Morphological and electrophysiological characteristics of neurons within identified subnuclei of the lateral habenula in rat brain slices

The lateral habenula: A hub for value-guided behavior

The habenula is an evolutionarily highly conserved diencephalic brain region divided into two major parts, medial and lateral. Over the past two decades, studies of the lateral habenula (LHb), in particular, have identified key functions in value-guided behavior in health and disease. In this review, we focus on recent insights into LHb connectivity and its functional relevance for different types of aversive and appetitive value-guided behavior. First, we give an overview of the anatomical organization of the LHb and its main cellular composition. Next, we elaborate on how distinct LHb neuronal subpopulations encode aversive and appetitive stimuli and on their involvement in more complex decision-making processes. Finally, we scrutinize the afferent and efferent connections of the LHb and discuss their functional implications for LHb-dependent behavior. A deepened understanding of distinct LHb circuit components will substantially contribute to our knowledge of value-guided behavior.

Input-output specific orchestration of aversive valence in lateral habenula during stress dynamics

Stress has been considered as a major risk factor for depressive disorders, triggering depression onset via inducing persistent dysfunctions in specialized brain regions and neural circuits. Among various regions across the brain, the lateral habenula (LHb) serves as a critical hub for processing aversive information during the dynamic process of stress accumulation, thus having been implicated in the pathogenesis of depression. LHb neurons integrate aversive valence conveyed by distinct upstream inputs, many of which selectively innervate the medial part (LHbM) or lateral part (LHbL) of LHb. LHb subregions also separately assign aversive valence via dissociable projections to the downstream targets in the midbrain which provides feedback loops. Despite these strides, the spatiotemporal dynamics of LHb-centric neural circuits remain elusive during the progression of depression-like state under stress. In this review, we attempt to describe a framework in which LHb orchestrates aversive valence via the input-output specific neuronal architecture. Notably, a physiological form of Hebbian plasticity in LHb under multiple stressors has been unveiled to incubate neuronal hyperactivity in an input-specific manner, which causally encodes chronic stress experience and drives depression onset. Collectively, the recent progress and future efforts in elucidating LHb circuits shed light on early interventions and circuit-specific antidepressant therapies.

The Neuroanatomy of the Habenular Complex and Its Role in the Regulation of Affective Behaviors

The habenular complex is a diencephalic structure divided into the medial and lateral divisions that lie within the epithalamus of most vertebrates. This brain structure, whose activities are mainly regulated via inputs/outputs from and to the stria medullaris and the fasciculus retroflexus, plays a significant role in the modulation of anti-reward behaviors in both the rodent and human brain. Such anti-reward circuits are regulated by dopaminergic and serotonergic projections with several other subcortical and cortical regions; therefore, it is plausible that impairment to this key subcortical structure or its connections contributes to the pathogenesis of affective disorders. Current literature reveals the existence of structural changes in the habenula complex in individuals afflicted by such disorders; however, there is a need for more comprehensive investigations to elucidate the underlying neuroanatomical connections that underpin disease development. In this review article, we aim to provide a comprehensive view of the neuroanatomical differences between the rodent and human habenular complex, the main circuitries, and provide an update on the emerging roles of this understudied subcortical structure in the control of affective behaviors, with special emphasis to morbid conditions of the affective sphere.

Potentiation of the lateral habenula-ventral tegmental area pathway underlines the susceptibility to depression in mice with chronic pain

Chronic pain often develops severe mood changes such as depression. However, how chronic pain leads to depression remains elusive and the mechanisms determining individuals' responses to depression are largely unexplored. Here we found that depression-like behaviors could only be observed in 67.9% of mice with chronic neuropathic pain, leaving 32.1% of mice with depression resilience. We determined that the spike discharges of the ventral tegmental area (VTA)-projecting lateral habenula (LHb) glutamatergic (Glu) neurons were sequentially increased in sham, resilient and susceptible mice, which consequently inhibited VTA dopaminergic (DA) neurons through a LHbGlu-VTAGABA-VTADA circuit. Furthermore, the LHbGlu-VTADA excitatory inputs were dampened via GABAB receptors in a pre-synaptic manner. Regulation of LHb-VTA pathway largely affected the development of depressive symptoms caused by chronic pain. Our study thus identifies a pivotal role of the LHb-VTA pathway in coupling chronic pain with depression and highlights the activity-dependent contribution of LHbGlu-to-VTADA inhibition in depressive behavioral regulation.

Stress-induced translation of KCNB1 contributes to the enhanced synaptic transmission of the lateral habenula

The lateral habenula (LHb) is a well-established brain region involved in depressive disorders. Synaptic transmission of the LHb neurons is known to be enhanced by stress exposure; however, little is known about genetic modulators within the LHb that respond to stress. Using recently developed molecular profiling methods by phosphorylated ribosome capture, we obtained transcriptome profiles of stress responsive LHb neurons during acute physical stress. Among such genes, we found that KCNB1 (Kv2.1 channel), a delayed rectifier and voltage-gated potassium channel, exhibited increased expression following acute stress exposure. To determine the roles of KCNB1 on LHb neurons during stress, we injected short hairpin RNA (shRNA) against the kcnb1 gene to block its expression prior to stress exposure. We observed that the knockdown of KCNB1 altered the basal firing pattern of LHb neurons. Although KCNB1 blockade did not rescue despair-like behaviors in acute learned helplessness (aLH) animals, we found that KCNB1 knockdown prevented the enhancement of synaptic strength in LHb neuron after stress exposure. This study suggests that KCNB1 may contribute to shape stress responses by regulating basal firing patterns and neurotransmission intensity of LHb neurons.

Recruitment of inhibitory neuronal pathways regulating dopaminergic activity for the control of cocaine seeking

Drug seeking is associated with the ventral tegmental area (VTA) dopaminergic (DA) activity. Previously, we have shown that brief optogenetic inhibition of VTA DA neurons with 1 s pulses delivered every 9 s attenuates cocaine seeking under extinction conditions in rats without producing overt signs of dysphoria or locomotor sedation. Whether recruitment of neuronal pathways inhibiting VTA neuronal activity would suppress drug seeking remains unknown. Here, we asked if optogenetic stimulation of the lateral habenula (LHb) efferents in the rostromedial tegmental nucleus (RMTg) as well as RMTg efferents in VTA would reduce drug seeking. To investigate this, we measured how recruitment of elements of this inhibitory pathway affects cocaine seeking in male rats under extinction conditions. The effectiveness of brief optogenetic manipulations was confirmed electrophysiologically at the level of electrical activity of VTA DA neurons. Real-time conditioned place aversion (RT-CPA) and open field tests were performed to control for potential dysphoric/sedating effects of brief optogenetic stimulation of LHb-RMTg-VTA circuitry. Optogenetic stimulation of either RMTg or LHb inhibited VTA DAergic neuron firing, whereas similar stimulation of RMTg efferents in VTA or LHb efferents in RMTg reduced cocaine seeking under extinction conditions. Moreover, stimulation of LHb-RMTg efferents produced an effect that was maintained 24 h later, during cocaine seeking test without stimulation. This effect was specific, as brief optogenetic stimulation did not affect locomotor activity and was not aversive. Our results indicate that defined inhibitory pathways can be recruited to inhibit cocaine seeking, providing potential new targets for non-pharmacological treatment of drug craving.

