Both anteromedial and anteroventral thalamic lesions impair radial-maze learning in rats

Projections from anteromedial thalamus nucleus to the midcingulate cortex mediate pain and anxiety-like behaviors in mice

Prior research has demonstrated the involvement of the midcingulate cortex (MCC) and its downstream pathway in pain regulation. However, the mechanism via which pain information is conveyed to the MCC remains unclear. The present study utilized immunohistochemistry, chemogenetics, optogenetics, and behavior detection methods to explore the involvement of MCC, anteromedial thalamus nucleus (AM), and AM-MCC pathway in pain and emotional regulation. Chemogenetics or optogenetics methods were employed to activate/inhibit MCCCaMKIIα, AMCaMKIIα, AMCaMKIIα-MCC pathway. This manipulation evokes/relieves mechanical and partial heat hyperalgesia, as well as anxiety-like behaviors. In the complete Freund,s adjuvant (CFA) inflammatory pain model, chemogenetic inhibition of the AMCaMKIIα-MCCCaMKIIα pathway contributed to pain relief. Notably, this study presented the first evidence implicating the AM in the regulation of nociception and negative emotions. Additionally, it was observed that the MCC primarily receives projections from the AM, highlighting the crucial role of this pathway in the transmission of pain and emotional information.

Convergence of location, direction, and theta in the rat anteroventral thalamic nucleus

The thalamus and cortex are anatomically interconnected, with the thalamus providing integral information for cortical functions. The anteroventral thalamic nucleus (AV) is reciprocally connected to retrosplenial cortex (RSC). Two distinct AV subfields, dorsomedial (AVDM) and ventrolateral (AVVL), project differentially to granular vs. dysgranular RSC, respectively. To probe if functional responses of AV neurons differ, we recorded single neurons and local field potentials from AVDM and AVVL in rats during foraging. We observed place cells (neurons modulated by spatial location) in both AVDM and AVVL. Additionally, we characterized neurons modulated by theta oscillations, heading direction, and a conjunction of these. Place cells and conjunctive Theta-by-Head direction cells were more prevalent in AVVL; more non-conjunctive theta and directional neurons were prevalent in AVDM. These findings add further evidence that there are two thalamocortical circuits connecting AV and RSC, and reveal that the signaling involves place information in addition to direction and theta.

Connectome-based prediction of functional impairment in experimental stroke models

Experimental rat models of stroke and hemorrhage are important tools to investigate cerebrovascular disease pathophysiology mechanisms, yet how significant patterns of functional impairment induced in various models of stroke are related to changes in connectivity at the level of neuronal populations and mesoscopic parcellations of rat brains remain unresolved. To address this gap in knowledge, we employed two middle cerebral artery occlusion models and one intracerebral hemorrhage model with variant extent and location of neuronal dysfunction. Motor and spatial memory function was assessed and the level of hippocampal activation via Fos immunohistochemistry. Contribution of connectivity change to functional impairment was analyzed for connection similarities, graph distances and spatial distances as well as the importance of regions in terms of network architecture based on the neuroVIISAS rat connectome. We found that functional impairment correlated with not only the extent but also the locations of the injury among the models. In addition, via coactivation analysis in dynamic rat brain models, we found that lesioned regions led to stronger coactivations with motor function and spatial learning regions than with other unaffected regions of the connectome. Dynamic modeling with the weighted bilateral connectome detected changes in signal propagation in the remote hippocampus in all 3 stroke types, predicting the extent of hippocampal hypoactivation and impairment in spatial learning and memory function. Our study provides a comprehensive analytical framework in predictive identification of remote regions not directly altered by stroke events and their functional implication.

Nonlinear Theta-Gamma Coupling between the Anterior Thalamus and Hippocampus Increases as a Function of Running Speed

The hippocampal theta rhythm strongly correlates to awake behavior leading to theories that it represents a cognitive state of the brain. As theta has been observed in other regions of the Papez circuit, it has been theorized that activity propagates in a reentrant manner. These observations complement the energy cascade hypothesis in which large-amplitude, slow-frequency oscillations reflect activity propagating across a large population of neurons. Higher frequency oscillations, such as gamma, are related to the speed with which inhibitory and excitatory neurons interact and distribute activity on the local level. The energy cascade hypothesis suggests that the larger anatomic loops, maintaining theta, drive the smaller loops. As hippocampal theta increases in power with running speed, so does the power and frequency of the gamma rhythm. If theta is propagated through the circuit, it stands to reason that the local field potential (LFP) recorded in other regions would be coupled to the hippocampal theta, with the coupling increasing with running speed. We explored this hypothesis using open-source simultaneous recorded data from the CA1 region of the hippocampus and the anterior dorsal and anterior ventral thalamus. Cross-regional theta coupling increased with running speed. Although the power of the gamma rhythm was lower in the anterior thalamus, there was an increase in the coupling of hippocampal theta to anterior thalamic gamma. Broadly, the data support models of how activity moves across the nervous system, suggesting that the brain uses large-scale volleys of activity to support higher cognitive processes.

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.

Time to retire the serial Papez circuit: Implications for space, memory, and attention

After more than 80 years, Papez serial circuit remains a hugely influential concept, initially for emotion, but in more recent decades, for memory. Here, we show how this circuit is anatomically and mechanistically naïve as well as outdated. We argue that a new conceptualisation is necessitated by recent anatomical and functional findings that emphasize the more equal, working partnerships between the anterior thalamic nuclei and the hippocampal formation, along with their neocortical interactions in supporting, episodic memory. Furthermore, despite the importance of the anterior thalamic for mnemonic processing, there is growing evidence that these nuclei support multiple aspects of cognition, only some of which are directly associated with hippocampal function. By viewing the anterior thalamic nuclei as a multifunctional hub, a clearer picture emerges of extra-hippocampal regions supporting memory. The reformulation presented here underlines the need to retire Papez serially processing circuit.

