An Evolutionary Hypothesis of Binary Opposition in Functional Incompatibility about Habenular Asymmetry in Vertebrates

Response Flexibility: The Role of the Lateral Habenula

The ability to make appropriate decisions that result in an optimal outcome is critical for survival. This process involves assessing the environment as well as integrating prior knowledge about the environment with information about one’s current internal state. There are many neural structures that play critical roles in mediating these processes, but it is not yet known how such information coalesces to influence behavioral output. The lateral habenula (LHb) has often been cited as a structure critical for adaptive and flexible responding when environmental contexts and internal state changes. A challenge, however, has been understanding how LHb promotes response flexibility. In this review, we hypothesize that the LHb enables flexible responding following the integration of context memory and internal state information by signaling downstream brainstem structures known to drive hippocampal theta. In this way, animals respond more flexibly in a task situation not because the LHb selects a particular action, but rather because LHb enhances a hippocampal neural state that is often associated with greater attention, arousal, and exploration. In freely navigating animals, these are essential conditions that are needed to discover and implement appropriate alternative choices and behaviors. As a corollary to our hypothesis, we describe short- and intermediate-term functions of the LHb. Finally, we discuss the effects on the behavior of LHb dysfunction in short- and intermediate-timescales, and then suggest that new therapies may act on the LHb to alleviate the behavioral impairments following long-term LHb disruption.

Decreased resting-state functional connectivity of the habenula-cerebellar in a major depressive disorder

BACKGROUND

In animal experiments, the habenula and septal nuclei are known as the key brain areas of depression. However, there are few magnetic resonance imaging (MRI) studies on the functional connectivity between these areas and the subcortical areas in humans with major depression. We aimed to investigate the difference in resting-state functional connectivity (RSFC) among the major regions of interest (ROI) in the subcortical areas, including both the habenula and septal nuclei.

METHODS

We performed the seed-to-voxel analysis to investigate the RSFC between both the habenula and septal nucleus, as well as other subcortical regions. Furthermore, ROI-to-ROI analysis was performed among the combinations of ROI pairs in the subcortical areas.

RESULTS

The seed-to-voxel analysis showed a lower RSFC between the left habenula and the cerebellum in major depressive disorder (MDD) than in healthy controls (HCs). As a result of ROI-to-ROI analysis in subcortical areas, a total of 31 pairs of FCs in the MDD group showed a lower RSFC than in the HCs group.

CONCLUSION

This study revealed a lower RSFC between the left habenula and cerebellum in patients with MDD and reduced RSFC among numerous subcortical areas. These new findings on the neural circuitry of MDD might contribute to an in-depth understanding of depression.

Regulation of habenular G-protein gamma 8 on learning and memory via modulation of the central acetylcholine system

Guanine nucleotide binding protein (G protein) gamma 8 (Gng8) is a subunit of G proteins and expressed in the medial habenula (MHb) and interpeduncular nucleus (IPN). Recent studies have demonstrated that Gng8 is involved in brain development; however, the roles of Gng8 on cognitive function have not yet been addressed. In the present study, we investigated the expression of Gng8 in the brain and found that Gng8 was predominantly expressed in the MHb-IPN circuit of the mouse brain. We generated Gng8 knockout (KO) mice by CRISPR/Cas9 system in order to assess the role of Gng8 on cognitive function. Gng8 KO mice exhibited deficiency in learning and memory in passive avoidance and Morris water maze tests. In addition, Gng8 KO mice significantly reduced long-term potentiation (LTP) in the hippocampus compared to that of wild-type (WT) mice. Furthermore, we observed that levels of acetylcholine (ACh) and choline acetyltransferase (ChAT) in the MHb and IPN of Gng8 KO mice were significantly decreased, compared to WT mice. The administration of nAChR α4β2 agonist A85380 rescued memory impairment in the Gng8 KO mice, suggesting that Gng8 regulates cognitive function via modulation of cholinergic activity. Taken together, Gng8 is a potential therapeutic target for memory-related diseases and/or neurodevelopmental diseases.

Thalamo-Habenular Connection Differences Between Patients With Major Depressive Disorder and Normal Controls

Background: The thalamus and habenula are thought to be key brain regions in the etiology of major depressive disorder (MDD); however, few studies have investigated the structural connection between them. We compared the number of white matter tracts between the thalamus and habenula between patient with MDD and normal controls (NCs). Methods: The habenula and thalamus region of interest masks were extracted from brain magnetic resonance imaging data and individual tractography analysis was performed. First, we compared the number of fiber connections from the habenula to the thalamus between the MDD (n = 34) and NC (n = 37) groups and also compared hemispherical differences to investigate possible asymmetries. Results: There was a significant difference in the number of tracts in the right habenula-left mediodorsal thalamus pair between the two groups. For hemispherical fiber connections, the waytotal ratio of the right ipsilateral tract between the thalamus and habenula was significantly higher than that of the left ipsilateral tract in both groups. Conclusion: The number of right habenula-left mediodorsal thalamus tracts was higher in patients with MDD than in NCs. These results indicate that MDD is related to the disintegration of the left thalamus-right habenula tract function with an increased number of tracts as a compensational mechanism.