Quantitative and qualitative sex difference in habenula-induced inhibition of midbrain dopamine neurons in the rat

Introduction

Clinically relevant sex differences have been noted in a number of affective, behavioral, cognitive, and neurological health disorders. Midbrain dopamine neurons are implicated in several of these same disorders and consequently are under investigation for their potential role in the manifestation of these sex differences. The lateral habenula exerts significant inhibitory control over dopamine neuronal firing, yet little is known about sex differences in this particular neurocircuit.

Methods

We performed in vivo, single unit, extracellular recordings of dopamine neurons in female and male anesthetized rats in response to single pulse stimulation of the lateral habenula. In addition, we assessed baseline firing properties of lateral habenula neurons and, by immunochemical means, assessed the distribution of estrogen receptor alpha cells in the lateral habenula.

Results

Habenula-induced inhibition of dopamine neuronal firing is reduced in female rats relative to male rats. In addition, male rats had a higher prevalence of rebound excitation. Furthermore, the firing pattern of lateral habenula neurons was less variable in female rats, and female rats had a higher density of estrogen receptor alpha positive cells in the lateral habenula.

Discussion

We found that the dopamine neuronal response to habenular stimulation is both qualitatively and quantitatively different in female and male rats. These novel findings together with reports in the contemporary literature lead us to posit that the sex difference in dopamine inhibition seen here relate to differential firing properties of lateral habenula neurons resulting from the presence of sex hormones. Further work is needed to test this hypothesis, which may have implications for understanding the etiology of several mental health disorders including depression, schizophrenia, and addiction.

Sustained antidepressant effect of ketamine through NMDAR trapping in the LHb

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist1, has revolutionized the treatment of depression because of its potent, rapid and sustained antidepressant effects2-4. Although the elimination half-life of ketamine is only 13 min in mice5, its antidepressant activities can last for at least 24 h6-9. This large discrepancy poses an interesting basic biological question and has strong clinical implications. Here we demonstrate that after a single systemic injection, ketamine continues to suppress burst firing and block NMDARs in the lateral habenula (LHb) for up to 24 h. This long inhibition of NMDARs is not due to endocytosis but depends on the use-dependent trapping of ketamine in NMDARs. The rate of untrapping is regulated by neural activity. Harnessing the dynamic equilibrium of ketamine-NMDAR interactions by activating the LHb and opening local NMDARs at different plasma ketamine concentrations, we were able to either shorten or prolong the antidepressant effects of ketamine in vivo. These results provide new insights into the causal mechanisms of the sustained antidepressant effects of ketamine. The ability to modulate the duration of ketamine action based on the biophysical properties of ketamine-NMDAR interactions opens up new opportunities for the therapeutic use of ketamine.

Pharmacological evaluation of lateral habenula and rostromedial tegmental nucleus in the expression of ethanol-induced place preference

Although ethanol administration produces a range of physiological effects, the rewarding aspect associated with its consumption is a major contributory factor to its abuse liability. Recently, lateral habenula (LHb) has been shown to be engaged by both rewarding and aversive stimuli. Its major glutamatergic output, the fasciculus retroflexus, projects to the rostromedial tegmental nucleus (RMTg) and controls the activity of the ventral tegmental area (VTA) dopaminergic system to promote reward circuitry. While several attempts have been made to understand the relationship between LHb and addiction, there is still a lack of knowledge in relation to ethanol addiction. In the present study, by pharmacologically exacerbating or inhibiting the LHb or RMTg neuronal activity during a post-conditioning test, we investigated the role of LHb-RMTg fasciculus retroflexus in ethanol-induced reward behavior using the conditioned place preference (CPP) test. We found that activation of LHb glutamatergic system by intra-LHb administration of l-trans-2,4-pyrrolidine dicarboxylate (PDC) (glutamate transporter inhibitor) significantly decreased CPP score; on the contrary, lamotrigine (inhibits glutamate release) significantly increased CPP score and showed a rewarding effect in CPP. Instead, intra-RMTg administration of muscimol (GABAA receptor agonist) significantly increased CPP score, whereas bicuculline (GABAA antagonist) treatment decreased CPP score. In immunohistochemistry, we found that PDC administration significantly decreased, whereas lamotrigine treatment significantly increased tyrosine hydroxylase immunoreactivity (TH-ir) in VTA and nucleus accumbens (NAc). Furthermore, while intra-RMTg administration of muscimol increased, the bicuculline treatment significantly decreased the TH-ir in VTA and NAc. Together, our behavioral and immunohistochemical results signify the role of LHb and RMTg in the expression of ethanol-conditioned reward behavior.

Deep brain stimulation in the lateral habenula reverses local neuronal hyperactivity and ameliorates depression-like behaviors in rats

Deep brain stimulation (DBS) is a promising therapy for treatment-resistant depression, while mechanisms underlying its therapeutic effects remain poorly defined. Increasing evidence has revealed an intimate association between the lateral habenula (LHb) and major depression, and suggests that the LHb might be an effective target of DBS therapy for depression. Here, we found that DBS in the LHb effectively decreased depression-like behaviors in rats experienced with chronic unpredictable mild stress (CUMS), a well-accepted paradigm for modeling depression in rodents. In vivo electrophysiological recording unveiled that CUMS increased neuronal burst firing, as well as the proportion of neurons showing hyperactivity to aversive stimuli in the LHb. Nevertheless, DBS downregulated local field potential power, reversed the CUMS-induced increase of LHb burst firing and neuronal hyperactivity to aversive stimuli, and decreased the coherence between LHb and ventral tegmental area (VTA). Our results demonstrate that DBS in the LHb exerts antidepressant-like effects and reverses local neural hyperactivity, supporting the LHb as a target of DBS therapy for depression.

Involvement of Lateral Habenula Dysfunction in Repetitive Mild Traumatic Brain Injury-Induced Motivational Deficits

Affective disorders including depression (characterized by reduced motivation, social withdrawal, and anhedonia), anxiety, and irritability are frequently reported as long-term consequences of mild traumatic brain injury (mTBI) in addition to cognitive deficits, suggesting a possible dysregulation within mood/motivational neural circuits. One of the important brain regions that control motivation and mood is the lateral habenula (LHb), whose hyperactivity is associated with depression. Here, we used a repetitive closed-head injury mTBI model that is associated with social deficits in adult male mice and explored the possible long-term alterations in LHb activity and motivated behavior 10-18 days post-injury. We found that mTBI increased the proportion of spontaneous tonically active LHb neurons yet decreased the proportion of LHb neurons displaying bursting activity. Additionally, mTBI diminished spontaneous glutamatergic and GABAergic synaptic activity onto LHb neurons, while synaptic excitation and inhibition (E/I) balance was shifted toward excitation through a greater suppression of GABAergic transmission. Behaviorally, mTBI increased the latency in grooming behavior in the sucrose splash test suggesting reduced self-care motivated behavior following mTBI. To show whether limiting LHb hyperactivity could restore motivational deficits in grooming behavior, we then tested the effects of Gi (hM4Di)-DREADD-mediated inhibition of LHb activity in the sucrose splash test. We found that chemogenetic inhibition of LHb glutamatergic neurons was sufficient to reverse mTBI-induced delays in grooming behavior. Overall, our study provides the first evidence for persistent LHb neuronal dysfunction due to an altered synaptic integration as causal neural correlates of dysregulated motivational states by mTBI.

