The behavioral relevance of a modular organization in the lateral habenula.

Behavioral strategies for survival rely on the updates the brain continuously makes based on the surrounding environment. External stimuli-neutral, positive, and negative-relay core information to the brain, where a complex anatomical network rapidly organizes actions, including approach or escape, and regulates emotions. Human neuroimaging and physiology in nonhuman primates, rodents, and teleosts suggest a pivotal role of the lateral habenula in translating external information into survival behaviors. Here, we review the literature describing how discrete habenular modules-reflecting the molecular signatures, anatomical connectivity, and functional components-are recruited by environmental stimuli and cooperate to prompt specific behavioral outcomes. We argue that integration of these findings in the context of valence processing for reinforcing or discouraging behaviors is necessary, offering a compelling model to guide future work.

Loss of microglial MCT4 leads to defective synaptic pruning and anxiety-like behavior in mice.

Microglia, the innate immune cells of the central nervous system, actively participate in brain development by supporting neuronal maturation and refining synaptic connections. These cells are emerging as highly metabolically flexible, able to oxidize different energetic substrates to meet their energy demand. Lactate is particularly abundant in the brain, but whether microglia use it as a metabolic fuel has been poorly explored. Here we show that microglia can import lactate, and this is coupled with increased lysosomal acidification. In vitro, loss of the monocarboxylate transporter MCT4 in microglia prevents lactate-induced lysosomal modulation and leads to defective cargo degradation. Microglial depletion of MCT4 in vivo leads to impaired synaptic pruning, associated with increased excitation in hippocampal neurons, enhanced AMPA/GABA ratio, vulnerability to seizures and anxiety-like phenotype. Overall, these findings show that selective disruption of the MCT4 transporter in microglia is sufficient to alter synapse refinement and to induce defects in mouse brain development and adult behavior.

Specialized astrocytes mediate glutamatergic gliotransmission in the CNS.

Multimodal astrocyte-neuron communications govern brain circuitry assembly and function1. For example, through rapid glutamate release, astrocytes can control excitability, plasticity and synchronous activity2,3 of synaptic networks, while also contributing to their dysregulation in neuropsychiatric conditions4-7. For astrocytes to communicate through fast focal glutamate release, they should possess an apparatus for Ca2+-dependent exocytosis similar to neurons8-10. However, the existence of this mechanism has been questioned11-13 owing to inconsistent data14-17 and a lack of direct supporting evidence. Here we revisited the astrocyte glutamate exocytosis hypothesis by considering the emerging molecular heterogeneity of astrocytes18-21 and using molecular, bioinformatic and imaging approaches, together with cell-specific genetic tools that interfere with glutamate exocytosis in vivo. By analysing existing single-cell RNA-sequencing databases and our patch-seq data, we identified nine molecularly distinct clusters of hippocampal astrocytes, among which we found a notable subpopulation that selectively expressed synaptic-like glutamate-release machinery and localized to discrete hippocampal sites. Using GluSnFR-based glutamate imaging22 in situ and in vivo, we identified a corresponding astrocyte subgroup that responds reliably to astrocyte-selective stimulations with subsecond glutamate release events at spatially precise hotspots, which were suppressed by astrocyte-targeted deletion of vesicular glutamate transporter 1 (VGLUT1). Furthermore, deletion of this transporter or its isoform VGLUT2 revealed specific contributions of glutamatergic astrocytes in cortico-hippocampal and nigrostriatal circuits during normal behaviour and pathological processes. By uncovering this atypical subpopulation of specialized astrocytes in the adult brain, we provide insights into the complex roles of astrocytes in central nervous system (CNS) physiology and diseases, and identify a potential therapeutic target.

Plasticity of neuronal dynamics in the lateral habenula for cue-punishment associative learning.

