Dissociable encoding of motivated behavior by parallel thalamo-striatal projections.

The successful pursuit of goals requires the coordinated execution and termination of actions that lead to positive outcomes. This process relies on motivational states that are guided by internal drivers, such as hunger or fear. However, the mechanisms by which the brain tracks motivational states to shape instrumental actions are not fully understood. The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus that shapes motivated behaviors via its projections to the nucleus accumbens (NAc)1,2,3,4,5,6,7,8 and monitors internal state via interoceptive inputs from the hypothalamus and brainstem.3,9,10,11,12,13,14 Recent studies indicate that the PVT can be subdivided into two major neuronal subpopulations, namely PVTD2(+) and PVTD2(-), which differ in genetic identity, functionality, and anatomical connectivity to other brain regions, including the NAc.4,15,16 In this study, we used fiber photometry to investigate the in vivo dynamics of these two distinct PVT neuronal types in mice performing a foraging-like behavioral task. We discovered that PVTD2(+) and PVTD2(-) neurons encode the execution and termination of goal-oriented actions, respectively. Furthermore, activity in the PVTD2(+) neuronal population mirrored motivation parameters such as vigor and satiety. Similarly, PVTD2(-) neurons also mirrored some of these parameters, but to a much lesser extent. Importantly, these features were largely preserved when activity in PVT projections to the NAc was selectively assessed. Collectively, our results highlight the existence of two parallel thalamo-striatal projections that participate in the dynamic regulation of goal pursuits and provide insight into the mechanisms by which the brain tracks motivational states to shape instrumental actions.

Disconnecting prefrontal cortical neurons from the ventral midline thalamus: Loss of specificity due to progressive neural toxicity of an AAV-Cre in the rat thalamus.

BACKGROUND: The thalamic reuniens (Re) and rhomboid (Rh) nuclei are bidirectionally connected with the medial prefrontal cortex (mPFC) and the hippocampus (Hip). Fiber-sparing N-methyl-D-aspartate lesions of the ReRh disrupt cognitive functions, including persistence of certain memories. Because such lesions irremediably damage neurons interconnecting the ReRh with the mPFC and the Hip, it is impossible to know if one or both pathways contribute to memory persistence. Addressing such an issue requires selective, pathway-restricted and direction-specific disconnections. NEW METHOD: A recent method associates a retrograde adeno-associated virus (AAV) expressing Cre recombinase with an anterograde AAV expressing a Cre-dependent caspase, making such disconnection feasible by caspase-triggered apoptosis when both constructs meet intracellularly. We injected an AAVrg-Cre-GFP into the ReRh and an AAV5-taCasp into the mPFC. As expected, part of mPFC neurons died, but massive neurotoxicity of the AAVrg-Cre-GFP was found in ReRh, contrasting with normal density of DAPI staining. Other stainings demonstrated increasing density of reactive astrocytes and microglia in the neurodegeneration site. COMPARISON WITH EXISTING METHODS: Reducing the viral titer (by a 4-fold dilution) and injection volume (to half) attenuated toxicity substantially, still with evidence for partial disconnection between mPFC and ReRh. CONCLUSIONS: There is an imperative need to verify potential collateral damage inherent in this type of approach, which is likely to distort interpretation of experimental data. Therefore, controls allowing to distinguish collateral phenotypic effects from those linked to the desired disconnection is essential. It is also crucial to know for how long neurons expressing the Cre-GFP protein remain operational post-infection.

Modulation of learning safety signals by acute stress: paraventricular thalamus and prefrontal inhibition.

Distinguishing between cues predicting safety and danger is crucial for survival. Impaired learning of safety cues is a central characteristic of anxiety-related disorders. Despite recent advances in dissecting the neural circuitry underlying the formation and extinction of conditioned fear, the neuronal basis mediating safety learning remains elusive. Here, we showed that safety learning reduces the responses of paraventricular thalamus (PVT) neurons to safety cues, while activation of these neurons controls both the formation and expression of safety memory. Additionally, the PVT preferentially activates prefrontal cortex somatostatin interneurons (SOM-INs), which subsequently inhibit parvalbumin interneurons (PV-INs) to modulate safety memory. Importantly, we demonstrate that acute stress impairs the expression of safety learning, and this impairment can be mitigated when the PVT is inhibited, indicating PVT mediates the stress effect. Altogether, our findings provide insights into the mechanism by which acute stress modulates safety learning.