Effects of GABAB receptor blockade on lateral habenula glutamatergic neuron activity following morphine injection in the rat: an electrophysiological study

Background and purpose

The lateral habenula (LHb), a key area in the regulation of the reward system, exerts a major influence on midbrain neurons. It has been shown that the gamma-aminobutyric acid (GABA)- ergic system plays the main role in morphine dependency. The role of GABA type B receptors (GABA_BR_s) in the regulation of LHb neural activity in response to morphine, remains unknown. In this study, the effect of GABA_BR_s blockade in response to morphine was assessed on the neuronal activity in the LHb.

Experimental approach

The baseline firing rate was recorded for 15 min, then morphine (5 mg/kg; s.c) and phaclofen (0, 0.5, 1, and 2 μg/rat), a GABA_BR_s' antagonist, were microinjected into the LHb. Their effects on firing LHb neurons were investigated using an extracellular single-unit recording in male rats.

Findings/Results

The results revealed that morphine decreased neuronal activity, and GABA_BR_s blockade alone did not have any effect on the neuronal activity of the LHb. A low dose of the antagonist had no significant effect on neuronal firing rate, while blockade with doses of 1 and 2 μg/rat of the antagonist could significantly prevent the inhibitory effects of morphine on the LHb neuronal activity.

Conclusion and implications

This result indicated that GABA_BR_s have a potential modulator effect, in response to morphine in the LHb.

Effects of maternal deprivation and environmental enrichment on anxiety-like and depression-like behaviors correlate with oxytocin system and CRH level in the medial-lateral habenula

The medial-lateral habenula (LHbM)'s role in anxiety and depression behaviors in female mice remains unclear. Here, we used neonatal maternal deprivation (MD) and post-weaning environmental enrichment (EE) to treat female BALB/c offspring and checked anxiety-like and depression-like behaviors as well as the corticotropin-releasing hormone (CRH), oxytocin receptor (OTR), estrogen receptor-beta (ERβ) levels in their LHbM at adulthood. We found that MD enhanced state anxiety-like behaviors in the elevated plus-maze test, and EE caused trait anxiety-like behaviors in the open field test and depression-like behaviors in the tail suspension test. The immunochemistry showed that MD reduced OT immunoreactive neuron numbers in the hypothalamic paraventricular nucleus but increased OTR levels in the LHbM; EE increased CRH levels in the LHbM but decreased OTR levels in the LHbM. The additive effects of EE and MD maintained the behavioral parameters, OT-ir neuronal numbers, CRH levels, and OTR levels similar to the additive of non-MD and non-EE. The correlation analysis showed that CRH levels correlated with synaptic connection levels, OTR levels correlated with nucleus densities, and ERβ levels correlated with Nissl body levels and body weights in female mice. Neither MD nor EE affected ERβ levels in the LHbM. Together, the study revealed the relationships between behaviors and neuroendocrine and neuronal alterations in female LHbM and the effects of experiences including MD and EE on them.

Transcriptomically-Guided Pharmacological Experiments in Neocortical and Hippocampal NPY-Positive GABAergic Interneurons

Cortical GABAergic interneurons have been shown to fulfil important roles by inhibiting excitatory principal neurons. Recent transcriptomic studies have confirmed seminal discoveries that used anatomic and electrophysiological methods highlighting the existence of multiple different classes of GABAergic interneurons. Although some of these studies have emphasized that inter-regional differences may exist for a given class, the extent of such differences remains unknown. To address this problem, we used single-cell Patch-RNAseq to characterize neuropeptide Y (NPY)-positive GABAergic interneurons in superficial layers of the primary auditory cortex (AC) and in distal layers of area CA3 in mice. We found that more than 300 genes are differentially expressed in NPY-positive neurons between these two brain regions. For example, the AMPA receptor (AMPAR) auxiliary subunit Shisa9/CKAMP44 and the 5HT2a receptor (5HT2aR) are significantly higher expressed in auditory NPY-positive neurons. These findings guided us to perform pharmacological experiments that revealed a role for 5HT2aRs in auditory NPY-positive neurons. Specifically, although the application of 5HT led to a depolarization of both auditory and CA3 NPY-positive neurons, the 5HT2aR antagonist ketanserin only reversed membrane potential changes in auditory NPY-positive neurons. Our study demonstrates the potential of single-cell transcriptomic studies in guiding directed pharmacological experiments.

A diencephalic circuit in rats for opioid analgesia but not positive reinforcement

Mu opioid receptor (MOR) agonists are potent analgesics, but also cause sedation, respiratory depression, and addiction risk. The epithalamic lateral habenula (LHb) signals aversive states including pain, and here we found that it is a potent site for MOR-agonist analgesia-like responses in rats. Importantly, LHb MOR activation is not reinforcing in the absence of noxious input. The LHb receives excitatory inputs from multiple sites including the ventral tegmental area, lateral hypothalamus, entopeduncular nucleus, and the lateral preoptic area of the hypothalamus (LPO). Here we report that LHb-projecting glutamatergic LPO neurons are excited by noxious stimulation and are preferentially inhibited by MOR selective agonists. Critically, optogenetic stimulation of LHb-projecting LPO neurons produces an aversive state that is relieved by LHb MOR activation, and optogenetic inhibition of LHb-projecting LPO neurons relieves the aversiveness of ongoing pain.

Opioids are potent analgesics but also have addiction risk. Here a lateral preoptic area to lateral habenula connection is identified by which opioids relieve ongoing pain but do not produce reward in animals that do not have ongoing pain.

Circadian Influences on the Habenula and Their Potential Contribution to Neuropsychiatric Disorders

The neural circadian system consists of the master circadian clock in the hypothalamic suprachiasmatic nuclei (SCN) communicating time of day cues to the rest of the body including other brain areas that also rhythmically express circadian clock genes. Over the past 16 years, evidence has emerged to indicate that the habenula of the epithalamus is a candidate extra-SCN circadian oscillator. When isolated from the SCN, the habenula sustains rhythms in clock gene expression and neuronal activity, with the lateral habenula expressing more robust rhythms than the adjacent medial habenula. The lateral habenula is responsive to putative SCN output factors as well as light information conveyed to the perihabenula area. Neuronal activity in the lateral habenula is altered in depression and intriguingly disruptions in circadian rhythms can elevate risk of developing mental health disorders including depression. In this review, we will principally focus on how circadian and light signals affect the lateral habenula and evaluate the possibility that alteration in these influences contribute to mental health disorders.