The brain's ability to associate threats with external stimuli is vital to execute essential behaviours including avoidance. Disruption of this process contributes instead to the emergence of pathological traits which are common in addiction and depression. However, the mechanisms and neural dynamics at the single-cell resolution underlying the encoding of associative learning remain elusive. Here, employing a Pavlovian discrimination task in mice we investigate how neuronal populations in the lateral habenula (LHb), a subcortical nucleus whose excitation underlies negative affect, encode the association between conditioned stimuli and a punishment (unconditioned stimulus). Large population single-unit recordings in the LHb reveal both excitatory and inhibitory responses to aversive stimuli. Additionally, local optical inhibition prevents the formation of cue discrimination during associative learning, demonstrating a critical role of LHb activity in this process. Accordingly, longitudinal in vivo two-photon imaging tracking LHb calcium neuronal dynamics during conditioning reveals an upward or downward shift of individual neurons' CS-evoked responses. While recordings in acute slices indicate strengthening of synaptic excitation after conditioning, support vector machine algorithms suggest that postsynaptic dynamics to punishment-predictive cues represent behavioral cue discrimination. To examine the presynaptic signaling in LHb participating in learning we monitored neurotransmitter dynamics with genetically-encoded indicators in behaving mice. While glutamate, GABA, and serotonin release in LHb remain stable across associative learning, we observe enhanced acetylcholine signaling developing throughout conditioning. In summary, converging presynaptic and postsynaptic mechanisms in the LHb underlie the transformation of neutral cues in valued signals supporting cue discrimination during learning.

Disruption of Astrocyte-Dependent Dopamine Control in the Developing Medial Prefrontal Cortex Leads to Excessive Grooming in Mice.

BACKGROUND: Astrocytes control synaptic activity by modulating perisynaptic concentrations of ions and neurotransmitters including dopamine (DA) and, as such, could be involved in the modulating aspects of mammalian behavior. METHODS: We produced a conditional deletion of the vesicular monoamine transporter 2 (VMAT2) specifically in astrocytes (aVMTA2cKO mice) and studied the effects of the lack of VMAT2 in prefrontal cortex (PFC) astrocytes on the regulation of DA levels, PFC circuit functions, and behavioral processes. RESULTS: We found a significant reduction of medial PFC (mPFC) DA levels and excessive grooming and compulsive repetitive behaviors in aVMAT2cKO mice. The mice also developed a synaptic pathology, expressed through increased relative AMPA versus NMDA receptor currents in synapses of the dorsal striatum receiving inputs from the mPFC. Importantly, behavioral and synaptic phenotypes were rescued by re-expression of mPFC VMAT2 and L-DOPA treatment, showing that the deficits were driven by mPFC astrocytes that are critically involved in developmental DA homeostasis. By analyzing human tissue samples, we found that VMAT2 is expressed in human PFC astrocytes, corroborating the potential translational relevance of our observations in mice. CONCLUSIONS: Our study shows that impairment of the astrocytic control of DA in the mPFC leads to symptoms resembling obsessive-compulsive spectrum disorders such as trichotillomania and has a profound impact on circuit function and behaviors.

A neural substrate for negative affect dictates female parental behavior.

Parental behaviors secure the well-being of newborns and concomitantly limit negative affective states in adults, which emerge when coping with neonatal distress becomes challenging. Whether negative-affect-related neuronal circuits orchestrate parental actions is unknown. Here, we identify parental signatures in lateral habenula neurons receiving bed nucleus of stria terminalis innervation (BNSTLHb). We find that LHb neurons of virgin female mice increase their activity following pup distress vocalization and are necessary for pup-call-driven aversive behaviors. LHb activity rises during pup retrieval, a behavior worsened by LHb inactivation. Intersectional cell identification and transcriptional profiling associate BNSTLHb cells to parenting and outline a gene expression in female virgins similar to that in mothers but different from that in non-parental virgin male mice. Finally, tracking and manipulating BNSTLHb cell activity demonstrates their specificity for encoding negative affect and pup retrieval. Thus, a negative affect neural circuit processes newborn distress signals and may limit them by guiding female parenting.

Partial MCT1 invalidation protects against diet-induced non-alcoholic fatty liver disease and the associated brain dysfunction.