Deep brain stimulation of thalamic nucleus reuniens promotes neuronal and cognitive resilience in an Alzheimer's disease mouse model.

The mechanisms that confer cognitive resilience to Alzheimer's Disease (AD) are not fully understood. Here, we describe a neural circuit mechanism underlying this resilience in a familial AD mouse model. In the prodromal disease stage, interictal epileptiform spikes (IESs) emerge during anesthesia in the CA1 and mPFC regions, leading to working memory disruptions. These IESs are driven by inputs from the thalamic nucleus reuniens (nRE). Indeed, tonic deep brain stimulation of the nRE (tDBS-nRE) effectively suppresses IESs and restores firing rate homeostasis under anesthesia, preventing further impairments in nRE-CA1 synaptic facilitation and working memory. Notably, applying tDBS-nRE during the prodromal phase in young APP/PS1 mice mitigates age-dependent memory decline. The IES rate during anesthesia in young APP/PS1 mice correlates with later working memory impairments. These findings highlight the nRE as a central hub of functional resilience and underscore the clinical promise of DBS in conferring resilience to AD pathology by restoring circuit-level homeostasis.

Thalamic nucleus reuniens coordinates prefrontal-hippocampal synchrony to suppress extinguished fear.

Traumatic events result in vivid and enduring fear memories. Suppressing the retrieval of these memories is central to behavioral therapies for pathological fear. The medial prefrontal cortex (mPFC) and hippocampus (HPC) have been implicated in retrieval suppression, but how mPFC-HPC activity is coordinated during extinction retrieval is unclear. Here we show that after extinction training, coherent theta oscillations (6-9 Hz) in the HPC and mPFC are correlated with the suppression of conditioned freezing in male and female rats. Inactivation of the nucleus reuniens (RE), a thalamic hub interconnecting the mPFC and HPC, reduces extinction-related Fos expression in both the mPFC and HPC, dampens mPFC-HPC theta coherence, and impairs extinction retrieval. Conversely, theta-paced optogenetic stimulation of RE augments fear suppression and reduces relapse of extinguished fear. Collectively, these results demonstrate a role for RE in coordinating mPFC-HPC interactions to suppress fear memories after extinction.

Distinctiveness and continuity in transcriptome and connectivity in the anterior-posterior axis of the paraventricular nucleus of the thalamus.

The paraventricular nucleus of the thalamus (PVT) projects axons to multiple areas, mediates a wide range of behaviors, and exhibits regional heterogeneity in both functions and axonal projections. Still, questions regarding the cell types present in the PVT and the extent of their differences remain inadequately addressed. We applied single-cell RNA sequencing to depict the transcriptomic characteristics of mouse PVT neurons. We found that one of the most significant variances in the PVT transcriptome corresponded to the anterior-posterior axis. While the single-cell transcriptome classified PVT neurons into five types, our transcriptomic and histological analyses showed continuity among the cell types. We discovered that anterior and posterior subpopulations had nearly non-overlapping projection patterns, while another population showed intermediate patterns. In addition, these subpopulations responded differently to appetite-related neuropeptides, with their activation showing opposing effects on food consumption. Our studies unveiled the contrasts and the continuity of PVT neurons that underpin their function.

Nucleus reuniens transiently synchronizes memory networks at beta frequencies.

Episodic memory-based decision-making requires top-down medial prefrontal cortex and hippocampal interactions. This integrated prefrontal-hippocampal memory state is thought to be organized by synchronized network oscillations and mediated by connectivity with the thalamic nucleus reuniens (RE). Whether and how the RE synchronizes prefrontal-hippocampal networks in memory, however, remains unknown. Here, we recorded local field potentials from the prefrontal-RE-hippocampal network while rats engaged in a nonspatial sequence memory task, thereby isolating memory-related activity from running-related oscillations. We found that synchronous prefrontal-hippocampal beta bursts (15-30 Hz) dominated during memory trials, whereas synchronous theta activity (6-12 Hz) dominated during non-memory-related running. Moreover, RE beta activity appeared first, followed by prefrontal and hippocampal synchronized beta, suggesting that prefrontal-hippocampal beta could be driven by the RE. To test whether the RE is capable of driving prefrontal-hippocampal beta synchrony, we used an optogenetic approach (retroAAV-ChR2). RE activation induced prefrontal-hippocampal beta coherence and reduced theta coherence, matching the observed memory-driven network state in the sequence task. These findings are the first to demonstrate that the RE contributes to memory by driving transient synchronized beta in the prefrontal-hippocampal system, thereby facilitating interactions that underlie memory-based decision-making.