Prevention of glutamate excitotoxicity in lateral habenula alleviates ethanol withdrawal-induced somatic and behavioral effects in ethanol dependent mice

Ethanol withdrawal commonly leads to anxiety-related disorder, a central factor toward negative reinforcement leading to relapse. The lateral habenula (LHb), an epithalamic nucleus, has emerged to be critical for both reward and aversion processing. Recent studies have also implicated the hyperactivity of LHb, adding to the emergence of negative emotional states during withdrawal from addictive drugs. Herein, we have studied the effects of glutamate transporter inhibitor (PDC), GluN2B-containing NMDAR antagonist (Ro25-6981), and intracellular calcium chelator (BAPTA-AM) injection in LHb on ethanol withdrawal symptoms. We found that ethanol 4 g/kg 20 % w/v intragastric (i.g.) for 10 days followed by 24 h of withdrawal showed a significant increase in somatic signs characterized by vocalization, shaking, and scratching. It also increased locomotor activity and anxiety-like behavior, collectively showing expression of ethanol withdrawal symptoms. The intra-LHb administration of PDC (0.5 ng) worsened the effect of ethanol withdrawal, whereas Ro25-6981 (2 and 4 ng) and BAPTA-AM (6.5 and 13 ng) significantly reversed ethanol withdrawal-induced behavior evident by a decrease in somatic signs, locomotor activity, and anxiety-like behavior. Further, pretreatment of Ro25-6981 and BAPTA-AM reduced the neuronal loss, whereas PDC increased it compared to the vehicle-treated group, as evidenced by NeuN staining. Altogether, our results suggest that increased glutamate, GluN2B activation, and likely calcium increase indicative of glutamate excitotoxicity-induced neuronal loss in LHb possibly endorse the emergence of ethanol withdrawal symptoms, while their inhibition might help in alleviating the ethanol withdrawal symptoms.

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Inhibition Within the Lateral Habenula-Implications for Affective Disorders

The lateral habenula (LHb) is a key brain region implicated in the pathology of major depressive disorder (MDD). Specifically, excitatory LHb neurons are known to be hyperactive in MDD, thus resulting in a greater excitatory output mainly to downstream inhibitory neurons in the rostromedial tegmental nucleus. This likely results in suppression of downstream dopaminergic ventral tegmental area neurons, therefore, resulting in an overall reduction in reward signalling. In line with this, increasing evidence implicates aberrant inhibitory signalling onto LHb neurons as a co-causative factor in MDD, likely as a result of disinhibition of excitatory neurons. Consistently, growing evidence now suggests that normalising inhibitory signalling within the LHb may be a potential therapeutic strategy for MDD. Despite these recent advances, however, the exact pharmacological and neural circuit mechanisms which control inhibitory signalling within the LHb are still incompletely understood. Thus, in this review article, we aim to provide an up-to-date summary of the current state of knowledge of the mechanisms by which inhibitory signalling is processed within the LHb, with a view of exploring how this may be targeted as a future therapy for MDD.

The circadian clock in the mouse habenula is set by catecholamines

Circadian rhythms are those variations in behavioral and molecular processes of organisms that follow roughly 24 h cycles in the absence of any external cue. The hypothalamic suprachiasmatic nucleus (SCN) harbors the principal brain pacemaker driving circadian rhythms. The epithalamic habenula (Hb) contains a self-sustained circadian clock functionally coupled to the SCN. Anatomically, the Hb projects to the midbrain dopamine (DA) and serotonin (5-HT) systems, and it receives inputs from the forebrain, midbrain, and brainstem. The SCN is set by internal signals such as 5-HT or melatonin from the raphe nuclei and pineal gland, respectively. However, how the Hb clock is set by internal cues is not well characterized. Hence, in the present study, we determined whether DA, noradrenaline (NA), 5-HT, and the neuropeptides orexin (ORX) and vasopressin influence the Hb circadian clock. Using PER2::Luciferase transgenic mice, we found that the amplitude of the PER2 protein circadian oscillations from Hb explants was strongly affected by DA and NA. Importantly, these effects were dose-and region (rostral vs. caudal) dependent for NA, with a main effect in the caudal part of the Hb. Furthermore, ORX also induced a significant change in the amplitude of PER2 protein oscillations in the caudal Hb. In conclusion, catecholaminergic (DA, NA) and ORXergic transmission impacts the clock properties of the Hb clock likely contributing to the circadian regulation of motivated behaviors. Accordingly, pathological conditions that lead in alterations of catecholamine or ORX activity (drug intake, compulsive feeding) might affect the Hb clock and conduct to circadian disturbances.

Microglial synaptic pruning on axon initial segment spines of dentate granule cells: Sexually dimorphic effects of early-life stress and consequences for adult fear response

Axon initial segments (AIS) of dentate granule cells in the hippocampus exhibit prominent spines (AISS) during early development that are associated with microglial contacts. In the present study, we investigated whether developmental changes in AISS could be modified by early-life stress (ELS), specifically neonatal maternal separation (MS), through stress hormones and microglial activation and examined the potential behavioural consequences. We examined AISS at postnatal day (PND)5, 15 and 50, using Golgi-Cox staining and anatomical analysis. Neurone-microglial interaction was assessed using antibodies against ankyrin-G, PSD-95 and Iba1, for AIS, AISS and microglia visualisation, respectively, in normally reared and neonatal maternally separated male and female rats. We observed a higher density of AISS in ELS rats at both PND15 and PND50 compared to controls. Effects were more pronounced in females than males. AIS-associated microglia in ELS rats showed a hyper-ramified morphology and less co-localisation with PSD-95 compared to controls at PND15. ELS-associated alteration in microglial morphology and synaptic pruning was mimicked by treatment of acute hippocampal slices of normally reared rats with vasopressin. ELS rats exhibited increased freezing behaviour during auditory fear memory testing, which was more pronounced in female subjects and corresponded with increased Fos expression in dorsal and ventral dentate granule cells. Thus, microglial synaptic pruning in dentate AIS of hippocampus is influenced by ELS, with demonstrable sex bias regarding its anatomical characteristics and subsequent fear-induced defensive behaviours.

NDNF is selectively expressed by neocortical, but not habenular neurogliaform cells

The lateral habenula (LHb) is a brain structure which is known to be pathologically hyperactive in depression, whereby it shuts down the brains' reward systems. Interestingly, inhibition of the LHb has been shown to have an antidepressant effect, hence making the LHb a fascinating subject of study for developing novel antidepressant therapies. Despite this however, the exact mechanisms by which inhibitory signalling is processed within the LHb remain incompletely understood. Some studies have proposed the existence of locally targeting inhibitory interneuron populations within the LHb. One such population is believed to be akin to neocortical neurogliaform cells, yet specific molecular markers for studying these neurons are sparse and hence their function remains elusive. Recently, neuron-derived neurotrophic factor (NDNF) has been proposed as one such marker for neocortical neurogliaform cells. Using a combination of histological, physiological and optogenetic tools, we hence sought to first validate if NDNF was selectively expressed by such inhibitory neurons within the neocortex, and then if it was confined to a similar population within the LHb. While we report this to be true for the neocortex, we find no such evidence within the LHb; rather that NDNF is expressed without restriction to a particular neuronal subpopulation. These results hence indicate that molecular markers can represent broadly diverse populations of neurons on a region-to-region basis and that therefore each population as defined by molecular marker expression should be validated in each brain structure.

