharmacological inactivation of the RE did not affect conditioned freezing measured in a drug-free retrieval test the following day. Importantly, we found in a subsequent test that MUS infusions immediately before retrieval testing increased conditioned freezing and impaired extinction retrieval, as we have previously reported. These results indicate that although RE inactivation impairs the expression of extinction, it does not impair either the consolidation or reconsolidation of extinction memories. We conclude that the RE may have a critical role in suppressing context-inappropriate fear memories in the extinction context.

The role of the nucleus reuniens in regulating contextual conditioning with the predator odor TMT in female rats

RATIONALE

Experiencing intrusive distressing memories of a traumatic event(s) is a prominent symptom profile for post-traumatic stress disorder (PTSD). Understanding the neurobiological mechanisms associated with this symptom profile can be invaluable for effective treatment for PTSD.

OBJECTIVES

Here, we investigated the functional role of the nucleus reuniens (RE), a midline thalamic in modulating stressor-related memory.

METHODS

Female Long Evans rats were implanted with a cannula aimed at the RE. The RE was pharmacologically inactivated via muscimol (0.5 mM) prior to exposure to the predator odor stressor trimethylthiazoline (TMT; synthetically derived fox feces component) or water (controls) in a distinct context with bedding material (experiment 1) or no bedding (experiment 2). To measure context reactivity, the index of the contextual memory, 2 weeks following exposure to TMT, rats were re-exposed to the TMT-paired context (in the absence of TMT).

RESULTS

In experiment 1, during context re-exposure (with bedding), inactivation of the RE had no effect on context reactivity. In experiment 2, during context re-exposure (no bedding), rats previously exposed to TMT showed decreased immobility compared to controls, indicating reactivity to the context and likely related to theincreased exploration of the environment. Rats in the TMT group that received RE inactivation showed increased immobility relative to rats that received aCSF, suggesting that muscimol pre-treatment blunted context reactivity.

CONCLUSION

In conclusion, recruitment of the RE in stressor-related contextual memory appears to be dependent on the contextual environment and whether the animal is able to engage in different stress coping strategies.

Chemogenetic inactivation of the nucleus reuniens impairs object placement memory in female mice

Episodic memory is a complex process requiring input from several regions of the brain. Emerging evidence suggests that coordinated activity between the dorsal hippocampus (DH) and medial prefrontal cortex (mPFC) is required for episodic memory consolidation. However, the mechanisms through which the DH and mPFC interact to promote memory consolidation remain poorly understood. A growing body of research suggests that the nucleus reuniens of the thalamus (RE) is one of several structures that facilitate communication between the DH and mPFC during memory and may do so through bidirectional excitatory projections to both regions. Furthermore, recent work from other labs indicates that the RE is necessary for spatial working memory. However, it is not clear to what extent the RE is necessary for memory of object locations. The goal of this study was to determine whether activity in the RE is necessary for spatial memory as measured by the object placement (OP) task in female mice. A kappa-opioid receptor DREADD (KORD) virus was used to inactivate excitatory neurons in the RE pre- or post-training to establish a role for the RE in spatial memory acquisition and consolidation, respectively. RE inactivation prior to, or immediately after, object training blocked OP memory formation relative to chance and to control mice. Moreover, expression of the immediate early gene EGR-1 was reduced in the RE 1 hour after an object training trial, supporting the conclusion that reduced neuronal activity in the RE impairs the formation of object location memories. In summary, the findings of this study support a key role for the RE in spatial memory acquisition and consolidation.

Midline Thalamic Damage Associated with Alcohol-Use Disorders: Disruption of Distinct Thalamocortical Pathways and Function

The thalamus, a significant part of the diencephalon, is a symmetrical and bilateral central brain structure. The thalamus is subdivided into three major groups of nuclei based on their function: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Anatomically, nuclei within the thalamus are described by their location, such as anterior, medial, lateral, ventral, and posterior. In this review, we summarize the role of medial and midline thalamus in cognition, ranging from learning and memory to flexible adaptation. We focus on the discoveries in animal models of alcohol-related brain damage, which identify the loss of neurons in the medial and midline thalamus as drivers of cognitive dysfunction associated with alcohol use disorders. Models of developmental ethanol exposure and models of adult alcohol-related brain damage and are compared and contrasted, and it was revealed that there are similar (anterior thalamus) and different (intralaminar [adult exposure] versus ventral midline [developmental exposure]) thalamic pathology, as well as disruptions of thalamo-hippocampal and thalamo-cortical circuits. The final part of the review summarizes approaches to recover alcohol-related brain damage and cognitive and behavioral outcomes. These approaches include pharmacological, nutritional and behavioral interventions that demonstrated the potential to mitigate alcohol-related damage. In summary, the medial/midline thalamus is a significant contributor to cognition function, which is also sensitive to alcohol-related brain damage across the life span, and plays a role in alcohol-related cognitive dysfunction.