Importance of the Habenula for Avoidance Learning Including Contextual Cues in the Human Brain: A Preliminary fMRI Study

Human habenula studies are gradually advancing, primarily through the use of functional magnetic resonance imaging (fMRI) analysis of passive (Pavlovian) conditioning tasks as well as probabilistic reinforcement learning tasks. However, no studies have particularly targeted aversive prediction errors, despite the essential importance for the habenula in the field. Complicated learned strategies including contextual contents are involved in making aversive prediction errors during the learning process. Therefore, we examined habenula activation during a contextual learning task. We performed fMRI on a group of 19 healthy controls. We assessed the manually traced habenula during negative outcomes during the contextual learning task. The Beck Depression Inventory-Second Edition (BDI-II), the State-Trait-Anxiety Inventory (STAI), and the Temperament and Character Inventory (TCI) were also administered. The left and right habenula were activated during aversive outcomes and the activation was associated with aversive prediction errors. There was also a positive correlation between TCI reward dependence scores and habenula activation. Furthermore, dynamic causal modeling (DCM) analyses demonstrated the left and right habenula to the left and right hippocampus connections during the presentation of contextual stimuli. These findings serve to highlight the neural mechanisms that may be relevant to understanding the broader relationship between the habenula and learning processes.

Stress-Related Neuronal Clusters in Sublenticular Extended Amygdala of Basal Forebrain Show Individual Differences of Positions

To understand functional neuronal circuits for emotion in the basal forebrain, patterns of neuronal activation were examined in mice by immunohistochemistry of immediate-early gene products (Zif268/Egr1 and c-Fos). In all mice examined, clusters of 30–50 neurons expressing Zif268 were found on both sides in the area between the extended amygdala (EA) and globus pallidus (GP), generally designated as sublenticular extended amygdala (SLEA). The clusters consisted of 79.9 ± 3.0% of GABAergic neurons in GAD65-mCherry mice. The expression of the cholinergic marker choline acetyltransferase and the GP markers parvalbumin, proenkephalin, and FoxP2 indicated that these neurons were different from known types of neurons in the EA and GP; therefore, we named them the sublenticular extended amygdalar Zif268/Egr1-expressing neuronal cluster (SLEA-zNC). Sublenticular extended amygdalar Zif268/Egr1-expressing neuronal clusters participated in stress processing because increasing numbers of cells were observed in SLEA-zNCs after exposure to restraint stress (RS), the induction of which was suppressed by diazepam treatment. Mapping SLEA-zNCs showed that their positions and arrangement varied individually; SLEA-zNCs were distributed asymmetrically and tended to be situated mainly in the middle region between the anterior commissure (AC) and posterior end of the GP. However, the total cell number in SLEA-zNCs was compatible between the right and left hemispheres after activation by RS. Therefore, SLEA-zNCs were distributed asymmetrically but were not lateralized. Because time courses of activation differed between the Zif268 and c-Fos, the sequential dual treatment of RSs enabled us to differentiate SLEA-zNCs activated by the first and second RS. The results supported that the same SLEA-zNCs responded to both the first and second RS, and this also applied for all SLEA-zNCs. Thus, we concluded that the cluster positions were invariable under RS in each mouse but were distributed differently between individual mice. We name these newly identified neuronal clusters as stress-related neuronal clusters, SLEA-zNCs, which are considered to be novel functional units of “islands of activation.” Moreover, SLEA-zNCs were situated at different positions in all mice examined, showing individual differences in their positions.

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

Transient Input-Specific Neural Plasticity in the Lateral Habenula Facilitates Learning

In this issue of Neuron, Trusel et al. (2019) demonstrate that circuit-specific plasticity in the lateral habenula is dynamically involved in translating CS-US contingencies into cue-driven avoidance behavior. Disruption of this plasticity prevents learning about CS-US relationships when they are uncertain.