Input of sensory, limbic, basal ganglia and pallial/cortical information into the ventral/lateral habenula: Functional principles in anuran amphibians

The habenula - a phylogenetically old brain structure present in all vertebrates - is involved in pain processing, reproductive behaviors, sleep-wake cycles, stress responses, reward, and learning. We performed intra- and extracellular recordings of ventral habenula (VHb) neurons in the isolated brain of anurans and revealed similar cell and response properties to those reported for the lateral habenula of mammals. We identified tonic regular, tonic irregular, rhythmic firing, and silent VHb neurons. Transitions between these firing patterns were observed during spontaneous activity. Electrical stimulation of various brain areas demonstrated VHb input of auditory, optic, limbic, basal ganglia, and pallial information. This resulted in three different response behaviors in VHb neurons: excitation, inhibition, or alternating facilitation and suppression of neuronal activity. Spontaneously changing activity patterns were observed to modulate, reset, or suppress the response behavior of VHb neurons, indicating a gating mechanism. This could be a network status or context dependent selection mechanism for which information are transmitted to task relevant brain areas (i.e., sensory system, limbic system, basal ganglia). Furthermore, alternating facilitation and suppression sequences upon auditory nerve stimulation correlated positively fictive motor activities recorded via the compound potential of the vagal nerve. Stimulation of the auditory nerve or the habenula led to facilitation, suppression, or alternating facilitation and suppression of neuronal activity in putative dopaminergic neurons. Due to complex habenula feedback loops with basal ganglia, limbic, and sensory systems, the habenula involvement in a variety of functions might therefore be explained by a modulatory effect on a task-relevant input stream.

Lycium barbarum polysaccharide-glycoprotein preventative treatment ameliorates aversive

Previous studies have shown that Lycium barbarum polysaccharide, the main active component of Lycium barbarum, exhibits anti-inflammatory and antioxidant effects in treating neurological diseases. However, the therapeutic action of Lycium barbarum polysaccharide on depression has not been studied. In this investigation, we established mouse models of depression using aversive stimuli including exposure to fox urine, air puff and foot shock and physical restraint. Concurrently, we administered 5 mg/kg per day Lycium barbarum polysaccharide-glycoprotein to each mouse intragastrically for the 28 days. Our results showed that long-term exposure to aversive stimuli significantly enhanced depressive-like behavior evaluated by the sucrose preference test and the forced swimming test and increased anxiety-like behaviors evaluated using the open field test. In addition, aversive stimuli-induced depressed mice exhibited aberrant neuronal activity in the lateral habenula. Importantly, concurrent Lycium barbarum polysaccharide-glycoprotein treatment significantly reduced these changes. These findings suggest that Lycium barbarum polysaccharide-glycoprotein is a potential preventative intervention for depression and may act by preventing aberrant neuronal activity and microglial activation in the lateral habenula. The study was approved by the Jinan University Institutional Animal Care and Use Committee (approval No. 20170301003) on March 1, 2017.

Lateral habenula dysfunctions in Tm4sf2-/y mice model for neurodevelopmental disorder

Mutations in the TM4SF2 gene, which encodes TSPAN7, cause a severe form of intellectual disability (ID) often comorbid with autism spectrum disorder (ASD). Recently, we found that TM4SF2 loss in mice affects cognition. Here, we report that Tm4sf2-/y mice, beyond an ID-like phenotype, display altered sociability, increased repetitive behaviors, anhedonic- and depressive-like states. Cognition relies on the integration of information from several brain areas. In this context, the lateral habenula (LHb) is strategically positioned to coordinate the brain regions involved in higher cognitive functions. Furthermore, in Tm4sf2-/y mice we found that LHb neurons present hypoexcitability, aberrant neuronal firing pattern and altered sodium and potassium voltage-gated ion channels function. Interestingly, we also found a reduced expression of voltage-gated sodium channel and a hyperactivity of the PKC-ERK pathway, a well-known modulator of ion channels activity, which might explain the functional phenotype showed by Tm4sf2-/y mice LHb neurons. These findings support Tm4sf2-/y mice as useful in modeling some ASD-like symptoms. Additionally, we can speculate that LHb functional alteration in Tm4sf2-/y mice might play a role in the disease pathophysiology.

Early life stress dysregulates kappa opioid receptor signaling within the lateral habenula

The lateral habenula (LHb) is an epithalamic brain region associated with value-based decision making and stress evasion through its modulation of dopamine (DA)-mediated reward circuitry. Specifically, increased activity of the LHb is associated with drug addiction, schizophrenia and stress-related disorders such as depression, anxiety and posttraumatic stress disorder. Dynorphin (Dyn)/Kappa opioid receptor (KOR) signaling is a mediator of stress response in reward circuitry. Previously, we have shown that maternal deprivation (MD), a severe early life stress, increases LHb spontaneous neuronal activity and intrinsic excitability while blunting the response of LHb neurons to extrahypothalamic corticotropin-releasing factor (CRF) signaling, another stress mediator. CRF pathways also interact with Dyn/KOR signaling. Surprisingly, there has been little study of direct KOR regulation of the LHb despite its distinct role in stress, reward and aversion processing. To test the functional role of Dyn/KOR signaling in the LHb, we utilized ex-vivo electrophysiology combined with pharmacological tools in rat LHb slices. We show that activation of KORs by a KOR agonist (U50,488) exerted differential effects on the excitability of two distinct sub-populations of LHb neurons that differed in their expression of hyperpolarization-activated cation currents (HCN, Ih). Specifically, KOR stimulation increased neuronal excitability in LHb neurons with large Ih currents (Ih+) while decreasing neuronal excitability in small/negative Ih (Ih-) neurons. We found that an intact fast-synaptic transmission was required for the effects of U50,488 on the excitability of both Ih- and Ih+ LHb neuronal subpopulations. While AMPAR-, GABAAR-, or NMDAR-mediated synaptic transmission alone was sufficient to mediate the effects of U50,488 on excitability of Ih- neurons, either GABAAR- or NMDAR-mediated synaptic transmission could mediate these effects in Ih+ neurons. Consistently, KOR activation also altered both glutamatergic and GABAergic synaptic transmission where stimulation of presynaptic KORs uniformly suppressed glutamate release onto LHb neurons while primarily decreased or in some cases increased GABA release. We also found that MD significantly increased immunolabeled Dyn (the endogenous KOR agonist) labeling in neuronal fibers in LHb while significantly decreasing mRNA levels of KORs in LHb tissues compared to those from non-maternally deprived (non-MD) control rats. Moreover, the U50,488-mediated increase in LHb neuronal firing observed in non-MD rats was absent following MD. Altogether, this is the first demonstration of the existence of functional Dyn/KOR signaling in the LHb that can be modulated in response to severe early life stressors such as MD.