Anterior thalamic circuits crucial for working memory

Alterations in the structure and functional connectivity of anterior thalamic nuclei (ATN) have been linked to reduced cognition during aging. However, ATN circuits that contribute to higher cognitive functions remain understudied. We found that the anteroventral (AV) subdivision of ATN is necessary specifically during the maintenance phase of a spatial working memory task. This function engages the AV→parasubiculum (PaS)→entorhinal cortex (EC) circuit. Aged mice showed a deficit in spatial working memory, which was associated with a decrease in the excitability of AV neurons. Activation of AV neurons or the AV→PaS circuit in aged mice was sufficient to rescue their working memory performance. Furthermore, rescued aged mice showed improved behavior-induced neuronal activity in prefrontal cortex (PFC), a critical site for working memory processes. Although the direct activation of PFC neurons in aged mice also rescued their working memory performance, we found that these animals exhibited increased levels of anxiety, which was not the case for AV→PaS circuit manipulations in aged mice. These results suggest that targeting AV thalamus in aging may not only be beneficial for cognitive functions but that this approach may have fewer unintended effects compared to direct PFC manipulations.

The anterior thalamic nuclei: core components of a tripartite episodic memory system

Standard models of episodic memory focus on hippocampal-parahippocampal interactions, with the neocortex supplying sensory information and providing a final repository of mnemonic representations. However, recent advances have shown that other regions make distinct and equally critical contributions to memory. In particular, there is growing evidence that the anterior thalamic nuclei have a number of key cognitive functions that support episodic memory. In this article, we describe these findings and argue for a core, tripartite memory system, comprising a 'temporal lobe' stream (centred on the hippocampus) and a 'medial diencephalic' stream (centred on the anterior thalamic nuclei) that together act on shared cortical areas. We demonstrate how these distributed brain regions form complementary and necessary partnerships in episodic memory formation.

Ventral midline thalamus activation is correlated with memory performance in a delayed spatial matching-to-sample task: A c-Fos imaging approach in the rat

The reuniens (Re) and rhomboid (Rh) nuclei of the ventral midline thalamus are bi-directionally connected with the hippocampus and the medial prefrontal cortex. They participate in a variety of cognitive functions, including information holding for seconds to minutes in working memory tasks. What about longer delays? To address this question, we used a spatial working memory task in which rats had to reach a platform submerged in water. The platform location was changed every 2-trial session and rats had to use allothetic cues to find it. Control rats received training in a typical response-memory task. We interposed a 6 h interval between instruction (locate platform) and evaluation (return to platform) trials in both tasks. After the last session, rats were killed for c-Fos imaging. A home-cage group was used as additional control of baseline levels of c-Fos expression. C-Fos expression was increased to comparable levels in the Re (not Rh) of both spatial memory and response-memory rats as compared to their home cage counterparts. However, in spatial memory rats, not in their response-memory controls, task performance was correlated with c-Fos expression in the Re: the higher this expression, the better the performance. Furthermore, we noticed an activation of hippocampal region CA1 and of the anteroventral nucleus of the rostral thalamus. This activation was specific to spatial memory. The data point to a possible performance-determinant participation of the Re nucleus in the delayed engagement of spatial information encoded in a temporary memory.

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.

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

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

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.

Anterior Thalamic Inputs Are Required for Subiculum Spatial Coding, with Associated Consequences for Hippocampal Spatial Memory

Just as hippocampal lesions are principally responsible for "temporal lobe" amnesia, lesions affecting the anterior thalamic nuclei seem principally responsible for a similar loss of memory, "diencephalic" amnesia. Compared with the former, the causes of diencephalic amnesia have remained elusive. A potential clue comes from how the two sites are interconnected, as within the hippocampal formation, only the subiculum has direct, reciprocal connections with the anterior thalamic nuclei. We found that both permanent and reversible anterior thalamic nuclei lesions in male rats cause a cessation of subicular spatial signaling, reduce spatial memory performance to chance, but leave hippocampal CA1 place cells largely unaffected. We suggest that a core element of diencephalic amnesia stems from the information loss in hippocampal output regions following anterior thalamic pathology.SIGNIFICANCE STATEMENT At present, we know little about interactions between temporal lobe and diencephalic memory systems. Here, we focused on the subiculum, as the sole hippocampal formation region directly interconnected with the anterior thalamic nuclei. We combined reversible and permanent lesions of the anterior thalamic nuclei, electrophysiological recordings of the subiculum, and behavioral analyses. Our results were striking and clear: following permanent thalamic lesions, the diverse spatial signals normally found in the subiculum (including place cells, grid cells, and head-direction cells) all disappeared. Anterior thalamic lesions had no discernible impact on hippocampal CA1 place fields. Thus, spatial firing activity within the subiculum requires anterior thalamic function, as does successful spatial memory performance. Our findings provide a key missing part of the much bigger puzzle concerning why anterior thalamic damage is so catastrophic for spatial memory in rodents and episodic memory in humans.

Time to put the mammillothalamic pathway into context

The medial diencephalon, in particular the mammillary bodies and anterior thalamic nuclei, has long been linked to memory and amnesia. The mammillary bodies provide a dense input into the anterior thalamic nuclei, via the mammillothalamic tract. In both animal models, and in patients, lesions of the mammillary bodies, mammillothalamic tract and anterior thalamic nuclei all produce severe impairments in temporal and contextual memory, yet it is uncertain why these regions are critical. Mounting evidence from electrophysiological and neural imaging studies suggests that mammillothalamic projections exercise considerable distal influence over thalamo-cortical and hippocampo-cortical interactions. Here, we outline how damage to the mammillary body-anterior thalamic axis, in both patients and animal models, disrupts behavioural performance on tasks that relate to contextual ("where") and temporal ("when") processing. Focusing on the medial mammillary nuclei as a possible 'theta-generator' (through their interconnections with the ventral tegmental nucleus of Gudden) we discuss how the mammillary body-anterior thalamic pathway may contribute to the mechanisms via which the hippocampus and neocortex encode representations of experience.

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.

Regulation of Cued Fear Expression via Corticotropin-Releasing-Factor Neurons in the Ventral Anteromedial Thalamic Nucleus

The ventral part of the anteromedial thalamic nucleus (AMv) is in a position to convey information to the cortico-hippocampal-amygdalar circuit involved in the processing of fear memory. Corticotropin-releasing-factor (CRF) neurons are closely associated with the regulation of stress and fear. However, few studies have focused on the role of thalamic CRF neurons in fear memory. In the present study, using a conditioned fear paradigm in CRF transgenic mice, we found that the c-Fos protein in the AMv CRF neurons was significantly increased after cued fear expression. Chemogenetic activation of AMv CRF neurons enhanced cued fear expression, whereas inhibition had the opposite effect on the cued fear response. Moreover, chemogenetic manipulation of AMv CRF neurons did not affect fear acquisition or contextual fear expression. In addition, anterograde tracing of projections revealed that AMv CRF neurons project to wide areas of the cerebral cortex and the limbic system. These results uncover a critical role of AMv CRF neurons in the regulation of conditioned fear memory.