Respiration organizes gamma synchrony in the prefronto-thalamic network.

Multiple cognitive operations are associated with the emergence of gamma oscillations in the medial prefrontal cortex (mPFC) although little is known about the mechanisms that control this rhythm. Using local field potential recordings from cats, we show that periodic bursts of gamma recur with 1 Hz regularity in the wake mPFC and are locked to the exhalation phase of the respiratory cycle. Respiration organizes long-range coherence in the gamma band between the mPFC and the nucleus reuniens the thalamus (Reu), linking the prefrontal cortex and the hippocampus. In vivo intracellular recordings of the mouse thalamus reveal that respiration timing is propagated by synaptic activity in Reu and likely underlies the emergence of gamma bursts in the prefrontal cortex. Our findings highlight breathing as an important substrate for long-range neuronal synchronization across the prefrontal circuit, a key network for cognitive operations.

Identifying the midline thalamus in humans in vivo.

The midline thalamus is critical for flexible cognition, memory, and stress regulation in humans and its dysfunction is associated with several neurological and psychiatric disorders, including Alzheimer's disease, schizophrenia, and depression. Despite the pervasive role of the midline thalamus in cognition and disease, there is a limited understanding of its function in humans, likely due to the absence of a rigorous noninvasive neuroimaging methodology to identify its location. Here, we introduce a new method for identifying the midline thalamus in vivo using probabilistic tractography and k-means clustering with diffusion weighted imaging data. This approach clusters thalamic voxels based on data-driven cortical and subcortical connectivity profiles and then segments the midline thalamus according to anatomical connectivity tracer studies in rodents and macaques. Results from two different diffusion weighted imaging sets, including adult data (22-35 years) from the Human Connectome Project (n = 127) and adolescent data (9-14 years) collected at Florida International University (n = 34) showed that this approach reliably classifies midline thalamic clusters. As expected, these clusters were most evident along the dorsal/ventral extent of the third ventricle and were primarily connected to the agranular medial prefrontal cortex (e.g., anterior cingulate cortex), nucleus accumbens, and medial temporal lobe regions. The midline thalamus was then bisected based on a human brain atlas into a dorsal midline thalamic cluster (paraventricular and paratenial nuclei) and a ventral midline thalamic cluster (rhomboid and reuniens nuclei). This anatomical connectivity-based identification of the midline thalamus offers the opportunity for necessary investigation of this region in vivo in the human brain and how it relates to cognitive functions in humans, and to psychiatric and neurological disorders.

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.

Collateral rostral thalamic projections to prelimbic, infralimbic, anterior cingulate and retrosplenial cortices in the rat brain.

As the functional properties of a cortical area partly reflect its thalamic inputs, the present study compared collateral projections arising from various rostral thalamic nuclei that terminate across prefrontal (including anterior cingulate) and retrosplenial areas in the rat brain. Two retrograde tracers, fast blue and cholera toxin B, were injected in pairs to different combinations of cortical areas. The research focused on the individual anterior thalamic nuclei, including the interanteromedial nucleus, nucleus reuniens and the laterodorsal nucleus. Of the principal anterior thalamic nuclei, only the anteromedial nucleus contained neurons reaching both the anterior cingulate cortex and adjacent cortical areas (prefrontal or retrosplenial), though the numbers were modest. For these same cortical pairings (medial prefrontal/anterior cingulate and anterior cingulate/retrosplenial), the interanteromedial nucleus and nucleus reuniens contained slightly higher proportions of bifurcating neurons (up to 11% of labelled cells). A contrasting picture was seen for collaterals reaching different areas within retrosplenial cortex. Here, the anterodorsal nucleus, typically provided the greatest proportion of bifurcating neurons (up to 15% of labelled cells). While individual neurons that terminate in different retrosplenial areas were also found in the other thalamic nuclei, they were infrequent. Consequently, these thalamo-cortical projections predominantly arise from separate populations of neurons with discrete cortical termination zones, consistent with the transmission of segregated information and influence. Overall, two contrasting medial-lateral patterns of collateral projections emerged, with more midline nuclei, for example, nucleus reuniens and the interoanteromedial nucleus innervating prefrontal areas, while more dorsal and lateral anterior thalamic collaterals innervated retrosplenial cortex.