Behavior: Local Lateral Habenula Interneurons Mediate Aggression

A recent study has shown that local inhibitory GAD2-positive neurons regulate the activity of lateral habenula neurons, thereby governing aggressive behavior in male mice.

Anti-depression effects of ketogenic diet are mediated via the restoration of microglial activation and neuronal excitability in the lateral habenula

Depression is a severe neuropsychiatric disorder, of which the underlying pathological mechanisms remain unclear. The ketogenic diet (KD) has been reported to exhibit preventative effects on depressive-like behaviors in rodents. However, the therapeutic effects of KD on depressive-like behaviors have not been illustrated thus far. Here, we found that KD treatment dramatically ameliorated depressive-like behaviors in both repeated social defeat stress (R-SDS) and lipopolysaccharide (LPS) models, indicating the potential therapeutic effects of KD on depression. Our electrophysiological studies further showed that neuronal excitability was increased in the lateral habenula (LHb) of mice exposed to R-SDS or LPS, which can be reversed in the presence of KD treatment. Moreover, R-SDS and LPS were also found to induce robust microglial inflammatory activation in the LHb. Importantly, these phenotypes were rescued in mice fed with KD. In addition, we found that the protein level of innate immune receptor Trem2 in the LHb was significantly decreased in depression models. Specific knockdown of Trem2 in LHb microglia induced depressive-like behaviors, increased neuronal excitability as well as robust microglial inflammatory activation. Altogether, we demonstrated the therapeutic effects of KD on depressive-like behaviors, which are probably mediated via the restoration of microglial inflammatory activation and neuronal excitability. Besides, we also proposed an unrecognized function of Trem2 in the LHb for depression. Our study sheds light on the pathogenesis of depression and thereby offers a potential therapeutic intervention.

Disentangling neuronal inhibition and inhibitory pathways in the lateral habenula

The lateral habenula (LHb) is hyperactive in depression, and thus potentiating inhibition of this structure makes an interesting target for future antidepressant therapies. However, the circuit mechanisms mediating inhibitory signalling within the LHb are not well-known. We addressed this issue by studying LHb neurons expressing either parvalbumin (PV) or somatostatin (SOM), two markers of particular sub-classes of neocortical inhibitory neurons. Here, we find that both PV and SOM are expressed by physiologically distinct sub-classes. Furthermore, we describe multiple sources of inhibitory input to the LHb arising from both local PV-positive neurons, from PV-positive neurons in the medial dorsal thalamic nucleus, and from SOM-positive neurons in the ventral pallidum. These findings hence provide new insight into inhibitory control within the LHb, and highlight that this structure is more neuronally diverse than previously thought.

Circuits and functions of the lateral habenula in health and in disease

The past decade has witnessed exponentially growing interest in the lateral habenula (LHb) owing to new discoveries relating to its critical role in regulating negatively motivated behaviour and its implication in major depression. The LHb, sometimes referred to as the brain's 'antireward centre', receives inputs from diverse limbic forebrain and basal ganglia structures, and targets essentially all midbrain neuromodulatory systems, including the noradrenergic, serotonergic and dopaminergic systems. Its unique anatomical position enables the LHb to act as a hub that integrates value-based, sensory and experience-dependent information to regulate various motivational, cognitive and motor processes. Dysfunction of the LHb may contribute to the pathophysiology of several psychiatric disorders, especially major depression. Recently, exciting progress has been made in identifying the molecular and cellular mechanisms in the LHb that underlie negative emotional state in animal models of drug withdrawal and major depression. A future challenge is to translate these advances into effective clinical treatments.

Volume-Conducted Origin of the Field Potential at the Lateral Habenula

Field potentials (FPs) are easily reached signals that provide information about the brain's processing. However, FP should be interpreted cautiously since their biophysical bases are complex. The lateral habenula (LHb) is a brain structure involved in the encoding of aversive motivational values. Previous work indicates that the activity of the LHb is relevant for hippocampal-dependent learning. Moreover, it has been proposed that the interaction of the LHb with the hippocampal network is evidenced by the synchronization of LHb and hippocampal FPs during theta rhythm. However, the origin of the habenular FP has not been analyzed. Hence, its validity as a measurement of LHb activity has not been proven. In this work, we used electrophysiological recordings in anesthetized rats and feed-forward modeling to investigate biophysical basis of the FP recorded in the LHb. Our results indicate that the FP in the LHb during theta rhythm is a volume-conducted signal from the hippocampus. This result highlight that FPs must be thoroughly analyzed before its biological interpretation and argues against the use of the habenular FP signal as a readout of the activity of the LHb.

Habenular connections with the dopaminergic and serotonergic system and their role in stress-related psychiatric disorders

The habenula (Hb) is a phylogenetically old epithalamic structure differentiated into two nuclear complexes, the medial (MHb) and lateral habenula (LHb). After decades of search for a great unifying function, interest in the Hb resurged when it was demonstrated that LHb plays a major role in the encoding of aversive stimuli ranging from noxious stimuli to the loss of predicted rewards. Consistent with a role as an anti-reward center, aberrant LHb activity has now been identified as a key factor in the pathogenesis of major depressive disorder. Moreover, both MHb and LHb emerged as new players in the reward circuitry by primarily mediating the aversive properties of distinct drugs of abuse. Anatomically, the Hb serves as a bridge that links basal forebrain structures with monoaminergic nuclei in the mid- and hindbrain. So far, research on Hb has focused on the role of the LHb in regulating midbrain dopamine release. However, LHb/MHb are also interconnected with the dorsal (DR) and median (MnR) raphe nucleus. Hence, it is conceivable that some of the habenular functions are at least partly mediated by the complex network that links MHb/LHb with pontomesencephalic monoaminergic nuclei. Here, we summarize research about the topography and transmitter phenotype of the reciprocal connections between the LHb and ventral tegmental area-nigra complex, as well as those between the LHb and DR/MnR. Indirect MHb outputs via interpeduncular nucleus to state-setting neuromodulatory networks will also be commented. Finally, we discuss the role of specific LHb-VTA and LHb/MHb-raphe circuits in anxiety and depression.