Individual Subnuclei of the Rat Anterior Thalamic Nuclei Differently affect Spatial Memory and Passive Avoidance Tasks

The role of the anterior thalamic nuclei (ATN) has been proven in different learning and memory tasks. The ATN consist of three main subnuclei, the anterodorsal (AD), anteroventral (AV) and anteromedial (AM), which have different biological characteristics such as distinct circuitry, cell population and neurotransmitter content. The role of ATN subnuclei in learning and memory has been shown in several studies. However, their probable role in different phases of memory including acquisition, consolidation and retrieval are not still well-known. For this purpose, the effect of reversible inactivation of each ATN subnucleus on different memory phases in two behavioral tasks including passive avoidance (PA) and Morris water maze (MWM) was studied. Wister male rats were bilaterally implanted with cannulas above the AD, AV or AM subnucleus in separate experimental groups in order to inject lidocaine (4%) for their temporal inactivation or, equal volume of saline. Animals were trained in the behavioral tasks and different phases of memory were investigated. Our findings indicated that the AV inactivation strongly disrupts all memory phases in the MWM, and consolidation and retrieval phases in the PA tasks. The AM inactivation had no effect on acquisition of both tasks while it impaired the PA consolidation and MWM retrieval. However, the AD inactivation could not disrupt memory phases in the PA task but impaired the MWM retrieval. In conclusion, it seems that the ATN distinct subnuclei differently affect different phases of memory in these two tasks.

Deconstructing the Direct Reciprocal Hippocampal-Anterior Thalamic Pathways for Spatial Learning

The hippocampus is essential for normal memory but does not act in isolation. The anterior thalamic nuclei may represent one vital partner. Using DREADDs, the behavioral consequences of transiently disrupting anterior thalamic function were examined, followed by inactivation of the dorsal subiculum. Next, the anterograde transport of an adeno-associated virus expressing DREADDs was paired with localized intracerebral infusions of a ligand to target specific input pathways. In this way, the direct projections from the anterior thalamic nuclei to the dorsal hippocampal formation were inhibited, followed by separate inhibition of the dorsal subiculum projections to the anterior thalamic nuclei. To assay spatial working memory, all animals performed a reinforced T-maze alternation task, then a more challenging version that nullifies intramaze cues. Across all four experiments, deficits emerged on the spatial alternation task that precluded the use of intramaze cues. Inhibiting dorsal subiculum projections to the anterior thalamic nuclei produced the severest spatial working memory deficit. This deficit revealed the key contribution of dorsal subiculum projections to the anteromedial and anteroventral thalamic nuclei for the processing of allocentric information, projections not associated with head-direction information. The overall pattern of results provides consistent causal evidence of the two-way functional significance of direct hippocampal-anterior thalamic interactions for spatial processing. At the same time, these findings are consistent with hypotheses that these same, reciprocal interactions underlie the common core symptoms of temporal lobe and diencephalic anterograde amnesia.

SIGNIFICANCE STATEMENT It has long been conjectured that the anterior thalamic nuclei might be key partners with the hippocampal formation and that, respectively, they are principally responsible for diencephalic and temporal lobe amnesia. However, direct causal evidence for this functional relationship is lacking. Here, we examined the behavioral consequences of transiently silencing the direct reciprocal interconnections between these two brain regions on tests of spatial learning. Disrupting information flow from the hippocampal formation to the anterior thalamic nuclei and vice versa impaired performance on tests of spatial learning. By revealing the conjoint importance of hippocampal-anterior thalamic pathways, these findings help explain why pathology in either the medial diencephalon or the medial temporal lobes can result in profound anterograde amnesic syndromes.

Inducing Angiogenesis, a Key Step in Cancer Vascularization, and Treatment Approaches

Angiogenesis is a term that describes the formation of new blood and lymphatic vessels from a pre-existing vasculature. This allows tumour cells to acquire sustenance in the form of nutrients and oxygen and the ability to evacuate metabolic waste. As one of the hallmarks of cancer, angiogenesis has been studied extensively in animal and human models to enable better understanding of cancer biology and the development of new anti-cancer treatments. Angiogenesis plays a crucial role in the process of tumour genesis, because solid tumour need a blood supply if they are to grow beyond a few millimeters in size. On the other hand, there is growing evidence that some solid tumour exploit existing normal blood supply and do not require a new vessel formation to grow and to undergo metastasis. This review of the literature will present the current understanding of this intricate process and the latest advances in the use of angiogenesis-targeting therapies in the fight against cancer.

Trajectory of hippocampal fibres to the contralateral anterior thalamus and mammillary bodies in rats, mice, and macaque monkeys

The routes by which the hippocampal formation projects bilaterally to the anterior thalamic nuclei and mammillary bodies were examined in the mouse, rat, and macaque monkey. Despite using different methods and different species, the principal pattern remained the same. For both target areas, the contralateral hippocampal (subiculum) projections arose via efferents in the postcommissural fornix ipsilateral to the tracer injection, which then crossed hemispheres both in or just prior to reaching the target site within the thalamus or hypothalamus. Precommissural fornix fibres could not be followed to the target areas. There was scant evidence that the ventral hippocampal commissure or decussating fornix fibres contribute to these crossed subiculum projections. Meanwhile, a small minority of postsubiculum projections in the mouse were seen to cross in the descending fornix at the level of the caudal septum to join the contralateral postcommissural fornix before reaching the anterior thalamus and lateral mammillary nucleus on that side. Although the rodent anterior thalamic nuclei also receive nonfornical inputs from the subiculum and postsubiculum via the ipsilateral internal capsule, few, if any, of these projections cross the midline. It was also apparent that nuclei within the head direction system (anterodorsal thalamic nucleus, laterodorsal thalamic nucleus, and lateral mammillary nucleus) receive far fewer crossed hippocampal inputs than the other anterior thalamic or mammillary nuclei. The present findings increase our understanding of the fornix and its component pathways while also informing disconnection analyses involving the hippocampal formation and diencephalon.