Selective Functional Interaction Between the Lateral Habenula and Hippocampus During Different Tests of Response Flexibility

The lateral habenula (LHb) has been shown to play critical roles in a variety of appetitive tasks (e.g., spatial memory and object recognition) that require animals to flexibly respond to changing task conditions. These types of tasks are known to be dependent on hippocampus (HPC) and/or medial prefrontal cortex (mPFC), suggesting that the LHb contributes to the limbic memory circuit. Here we provide new evidence that the LHb and HPC play distinct but complimentary roles in tasks that require flexible responding to changing task conditions. Experiment 1 tested whether the LHb is needed for the performance of a HPC-dependent maze-based spatial delayed alternation task. The importance of interactions between the LHb and HPC to accomplish the same spatial delayed alternation task was tested in Experiment 2 where the LHb and HPC were disconnected both ipsilaterally and contralaterally. Experiment 3 tested LHb's involvement in a standard behavioral economic task that requires flexible responding (maze-based delayed discounting), a task previously shown to rely on HPC. Results of Experiment 1, revealed that LHb inactivation impairs spatial delayed alternation during asymptotic performance but not during initial learning. Importantly, working memory did not appear to be affected as performance remained above chance levels both during initial learning and asymptotic testing. Experiment 2 showed that ipsilateral and contralateral disconnection of the LHb and HPC led to impaired performance on the spatial delayed alternation task. Impairments were not observed after unilateral inactivation of only one structure. Results of Experiment 3 were similar to our previous report of the effects of HPC inactivation: LHb inactivation impaired delayed discounting. All effects could not be accounted for by changes in reward magnitude discrimination, reward location per se, or sex of the animal. These findings, combined with other recent publications confirms and extends our working hypothesis that the LHb enables adaptive and flexible responding, particularly when established rules must be flexibly applied on a trial by trial basis. Since there are no known direct anatomical connections between LHb and HPC, future research is needed to understand how these structures communicate to enable flexible and rapid responding.

Sox1a mediates the ability of the parapineal to impart habenular left-right asymmetry

Left-right asymmetries in the zebrafish habenular nuclei are dependent upon the formation of the parapineal, a unilateral group of neurons that arise from the medially positioned pineal complex. In this study, we show that both the left and right habenula are competent to adopt left-type molecular character and efferent connectivity upon the presence of only a few parapineal cells. This ability to impart left-sided character is lost in parapineal cells lacking Sox1a function, despite the normal specification of the parapineal itself. Precisely timed laser ablation experiments demonstrate that the parapineal influences neurogenesis in the left habenula at early developmental stages as well as neurotransmitter phenotype and efferent connectivity during subsequent stages of habenular differentiation. These results reveal a tight coordination between the formation of the unilateral parapineal nucleus and emergence of asymmetric habenulae, ensuring that appropriate lateralised character is propagated within left and right-sided circuitry.

Lateralized Feeding Behavior is Associated with Asymmetrical Neuroanatomy and Lateralized Gene Expressions in the Brain in Scale-Eating Cichlid Fish

Lateralized behavior ("handedness") is unusual, but consistently found across diverse animal lineages, including humans. It is thought to reflect brain anatomical and/or functional asymmetries, but its neuro-molecular mechanisms remain largely unknown. Lake Tanganyika scale-eating cichlid fish, Perissodus microlepis show pronounced asymmetry in their jaw morphology as well as handedness in feeding behavior-biting scales preferentially only from one or the other side of their victims. This makes them an ideal model in which to investigate potential laterality in neuroanatomy and transcription in the brain in relation to behavioral handedness. After determining behavioral handedness in P. microlepis (preferred attack side), we estimated the volume of the hemispheres of brain regions and captured their gene expression profiles. Our analyses revealed that the degree of behavioral handedness is mirrored at the level of neuroanatomical asymmetry, particularly in the tectum opticum. Transcriptome analyses showed that different brain regions (tectum opticum, telencephalon, hypothalamus, and cerebellum) display distinct expression patterns, potentially reflecting their developmental interrelationships. For numerous genes in each brain region, their extent of expression differences between hemispheres was found to be correlated with the degree of behavioral lateralization. Interestingly, the tectum opticum and telencephalon showed divergent biases on the direction of up- or down-regulation of the laterality candidate genes (e.g., grm2) in the hemispheres, highlighting the connection of handedness with gene expression profiles and the different roles of these brain regions. Hence, handedness in predation behavior may be caused by asymmetric size of brain hemispheres and also by lateralized gene expressions in the brain.

The molecular mechanisms controlling morphogenesis and wiring of the habenula

The habenula is an evolutionarily conserved brain region comprising bilaterally paired nuclei that plays a key role in processing reward information and mediating aversive responses to negative stimuli. An important aspect underlying habenula function is relaying information between forebrain and mid- and hindbrain areas. This is mediated by its complex organization into multiple subdomains and corresponding complexity in circuit organization. Additionally, in many species habenular nuclei display left-right differences at the anatomical and functional level. In order to ensure proper functional organization of habenular circuitry, sophisticated molecular programs control the morphogenesis and wiring of the habenula during development. Knowledge of how these mechanisms shape the habenula is crucial for obtaining a complete understanding of this brain region and can provide invaluable tools to study habenula evolution and function. In this review we will discuss how these molecular mechanisms pattern the early embryonic nervous system and control the formation of the habenula, how they shape its asymmetric organization, and how these mechanisms ensure proper wiring of the habenular circuit. Finally, we will address unexplored aspects of habenula development and how these may direct future research.