Chronic Stress Induces Activity, Synaptic, and Transcriptional Remodeling of the Lateral Habenula Associated with Deficits in Motivated Behaviors

Chronic stress (CS) is a major risk factor for the development of depression. Here, we demonstrate that CS-induced hyperactivity in ventral tegmental area (VTA)-projecting lateral habenula (LHb) neurons is associated with increased passive coping (PC), but not anxiety or anhedonia. LHb→VTA neurons in mice with increased PC show increased burst and tonic firing as well as synaptic adaptations in excitatory inputs from the entopeduncular nucleus (EP). In vivo manipulations of EP→LHb or LHb→VTA neurons selectively alter PC and effort-related motivation. Conversely, dorsal raphe (DR)-projecting LHb neurons do not show CS-induced hyperactivity and are targeted indirectly by the EP. Using single-cell transcriptomics, we reveal a set of genes that can collectively serve as biomarkers to identify mice with increased PC and differentiate LHb→VTA from LHb→DR neurons. Together, we provide a set of biological markers at the level of genes, synapses, cells, and circuits that define a distinctive CS-induced behavioral phenotype.

A Visual Circuit Related to Habenula Underlies the Antidepressive Effects of Light Therapy

Light plays a pivotal role in the regulation of affective behaviors. However, the precise circuits that mediate the impact of light on depressive-like behaviors are not well understood. Here, we show that light influences depressive-like behaviors through a disynaptic circuit linking the retina and the lateral habenula (LHb). Specifically, M4-type melanopsin-expressing retinal ganglion cells (RGCs) innervate GABA neurons in the thalamic ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL), which in turn inhibit CaMKIIα neurons in the LHb. Specific activation of vLGN/IGL-projecting RGCs, activation of LHb-projecting vLGN/IGL neurons, or inhibition of postsynaptic LHb neurons is sufficient to decrease the depressive-like behaviors evoked by long-term exposure to aversive stimuli or chronic social defeat stress. Furthermore, we demonstrate that the antidepressive effects of light therapy require activation of the retina-vLGN/IGL-LHb pathway. These results reveal a dedicated retina-vLGN/IGL-LHb circuit that regulates depressive-like behaviors and provide a potential mechanistic explanation for light treatment of depression.

Decoding Depression: Insights from Glial and Ketamine Regulation of Neuronal Burst Firing in Lateral Habenula

The rapid antidepressant effect of ketamine is arguably one of the most significant advances in the mental health field in the last half century. However, its mechanism of action has remained elusive. Here, we describe our latest discovery on how ketamine blocks N-methyl-D-aspartate receptor (NMDAR)-dependent burst firing of an "antireward" center in the brain, the lateral habenula (LHb), to mediate its antidepressant effects. We also discuss a novel structure-function mechanism at the glia-neuron interface to account for the enhanced LHb bursting during depression. These results reveal new molecular targets for the therapeutic intervention of major depression.

Ketamine blocks bursting in the lateral habenula to rapidly relieve depression

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine has attracted enormous interest in mental health research owing to its rapid antidepressant actions, but its mechanism of action has remained elusive. Here we show that blockade of NMDAR-dependent bursting activity in the 'anti-reward center', the lateral habenula (LHb), mediates the rapid antidepressant actions of ketamine in rat and mouse models of depression. LHb neurons show a significant increase in burst activity and theta-band synchronization in depressive-like animals, which is reversed by ketamine. Burst-evoking photostimulation of LHb drives behavioural despair and anhedonia. Pharmacology and modelling experiments reveal that LHb bursting requires both NMDARs and low-voltage-sensitive T-type calcium channels (T-VSCCs). Furthermore, local blockade of NMDAR or T-VSCCs in the LHb is sufficient to induce rapid antidepressant effects. Our results suggest a simple model whereby ketamine quickly elevates mood by blocking NMDAR-dependent bursting activity of LHb neurons to disinhibit downstream monoaminergic reward centres, and provide a framework for developing new rapid-acting antidepressants.

The dorsal diencephalic conduction system in reward processing: Spotlight on the anatomy and functions of the habenular complex

The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.

Review of the cytology and connections of the lateral habenula, an avatar of adaptive behaving

The cytology and connections of the lateral habenula (LHb) are reviewed. The habenula is first introduced, after which the cytology of the LHb is discussed mainly with reference to cell types, general topography and descriptions of subnuclei. An overview of LHb afferent connections is given followed by some details about the projections to LHb from a number of structures. An overview of lateral habenula efferent connections is given followed by some details about the projections from LHb to a number of structures. In considering the afferent and efferent connections of the LHb some attention is given to the relative validity of regarding it as a bi-partite structure featuring 'limbic' and 'pallidal' parts. The paper ends with some concluding remarks about the relative place of the LHb in adaptive behaving.

Ethanol Withdrawal Drives Anxiety-Related Behaviors by Reducing M-type Potassium Channel Activity in the Lateral Habenula

Alcohol use disorders (AUDs) and anxiety disorders (ADs) are often seen concurrently, but their underlying cellular basis is unclear. For unclear reasons, the lateral habenula (LHb), a key brain region involved in the pathophysiology of ADs, becomes hyperactive after ethanol withdrawal. M-type K⁺ channels (M-channels), important regulators of neuronal activity, are abundant in the LHb, yet little is known about their role in AUDs and associated ADs. We report here that in rats at 24 h withdrawal from systemic ethanol administration (either by intraperitoneal injection, 2 g/kg, twice/day, for 7 days; or intermittent drinking 20% ethanol in a two-bottle free choice protocol for 8 weeks), the basal firing rate and the excitability of LHb neurons in brain slices was higher, whereas the amplitude of medium afterhyperpolarization and M-type K⁺ currents were smaller, when compared to ethanol naive rats. Concordantly, M-channel blocker (XE991)-induced increase in the spontaneous firing rate in LHb neurons was smaller. The protein expression of M-channel subunits, KCNQ2/3 in the LHb was also smaller. Moreover, anxiety levels (tested in open field, marble burying, and elevated plus maze) were higher, which were alleviated by LHb inhibition either chemogenetically or by local infusion of the M-channel opener, retigabine. Intra-LHb infusion of retigabine also reduced ethanol consumption and preference. These findings reveal an important role of LHb M-channels in the expression of AUDs and ADs, and suggest that the M-channels could be a potential therapeutic target for alcoholics.

The lateral habenula and alcohol: Role of glutamate and M-type potassium channels

Alcohol use disorder (AUD) or alcoholism is a chronic relapsing disorder. Our knowledge of alcoholism hinges on our understanding of its effects on the brain. This review will center on the effects of alcohol in the lateral habenula (LHb), an epithalamic structure that connects the forebrain with the midbrain and encodes aversive signaling. Like many addictive drugs, alcohol has both rewarding and aversive properties. While alcohol's euphoric property is believed to be important for the initiation of drinking, increasing evidence suggests that alcohol's negative affect plays a critical role in excessive drinking and alcohol dependence. During withdrawal and abstinence, alcoholics often experience anxiety and depressions, both of which have been implicated in relapse drinking. This review focuses on the recent accumulation of knowledge about the effects of acute and chronic alcohol exposure on the activity of and synaptic transmissions on LHb neurons, as well as the effects of manipulation of LHb function on alcohol consumption and related behaviors. Recent evidence highlights a critical role for the LHb in AUD and related psychiatric ailments. Multidisciplinary work in animals collectively suggests that LHb function and activity, including M-type potassium channels and glutamatergic transmission are altered by acute and repeated chronic alcohol exposure. We will also discuss how functional, pharmacological, and chemogenetic manipulation of the LHb affects ethanol drinking and psychiatric disorders occurring in animals withdrawn from chronic alcohol exposure. Conceivable mechanisms behind these effects and their potential as targets for therapies will also be discussed.