The effect of pharmacological inactivation of the mammillary body and anterior thalamic nuclei on hippocampal theta rhythm in urethane-anesthetized rats

The mammillary body (MB) and the anterior thalamic nuclei (ATN) are closely related structures, which take part in learning and memory processes. However, the exact role of these structures has remained unclear. In both structures neurons firing according to hippocampal theta rhythm have been found, mainly in the medial mammillary nucleus (MM) and anteroventral thalamic nucleus (AV). These neurons are driven by descending projections from the hippocampal formation and are thought to convey theta rhythm back to the hippocampus (HP). We argue that the MB-ATN axis not only relays theta signal, but may also modulate it. To examine it, we performed a pharmacological inactivation of the MM and AV by local infusion of procaine, and measured changes in theta activity in selected structures of the extended hippocampal system in urethane-anesthetized rats. The inactivation of the MM resulted in decrease in EEG power in the HP and AV, the most evidently in the lower theta frequency bands, i.e. 3-5Hz in the HP (down to 9.2% in 3- to 4-Hz band and 37.6% in 4- to 5-Hz band, in comparison to the power in the control conditions) and 3-4Hz in the AV (down to 24.9%). After the AV inactivation, hippocampal EEG power decreased in theta frequency bands of 3-8Hz (down to 61.6% in 6- to 7-Hz band and 69.4% in 7- to 8-Hz band). Our results suggest that the role of the MB-ATN axis in regulating theta rhythm signaling may be much more important than has been speculated so far.

Asymmetric cross-hemispheric connections link the rat anterior thalamic nuclei with the cortex and hippocampal formation

Dense reciprocal connections link the rat anterior thalamic nuclei with the prelimbic, anterior cingulate and retrosplenial cortices, as well as with the subiculum and postsubiculum. The present study compared the ipsilateral thalamic-cortical connections with the corresponding crossed, contralateral connections between these same sets of regions. All efferents from the anteromedial thalamic nucleus to the cortex, as well as those to the subiculum, remained ipsilateral. In contrast, all of these target sites provided reciprocal, bilateral projections to the anteromedial nucleus. While the anteroventral thalamic nucleus often shared this same asymmetric pattern of cortical connections, it received relatively fewer crossed inputs than the anteromedial nucleus. This difference was most marked for the anterior cingulate projections, as those to the anteroventral nucleus remained almost entirely ipsilateral. Unlike the anteromedial nucleus, the anteroventral nucleus also appeared to provide a restricted, crossed projection to the contralateral retrosplenial cortex. Meanwhile, the closely related laterodorsal thalamic nucleus had almost exclusively ipsilateral efferent and afferent cortical connections. Likewise, within the hippocampus, the postsubiculum seemingly had only ipsilateral efferent and afferent connections with the anterior thalamic and laterodorsal nuclei. While the bilateral cortical projections to the anterior thalamic nuclei originated predominantly from layer VI, the accompanying sparse projections from layer V largely gave rise to ipsilateral thalamic inputs. In testing a potentially unifying principle of anterior thalamic - cortical interactions, a slightly more individual pattern emerged that reinforces other evidence of functional differences within the anterior thalamic and also helps to explain the consequences of unilateral interventions involving these nuclei.

Effects of thalamic hemorrhagic lesions on explicit and implicit learning during the acquisition and retrieval phases in an animal model of central post-stroke pain

Hemorrhagic stroke has many symptoms, including central pain, learning and memory impairments, motor deficits, language problems, emotional disturbances, and social maladjustment. Lesions of the ventral basal complex (VBC) of the thalamus elicit thermal and mechanical hyperalgesia, forming an animal model of central post-stroke pain (CPSP). However, no research has yet examined the involvement of learning and memory in CPSP using an animal model. The present study examined whether VBC lesions affect motor function, conditioned place preference (CPP; implicit memory), and spatial learning (explicit memory) in the acquisition and retrieval phases. The results showed that rats with VBC lesions exhibited thermal hyperalgesia in the acquisition and retrieval phases, indicating that these lesions can induce CPSP. During these phases, the rats with VBC lesions exhibited enhanced (morphine-induced) CPP learning. These lesions did not affect the rats' total distance travelled, time spent, or velocity in the spatial learning tasks. The lesions also did not affect motor function in the rotarod task. Altogether, VBC lesions resulted in CPSP and facilitated CPP (implicit memory). However, the lesions did not affect spatial learning (explicit memory) or motor function. The relationship between CPSP and learning and memory is important for patients who suffer from such central pain. The implications of the present study may provide insights into helping reduce CPSP and its associated symptoms.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

A role for the anteromedial thalamic nucleus in the acquisition of contextual fear memory to predatory threats

Previous studies from our group have shown that cytotoxic lesions in the ventral portion of the anteromedial thalamic nucleus (AMv), one of the main targets of the hypothalamic predator-responsive circuit, strongly impairs contextual fear responses to an environment previously associated with a predator. The AMv is in a position to convey information to cortico-hippocampal-amygdalar circuits involved in the processing of fear memory. However, it remains to be determined whether the nucleus is involved in the acquisition or subsequent expression of contextual fear. In the present investigation, we addressed this question by inactivating the rat AMv with muscimol either prior to cat exposure or prior to exposure to the cat-related context. Accordingly, AMv pharmacological inactivation prior to cat exposure did not interfere with innate fear responses, but it drastically reduced contextual conditioning to the predator-associated environment. On the other hand, AMv inactivation prior to exposure to the environment associated with the predator threat did not affect contextual fear responses. The behavioral results were further supported by the demonstration that AMv inactivation prior to cat exposure also blocked the activation of sites critically involved in the expression of anti-predatory contextual defensive responses (i.e., the dorsal premammillary nucleus and the dorsolateral periaqueductal gray) in animals exposed to the predator-associated context. The AMv projections were also examined, and the results of this investigation outline important paths that can influence hippocampal circuitry and raise new ideas for anterior thalamic-hippocampal paths involved in emotional learning.