Sex differences in electrophysiological properties and voltage-gated ion channel expression in the paraventricular thalamic nucleus following repeated stress.

BACKGROUND: Habituation to repeated stress refers to a progressive reduction in the stress response following multiple exposures to the same, predictable stressor. We previously demonstrated that the posterior division of the paraventricular thalamic nucleus (pPVT) nucleus regulates habituation to 5 days of repeated restraint stress in male rats. Compared to males, female rats display impaired habituation to 5 days of restraint. To better understand how activity of pPVT neurons is differentially impacted in stressed males and females, we examined the electrophysiological properties of pPVT neurons under baseline conditions or following restraint. METHODS: Adult male and female rats were exposed to no stress (handling only), a single period of 30 min restraint or 5 daily exposures to 30 min restraint. 24 h later, pPVT tissue was prepared for recordings. RESULTS: We report here that spontaneous excitatory post-synaptic current (sEPSC) amplitude was increased in males, but not females, following restraint. Furthermore, resting membrane potential of pPVT neurons was more depolarized in males. This may be partially due to reduced potassium leakage in restrained males as input resistance was increased in male, but not female, rats 24 h following 1 or 5 days of 30-min restraint. Reduced potassium efflux during action potential firing also occurred in males following a single restraint as action potential half-width was increased following a single restraint. Restraint had limited effects on electrophysiological properties in females, although the mRNA for 10 voltage-gated ion channel subunits was altered in the pPVT of female rats. CONCLUSIONS: The results suggest that restraint-induced changes in pPVT activation promote habituation in males. These findings are the first to describe a sexual dimorphism in stress-induced electrophysiological properties and voltage-gated ion channel expression in the pPVT. These results may explain, at least in part, why habituation to 5 days of restraint is disrupted in female rats.

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.

Prefrontal projections to the nucleus reuniens signal behavioral relevance of stimuli during associative learning.

The nucleus reuniens (RE) is necessary for memories dependent on the interaction between the medial prefrontal cortex (mPFC) and hippocampus (HPC). One example is trace eyeblink conditioning, in which the mPFC exhibits differential activity to neutral conditioned stimuli (CS) depending on their contingency with an aversive unconditioned stimulus (US). To test if this relevancy signal is routed to the RE, we photometrically recorded mPFC axon terminals within the RE and tracked their changes with learning. As a comparison, we measured prefrontal terminal activity in the mediodorsal thalamus (MD), which lacks connectivity with the HPC. In naïve male rats, prefrontal terminals within the RE were not strongly activated by tone or light. As the rats associated one of the stimuli (CS+) with the US, terminals gradually increased their response to the CS+ but not the other stimulus (CS-). In contrast, stimulus-evoked responses of prefrontal terminals within the MD were strong even before conditioning. They also became augmented only to the CS+ in the first conditioning session; however, the degree of activity differentiation did not improve with learning. These findings suggest that associative learning selectively increased mPFC output to the RE, signaling the behavioral relevance of sensory stimuli.

Neuropeptide S Encodes Stimulus Salience in the Paraventricular Thalamus.

Evaluation of stimulus salience is critical for any higher organism, as it allows for prioritizing of vital information, preparation of responses, and formation of valuable memory. The paraventricular nucleus of the thalamus (PVT) has recently been identified as an integrator of stimulus salience but the neurochemical basis and afferent input regarding salience signaling have remained elusive. Here we report that neuropeptide S (NPS) signaling in the PVT is necessary for stimulus salience encoding, including aversive, neutral and reinforcing sensory input. Taking advantage of a striking deficit of both NPS receptor (NPSR1) and NPS precursor knockout mice in fear extinction or novel object memory formation, we demonstrate that intra-PVT injections of NPS can rescue the phenotype in NPS precursor knockout mice by increasing the salience of otherwise low-intensity stimuli, while intra-PVT injections of NPSR1 antagonist in wild type mice partially replicates the knockout phenotype. The PVT appears to provide stimulus salience encoding in a dose- and NPS-dependent manner. PVT NPSR1 neurons recruit the nucleus accumbens shell and structures in the prefrontal cortex and amygdala, which were previously linked to the brain salience network. Overall, these results demonstrate that stimulus salience encoding is critically associated with NPS activity in the PVT.

Tonic excitation of nucleus reuniens decreases prefrontal-hippocampal coordination during slow-wave states.