BACKGROUND & AIMS: Non-alcoholic fatty liver disease (NAFLD) has been associated with mild cerebral dysfunction and cognitive decline, although the exact pathophysiological mechanism remains ambiguous. Using a diet-induced model of NAFLD and monocarboxylate transporter-1 (Mct1+/-) haploinsufficient mice, which resist high-fat diet-induced hepatic steatosis, we investigated the hypothesis that NAFLD leads to an encephalopathy by altering cognition, behaviour, and cerebral physiology. We also proposed that global MCT1 downregulation offers cerebral protection. METHODS: Behavioural tests were performed in mice following 16 weeks of control diet (normal chow) or high-fat diet with high fructose/glucose in water. Tissue oxygenation, cerebrovascular reactivity, and cerebral blood volume were monitored under anaesthesia by multispectral optoacoustic tomography and optical fluorescence. Cortical mitochondrial oxygen consumption and respiratory capacities were measured using ex vivo high-resolution respirometry. Microglial and astrocytic changes were evaluated by immunofluorescence and 3D reconstructions. Body composition was assessed using EchoMRI, and liver steatosis was confirmed by histology. RESULTS: NAFLD concomitant with obesity is associated with anxiety- and depression-related behaviour. Low-grade brain tissue hypoxia was observed, likely attributed to the low-grade brain inflammation and decreased cerebral blood volume. It is also accompanied by microglial and astrocytic morphological and metabolic alterations (higher oxygen consumption), suggesting the early stages of an obesogenic diet-induced encephalopathy. Mct1 haploinsufficient mice, despite fat accumulation in adipose tissue, were protected from NAFLD and associated cerebral alterations. CONCLUSIONS: This study provides evidence of compromised brain health in obesity and NAFLD, emphasising the importance of the liver-brain axis. The protective effect of Mct1 haploinsufficiency points to this protein as a novel therapeutic target for preventing and/or treating NAFLD and the associated brain dysfunction. IMPACT AND IMPLICATIONS: This study is focused on unravelling the pathophysiological mechanism by which cerebral dysfunction and cognitive decline occurs during NAFLD and exploring the potential of monocarboxylate transporter-1 (MCT1) as a novel preventive or therapeutic target. Our findings point to NAFLD as a serious health risk and its adverse impact on the brain as a potential global health system and economic burden. These results highlight the utility of Mct1 transgenic mice as a model for NAFLD and associated brain dysfunction and call for systematic screening by physicians for early signs of psychological symptoms, and an awareness by individuals at risk of these potential neurological effects. This study is expected to bring attention to the need for early diagnosis and treatment of NAFLD, while having a direct impact on policies worldwide regarding the health risk associated with NAFLD, and its prevention and treatment.

Cerebellar dopamine D2 receptors regulate social behaviors.

The cerebellum, a primary brain structure involved in the control of sensorimotor tasks, also contributes to higher cognitive functions including reward, emotion and social interaction. Although the regulation of these behaviors has been largely ascribed to the monoaminergic system in limbic regions, the contribution of cerebellar dopamine signaling in the modulation of these functions remains largely unknown. By combining cell-type-specific transcriptomics, histological analyses, three-dimensional imaging and patch-clamp recordings, we demonstrate that cerebellar dopamine D2 receptors (D2Rs) in mice are preferentially expressed in Purkinje cells (PCs) and regulate synaptic efficacy onto PCs. Moreover, we found that changes in D2R levels in PCs of male mice during adulthood alter sociability and preference for social novelty without affecting motor functions. Altogether, these findings demonstrate novel roles for D2R in PC function and causally link cerebellar D2R levels of expression to social behaviors.

Why stay lonely packaged in a synaptic vesicle when you can be in good company? A GABA-glutamate gathering.

Neurons can release multiple neurotransmitters. Are they packaged in segregated pools of vesicles or within the same ones? In this issue of Neuron, Kim et al., 2022, examined features of GABA-glutamate co-release at basal ganglia to habenula synapses.

Synaptic inhibition in the lateral habenula shapes reward anticipation.

The lateral habenula (LHb) supports learning processes enabling the prediction of upcoming rewards. While reward-related stimuli decrease the activity of LHb neurons, whether this anchors on synaptic inhibition to guide reward-driven behaviors remains poorly understood. Here, we combine in vivo two-photon calcium imaging with Pavlovian conditioning in mice and report that anticipatory licking emerges along with decreases in cue-evoked calcium signals in individual LHb neurons. In vivo multiunit recordings and pharmacology reveal that the cue-evoked reduction in LHb neuronal firing relies on GABAA-receptor activation. In parallel, we observe a postsynaptic potentiation of GABAA-receptor-mediated inhibition, but not excitation, onto LHb neurons together with the establishment of anticipatory licking. Finally, strengthening or weakening postsynaptic inhibition with optogenetics and GABAA-receptor manipulations enhances or reduces anticipatory licking, respectively. Hence, synaptic inhibition in the LHb shapes reward anticipation.

Reward and aversion encoding in the lateral habenula for innate and learned behaviours.