Complementary subicular pathways to the anterior thalamic nuclei and mammillary bodies in the rat and macaque monkey brain

The origins of the hippocampal (subicular) projections to the anterior thalamic nuclei and mammillary bodies were compared in rats and macaque monkeys using retrograde tracers. These projections form core components of the Papez circuit, which is vital for normal memory. The study revealed a complex pattern of subicular efferents, consistent with the presence of different, parallel information streams, whose segregation appears more marked in the rat brain. In both species, the cells projecting to the mammillary bodies and anterior thalamic nuclei showed laminar separation but also differed along other hippocampal axes. In the rat, these diencephalic inputs showed complementary topographies in the proximal–distal (columnar) plane, consistent with differential involvement in object‐based (proximal subiculum) and context‐based (distal subiculum) information. The medial mammillary inputs, which arose along the anterior–posterior extent of the rat subiculum, favoured the central subiculum (septal hippocampus) and the more proximal subiculum (temporal hippocampus). In contrast, anterior thalamic inputs were largely confined to the dorsal (i.e. septal and intermediate) subiculum, where projections to the anteromedial nucleus favoured the proximal subiculum while those to the anteroventral nucleus predominantly arose in the distal subiculum. In the macaque, the corresponding diencephalic inputs were again distinguished by anterior–posterior topographies, as subicular inputs to the medial mammillary bodies predominantly arose from the posterior hippocampus while subicular inputs to the anteromedial thalamic nucleus predominantly arose from the anterior hippocampus. Unlike the rat, there was no clear evidence of proximal–distal separation as all of these medial diencephalic projections preferentially arose from the more distal subiculum.

Evidence for spatially-responsive neurons in the rostral thalamus

Damage involving the anterior thalamic and adjacent rostral thalamic nuclei may result in a severe anterograde amnesia, similar to the amnesia resulting from damage to the hippocampal formation. Little is known, however, about the information represented in these nuclei. To redress this deficit, we recorded units in three rostral thalamic nuclei in freely-moving rats [the parataenial nucleus (PT), the anteromedial nucleus (AM) and nucleus reuniens NRe]. We found units in these nuclei possessing previously unsuspected spatial properties. The various cell types show clear similarities to place cells, head direction cells, and perimeter/border cells described in hippocampal and parahippocampal regions. Based on their connectivity, it had been predicted that the anterior thalamic nuclei process information with high spatial and temporal resolution while the midline nuclei have more diffuse roles in attention and arousal. Our current findings strongly support the first prediction but directly challenge or substantially moderate the second prediction. The rostral thalamic spatial cells described here may reflect direct hippocampal/parahippocampal inputs, a striking finding of itself, given the relative lack of place cells in other sites receiving direct hippocampal formation inputs. Alternatively, they may provide elemental thalamic spatial inputs to assist hippocampal spatial computations. Finally, they could represent a parallel spatial system in the brain.

The mammillary bodies and memory: more than a hippocampal relay

Although the mammillary bodies were one of the first neural structures to be implicated in memory, it has long been assumed that their main function was to act primarily as a hippocampal relay, passing information on to the anterior thalamic nuclei and from there to the cingulate cortex. This view not only afforded the mammillary bodies no independent role in memory, it also neglected the potential significance of other, nonhippocampal, inputs to the mammillary bodies. Recent advances have transformed the picture, revealing that projections from the tegmental nuclei of Gudden, and not the hippocampal formation, are critical for sustaining mammillary body function. By uncovering a role for the mammillary bodies that is independent of its subicular inputs, this work signals the need to consider a wider network of structures that form the neural bases of episodic memory.

Looking beyond the hippocampus: old and new neurological targets for understanding memory disorders

Although anterograde amnesia can occur after damage in various brain sites, hippocampal dysfunction is usually seen as the ultimate cause of the failure to learn new episodic information. This assumption is supported by anatomical evidence showing direct hippocampal connections with all other sites implicated in causing anterograde amnesia. Likewise, behavioural and clinical evidence would seem to strengthen the established notion of an episodic memory system emanating from the hippocampus. There is, however, growing evidence that key, interconnected sites may also regulate the hippocampus, reflecting a more balanced, integrated network that enables learning. Recent behavioural evidence strongly suggests that medial diencephalic structures have some mnemonic functions independent of the hippocampus, which can then act upon the hippocampus. Anatomical findings now reveal that nucleus reuniens and the retrosplenial cortex provide parallel, disynaptic routes for prefrontal control of hippocampal activity. There is also growing clinical evidence that retrosplenial cortex dysfunctions contribute to both anterograde amnesia and the earliest stages of Alzheimer's disease, revealing the potential significance of this area for clinical studies. This array of findings underlines the importance of redressing the balance and the value of looking beyond the hippocampus when seeking to explain failures in learning new episodic information.

Decoding signal processing in thalamo-hippocampal circuitry: implications for theories of memory and spatial processing

A major tool in understanding how information is processed in the brain is the analysis of neuronal output at each hierarchical level through which neurophysiological signals are propagated. Since the experimental brain operation performed on Henry Gustav Molaison (known as patient H.M.) in 1953, the hippocampal formation has gained special attention, resulting in a very large number of studies investigating signals processed by the hippocampal formation. One of the main information streams to the hippocampal formation, vital for episodic memory formation, arises from thalamo-hippocampal projections, as there is extensive connectivity between these structures. This connectivity is sometimes overlooked by theories of memory formation by the brain, in favour of theories with a strong cortico-hippocampal flavour. In this review, we attempt to address some of the complexity of the signals processed within the thalamo-hippocampal circuitry. To understand the signals encoded by the anterior thalamic nuclei in particular, we review key findings from electrophysiological, anatomical, behavioural and computational studies. We include recent findings elucidating the integration of different signal modalities by single thalamic neurons; we focus in particular on the propagation of two prominent signals: head directionality and theta rhythm. We conclude that thalamo-hippocampal processing provides a centrally important, substantive, and dynamic input modulating and moderating hippocampal spatial and mnemonic processing. This article is part of a Special Issue entitled SI: Brain and Memory.

Functional heterogeneity of the limbic thalamus: From hippocampal to cortical functions

Today, the idea that the integrity of the limbic thalamus is necessary for normal memory functions is well established. However, if the study of thalamic patients emphasized the anterior and the mediodorsal thalamus as the critical thalamic loci supporting cognitive functions, clinical studies have so far failed to attribute a specific role to each of these regions. In view of these difficulties, we review here the experimental data conducted in rodents harboring specific lesions of each thalamic region. These data clearly indicate a major functional dissociation within the limbic thalamus. The anterior thalamus provides critical support for hippocampal functions due to its cardinal location in the Papez circuit, while the mediodorsal thalamus may signal relevant information in a circuit encompassing the basolateral amygdala and the prefrontal cortex. Interestingly, while clinical studies have suggested that diencephalic pathologies may disconnect the medial temporal lobe from the cortex, experimental studies conducted in rodent show how this may differently affect distinct temporo-thalamo-cortical circuits, sharing the same general organization but supporting dissociable functions.