The nucleus reuniens, a thalamic relay for cortico-hippocampal interaction in recent and remote memory consolidation

The consolidation of declarative memories is believed to occur mostly during sleep and involves a dialogue between two brain regions, the hippocampus and the medial prefrontal cortex. The information encoded during experience by neuronal assemblies is replayed during sleep leading to the progressive strengthening and integration of the memory trace in the prefrontal cortex. The gradual transfer of information from the hippocampus to the medial prefrontal cortex for long-term storage requires the synchronization of cortico-hippocampal networks by different oscillations, like ripples, spindles, and slow oscillations. Recent studies suggest the involvement of a third partner, the nucleus reuniens, in memory consolidation. Its bidirectional connections with the hippocampus and medial prefrontal cortex place the reuniens in a key position to relay information between the two structures. Indeed, many topical works reveal the original role that the nucleus reuniens occupies in different recent and remote memories consolidation. This review aimed to examine these contributions, as well as its functional embedment in this complex memory network, and provide some insights on the possible mechanisms.

Recovering Spatial Information through Reactivation: Brain Oxidative Metabolism Involvement in Males and Females

Memory involves a complex network system of interconnected brain areas in which labile trace memories are transformed into enduring ones and reorganized in a time-dependant manner. Although it has been observed that remote memories are less prone to destabilizing, they can become fragile and lead to behavioural decline. We explored the behavioural outcomes of male and female rats in response to the reactivation of a previously acquired allocentric spatial reference memory, under conditions in which animals have shown a retrieval decay. In addition, we assessed their brain metabolic activity through cytochrome c oxidase (CCO) histochemistry. Our results show that a spatial memory amnesia-like behaviour with a time interval of 45 days can be recovered after re-exposure to the environmental configuration with the reinforced contingencies. Moreover, we observed that, following reactivation, male rats reveal a decrease in metabolic activity in septal nuclei and thalamic structures, whereas female rats add a metabolic reduction in the hippocampus, amygdala, mPFC, and retrosplenial, parietal and rhinal cortices, suggesting that they efficiently employ these brain areas when reactivation a memory that has suffered a decay with time. Finally, although male and female rats perform the behavioural task equally, we found sex differences at the brain metabolism level, revealing the differential contribution of brain limbic system energy demands by sex, even when their performance is similar. In conclusion, our work provides behavioural and brain data about remote spatial retrieval and memory reactivation processes.

Optogenetic perturbation of projections from thalamic nucleus reuniens to hippocampus disrupts spatial working memory retrieval more than encoding

BACKGROUND

Working memory deficits are key cognitive symptoms of schizophrenia. Elevated delta oscillations, which are uniquely associated with the presence of the illness, may be the proximal cause of these deficits. Spatial working memory (SWM) is impaired by elevated delta oscillations projecting from thalamic nucleus reuniens (RE) to the hippocampus (HPC); these findings imply a role of the RE-HPC circuit in working memory deficits in schizophrenia, but questions remain as to whether the affected process is the encoding of working memory, recall, or both. Here, we answered this question by optogenetically inducing delta oscillations in the HPC terminals of RE axons in mice during either the encoding or retrieval phase (or both) of an SWM task.

METHODS

We transduced cells in RE to express channelrhodopsin-2 through bilateral injection of adeno-associated virus, and bilaterally implanted optical fibers dorsal to the hippocampus (HPC). While mice performed a spatial memory task on a Y-maze, the RE-HPC projections were optogenetically stimulated at delta frequency during distinct phases of the task.

RESULTS

Full-trial stimulation successfully impaired SWM performance, replicating the results of the previous study in a mouse model. Task-phase-specific stimulation significantly impaired performance during retrieval but not encoding.