Throughout life, individuals experience a vast array of positive and aversive events that trigger adaptive behavioural responses. These events are often unpredicted and engage actions that are likely anchored on innate behavioural programs expressed by each individual member of virtually all animal species. In a second step, environmental cues, that are initially neutral, acquire value through the association with external sensory stimuli, and become instrumental to predict upcoming positive or negative events. This process ultimately prompts learned goal-directed actions allowing the pursuit of rewarding experience or the avoidance of a danger. Both innate and learned behavioural programs are evolutionarily conserved and fundamental for survival. Among the brain structures participating in the encoding of positive/negative stimuli and contributing to innate and learned behaviours is the epithalamic lateral habenula (LHb). The LHb provides top-down control of monoaminergic systems, responds to unexpected appetitive/aversive stimuli as well as external cues that predict the upcoming rewards or punishments. Accordingly, the LHb controls a number of behaviours that are innate (originating from unpredicted stimuli), and learned (stemming from predictive cues). In this review, we will discuss the progresses that rodent's experimental work made in identifying how LHb activity governs these vital processes, and we will provide a view on how these findings integrate within a complex circuit connectivity.

Mild stress accumulation limits GABAergic synaptic plasticity in the lateral habenula.

Animals can cope with isolated stressful situations without enduring long-term consequences. However, when exposure to stressors becomes recurrent, behavioural symptoms of anxiety and depression can emerge. Yet, the neuronal mechanisms governing responsivity to isolated stressor remain elusive. Here, we investigate synaptic adaptations following mild stress in the lateral habenula (LHb), a structure engaged in aversion encoding and dysfunctional in depression. We describe that neuronal depolarization in the LHb drives long-term depression of inhibitory, but not excitatory, synaptic transmission (GABA LTD). This plasticity requires nitric oxide and presynaptic GABAB receptors, leading to a decrease in probability of GABA release. Mild stressors such as brief social isolation, or exposure to novel environment in the company of littermates, do not alter GABA LTD. In contrast, GABA LTD is absent after mice experience a novel environment in social isolation. Altogether, our results suggest that LHb GABAergic plasticity is sensitive to stress accumulation, which could represent a threshold mechanism for long-term alterations of LHb function.

Deep Brain Stimulation of the Habenula: Systematic Review of the Literature and Clinical Trial Registries.

The habenula is a small bilateral epithalamic structure that plays a key role in the regulation of the main monoaminergic systems. It is implicated in many aspects of behavior such as reward processing, motivational behavior, behavioral adaptation, and sensory integration. A role of the habenula has been indicated in the pathophysiology of a number of neuropsychiatric disorders such as depression, addiction, obsessive-compulsive disorder, and bipolar disorder. Neuromodulation of the habenula using deep brain stimulation (DBS) as potential treatment has been proposed and a first successful case of habenula DBS was reported a decade ago. To provide an overview of the current state of habenula DBS in human subjects for the treatment of neuropsychiatric disorders we conducted a systematic review of both the published literature using PUBMED and current and past registered clinical trials using ClinicalTrials.gov as well as the International Clinical Trials Registry Platform. Using PRISMA guidelines five articles and five registered clinical trials were identified. The published articles detailed the results of habenula DBS for the treatment of schizophrenia, depression, obsessive-compulsive disorder, and bipolar disorder. Four are single case studies; one reports findings in two patients and positive clinical outcome is described in five of the six patients. Of the five registered clinical trials identified, four investigate habenula DBS for the treatment of depression and one for obsessive-compulsive disorder. One trial is listed as terminated, one is recruiting, two are not yet recruiting and the status of the fifth is unknown. The planned enrollment varies between 2 to 13 subjects and four of the five are open label trials. While the published studies suggest a potential role of habenula DBS for a number of indications, future trials and studies are necessary. The outcomes of the ongoing clinical trials will provide further valuable insights. Establishing habenula DBS, however, will depend on successful randomized clinical trials to confirm application and clinical benefit of this promising intervention.

Biophysical and synaptic properties of NMDA receptors in the lateral habenula.

Excitatory synaptic transmission in the lateral habenula (LHb), an evolutionarily ancient subcortical structure, encodes aversive stimuli and affective states. Habenular glutamatergic synapses contribute to these processes partly through the activation of AMPA receptors. Yet, N-methyl-d-aspartate receptors (NMDARs) are also expressed in the LHb and support the emergence of depressive symptoms. Indeed, local NMDAR blockade in the LHb rescues anhedonia and behavioral despair in rodent models of depression. However, the subunit composition and biophysical properties of habenular NMDARs remain unknown, thereby hindering their study in the context of mental health. Here, we performed electrophysiological recordings and optogenetic-assisted circuit mapping in mice, to study pharmacologically-isolated NMDAR currents in LHb neurons that receive innervation from different brain regions (entopeduncular nucleus, lateral hypothalamic area, bed nucleus of the stria terminalis, or ventral tegmental area). This systematic approach revealed that habenular NMDAR currents are sensitive to TCN and ifenprodil - drugs that specifically inhibit GluN2A- and GluN2B-containing NMDARs, respectively. Whilst these pharmacological effects were consistently observed across inputs, we detected region-specific differences in the current-voltage relationship and decay time of NMDAR currents. Finally, inspired by the firing of LHb neurons in vivo, we designed a burst protocol capable of eliciting calcium-dependent long-term potentiation of habenular NMDAR transmission ex vivo. Altogether, we define basic biophysical and synaptic properties of NMDARs in LHb neurons, opening new avenues for studying their plasticity processes in physiological as well as pathological contexts.