The impact of fornix lesions in rats on spatial learning tasks sensitive to anterior thalamic and hippocampal damage

•

Fornix damage mildly impair spatial biconditional and passive place learning tasks.

•

Fornix lesions impair spatial go/no-go and alternation problems.

•

Fornix lesions impair tests making flexible demands on spatial memory.

•

Fornix connections are not always required for learning fixed spatial responses.

The present study sought to understand how the hippocampus and anterior thalamic nuclei are conjointly required for spatial learning by examining the impact of cutting a major tract (the fornix) that interconnects these two sites. The initial experiments examined the consequences of fornix lesions in rats on spatial biconditional discrimination learning. The rationale arose from previous findings showing that fornix lesions spare the learning of spatial biconditional tasks, despite the same task being highly sensitive to both hippocampal and anterior thalamic nuclei lesions. In the present study, fornix lesions only delayed acquisition of the spatial biconditional task, pointing to additional contributions from non-fornical routes linking the hippocampus with the anterior thalamic nuclei. The same fornix lesions spared the learning of an analogous nonspatial biconditional task that used local contextual cues. Subsequent tests, including T-maze place alternation, place learning in a cross-maze, and a go/no-go place discrimination, highlighted the impact of fornix lesions when distal spatial information is used flexibly to guide behaviour. The final experiment examined the ability to learn incidentally the spatial features of a square water-maze that had differently patterned walls. Fornix lesions disrupted performance but did not stop the rats from distinguishing the various corners of the maze. Overall, the results indicate that interconnections between the hippocampus and anterior thalamus, via the fornix, help to resolve problems with flexible spatial and temporal cues, but the results also signal the importance of additional, non-fornical contributions to hippocampal-anterior thalamic spatial processing, particularly for problems with more stable spatial solutions.

Why do lesions in the rodent anterior thalamic nuclei cause such severe spatial deficits?

•

A dual hypothesis is introduced to explain the importance of these thalamic nuclei.

•

ATN are vital for multiple spatial functions.

•

ATN damage disrupts processing across distal limbic sites.

•

Distal pathology caused by ATN damage disrupts plasticity and metabolic activity.

•

ATN lesion effects reflect both their intrinsic importance and distal dysfunctions.

Lesions of the rodent anterior thalamic nuclei cause severe deficits to multiple spatial learning tasks. Possible explanations for these effects are examined, with particular reference to T-maze alternation. Anterior thalamic lesions not only impair allocentric place learning but also disrupt other spatial processes, including direction learning, path integration, and relative length discriminations, as well as aspects of nonspatial learning, e.g., temporal discriminations. Working memory tasks, such as T-maze alternation, appear particularly sensitive as they combine an array of these spatial and nonspatial demands. This sensitivity partly reflects the different functions supported by individual anterior thalamic nuclei, though it is argued that anterior thalamic lesion effects also arise from covert pathology in sites distal to the thalamus, most critically in the retrosplenial cortex and hippocampus. This two-level account, involving both local and distal lesion effects, explains the range and severity of the spatial deficits following anterior thalamic lesions. These findings highlight how the anterior thalamic nuclei form a key component in a series of interdependent systems that support multiple spatial functions.

How do mammillary body inputs contribute to anterior thalamic function?

It has long been assumed that the main function of the mammillary bodies is to provide a relay for indirect hippocampal inputs to the anterior thalamic nuclei. Such models afford the mammillary bodies no independent role in memory and overlook the importance of their other, non-hippocampal, inputs. This review focuses on recent advances that herald a new understanding of the importance of the mammillary bodies, and their inputs from the limbic midbrain, for anterior thalamic function. It has become apparent that the mammillary bodies' contribution to memory is not dependent on afferents from the subicular complex. Rather, the ventral tegmental nucleus of Gudden is a vital source of inputs that support memory processes within the medial mammillary bodies. In parallel, the lateral mammillary bodies, via their connections with the dorsal tegmental nucleus of Gudden, are critical for generating head-direction signals. These two parallel, but distinct, information streams converge on the anterior thalamic nuclei and support different aspects of spatial memory.

Thalamic control of human attention driven by memory and learning

The role of the thalamus in high-level cognition—attention, working memory (WM), rule-based learning, and decision making—remains poorly understood, especially in comparison to that of cortical frontoparietal networks [1–3]. Studies of visual thalamus have revealed important roles for pulvinar and lateral geniculate nucleus in visuospatial perception and attention [4–10] and for mediodorsal thalamus in oculomotor control [11]. Ventrolateral thalamus contains subdivisions devoted to action control as part of a circuit involving the basal ganglia [12, 13] and motor, premotor, and prefrontal cortices [14], whereas anterior thalamus forms a memory network in connection with the hippocampus [15]. This connectivity profile suggests that ventrolateral and anterior thalamus may represent a nexus between mnemonic and control functions, such as action or attentional selection. Here, we characterize the role of thalamus in the interplay between memory and visual attention. We show that ventrolateral lesions impair the influence of WM representations on attentional deployment. A subsequent fMRI study in healthy volunteers demonstrates involvement of ventrolateral and, notably, anterior thalamus in biasing attention through WM contents. To further characterize the memory types used by the thalamus to bias attention, we performed a second fMRI study that involved learning of stimulus-stimulus associations and their retrieval from long-term memory to optimize attention in search. Responses in ventrolateral and anterior thalamic nuclei tracked learning of the predictiveness of these abstract associations and their use in directing attention. These findings demonstrate a key role for human thalamus in higher-level cognition, notably, in mnemonic biasing of attention.