The nucleus reuniens of the thalamus (RE) is an important node between the medial prefrontal cortex (mPFC) and the hippocampus (HPC). Previously, we have shown that its mode of activity and its influence in mPFC-HPC communication is dependent upon brain state. During slow-wave states, RE units are closely and rhythmically coupled to the ongoing mPFC-slow oscillation (SO), while during activated (theta) states, RE neurons fire in an arrhythmic and tonically active manner. Inactivating the RE selectively impoverishes coordination of the SO between mPFC and HPC and interestingly, both mPFC and RE stimulation during the SO cause larger responses in the HPC than during theta. It is unclear if the activity patterns within the RE across states may play a role in both phenomena. Here, we optogenetically excited RE neurons in a tonic fashion to assess the impact on mPFC-HPC coupling. This stimulation decreased the influence of mPFC stimulation in the HPC during SO states, in a manner similar to what is observed across state changes into theta. Importantly, this type of stimulation had no effect on evoked responses during theta. Perhaps more interestingly, tonic optogenetic excitation of the RE also decreased mPFC-HPC SO coherence. Thus, it may not be the integrity of the RE per se that is responsible for efficient communication between mPFC and HPC, but rather the particular state in which RE neurons find themselves. Our results have direct implications for how distant brain regions can communicate most effectively, an issue that is ultimately important for activity-dependent processes occurring during slow-wave sleep-dependent memory consolidation.

Dual medial prefrontal cortex and hippocampus projecting neurons in the paraventricular nucleus of the thalamus.

The paraventricular nucleus (PVT) of the midline thalamus is a critical higher-order cortico-thalamo-cortical integration site that plays a critical role in various behaviors including reward seeking, cue saliency, and emotional memory. Anatomical studies have shown that PVT projects to both medial prefrontal cortex (mPFC) and hippocampus (HC). However, dual mPFC-HC projecting neurons which could serve a role in synchronizing mPFC and HC activity during PVT-dependent behaviors, have not been explored. Here we used a dual retrograde adenoassociated virus (AAV) tracing approach to characterize the location and proportion of different projection populations that send collaterals to mPFC and/or ventral hippocampus (vHC) in rats. Additionally, we examined the distribution of calcium binding proteins calretinin (CR) and calbindin (CB) with respect to these projection populations in PVT. We found that PVT contains separate populations of cells that project to mPFC, vHC, and those that innervate both regions. Interestingly, dual mPFC-HC projecting cells expressed neither CR nor CB. Topographically, CB+ and CR+ containing cells clustered around dual projecting neurons in PVT. These results are consistent with the features of dual mPFC-vHC projecting cells in the nucleus reuniens (RE) and suggestive of a functional mPFC-PVT-vHC system that may support mPFC-vHC interactions in PVT-dependent behaviors.

Exposure to hot and cold environments activates neurons projecting from the paraventricular thalamic nucleus to brain regions related to approach and avoidance behaviors.

Although cool- and warm-seeking behaviors for behavioral thermoregulation are considered to be appetitive/approach and aversive/avoidance behaviors, the neuronal circuits mediating such behaviors remain to be elucidated. A growing body of evidence suggests that the paraventricular thalamic nucleus (PVT) is a key brain region in a neuronal circuit that mediates appetitive/approach and aversive/avoidance behaviors. In this study, to elucidate the neuronal circuits mediating behavioral thermoregulatory responses, we examined whether neuronal pathways from the PVT to the nucleus accumbens (NAc), bed nucleus of the stria terminalis (BNST), and central nucleus of the amygdala (CeA), which are brain regions implicated in mediating appetitive/approach and aversive/avoidance behaviors, are activated during exposure to hot (38°C) and cold (8°C) environments using c-Fos immunostaining and retrograde tracing. Our results showed activation of neuronal pathways from the PVT to the NAc, BNST, and CeA during exposure to hot and cold environments, suggesting that activation of these pathways may be involved in avoidance behaviors from hot and cold environments for behavioral thermoregulation.

Dual projecting cells linking thalamic and cortical communication routes between the medial prefrontal cortex and hippocampus.