Neuronal adaptations in the lateral habenula during drug withdrawal: Preclinical evidence for addiction therapy.

The epithalamic lateral habenula (LHb) regulates monoaminergic systems and contributes to the expression of both appetitive and aversive behaviours. Over the past years, the LHb has emerged as a vulnerable brain structure in mental illnesses including addiction. Behavioural and functional evidence in humans and rodents provide substantial support for a role of LHb in the negative affective symptoms emerging during withdrawal from addictive substances. Multiple forms of cellular and synaptic adaptations that take hold during drug withdrawal within the LHb are causally linked with the emergence of negative affective symptoms. These results indicate that targeting drug withdrawal-driven adaptations in the LHb may represent a potential strategy to normalize drug-related behavioural adaptations. In the current review we describe the mechanisms leading to functional alterations in the LHb, as well as the existing interventions used to counteract addictive behaviours. Finally, closing this loop we discuss and propose new avenues to potentially target the LHb in humans in light of the mechanistic understanding stemming from pre-clinical studies. Altogether, we provide an overview on how to leverage cellular-level understanding to envision clinically-relevant approaches for the treatment of specific aspects in drug addiction.

Output-Specific Adaptation of Habenula-Midbrain Excitatory Synapses During Cocaine Withdrawal.

Projections from the lateral habenula (LHb) control ventral tegmental area (VTA) neuronal populations' activity and both nuclei shape the pathological behaviors emerging during cocaine withdrawal. However, it is unknown whether cocaine withdrawal modulates LHb neurotransmission onto subsets of VTA neurons that are part of distinct neuronal circuits. Here we show that, in mice, cocaine withdrawal, drives discrete and opposing synaptic adaptations at LHb inputs onto VTA neurons defined by their output synaptic connectivity. LHb axons innervate the medial aspect of VTA, release glutamate and synapse on to dopamine and non-dopamine neuronal populations. VTA neurons receiving LHb inputs project their axons to medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and lateral hypothalamus (LH). While cocaine withdrawal increases glutamate release from LHb onto VTA-mPFC projectors, it reduces presynaptic release onto VTA-NAc projectors, leaving LHb synapses onto VTA-to-LH unaffected. Altogether, cocaine withdrawal promotes distinct adaptations at identified LHb-to-VTA circuits, which provide a framework for understanding the circuit basis of the negative states emerging during abstinence of drug intake.

Stress undermines reward-guided cognitive performance through synaptic depression in the lateral habenula.

Weighing alternatives during reward pursuit is a vital cognitive computation that, when disrupted by stress, yields aspects of neuropsychiatric disorders. To examine the neural mechanisms underlying these phenomena, we employed a behavioral task in which mice were confronted by a reward and its omission (i.e., error). The experience of error outcomes engaged neuronal dynamics within the lateral habenula (LHb), a subcortical structure that supports appetitive behaviors and is susceptible to stress. A high incidence of errors predicted low strength of habenular excitatory synapses. Accordingly, stressful experiences increased error choices while decreasing glutamatergic neurotransmission onto LHb neurons. This synaptic adaptation required a reduction in postsynaptic AMPA receptors (AMPARs), irrespective of the anatomical source of glutamate. Bidirectional control of habenular AMPAR transmission recapitulated and averted stress-driven cognitive deficits. Thus, a subcortical synaptic mechanism vulnerable to stress underlies behavioral efficiency during cognitive performance.

In DA Club of Reinforcement: Glutamate, It's Your Birthday.

Dopamine release guides reward encoding, but the contribution of glutamate remains unclear. In this issue of Neuron, Zell et al. leverage the genetic ablation of dopamine synthesis from midbrain VGluT2 neurons to assess how glutamate shapes positive reinforcement.