•

Pivotal role of human thalamus in linking memory and attention in vision

•

Thalamus lesions disrupted attention biases by working memory contents

•

Ventrolateral (VL) thalamic lesions reversed the normal direction of these biases

•

Flexible scope of memory types biasing attention through VL and anterior thalamus

Colocalization pattern of calbindin and cocaine- and amphetamine-regulated transcript in the mammillary body-anterior thalamic nuclei axis of the guinea pig

The study describes for the first time the colocalization pattern of calbindin (CB) and cocaine- and amphetamine-regulated transcript (CART) in the mammillary body (MB) and anterior thalamic nuclei (ATN) - structures connected in a topographically organized manner by the mammillothalamic tract (mtt). Immunohistochemical study was performed on fetal (E40, E50, E60), newborn (P0) and postnatal (P20, P80) brains of the guinea pig, but the coexistence pattern of the substances was invariable throughout the examined developmental stages. CB and CART colocalized in the perikarya of the lateral part of the medial mammillary nucleus (MMl), whereas in its medial part (MMm) only CB was detected. In the mtt, which originates from the MB, both the substances were present and colocalized in single fibers. Next, fibers from the mtt spread toward the ATN in a particular way: fibers containing CB ran to both the anteromedial thalamic nucleus (AM) and anteroventral thalamic nucleus (AV), while fibers containing CART ran mostly to the latter one. In the ventral part of AV, CB and CART colocalized vastly in the neuropil. The lateral mammillary nucleus and anterodorsal thalamic nucleus were virtually devoid of CB- and CART-positive structures. Based on the known connections between the MB and ATN, we conclude that the studied substances may cooperate in the MMl-AV part of the axis and CB plays a significant role in the MMm-AM part.

Selective importance of the rat anterior thalamic nuclei for configural learning involving distal spatial cues

To test potential parallels between hippocampal and anterior thalamic function, rats with anterior thalamic lesions were trained on a series of biconditional learning tasks. The anterior thalamic lesions did not disrupt learning two biconditional associations in operant chambers where a specific auditory stimulus (tone or click) had a differential outcome depending on whether it was paired with a particular visual context (spot or checkered wall-paper) or a particular thermal context (warm or cool). Likewise, rats with anterior thalamic lesions successfully learnt a biconditional task when they were reinforced for digging in one of two distinct cups (containing either beads or shredded paper), depending on the particular appearance of the local context on which the cup was placed (one of two textured floors). In contrast, the same rats were severely impaired at learning the biconditional rule to select a specific cup when in a particular location within the test room. Place learning was then tested with a series of go/no-go discriminations. Rats with anterior thalamic nuclei lesions could learn to discriminate between two locations when they were approached from a constant direction. They could not, however, use this acquired location information to solve a subsequent spatial biconditional task where those same places dictated the correct choice of digging cup. Anterior thalamic lesions produced a selective, but severe, biconditional learning deficit when the task incorporated distal spatial cues. This deficit mirrors that seen in rats with hippocampal lesions, so extending potential interdependencies between the two sites.

Embryonic and postnatal development of calcium-binding proteins immunoreactivity in the anterior thalamus of the guinea pig

Our recent studies have shown that the distribution of calretinin (CR) in the anterior thalamic nuclei (ATN) changes significantly during the development of the guinea pig. The present study was designed to reveal the distribution pattern of calcium-binding proteins, i.e. calbindin (CB) and parvalbumin (PV), as well as the colocalization pattern of all three proteins, including CR, in the ATN of guinea pigs ranging from the 40th embryonic day (E40) to the 80th postnatal day (P80). According to these patterns, CB appears exclusively in the perikarya of the anteromedial nucleus (AM) not before P20 and always colocalizes with CR. Moreover, CB and CR colocalize in fibers of thin bundles traversing the anteroventral nucleus (AV) since E50. The ATN also display CB-positive neuropil in all studied stages, especially a strong one in the ventral part of the AV. PV was not observed in the perikarya of the ATN in all the stages, but was abundantly present in the neuropil of the anterodorsal nucleus (AD). No colocalizations exist between PV and the rest of the studied proteins. In conclusion, our study reveals that the distribution of the studied proteins differs greatly. Nevertheless, the postnatal coexistence of CB and CR in the AM perikarya may indicate the cooperation of both of the proteins in some functions of the nucleus. Parvalbumin is limited mostly to the neuropil of the AD, suggesting different functions in comparison to CB and CR.

Segregation of parallel inputs to the anteromedial and anteroventral thalamic nuclei of the rat

Many brain structures project to both the anteroventral thalamic nucleus and the anteromedial thalamic nucleus. In the present study, pairs of different tracers were placed into these two thalamic sites in the same rats to determine the extent to which these nuclei receive segregated inputs. Only inputs from the laterodorsal tegmental nucleus, the principal extrinsic cholinergic source for these thalamic nuclei, showed a marked degree of collateralization, with approximately 13% of all cells labeled in this tegmental area projecting to both nuclei. Elsewhere, double-labeled cells were very scarce, making up ∼1% of all labeled cells. Three general patterns of anterior thalamic innervation were detected in these other areas. In some sites, e.g., prelimbic cortex, anterior cingulate cortex, and secondary motor area, cells projecting to the anteromedial and anteroventral thalamic nuclei were closely intermingled, with often only subtle distribution differences. These same projections were also often intermingled with inputs to the mediodorsal thalamic nucleus, but again there was little or no collateralization. In other sites, e.g., the subiculum and retrosplenial cortex, there was often less overlap of cells projecting to the two anterior thalamic nuclei. A third pattern related to the dense inputs from the medial mammillary nucleus, where well-defined topographies ensured little intermingling of the neurons that innervate the two thalamic nuclei. The finding that a very small minority of cortical and limbic inputs bifurcates to innervate both anterior thalamic nuclei highlights the potential for parallel information streams to control their functions, despite arising from common regions.

The anterior thalamus provides a subcortical circuit supporting memory and spatial navigation

The anterior thalamic nuclei (ATN), a central component of Papez' circuit, are generally assumed to be key constituents of the neural circuits responsible for certain categories of learning and memory. Supporting evidence for this contention is that damage to either of two brain regions, the medial temporal lobe and the medial diencephalon, is most consistently associated with anterograde amnesia. Within these respective regions, the hippocampal formation and the ATN (anteromedial, anteroventral, and anterodorsal) are the particular structures of interest. The extensive direct and indirect hippocampal-anterior thalamic interconnections and the presence of theta-modulated cells in both sites further support the hypothesis that these structures constitute a neuronal network crucial for memory and cognition. The major tool in understanding how the brain processes information is the analysis of neuronal output at each hierarchical level along the pathway of signal propagation coupled with neuroanatomical studies. Here, we discuss the electrophysiological properties of cells in the ATN with an emphasis on their role in spatial navigation. In addition, we describe neuroanatomical and functional relationships between the ATN and hippocampal formation.