CONCLUSIONS

Our results indicate that perturbations in the RE-HPC circuit specifically impair the retrieval phase of working memory. This finding supports the hypothesis that abnormal delta frequency bursting in the thalamus could have a causal role in producing the WM deficits seen in schizophrenia.

The anterior thalamic nuclei and nucleus reuniens: So similar but so different

Two thalamic sites are of especial significance for understanding hippocampal - diencephalic interactions: the anterior thalamic nuclei and nucleus reuniens. Both nuclei have dense, direct interconnections with the hippocampal formation, and both are directly connected with many of the same cortical and subcortical areas. These two thalamic sites also contain neurons responsive to spatial stimuli while lesions within these two same areas can disrupt spatial learning tasks that are hippocampal dependent. Despite these many similarities, closer analysis reveals important differences in the details of their connectivity and the behavioural impact of lesions in these two thalamic sites. These nuclei play qualitatively different roles that largely reflect the contrasting relative importance of their medial frontal cortex interactions (nucleus reuniens) compared with their retrosplenial, cingulate, and mammillary body interactions (anterior thalamic nuclei). While the anterior thalamic nuclei are critical for multiple aspects of hippocampal spatial encoding and performance, nucleus reuniens contributes, as required, to aid cognitive control and help select correct from competing memories.

Characterizing the human APOE epsilon 4 knock-in transgene in female and male rats with multimodal magnetic resonance imaging

The APOE Ɛ4 genotype is the most prevalent genetic risk for Alzheimer's disease (AD). Women carriers of Ɛ4 have higher risk for an early onset of AD than men. Human imaging studies suggest apolipoprotein Ɛ4 may affect brain structures associated with cognitive decline in AD many years before disease onset. It was hypothesized that female APOE Ɛ4 carriers would present with decreased cognitive function and neuroradiological evidence of early changes in brain structure and function as compared to male carriers. Six-month old wild-type (WT) and human APOE Ɛ4 knock-in (TGRA8960), male and female Sprague Dawley rats were studied for changes in brain structure using voxel-based morphometry, alteration in white and gray matter microarchitecture using diffusion weighted imaging with indices of anisotropy, and functional coupling using resting state BOLD functional connectivity. Images from each modality were registered to, and analyzed, using a 3D MRI rat atlas providing site-specific data on over 168 different brain areas. Quantitative volumetric analysis revealed areas involved in memory and arousal were significantly different between Ɛ4 and wild-type (WT) females, with few differences between male genotypes. Diffusion weighted imaging showed few differences between WT and Ɛ4 females, while male genotypes showed significant different measures in fractional anisotropy and apparent diffusion coefficient. Resting state functional connectivity showed Ɛ4 females had greater connectivity between areas involved in cognition, emotion, and arousal compared to WT females, with male Ɛ4 showing few differences from controls. Interestingly, male Ɛ4 showed increased anxiety and decreased performance in spatial and episodic memory tasks compared to WT males, with female genotypes showing little difference across behavioral tests. The sex differences in behavior and diffusion weighted imaging suggest male carriers of the Ɛ4 allele may be more vulnerable to cognitive and emotional complications compared to female carriers early in life. Conversely, the data may also suggest that female carriers are more resilient to cognitive/emotional problems at this stage of life perhaps due to altered brain volumes and enhanced connectivity.

The Reuniens and Rhomboid Nuclei Are Required for Acquisition of Pavlovian Trace Fear Conditioning in Rats

The reuniens (Re) and rhomboid (Rh) nuclei (ReRh) of the midline thalamus interconnects the hippocampus (HPC) and the medial prefrontal cortex (mPFC). Several studies have suggested that the ReRh participates in various cognitive tasks. However, little is known about the contribution of the ReRh in Pavlovian trace fear conditioning, a procedure with a temporal gap between the conditioned stimulus (CS) and the unconditioned stimulus (US), and therefore making it harder for the animals to acquire. Because the HPC and mPFC are involved in trace, but not delay, fear conditioning and given the role of the ReRh in mediating this neurocircuitry, we hypothesized that ReRh inactivation leads to a learning deficit only in trace conditioning. In a series of experiments, we first examined the c-Fos expression in male Long-Evans rats and established that the ReRh was recruited in the encoding, but not the retrieval phase, of fear memory. Next, we performed behavioral pharmacology experiments and found that ReRh inactivation impaired only the acquisition, but not the consolidation or retrieval, of trace fear. However, although the ReRh was recruited during the encoding of delay fear demonstrated by c-Fos results, ReRh inactivation in any phases did not interfere with delay conditioning. Finally, we found that trace fear acquired under ReRh inactivation reprised when the ReRh was brought off-line during retrieval. Together, our data revealed the essential role of the ReRh in a learning task with temporally discontinuous stimuli.