The role of the habenula in the transition from reward to misery in substance use and mood disorders

The habenula (Hb) is an evolutionary well-conserved structure located in the epithalamus. The Hb receives inputs from the septum, basal ganglia, hypothalamus, anterior cingulate and medial prefrontal cortex, and projects to several midbrain centers, most importantly the inhibitory rostromedial tegmental nucleus (RMTg) and the excitatory interpeduncular nucleus (IPN), which regulate the activity of midbrain monoaminergic nuclei. The Hb is postulated to play a key role in reward and aversion processing across species, including humans, and to be implicated in the different stages of transition from recreational drug intake to addiction and co-morbid mood disorders. The Hb is divided into two anatomically and functionally distinct nuclei, the lateral (LHb) and the medial (MHb), which are primarily involved in reward-seeking (LHb) and misery-fleeing (MHb) behavior by controlling the RMTg and IPN, respectively. This review provides a neuroanatomical description of the Hb, discusses preclinical and human findings regarding its role in the development of addiction and co-morbid mood disorders, and addresses future directions in this area.

The lateral habenula and the serotonergic system

The habenula (Hb) is an epithalamic structure differentiated into two nuclear complexes, medial (MHb) and lateral habenula (LHb). After decades of relative neglect, interest in the Hb resurged when it was demonstrated that LHb neurons play a key role in encoding disappointments and expectation of punishments. Consistent with such a role, the LHb has been implicated in a broad array of functions and pathologic conditions, notably in mechanisms of stress and pain, as well as in the pathophysiology of mood disorders. So far, the vast majority of research involving the LHb has focused on its role in regulating midbrain dopamine release. However, the LHb is also robustly interconnected in a reciprocal manner with a set of rostral serotonin (5-HT) nuclei. Thus, there is increasing evidence that the LHb is amply linked to the dorsal (DR) and median raphe nucleus (MnR) by a complex network of parallel topographically organized direct and indirect pathways. Here, we summarize research about the interconnections of the LHb with different subregions of the DR and MnR, as well as findings about 5-HT-dependent modulation of LHb neurons. Finally, we discuss the contribution of distinct LHb-raphe loops to stress and stress-related psychiatric disorders including anxiety and depression.

An emerging role for the lateral habenula in aggressive behavior

Inter-male aggression is an essential component of social behavior in organisms from insects to humans. However, when expressed inappropriately, aggression poses significant threats to the mental and physical health of both the aggressor and the target. Inappropriate aggression is a common feature of numerous neuropsychiatric disorders in humans and has been hypothesized to result from the atypical activation of reward circuitry in response to social targets. The lateral habenula (LHb) has recently been identified as a major node of the classical reward circuitry and inhibits the release of dopamine from the midbrain to signal negative valence. Here, we discuss the evidence linking LHb function to aggression and its valence, arguing that strong LHb outputs to the ventral tegmental area (VTA) and the dorsal raphe nucleus (DRN) are likely to play roles in aggression and its rewarding components. Future studies should aim to elucidate how various inputs and outputs of the LHb shape motivation and reward in the context of aggression.

Translating the Habenula-From Rodents to Humans

The habenula (Hb) is a central structure connecting forebrain to midbrain regions. This microstructure regulates monoaminergic systems, notably dopamine and serotonin, and integrates cognitive with emotional and sensory processing. Early preclinical data have described Hb as a brain nucleus activated in anticipation of aversive outcomes. Evidence has now accumulated to show that the Hb encodes both rewarding and aversive aspects of external stimuli, thus driving motivated behaviors and decision making. Human Hb research is still nascent but develops rapidly, alongside with the growth of neuroimaging and deep brain stimulation techniques. Not surprisingly, Hb dysfunction has been associated with psychiatric disorders, and studies in patients have established evidence for Hb involvement in major depression, addiction, and schizophrenia, as well as in pain and analgesia. Here, we summarize current knowledge from animal research and overview the existing human literature on anatomy and function of the Hb. We also discuss challenges and future directions in targeting this small brain structure in both rodents and humans. By combining animal data and human experimental studies, this review addresses the translational potential of preclinical Hb research.

Temporal variations in presynaptic release probability in the lateral habenula

Rhythmicity plays an important role in a number of biological systems. The habenular complex is reported to contain an intrinsic molecular clock and to show rhythmic expression of circadian clock genes and proteins including per2/PER2. In this study, we observed that there is a temporal rhythmicity in the presynaptic efficacy of the lateral habenula (LHb) neurons. We collected a substantial number of recordings at different time points of the day during the light phase. The frequency and amplitude of spontaneous excitatory transmission were increased in the afternoon compared to recordings performed in the morning. In addition, the paired-pulse ratio and the success rate of minimal stimulation were also significantly different depending on the time of the recording. We did not see any significant differences in recordings obtained from pyramidal neurons of the hippocampus in the same brain slices. Taken together, our data indicates that the LHb exhibits intrinsic temporal oscillation in basal neurotransmission and in presynaptic release probability. Given the rapidly growing interest on the function of the LHb, more careful examination of synaptic transmission in the LHb is thus required.

Ethanol drives aversive conditioning through dopamine 1 receptor and glutamate receptor-mediated activation of lateral habenula neurons

There has been increasing interest in the lateral habenula (LHb) given its potent regulatory role in many aversion-related behaviors. Interestingly, ethanol can be rewarding as well as aversive; we therefore investigated whether ethanol exposure alters pacemaker firing or glutamate receptor signaling in LHb neurons in vitro and also whether LHb activity in vivo might contribute to the acquisition of conditioned place aversion to ethanol. Surprisingly, in epithalamic slices, low doses of ethanol (1.4 mM) strongly accelerated LHb neuron firing (by ~60%), and ethanol's effects were much reduced by blocking glutamate receptors. Ethanol increased presynaptic glutamate release, and about half of this effect was mediated by dopamine subtype 1 receptors (D1Rs) and cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. In agreement with these findings, c-Fos immunoreactivity in LHb regions was enhanced after a single administration of a low dose of ethanol (0.25 g/kg i.p.). Importantly, the same dose of ethanol in vivo also produced strong conditioned place aversion, and this was prevented by inhibiting D1Rs or neuronal activity within the LHb. By contrast, a higher dose (2 g/kg) led to ethanol conditioned place preference, which was enhanced by inhibiting neuronal activity or D1Rs within the LHb and suppressed by infusing aminomethylphosphonic acid or the D1R agonist SKF38393 within the LHb. Our in vitro and in vivo observations show, for the first time, that ethanol increases LHb excitation, mediated by D1R and glutamate receptors, and may underlie a LHb aversive signal that contributes to ethanol-related aversion.