The anterior thalamus is critical for overcoming interference in a context-dependent odor discrimination task

The anterior thalamus (AT) is anatomically interconnected with the hippocampus and other structures known to be involved in memory, and the AT is involved in many of the same learning and memory functions as the hippocampus. For example, like the hippocampus, the AT is involved in spatial cognition and episodic memory. The hippocampus also has a well-documented role in contextual memory processes, but it is not known whether the AT is similarly involved in contextual memory. In the present study, we assessed the role of the AT in contextual memory processes by temporarily inactivating the AT and training rats on a recently developed context-based olfactory list learning task, which was designed to assess the use of contextual information to resolve interference. Rats were trained on one list of odor discrimination problems, followed by training on a second list in either the same context or a different context. In order to induce interference, some of the odors appeared on both lists with their predictive value reversed. Control rats that learned the two lists in different contexts performed significantly better than rats that learned the two lists in the same context. However, AT lesions completely abolished this contextual learning advantage, a result that is very similar to the effects of hippocampal inactivation. These findings demonstrate that the AT, like the hippocampus, is involved in contextual memory and suggest that the hippocampus and AT are part of a functional circuit involved in contextual memory.

Anterior thalamic nuclei lesions in rats disrupt markers of neural plasticity in distal limbic brain regions

In two related experiments, neurotoxic lesions were placed in the anterior thalamic nuclei of adult rats. The rats were then trained on behavioral tasks, immediately followed by the immunohistochemical measurement of molecules linked to neural plasticity. These measurements were made in limbic sites including the retrosplenial cortex, the hippocampal formation, and parahippocampal areas. In Experiment 1, rats with unilateral anterior thalamic lesions explored either novel or familiar objects prior to analysis of the immediate-early gene zif268. The lesions reduced zif268 activity in the granular retrosplenial cortex and postsubiculum. Exploring novel objects resulted in local changes of hippocampal zif268, but this change was not moderated by anterior thalamic lesions. In Experiment 2, rats that had received either bilateral anterior thalamic lesions or control surgeries were exposed to novel room cues while running in the arms of a radial maze. In addition to zif268, measurements of c-AMP response element binding protein (CREB), phosphorylated CREB (pCREB), and growth associated protein43 (GAP-43) were made. As before, anterior thalamic lesions reduced zif268 in retrosplenial cortex and postsubiculum, but there were also reductions of pCREB in granular retrosplenial cortex. Again, the hippocampus did not show lesion-induced changes in zif268, but there were differential effects on CREB and pCREB consistent with reduced levels of hippocampal CREB phosphorylation following anterior thalamic damage. No changes in GAP-43 were detected. The results not only point to changes in several limbic sites (retrosplenial cortex and hippocampus) following anterior thalamic damage, but also indicate that these changes include decreased levels of pCREB. As pCREB is required for neuronal plasticity, partly because of its regulation of immediate early-gene expression, the present findings reinforce the concept of an 'extended hippocampal system' in which hippocampal function is dependent on distal sites such as the anterior thalamic nuclei.

Lesions of the anterior thalamic nuclei and intralaminar thalamic nuclei: place and visual discrimination learning in the water maze

Medial thalamic damage produces memory deficits in humans (e.g., Korsakoff's syndrome) and experimental animals. Both the anterior thalamic nuclei (ATN) and rostral intralaminar plus adjacent lateral thalamic nuclei (ILN/LT) have been implicated. Based on the differences in their main connections with other neural structures, we tested the prediction that ATN lesions would selectively impair acquisition of spatial location discrimination, reflecting a hippocampal system deficit, whereas ILN/LT lesions would impair acquisition of visual pattern discrimination, reflecting a striatal system deficit. Half the rats were first trained in a spatial task in a water maze before switching to a visual task in the same maze, while the remainder were tested with the reverse order of tasks. Compared with sham-operated controls, (1) rats with ATN lesions showed impaired place learning, but normal visual discrimination learning, (2) rats with ILN/LT lesions showed no deficit on either task. Rats with ATN lesions were also hyperactive when their home cage was placed in a novel room and remained more active than ILN/LT or SHAM rats for the subsequent 21 h, especially during the nocturnal phase. These findings confirmed the influence of ATN lesions on spatial learning, but failed to support the view that ILN/LT lesions disrupt striatal-dependent memory.

Brain and behavioral pathology in an animal model of Wernicke's encephalopathy and Wernicke-Korsakoff Syndrome

Animal models provide the opportunity for in-depth and experimental investigation into the anatomical and physiological underpinnings of human neurological disorders. Rodent models of thiamine deficiency have yielded significant insight into the structural, neurochemical and cognitive deficits associated with thiamine deficiency as well as proven useful toward greater understanding of memory function in the intact brain. In this review, we discuss the anatomical, neurochemical and behavioral changes that occur during the acute and chronic phases of thiamine deficiency and describe how rodent models of Wernicke-Korsakoff Syndrome aid in developing a more detailed picture of brain structures involved in learning and memory.

Anterior thalamic lesions alter both hippocampal-dependent behavior and hippocampal acetylcholine release in the rat

The anterior thalamic nuclei (ATN) are important for learning and memory as damage to this region produces a persistent amnestic syndrome. Dense connections between the ATN and the hippocampus exist, and importantly, damage to the ATN can impair hippocampal functioning. Acetylcholine (ACh) is a key neurotransmitter in the hippocampus, and in vivo measures of ACh are correlated to learning and memory performance. In the present study, complete lesions of the ATN impaired performance on two measures of hippocampal-dependent learning and memory (spontaneous alternation and delayed alternation) and severely disrupted behaviorally evoked ACh efflux within the hippocampus of adult male rats. In contrast, incomplete ATN lesions did not impair spontaneous alternation performance but did impair delayed alternation performance while blunting hippocampal ACh efflux. Interestingly, ATN lesions of any size did not affect basal concentrations of ACh in the hippocampus. These results demonstrate that the ATN have the capacity to modulate behaviorally relevant neuronal transmission within the hippocampus.