The interactions between the medial prefrontal cortex (mPFC) and the hippocampus (HC) are critical for memory and decision making and have been specifically implicated in several neurological disorders including schizophrenia, epilepsy, frontotemporal dementia, and Alzheimer's disease. The ventral midline thalamus (vmThal), and lateral entorhinal cortex and perirhinal cortex (LEC/PER) constitute major communication pathways that facilitate mPFC-HC interactions in memory. Although vmThal and LEC/PER circuits have been delineated separately we sought to determine whether these two regions share cell-specific inputs that could influence both routes simultaneously. To do this we used a dual fluorescent retrograde tracing approach using cholera toxin subunit-B (CTB-488 and CTB-594) with injections targeting vmThal and the LEC/PER in rats. Retrograde cell body labeling was examined in key regions of interest within the mPFC-HC system including: (1) mPFC, specifically anterior cingulate cortex (ACC), dorsal and ventral prelimbic cortex (dPL, vPL), and infralimbic cortex (IL); (2) medial and lateral septum (MS, LS); (3) subiculum (Sub) along the dorsal-ventral and proximal-distal axes; and (4) LEC and medial entorhinal cortex (MEC). Results showed that dual vmThal-LEC/PER-projecting cell populations are found in MS, vSub, and the shallow layers II/III of LEC and MEC. We did not find any dual projecting cells in mPFC or in the cornu ammonis (CA) subfields of the HC. Thus, mPFC and HC activity is sent to vmThal and LEC/PER via non-overlapping projection cell populations. Importantly, the dual projecting cell populations in MS, vSub, and EC are in a unique position to simultaneously influence both cortical and thalamic mPFC-HC pathways critical to memory. SIGNIFICANCE STATEMENT: The interactions between mPFC and HC are critical for learning and memory, and dysfunction within this circuit is implicated in various neurodegenerative and psychiatric diseases. mPFC-HC interactions are mediated through multiple communication pathways including a thalamic hub through the vmThal and a cortical hub through lateral entorhinal cortex and perirhinal cortex. Our data highlight newly identified dual projecting cell populations in the septum, Sub, and EC of the rat brain. These dual projecting cells may have the ability to modify the information flow within the mPFC-HC circuit through synchronous activity, and thus offer new cell-specific circuit targets for basic and translational studies in memory.

Pathways for Memory, Cognition and Emotional Context: Hippocampal, Subgenual Area 25, and Amygdalar Axons Show Unique Interactions in the Primate Thalamic Reuniens Nucleus.

The reuniens nucleus (RE) is situated at the most ventral position of the midline thalamus. In rats and mice RE is distinguished by bidirectional connections with the hippocampus and medial prefrontal cortex (mPFC) and a role in memory and cognition. In primates, many foundational questions pertaining to RE remain unresolved. We addressed these issues by investigating the composition of the rhesus monkey RE in both sexes by labeling for GABA, a marker of inhibitory neurons, and for the calcium-binding proteins parvalbumin (PV), calbindin (CB), and calretinin (CR), which label thalamic excitatory neurons that project to cortex. As in rats and mice, the macaque RE was mostly populated by CB and CR neurons, characteristic of matrix-dominant nuclei, and had bidirectional connections with hippocampus and mPFC area 25 (A25). Unlike rodents, we found GABAergic neurons in the monkey RE and a sparser but consistent population of core-associated thalamocortical PV neurons. RE had stronger connections with the basal amygdalar complex than in rats or mice. Amygdalar terminations were enriched with mitochondria and frequently formed successive synapses with the same postsynaptic structures, suggesting an active and robust pathway to RE. Significantly, hippocampal pathways formed multisynaptic complexes that uniquely involved excitatory projection neurons and dendrites of local inhibitory neurons in RE, extending this synaptic principle beyond sensory to high-order thalamic nuclei. Convergent pathways from hippocampus, A25, and amygdala in RE position it to flexibly coordinate activity for memory, cognition, and emotional context, which are disrupted in several psychiatric and neurologic diseases in humans.SIGNIFICANCE STATEMENT The primate RE is a central node for memory and cognition through connections with the hippocampus and mPFC. As in rats or mice, the primate RE is a matrix-dominant thalamic nucleus, suggesting signal traffic to the upper cortical layers. Unlike rats or mice, the primate RE contains inhibitory neurons, synaptic specializations with the hippocampal pathway, and robust connections with the amygdala, suggesting unique adaptations. Convergence of hippocampal, mPFC, and amygdalar pathways in RE may help unravel a circuit basis for binding diverse signals for conscious flexible behaviors and the synthesis of memory with affective significance in primates, whereas disruption of distinct circuit nodes may occur in psychiatric disorders in humans.