Dorsomedial prefrontal cortex and hippocampus represent strategic context even while simultaneously changing representation throughout a task session

Dorsomedial prefrontal cortex (dmPFC) and hippocampus (HPC) are thought to play complementary roles in a spatial working memory and decision-making network, where spatial information from HPC informs representations in dmPFC, and contextual information from dmPFC biases how HPC recalls that information. We recorded simultaneously from neural ensembles in rodent dmPFC and HPC as rats performed a rule-switching task, and found that ensembles in dmPFC and HPC simultaneously encoded task contingencies and other time-varying information. While ensembles in HPC transitioned to represent new contingencies at the same time as rats updated their strategies to be consistent with the new contingency, dmPFC ensembles transitioned earlier. Neural representations of other time-varying information also changed faster in dmPFC than in HPC. Our results suggest that HPC and dmPFC represent contingencies while simultaneously representing other information which changes over time, and that this contextual information is integrated into hippocampal representations more slowly than in dmPFC.

The reuniens and rhomboid nuclei are necessary for contextual fear memory persistence in rats

Memory persistence refers to the process by which a temporary, labile memory is transformed into a stable and long-lasting state. This process involves a reorganization of brain networks at systems level, which requires functional interactions between the hippocampus (HP) and medial prefrontal cortex (mPFC). The reuniens (Re) and rhomboid (Rh) nuclei of the ventral midline thalamus are bidirectionally connected with both regions, and we previously demonstrated their crucial role in spatial memory persistence. We now investigated, in male rats, whether specific manipulations of ReRh activity also affected contextual and cued fear memory persistence. We showed that the permanent ReRh lesion impaired remote, but not recent contextual fear memory. Tone-cued recent and remote fear memory were spared by the lesion. In intact rats, acute chemogenetic ReRh inhibition conducted before recall of either recent or remote contextual fear memories produced no effect, indicating that the ReRh nuclei are not required for retrieval of such memories. This was also suggested by a functional cellular imaging approach, as retrieval did not alter c-fos expression in the ReRh. Collectively, these data are compatible with a role for the ReRh in 'off-line' consolidation of a contextual fear memory and support the crucial importance of ventral midline thalamic nuclei in systems consolidation of memories.

Naïve to expert: Considering the role of previous knowledge in memory

In humans, most of our new memories are in some way or another related to what we have already experienced. However, in memory research, especially in non-human animal research, subjects are often mostly naïve to the world. But we know that previous knowledge will change how memories are processed and which brain areas are critical at which time point. Each process from encoding, consolidation, to memory retrieval will be affected. Here, we summarise previous knowledge effects on the neurobiology of memory in both humans and non-human animals, with a special focus on schemas - associative network structures. Furthermore, we propose a new theory on how there may be a continuous gradient from naïve to expert, which would modulate the importance and role of brain areas, such as the hippocampus and prefrontal cortex.

The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior

The nucleus reuniens of the thalamus (RE) is a key component of an extensive network of hippocampal and cortical structures and is a fundamental substrate for cognition. A common misconception is that RE is a simple relay structure. Instead, a better conceptualization is that RE is a critical component of a canonical higher-order cortico-thalamo-cortical circuit that supports communication between the medial prefrontal cortex (mPFC) and the hippocampus (HC). RE dysfunction is implicated in several clinical disorders including, but not limited to Alzheimer's disease, schizophrenia, and epilepsy. Here, we review key anatomical and physiological features of the RE based primarily on studies in rodents. We present a conceptual model of RE circuitry within the mPFC-RE-HC system and speculate on the computations RE enables. We review the rapidly growing literature demonstrating that RE is critical to, and its neurons represent, aspects of behavioral tasks that place demands on memory focusing on its role in navigation, spatial working memory, the temporal organization of memory, and executive functions.